No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Comparison of the Permeability Surface Product (PS) of the Blood Capillary Wall in Skeletal Muscle Tissue of Various Species and In Vitro Porous Membranes Using Hydrophilic Drugs
Abstract
The aim of this study was twofold: Firstly, to compare the PS, the permeability surface product or organ clearance, of various hydrophilic molecules in the muscle blood capillary wall of different animal species, including humans and, secondly, to design an in vitro diffusion cell to identify similar PS values using the respective membranes. The flux rates of solutes with a wide range of molecular weights measured across porous membranes (Millipore(R) VM (MVM), and Nadir(R) UFC (NUFC)) was associated with high reproducibility (<+/-10% SD). The findings were as follows: (1) For the first time ever it was demonstrated that the log molecular weight dependency (between 180 and 10,000 Dalton) of the log of the PS product of the animal muscle capillary wall (slope = -0.51 +/- 0. 16, and a regression coefficient r(2) = 0.90) reported in the literature was similar to that observed by various authors in humans (slope = -0.50 +/- 0.25 r(2) = 0.77). (2) PS of the MVM membrane with a surface area of 3.8 cm(2) recorded for the newly developed diffusion cell ranged at approximately 50% below (slope -0.58 +/- 0. 12, r(2) = 0.85) the value observed in the in vivo human experiments after intramuscular injection. No linear relationship was established for the NUFC membrane due to solute exclusion effects of the membrane. (3) The diameter of the inner donor chamber (resembling the interstice between the muscle fiber membrane and the capillary wall membrane) was calculated to be sufficiently small to eliminate the possibility of creating a statistically significant diffusion barrier to the test membranes, which was corroborated by theoretical modeling. The structure of the in vitro diffusion cell prevents any significant contribution from the unstirred water layer (UWL) to overall resistance, thus reflecting in vivo diffusion properties of hydrophilic solutes deposited intramuscularly to enter the systemic circulation. It may be concluded, that MVM membrane in the vitro diffusion cell mimics the in vivo blood capillary PS for hydrophilic solutes of up to 10,000 Da.
... With a surface area of 70 cm 2 /g [27], we deduce that the permeability between the normal tissue and the blood is 4.3 × 10 -8 cm/ s. A similar value was found using the permeability-surface area product using Schmittmann and Rohr's study [28]. Because there is paucity of experimental data on VEGF-dependence of macromolecular permeability [29,30], we consider a permeability range from 4 × 10 -9 to 4 × 10 -6 cm/s for sensitivity analyses; this range includes the 2–3 fold VEGF-dependent increase in permeability reported in [29,30]. ...
... dothelial cell is qualitatively similar to that in the healthy tissue (Figure 5B). In these tissues, most VEGFR1 and most NRP1 are present as the VEGFR1- NRP1 complex. Most VEGFR2 is unbound. The majority of ligated VEGFR2 is in the VEGF 165 -VEGFR2-NRP1 complex . A ten-fold increase in NRP1 density in the tumor causes uncomplexed NRP1 to dominate. [28]; vascular permeability for VEGF k p = 4 × 10 -8 cm/s; VEGFR1 = 10,000, VEGFR2 = 10,000, NRP1 = 10,000 molecules/endothelial cell. Simulation of a 3-hour two-legged knee extension that has been shown to upregulate VEGF mRNA by about 3.5 fold for at least 6 hours [13]. A, Free VEGF concentration in tissue and blood. Six hours after upregu ...
... This configuration corresponds to low vascular permeability in healthy tissue inFigure 4D. The diseased compartment represents a 4-cm diameter tumor; vascular permeability of the healthy tissue, = 4 × 10 -8 cm/s; vascular permeability in the tumor = 4 × 10 -7 cm/s; VEGF 165 secretion rate in tumor q D = 0.076 molecule/cell/s; VEGF plasma clearance c V = 0.0206 min -1 [28]; NRP1 = 100,000 molecules/endothelial cell in the tumor. C, At higher vascular permeability in the healthy tissue = 4 × 10 - 6 cm/s, the net flow of VEGF entering the bloodstream from the healthy tissue increases to 0.580%. ...
Angiogenesis is a process by which new capillaries are formed from pre-existing blood vessels in physiological (e.g., exercise, wound healing) or pathological (e.g., ischemic limb as in peripheral arterial disease, cancer) contexts. This neovascular mechanism is mediated by the vascular endothelial growth factor (VEGF) family of cytokines. Although VEGF is often targeted in anti-angiogenic therapies, there is little knowledge about how its concentration may vary between tissues and the vascular system. A compartment model is constructed to study the VEGF distribution in the tissue (including matrix-bound, cell surface receptor-bound and free VEGF isoforms) and in the blood. We analyze the sensitivity of this distribution to the secretion rate, clearance rate and vascular permeability of VEGF.
We find that, in a physiological context, VEGF concentration varies approximately linearly with the VEGF secretion rate. VEGF concentration in blood but not in tissue is dependent on the vascular permeability of healthy tissue. Model simulations suggest that relative VEGF increases are similar in blood and tissue during exercise and return to baseline within several hours. In a pathological context (tumor), we find that blood VEGF concentration is relatively insensitive to increased vascular permeability in tumors, to the secretion rate of VEGF by tumors and to the clearance. However, it is sensitive to the vascular permeability in the healthy tissue. Finally, the VEGF distribution profile in healthy tissue reveals that about half of the VEGF is complexed with the receptor tyrosine kinase VEGFR2 and the co-receptor Neuropilin-1. In diseased tissues, this binding can be reduced to 15% while VEGF bound to the extracellular matrix and basement membranes increases.
The results are of importance for physiological conditions (e.g., exercise) and pathological conditions (e.g., peripheral arterial disease, coronary artery disease, cancer). This mathematical model can serve as a tool for understanding the VEGF distribution in physiological and pathological contexts as well as a foundation to investigate pro- or anti-angiogenic strategies.
... 20 One aim of this study was to determine the permeability surface product or organ clearance for melphalan during ILI. The rate and extent of systemic bioavailability of most hydrophilic solutes is primarily restricted by the use of the paracellular (porous) pathway, 21 and there is a marked molecular weight dependency. 21 In humans, this relationship is demonstrated by PS values of 3.31, 0.9 and 0.05 ml/min per 100 g tissue for Cr-ethylene diamine tetra-acetic acid (EDTA) (molecular weight 341), inulin (molecular weight 3600) and albumin (molecular weight 36000), respectively. ...
... The rate and extent of systemic bioavailability of most hydrophilic solutes is primarily restricted by the use of the paracellular (porous) pathway, 21 and there is a marked molecular weight dependency. 21 In humans, this relationship is demonstrated by PS values of 3.31, 0.9 and 0.05 ml/min per 100 g tissue for Cr-ethylene diamine tetra-acetic acid (EDTA) (molecular weight 341), inulin (molecular weight 3600) and albumin (molecular weight 36000), respectively. 22±24 The PS values calculated with the help of the mathematical model varied, with values ranging from 1.6±8.1 ml/min per 100 g infused tissue, with a mean of 4.3 AE 1.7 ml/min per 100 g (Table 2). ...
Isolated limb infusion (ILI) is an attractive, less complex alternative to isolated limb perfusion (ILP). It has a lower morbidity in treating localized recurrences and in transit metastases of the limb for tumours such as melanoma, Merkel cell tumour and Kaposi's sarcoma, allowing administration of high concentrations of cytotoxic agent to the affected limb under hypoxic conditions. Melphalan is the preferred cytotoxic agent for the treatment of melanoma by ILP or ILI. We report pharmacokinetic data from 12 patients treated by ILI for tumours of the limb in Brisbane. The kinetics of drug distribution in the limb was calculated using a two-compartment vascular model, where both tissue and infusate act as well-stirred compartments. Analysis of melphalan concentrations in the perfusate during ILI showed good agreement between the values measured and the concentrations predicted by the model. Recirculation and wash-out flow rates, tissue concentrations and the permeability surface area product (PS) were calculated. Correlations between the PS value and the drug concentrations in the perfusate and tissue were supported by the results. These data contribute to a better understanding of the distribution of melphalan during ILI in the limb, and offer the opportunity to optimize the drug regimen for patients undergoing ILI.
... Although interspecies differences in body composition may likewise lead to differences in blood-tissue drug exchange, less variability may be anticipated when comparisons are considered relative to lean body mass. Schmittmann and Rohr (31) suggested that the PS of skeletal muscle capillaries tends to be very similar across several species. These investigators observed that in humans and animal species, there is virtually the same negative linear correlation between PS and molecular weight. ...
Depending upon the drug and drug delivery platform, species-specific physiological differences can lead to errors in the interspecies extrapolation of drug performance. This manuscript provides an overview of the species-specific physiological variables that can influence the performance of parenteral dosage forms such as in situ forming delivery systems, nanoparticles, microspheres, liposomes, targeted delivery systems, lipophilic solutions, and aqueous suspensions. Also discussed are those factors that can influence the partitioning of therapeutic compounds into tumors, the central nervous system and the lymphatics. Understanding interspecies differences in the movement and absorption of molecules is important to the interpretation of data generated through the use of animal models when studying parenteral drug delivery.
... Experiments demonstrate an inverse relationship between microvascular permeability and the size of a molecule (molecular weight or Stokes-Einstein radius; refs. [22][23][24]. Therefore, in the absence of active transport, large proteins, such as anti-VEGF agents (150 kDa for bevacizumab and 110 kDa for aflibercept), should extravasate relatively slowly. ...
Vascular endothelial growth factor (VEGF) is one of the most potent cytokines targeted in antiangiogenic therapies. Bevacizumab, a recombinant humanized monoclonal antibody to VEGF, is being used clinically in combination with chemotherapy for colorectal, non-small cell lung and breast cancers, and as a single agent for glioblastoma and is being tested for other types of cancer in numerous clinical trials. It has been reported that the intravenous injection of bevacizumab leads to an increase of plasma VEGF concentration in cancer patients. The mechanism responsible for this counterintuitive increase has not been elucidated, although several hypotheses have been proposed. We use a multiscale systems biology approach to address this problem. We have constructed a whole-body pharmacokinetic model comprising three compartments: blood, normal tissue, and tumor tissue. Molecular interactions among VEGF-A family members, their major receptors, the extracellular matrix, and an anti-VEGF ligand are considered for each compartment. Diffusible molecules extravasate, intravasate, are removed from the healthy tissue through the lymphatics, and are cleared from the blood.
... The predictions for MIN concentrations in the different organs are shown in Figure 4, for the CCND1'04 (Wish et al., 1950;Crispens, 1975a) 589.09 (Gjedde and Gjedde, 1980;Sarin et al., 1990;Lindstedt and Schaeffer, 2002;Kober et al., 2004) Arterial 0.56 0.56 (Wish et al., 1950;Crispens, 1975a) Same as venous compartment Lung 0.04 0.11 0.14 (Plata and Murphy, 1972;Crispens, 1975b) 589.09 (Peng et al., 2001) Stomach 0.02 0.10 0.12 (Barnett and Widdowson, 1965) 11.66 (Stott et al., 1983) Spleen 0.02 0.06 0.08 (Tsai et al., 2002) 5.45 (Gerlowski and Jain, 1983;Baxter et al., 1994 (Chakrabarti et al., 2007) 17.67 (Raczka et al., 1983) Figure 5. Overall, the physiologically based model can satisfactorily describe the time course behavior of the tissue, and in particular the tumor, distribution of the variants of MINs with one common set of transport and partition values. A sensitivity analysis of the fit in response to the value of the permeability surface area product, based on the work by Schmittmann and Rohr (2000), shown in the Supplemental Material, shows that the results are robust to variations in the PS T values. ...
Disease detection and management might benefit from external imaging of disease gene mRNAs. Previously we designed molecular imaging nanoparticles (MINs) based on peptide nucleic acids complementary to cancer gene mRNAs. The MINs included contrast agents and analogs of insulin-like growth factor 1 (IGF-1). Analysis of MIN tumor uptake data showed stronger binding in tumors than in surrounding tissues. We hypothesized that MINs with an IGF-1 analog stay in circulation by binding to IGF-binding proteins. To test that hypothesis, we fit the tissue distribution results of several MINs in xenograft-bearing mice to a physiological pharmacokinetics model. Fitting experimental tissue distribution data to model-predicted mass transfer of MINs from blood into organs and tumors converged only when the parameter for MINs bound to circulating IGF-binding proteins was set to 10%-20% of the injected MIN dose. This result suggests that previous mouse imaging trials used more MINs than necessary. This prediction can be tested by a ramp of decreasing doses.
... The corresponding permeability-surface area product (PS) were then determined from a set of theoretical curves for PS vs. a E [110]. Finally, assuming a capillary surface area (S) of 70 cm 2 /g muscle tissue [111], the molecular species-specific basal permeabilities (k p = PS/S) were obtained (Table 4). The posture and activity-dependent surface area recruitment factors (Table 5) were approximated as follows. ...
Vascular endothelial growth factor (VEGF), through its activation of cell surface receptor tyrosine kinases including VEGFR1 and VEGFR2, is a vital regulator of stimulatory and inhibitory processes that keep angiogenesis--new capillary growth from existing microvasculature--at a dynamic balance in normal physiology. Soluble VEGF receptor-1 (sVEGFR1)--a naturally-occurring truncated version of VEGFR1 lacking the transmembrane and intracellular signaling domains--has been postulated to exert inhibitory effects on angiogenic signaling via two mechanisms: direct sequestration of angiogenic ligands such as VEGF; or dominant-negative heterodimerization with surface VEGFRs. In pre-clinical studies, sVEGFR1 gene and protein therapy have demonstrated efficacy in inhibiting tumor angiogenesis; while in clinical studies, sVEGFR1 has shown utility as a diagnostic or prognostic marker in a widening array of angiogenesis-dependent diseases. Here we developed a novel computational multi-tissue model for recapitulating the dynamic systemic distributions of VEGF and sVEGFR1. Model features included: physiologically-based multi-scale compartmentalization of the human body; inter-compartmental macromolecular biotransport processes (vascular permeability, lymphatic drainage); and molecularly-detailed binding interactions between the ligand isoforms VEGF(121) and VEGF(165), signaling receptors VEGFR1 and VEGFR2, non-signaling co-receptor neuropilin-1 (NRP1), as well as sVEGFR1. The model was parameterized to represent a healthy human subject, whereupon we investigated the effects of sVEGFR1 on the distribution and activation of VEGF ligands and receptors. We assessed the healthy baseline stability of circulating VEGF and sVEGFR1 levels in plasma, as well as their reliability in indicating tissue-level angiogenic signaling potential. Unexpectedly, simulated results showed that sVEGFR1 - acting as a diffusible VEGF sink alone, i.e., without sVEGFR1-VEGFR heterodimerization--did not significantly lower interstitial VEGF, nor inhibit signaling potential in tissues. Additionally, the sensitivity of plasma VEGF and sVEGFR1 to physiological fluctuations in transport rates may partially account for the heterogeneity in clinical measurements of these circulating angiogenic markers, potentially hindering their diagnostic reliability for diseases.
... 18 It is unknown whether these pore sizes are different in canines, but capillary permeability in the muscle tissue was found similar across mammalian species. 19 In addition, microperoxidase (molecular weight of 1900, and molecular span of 2 nm) was found to diffuse readily through the small pore of muscle capillary in rats. 20 Since the molecular span of many NMBAs is $2 nm between both quaternary amines, 21 transcapillary exchange should not be a ratelimiting step for cisatracurium. ...
Pharmacokinetic/pharmacodynamic (PK/PD) parameters of neuromuscular blocking agents (NMBAs) are generally assumed to be dose-independent. To our knowledge, there are very few clinical reports where the PK/PD parameters of a NMBA were derived separately for each dose group during a formal dose-ranging study. The primary objective of this study was to challenge a potential dose-dependency of cisatracurium PK/PD parameters by conducting a well-controlled experimental study.
Eight dogs were anaesthetized with pentobarbital and mechanically ventilated. Two doses of cisatracurium (1.5xED(95) and 6xED(95)) were administered in a randomized cross-over design after an appropriate washout period. Neuromuscular function was monitored using train-of-four (TOF) stimulation. Arterial blood was sampled continuously for the first minute after cisatracurium injection and at frequent intervals thereafter. Cisatracurium plasma concentrations were determined by high performance liquid chromatography analysis. PK/PD modelling of individual data sets was performed with NONMEM using a non-parametric approach and a descriptive sigmoid E(max) model.
Cisatracurium PKs were linear over the dose range studied. Using non-parametric PK/PD analysis, mean values for plasma-effect compartment equilibration delay (k(e0)) were 0.0600 vs 0.1278 min(-1) (P<0.05) and sensitivity (EC(50)) were 323 vs 235 ng ml(-1) (P<0.05) for the high and low doses, respectively.
A dose-dependent effect on the PK/PD parameters of cisatracurium has important clinical implications as an accurate estimate of the EC(50) is desirable. PK/PD parameters derived after intubating bolus doses of cisatracurium would be more reliable.
... Consistent with expectation, the M r dependence (M r -0.5 ) was similar to that observed in aqueous media (M r -0.37 ) and in good agreement with the observed molecular weight dependence of the permeability surface area product of animal muscle capillary wall (M r -0.51 ) and of human muscle (M r -0.50 ). 32 To clarify the log P dependence of D MCL , we normalized for differences in size of the molecules by calculating the expected diffusion coefficient at the same M r as TPZ (D′ MCL ), using the M r dependence determined in eq 1. These size-corrected values, plotted in Figure 4, showed an increase from approximately 0.3 × 10 -6 cm 2 s -1 for hydrophilic molecules to a 15-fold higher value for the most lipophilic examples. ...
The extravascular diffusion of antitumor agents is a key determinant of their therapeutic activity, but the relationships between physicochemical properties of drugs and their extravascular transport are poorly understood. It is well-known that drug lipophilicity plays an important role in transport across biological membranes, but the net effect of lipophilicity on transport through multiple layers of tumor cells is less clear. This study examines the influence of lipophilicity (measured as the octanol-water partition coefficient P) on the extravascular transport properties of the hypoxic cytotoxin tirapazamine (TPZ, 1) and a series of 13 neutral analogues, using multicellular layers (MCLs) of HT29 human colon carcinoma cells as an in vitro model for the extravascular compartment of tumors. Flux of drugs across MCLs was determined using diffusion chambers, with the concentration-time profile on both sides of the MCL measured by HPLC. Diffusion coefficients in the MCLs (D(MCL)) were inversely proportional to M(r)(0.5) (M(r), relative molecular weight), although this was a minor contributor to differences between compounds over the narrow M(r) range investigated. Differences in lipophilicity had a larger effect, with a sigmoidal dependence of D(MCL) on log P. Correcting for M(r) differences, lipophilic compounds (log P > 1.5) had ca. 15-fold higher D(MCL) than hydrophilic compounds (log P < -1). Using a pharmacokinetic/pharmacodynamic (PK/PD) model in which diffusion in the extravascular compartment of tumors is considered explicitly, we demonstrated that hypoxic cell kill is very sensitive to changes in extravascular diffusion coefficient of TPZ analogues within this range. This study shows that simple monosubstitution of TPZ can alter log P enough to markedly improve extravascular transport and activity against target cells, especially if rates of metabolic activation are also optimized.
We describe a problem of brain image registration during image- guided procedures using a framework of solid mechanics. We show that the registration of pre-operative MRIs on sparse intra-operative images of the brain deformed due to craniotomy can be reasonably described as pure displacement or displacement - zero traction problems of solid mechanics. We discuss available solution methods for such problems and suggest using explicit dynamics finite element scheme. We present a computational example using clinical data confirming the appropriateness and accuracy of proposed methods.
Radionuclide bone imaging using polynuclear 99mTc complexes of bisphosphonates is the most common clinical practice in nuclear medicine. However, the improvement in the contrast between normal and osteogenic bone regions has been required. Herein we reported a new 99mTc-labeled compound considering the increased vascular permeability of osteogenic region. We selected penta-D-Asp as both a targeting motif to hydroxyapatite (HA) and a molecular size modifier, and two penta-D-Asp molecules were conjugated with the two carboxylate residues of ethylene dicysteine (EC) selected as the 99mTc chelating moiety to prepare EC-[(D-Asp)5]2. The molecular size, HA binding and pharmacokinetics of 99mTc-EC-[(D-Asp)5]2 in normal mice and model rats bearing osteogenic tumor were compared to those of 99mTc-MDP and 99mTc-EC with one (D-Asp)5 motif, 99mTc-EC-(D-Asp)5. The molecular size of 99mTc-EC-[(D-Asp)5]2 was higher than that of 99mTc-MDP and 99mTc-EC-(D-Asp)5 when determined by permeability of the 99mTc-compounds through a membrane filter (10 kDa). The HA binding of 99mTc-EC-[(D-Asp)5]2 was higher than and similar to that of 99mTc-EC-(D-Asp)5 and 99mTc-MDP. 99mTc-EC-[(D-Asp)5]2 exhibited significantly lower accumulation in normal bone of mice than did 99mTc-MDP. In osteogenic tumor bearing model rats, 99mTc-EC-[(D-Asp)5]2 accumulated in the osteogenic and normal bone region similar to and lower than 99mTc-MDP, respectively. Although further studies including the chain length of D-Asp are required, these findings indicated that the present chemical design of 99mTc-labeled probe would be applicable to develop 99mTc-labeled probes for selective imaging of osteogenic bone region as well as develop therapeutic agents using therapeutic radionuclides such as 90Y, 177Lu, 186Re or 188Re and cytotoxic agents to osteogenic bone tumor region.
Mathematical modelling and computer simulation have proved tremendously successful in engineering. One of the greatest challenges for mechanists is to extend the success of computational mechanics to fields outside traditional engineering, in particular to biology, biomedical sciences, and medicine. The proposed workshop will provide an opportunity for computational biomechanics specialists to present and exchange opinions on the opportunities of applying their techniques to computer-integrated medicine. For example, continuum mechanics models provide a rational basis for analysing biomedical images by constraining the solution to biologically reasonable motions and processes. Biomechanical modelling can also provide clinically important information about the physical status of the underlying biology, integrating information across molecular, tissue, organ, and organism scales. The main goal of this workshop is to showcase the clinical and scientific utility of computational biomechanics in computer-integrated medicine. © Springer Science+Business Media, LLC 2010. All rights reserved.
One of the greatest challenges for mechanical engineers is to extend the success of computational mechanics to fields outside traditional engineering, in particular to biology, biomedical sciences, and medicine. This book is an opportunity for computational biomechanics specialists to present and exchange opinions on the opportunities of applying their techniques to computer-integrated medicine. Computational Biomechanics for Medicine: Deformation and Flow collects the papers from the Medical Image Computing and Computer Assisted Intervention conference (MICCAI 2011) dedicated to research in the field of medical image computing and computer assisted medical interventions. The topics covered include: medical image analysis, image-guided surgery, surgical simulation, surgical intervention planning, disease prognosis and diagnostics, injury mechanism analysis, implant and prostheses design, and medical robotics. © Springer Science+Business Media New York 2012. All rights reserved.
The pharmacokinetic performance of a matrix system for transdermal beta-estradiol (E(2)) delivery after multiple consecutive dosing in postmenopausal women undergoing hormone replacement therapy was investigated. The E(2) plasma profiles determined during the third application in 16 postmenopausal women were compared with results obtained in a published clinical study using the same patch in 24 postmenopausal women without E(2) pretreatment; they were compared with a theoretical diffusion/pharmacokinetic model. A conventional theoretical model with constant model parameter (CPM) obtained from in vitro mass balance experiments in a Franz cell type set up described successfully the transdermal E(2) bioavailability parameter AUC(0-96h) (4341.9 +/- 1513.1; calculated 4250.8) and C(average) (45.0 +/- 13.2; calculated 41.2). Also, experimentally, there was no significant drop in E(2) plasma values after patch removal and reapplication; this was corroborated by calculations. Accumulation of E(2) did not occur when several patches were applied consecutively over a period of 3 weeks. Steady state was achieved following application of the first patch. However, the differences between recorded E(2) plasma profiles and theoretical results detected at specific measurement points cannot be explained by the CPM model. Experimentally obtained plasma profiles were always lower in the morning and higher in the evening than predicted on the basis of the model. Measurements of in vivo skin temperature in the postmenopausal women showed oscillating temperature profiles in the form of a cosinor function: The temperature mesor of untreated postmenopausal women was 34.8 degrees C with an acrophase at 17.0 o'clock (95% CI: 14.30-19.30) and an amplitude of +/- 0.4 degrees C (p = 0.1). During the application of the patch the average temperature next to a patch rose 0.3 degrees C, which was statistically significant (p = 0.1). In the skin under the application of the matrix patch a mesor temperature was detected as 35.6 degrees C with an amplitude of +/- 0.5 degrees C with an acrophase at 17.51 o'clock (95% CI: 14.30-21.00) (p = 0.05). The temperature period was 24 h for all measurements and the maximum temperature was observed at about 16.30 h, and a minimum at about 5.00 h. A linear dependency was detected in in vitro experiments between the log of E(2) permeability and the temperature for stripped skin, epidermis/dermis layer, as well as for the matrix. Modeling of E(2) plasma profiles with oscillating diffusion coefficients (ODM1) with a sine wave function results in this equation: D(1) = D(0x) + Da(x).sin(k.t). D(0x) is the diffusion coefficient determined at 35.6 degrees C, k is 1/24 h, D(a) is the diffusion coefficient of the temperature amplitude, h is hour, and x stands for the respective diffusion layer. It was shown that the experimental E(2) plasma profile variations are more pronounced than can simply be explained by skin temperature variations alone (ODM1 model). A simplex fit with an oscillating diffusion coefficient in the form of a sine wave function for the stratum corneum (ODM2 model) resulted in a temperature amplitude of 1.1 degrees C, about twice as high as was determined in the in vivo measurements (ODM2 model). Therefore, other circadian parameterlike blood flow might superimpose the temperature profile. The improvement in data analysis by incorporating oscillating diffusion coefficients (ODM1 and 2) over CPM was judged from a comparison of experimental data with the calculated plasma profiles with the AIC, Akaikes model selection criterion, which allows ranking between models because it is independent of the scaling of the data points. ODM1 and ODM2 improved the data analysis over CPM by allowing better calculation of experimental C(max), t(max), the time to reach to C(max), and the fluctuation, f. No difference between CPM, ODM1, or ODM2 was found for the bioavailability parameter C(average) and AUC(0-96h).
We have tested the hypothesis that transit through the interstitial fluid, rather than across cell membranes, is rate limiting for amino acid uptake from blood into muscle in human subjects. To quantify muscle transmembrane transport of naturally occurring amino acids, we developed a novel 4-pool model that distinguishes between the interstitial and intracellular fluid compartments. Transport kinetics of phenylalanine, leucine, lysine, and alanine were quantified using tracers labeled with stable isotopes. The results indicate that interstitial fluid is a functional compartment insofar as amino acid kinetics are concerned. In the case of leucine and alanine, transit between blood and interstitial fluid was potentially rate limiting for muscle amino acid uptake and release in the postabsorptive state. For example, in the case of leucine, the rate of transport between blood and interstitial fluid compared with the corresponding rate between interstitial fluid and muscle was 247 +/- 36 vs. 610 +/- 95 nmol.min(-1).100 ml leg(-1), respectively (P < 0.05). Our results are consistent with the process of diffusion governing transit from blood to interstitial fluid without selectivity, and of specific amino acid transport systems with varying degrees of efficiency governing transit from interstitial fluid to muscle. These results imply that changes in factors that affect the transit of amino acids from blood through interstitial fluid, such as muscle blood flow or edema, could play a major role in controlling the rate of muscle amino acid uptake.
The regional distribution of skeletal muscle blood flow was measured during postischemic reactive hyperemia using Gd-DTPA contrast-enhanced (CE) MRI. The release of an occlusive thigh cuff was used to deliver a step-input of contrast concentration that was coincident with the onset of reactive hyperemia. A first-order tracer kinetic equation was used to estimate the unidirectional influx constant, Ki (ml/100 g/min), and the distribution volume of Gd-DTPA in the tissue, v(e), from T1-weighted images acquired with saturation recovery (SR) steady-state free precession (SSFP) and spoiled gradient-echo (SPGR) protocols. The capillary permeability surface (PS) area increased significantly during reactive hyperemia, which facilitated rapid extraction of Gd-DTPA during the first pass. Regional muscle group studies from 11 normal volunteers yielded blood flow (Ki) values of 108.3 +/- 34.1 ml/100 g/min in the gastrocnemius, 184.3 +/- 41.3 ml/100 g/min in the soleus, and 122.4 +/- 34.4 ml/100 g/min in the tibialis anterior. The distribution volumes (v(e)) in the corresponding muscle groups were respectively 8.3% +/- 2.1%, 9.3% +/- 1.9%, and 7.9% +/- 1.8% from the kinetic model, and 8.8% +/- 2.4%, 9.1% +/- 1.9%, and 7.2% +/- 1.4% from tissue relaxometry studies. Bulk blood flow studies in the same volunteers using phase-contrast velocimetry (popliteal artery) yielded significantly lower flow values, but with a correlation coefficient R2 = 0.62 and P = 0.004.
Anatomic, velocimetric, and brain motion MRI scans were combined with a computational fluid dynamics model to investigate cerebrospinal fluid (CSF) mixing in the third cerebral ventricle of a healthy male adult. It was found that advection dominates over diffusion in most of the third ventricle. Three zones where diffusion plays an important role in the mixing process were identified. One of these zones, consisting of recessus infundibulus, recessus opticus and the adjacent regions up to commissura anterior, is likely to exist in the general population. We hypothesize that this zone may act as a buffer to flatten concentration peaks of pituitary gland hormones released into the CSF of the third ventricle. We further hypothesize that this zone may facilitate the communication between hypothalamus and the pituitary gland through the third ventricle cerebrospinal fluid by prolonging residence times of the communicated hormones.
Analyses of data on the transcapillary exchange and cellular uptake in the normal heart have generally been based on the assumption that local membrane conductances and volumes of distribution are everywhere the same. The question is whether such an assumption is justified in view of the marked (sixfold) heterogeneity of local blood flows per gram tissue. The method was to estimate both flow and capillary membrane permeability-surface area products (PS) locally in the heart. For each of five dogs running on a sloped treadmill, the deposition of tracer microspheres and of [131I]iodophenylpentadecanoic acid (IPPA), after left atrial injection, was determined in 256 pieces of left ventricular myocardium by killing the animals at approximately 100 s after radiotracer injection. A hydraulic occluder stopped the flow to a portion of the myocardium supplied by the left circumflex coronary artery 30 s before tracer injection. Regional flows ranged from 0.1 to 7.0 ml.g-1.min-1. IPPA extractions ranged from 20 to 49%. Using the known flows, we assumed the applicability of an axially distributed blood-tissue exchange model to estimate the PS for the capillary (PSc) and the parenchymal cell. It was impossible to explain the data if the PSc values for membrane transport were uniform throughout the organ. Rather, the only reasonable descriptors of the data required that local PSc values increase with local flow, almost in proportion. Current methods of analysis using data based on deposition methods need to be revised to take into account the near proportionality of PS to flow for at least some substrates.
Kinetic analysis of residue and outflow curves of y-emitting indicators such as chromium-51-EDTA and iodide-131-thalamate from skeletal muscle gives the possibility to determine the extraction fraction and the plasma flow, and from these two values the capillary diffusion capacity can be calculated (Sejrsen 1970, preliminary report). This is possible both for the transport from blood to tissue and from tissue to blood. This alternative method has been compared in the autoperfused cat gastrocnemius preparation with the indicator diffusion method based on venous registration of a diffusible test indicator and an intravascular reference indicator (Chinard et at. 1955, Crone 1963). The results of the five independent measurements show good agreement. Calculation of the permeability Pd based on a capillary surface area of 7 000 cm2/100 g of tissue gives values of 1.05–10-5, 1.10–10-5, and 1.16–10-5 cm/s, which is in agreement with results obtained by other investigators. The permeability was equal in both directions, and thus the capillary membrane seems to function as a symmetrical membrane. Using an area of 5 000 cm2/100 g which presumably is more realistic at the plasma flow range used gives Pd values around 1.5–10-6 cm/s. The effective pore area is calculated to constitute 1/50 000 of the capillary surface area. Calculation of volumes of distribution in the muscle tissue gave intravascular plasma volumes of 1.8 to 2.0 ml/100 g, an extravascular volume of 12.4 and 15.2 ml/100 g and a final monoexponential component constituting a compartment of 5.4 and 7.0 ml/100 g from residue and venous curves, respectively. The last mentioned compartment constitutes nearly 50 per cent of the extravascular space, and it is suggested, that it is located inside the sarcoplasmic reticulum, which anatomically constitutes about 50 per cent of the interstitial space. The total area of contact between the longitudinal and the transversal tubules in this subsystem, which is the membrane of the lateral saccus, is estimated to about 6 times the capillary surface area at a plasma flow of 15 ml/100 g min which gives a permeability about 60 times lower for this membrane compared to the capillary membrane.
Capillary permeability of 51Cr-EDTA in the canine myocardium was determined by applying: (A) the single injection, external registration method, and (B) the local tissue clearance method, to the intact dog heart of open-chest anesthetized dogs. (A) 51Cr-EDTA was administered into the left anterior descending coronary artery as a bolus injection, and the response curve was recorded by external registration. The capillary diffusion capacity (the permeability-surface area product) for 51Cr-EDTA amounted to 32.5 ml/100 g·min at a capillary extraction of 0.40 and a plasma flow of 73.3 ml/100 g·min. The diffusional permeability coefficient of 51Cr-EDTA was calculated to 1.08 × 10-5 cm/s, which indicates that the permeability of the capillaries in the myocardium for 51Cr-EDTA is similar to that of continuous capillaries in other tissues. (B) 51Cr-EDTA (3–100 μl) was injected at a depth of 5 mm into the myocardium of the left ventricular free wall and the residue curve was recorded. The capillary extraction, as determined by the tissue clearance method, was calculated to values 6 times smaller than determined with the single injection, residue detection method. This unreasonably low extraction was probably due to methodological errors inherent to the tissue clearance technique.
1. As the majority of drug molecules are relatively small and lipophilic, capillary uptake in those regions with a continuous endothelium, such as the heart, has generally been regarded as taking place by flow-limited transcellular capillary transport. Little attention has been paid to myocardial paracellular capillary transport of hydrophilic drugs, despite efficient transport of hydrophilic solutes, such as sucrose, inulin and EDTA, by this route.
2. The paracellular pathway is formed by the cleft between adjacent endothelial cells and its permeability properties are determined by the tight junction, a region of restricted diameter within the cleft. Paracellular capillary permeability is modulated by circulating macromolecules, such as albumin, by an unknown mechanism thought to involve the intra-endothelial cell Ca2+ concentration.
3. Studies in perfused rat heart have shown that capillary permeability of quinidine does not vary when perfusate pH is varied from 7.0-8.0, suggesting that capillary transport involves the ionized as well as unionized moiety. Addition of albumin to the perfusion medium reduced quindine capillary permeability, which is consistent with paracellular transport of quinidine ions.
4. Paracellular transport increases in capillary inflammation. Therefore, if there is significant paracellular transport of hydrophilic drugs, an increase in uptake of such drugs in areas of inflamed myocardium associated with acute myocardial infarction would be expected to result.
Atom/fragment contribution values, used to estimate the log octanol–water partition coefficient (log P) of organic compounds, have been determined for 130 simple chemical substructures by a multiple linear regression of 1120 compounds with measured log P values. An additional 1231 compounds were used to determine 235 “correction factors” for various substructure orientations. The log P of a compound is estimated by simply summing all atom/fragment contribution values and correction factors occurring in a chemical structure. For the 2351 compound training set the correlation coefficient (r2) for the estimated vs measured log P values is 0.98 with a standard deviation (SD) of 0.22 and an absolute mean error (ME) of 0.16 log units. This atom/fragment contribution (AFC) method was then tested on a separate validation set of 6055 measured tog P values that were not used to derive the methodology and yielded an r2 of 0.943, an SD of 0.408, and an ME of 0.31. The method is able to predict tog P within ±0.8 log units for over 96% of the experimental dataset of 8406 compounds. Because of the simple atom/fragment methodology, “missing fragments” (a problem encountered in other methods) do not occur in the AFC method. Statistically, it is superior to other comprehensive estimation methods.
The permeability characteristics of cat small intestine capillaries were studied using both osmotic transient and lymph-to-plasma protein concentration ratio (L/P) techniques. A vascularly perfused segment of ileum in which superior mesenteric arterial pressure, blood flow, superior mesenteric venous pressure, and lymph flow were monitored was used for both determinations. Intestinal volume was continuously monitored during the osmotic transient experiments. Hyperosmotic solutions of various test solutes were infused at a constant rate directly into the superior mesenteric artery after the establishment of an isovolumetric state. Osmotic conductances were calculated from the initial rate of volume loss and the calculated plasma osmotic pressure change. Reflection coefficients for the various solutes were determined from the osmotic conductance and filtration coefficients within the same preparation. The predicted equivalent pore radius for intestinal capillaries using this approach is 200-350 λ. In another group of cats, the L/P ratio for total plasma proteins was determined prior to and following graded increases in intestinal venous outflow pressure (10-30 mm Hg). At the greatest dilution of lymph proteins, the minimum osmotic reflection coefficient for total plasma proteins (σ(p)) was estimated assuming σ(p)= 1 - (C 1/C(p)), and values of σ(p) between 0.87 and 0.92 were acquired. Minimum osmotic reflection coefficients for total plasma proteins were then determined during osmotic transients induced by hypertonic glucose (20-70 mM). Values of σ(p) between 0.56 and 0.74 were acquired during the osmotic transients. The results of these studies suggest that intestinal capillaries are relatively impermeable to endogenous plasma proteins. The discrepancy between the L/P ratio and osmotic transient techniques results, at least in part, from an increased capillary permeability during the osmotic transient.
P aaske , W. P. Capillary permeability in cutaneous tissue . Acta physiol. scand. 1976. 98 . 492–499.
Capillary permeability in cutaneous tissue was determined by the single injection, external counting method. A rabbit abdominal skin preparation was developed. After local vasodilatation, 5 μ1 (400 μCi ) ⁵¹ Cr‐EDTA was administered intraarterially by a special bolus injection technique, and the response curve was recorded by external registration. The capillary diffusion capacity (the permeability‐surface area product) for ⁵¹ Cr‐EDTA amounted to 3.7 m1/100 g·min at a capillary extraction of 0.59 and a plasma flow of 4.8 m1/100 gemin. The permeability coefficient for ⁵¹ Cr‐EDTA in cutaneous tissue was calculated to 0.9.10 ‐5 cm/s assuming a capillary surface area of 70 cm ² /g.
Only a very limited number of clinical studies have been reported on the measurement of endothelial permeability to hydrophilic solutes (molecular weight less than 6000 Daltons) that normally gain free access to the extravascular space. Because of high extraction efficiencies into the extravascular space, the transfer rates of small solutes of molecular weight about 500 Daltons, like 99Tcm DTPA, are perfusion-dependent as well as diffusion-dependent. We describe non-invasive techniques for measurement of clearance and extraction fraction of 99Tcm DTPA into the extravascular space of the resting forearm using a scintillation probe, from which we then calculated permeability surface area (PS) product. Our values for extraction fraction of about 0.5 and for PS product of about 3 ml per minute per 100 ml tissue are comparable to the values reported in the literature for resting skeletal muscle using more invasive techniques.
The steady-state dialysis kinetics in buffer, erythrocyte suspension and muscle have been analyzed by clearance theory in the microdialysis study. "Tube" model has been demonstrated to be a useful model to relate the dialysis clearance, CLD, the dialysis flow rate, F, and the permeability rate constant, PA, for microdialysis employing the transcranial type microdialysis probe. The effective dialysis coefficient (Rd), defined as the ratio of the in vivo PA and in vitro PA, was introduced to account for the differences between in vivo and in vitro microdialyses. The Arrhenius plot of the antipyrine permeability rate constant presented a single straight line in the range of 15-37 degrees C with an activation energy of 5.49 kcal/mol. A fairly good correlation was observed between the reciprocal of the permeability rate constant and the root of the molecular weight in the range of 18-1039. On the contrary, the molecular weight and the plasma membrane permeability were not determinant factors for Rd value determined in the erythrocyte suspension (Rd,erythrocyte), while the interstitial fluid space (100-hematocrit)% of erythrocyte suspension plays a dominant factor to change Rd,erythrocyte. The in vivo permeability rate constant was determined in the muscle for [3H]water, [14C]urea, antipyrine and [14C]sucrose under the steady-state condition. No significant difference of Rd in muscle tissue was demonstrated for these four model substances. By using the Rd value, a hypothetical equation has been proposed to relate the concentration in the dialysate and the interstitial fluid at steady-state.
To measure local capillary permeability to lipid-insoluble substances, we developed a microscopic tissue clearance method. It has been theoretically predicted that, when a tissue is stained with a dye by suffusing its solution around the tissue, subsequent concentration changes of the dye in the tissue due to adequate capillary flow washout takes a monoexponential time course of which decay constant is equal to the local capillary permeability surface area product (PS) per unit tissue volume. Therefore, when the capillary surface area (S) is calculated from the open capillary density in the adjacent tissue, it is possible to estimate the local permeability (P). This method was applied to the rabbit tenuissimus muscle under maximum vasodilatation, using Cr-EDTA (M.W. = 341) as a tracer. The correlation coefficient of the obtained clearance curves to the monoexponential decay was averaged to be 0.958 +/- 0.029 in 12 curves. The calculated values of Cr-EDTA permeability, 6.0 +/- 0.7 x 10(-6) cm/s, fairly well agreed with those reported for sucrose (M.W. = 342). It was concluded that this method is useful to measure local capillary permeability of small lipid-insoluble tracers.
In 11 anesthetized pigs, the left anterior descending coronary artery (LAD) was cannulated and pump perfused with blood before and during maximum adenosine vasodilation. For LAD plasma flows (F) ranging from 0.42 to 3.6 ml.min-1.g perfused tissue-1, we injected radiolabeled microspheres to measure heterogeneity and used the multiple indicator-dilution method to measure permeability-surface area product (PS) for EDTA. Heterogeneity of flow from the LAD was expressed as relative dispersion (RD) = standard deviation of flow/mean flow. Values of RD, corrected for tissue sample size using fractal theory, ranged from 13 to 87%, approaching 16-35% at high F. We developed a "variable-recruitment model" of regional heterogeneous capillary transport to correct PS for flow heterogeneity and capillary surface area recruitment. Values of PS ranged from 0.14 to 0.96 ml.min-1.g-1. Accounting for heterogeneity increased PS values by 0-18% compared with homogeneous values. Results revealed PS to be proportional to flow up to F = 1.5-2.1 ml.min-1.g-1 and then was constant at higher flows. The initial increase of PS with F may be due to capillary recruitment. When full recruitment is reached, PS becomes independent of F. We conclude that flow heterogeneity is significant but not readily predictable in the pig myocardium and that the use of microspheres to correct indicator-dilution data for flow heterogeneity improves the interpretation of multiple-tracer studies, particularly when tracers are used to study interventions that may alter flow distribution.
The aim was to examine capillary permeability of 131I-albumin in the normal, resting human forearm.
A bolus injection of 131I-albumin was injected into the brachial artery, and the residue function was measured by external registration over the forearm tissues. The results were analysed by indicator kinetic "black box" theory using the single injection, residue detection method which is based on indicator diffusion principles.
Seven normal volunteers participated in the study.
The mean capillary extraction fraction of 131I-albumin was 0.023 (SD 0.0056), n = 7, at a mean plasma flow rate of 2.1 (0.34) ml.100 g-1.min-1. At an estimated capillary surface area of 70 cm.g-1 the permeability coefficient was 11.10(-8) cm.s-1.
According to the theories of restricted diffusion and equivalent pores the results are compatible with an equivalent pore radius estimate of 113 A using additional previously published results from experiments with 51Cr-EDTA.
This study was aimed at assessing the pharmacokinetics of a single dose of theophylline solution (aminophylline 480 mg) administered by intramuscular (i.m.) route to 16 subjects (age 28-61 years, body weight 52-75 kg). The same dose was given a week apart by oral route in fasting conditions. The intraindividual comparison shows that by i.m. route the rate, but not the extent, of absorption may be somewhat lower. From the first hour on, until hour 3-4 after i.m. dosing, all subjects achieved a safe and effective serum theophylline concentration (9-17 mg/liter). This suggests that, although usually not recommended, self-administration of a single dose of aminophylline by i.m. route may temporarily help theophylline-responsive patients in distress, when other routes of administration are not available and facilities for intensive conventional treatment are lacking.
Capillary permeability of [14C]inulin and [51Cr]EDTA was examined in human forearm in five healthy, subjects by indicator diffusion technique. Injections of, initially [125I]albumin and [14C]inulin, and after 30 min resting, of [125I]albumin and [51Cr]EDTA, were given in a brachial artery. During light exercise of the forearm, blood was sampled in 2-s periods from a deep cubital vein primarily draining muscles. The plasma flow rate, calculated as the dose of [125I]albumin in the injectate divided by the area under the curve for the venous concentration of 125I, was, on average, 8.5 ml min-1 100 g-1 forearm. Assuming [125I]albumin is a partially permeable tracer, a correction for extraction of albumin was performed. This gave extraction fractions of 0.107 +/- 0.015 (mean +/- SEM) for [14C]inulin and 0.377 +/- 0.033 for [51Cr]EDTA, respectively. The capillary permeability surface area product per 100 g tissue (CDC) was for [14C]inulin 0.90 +/- 0.19, and for [51Cr]EDTA 3.31 +/- 0.38 ml min-1 100 g-1 forearm. The average of the ratios of the CDC values of [51Cr]EDTA to those of [14C]inulin, 4.0 +/- 0.5, is significantly higher than the corresponding ratio between the measured free diffusion coefficients in water at 37 degrees C, 3.07 +/- 0.002 (N = 36 and 17, respectively). This indicates that there is some degree of restriction for [14C]inulin (MW 5200) relative to [51Cr]EDTA (MW 340.2) and it points to an 'equivalent pore radius estimate' of about 160 A in human muscle capillaries.
The transport of radiolabelled albumin from tissue to blood was measured with an external detection technique in isolated, maximally vasodilated rat skeletal muscles. Initially, rat hindlimbs were perfused with albumin-serum solutions containing [99mTc]albumin for at least 2 h, during which time the tracer accumulated interstitially. The accumulated tracer albumin was then washed out over a period of 1 h, using a tracer-free, otherwise identical, perfusate. The wash-out curve was multi-exponential and the last 30-min period was used to calculate the turnover rate constant (k), which was 7.5 x 10(-4) min-1, (+/- 0.7 x 10(-4), n = 5). Moreover, if albumin was assumed to be distributed homogeneously within the interstitium, with a distribution volume (Vi) of 10 ml 100 g-1, a tissue-to-blood clearance of albumin (ClT-B) of 0.0075 ml min-1 100 g-1 could be calculated. By this approach ClT-B is probably slightly overestimated, but is still only 30% of the clearance from blood to tissue (ClB-T), as determined in several previous studies under similar conditions. Thus, transcapillary passage of albumin is highly asymmetrical, being at least three times greater from blood to tissue than in the opposite direction. This is in agreement with the concept of the capillary walls being composed of two populations of functional pores, where macromolecules are transported from blood to tissue mainly by convection through large pores, even at low filtration rates.
Osmotic reflection coefficients (sigma) for a variety of solutes ranging from NaCl to albumin were determined in perfused maximally vasodilated rat hindquarters employing the osmotic transient method (Vargas & Johnson 1964). Measurements were performed at high flows and using short tubings with small volumes. Intracapillary solute concentrations of the osmotic transients were measured or estimated for solutes of the size of inulin or smaller. The PS for Cr-EDTA and cyanocobalamine were determined repeatedly in half of the experiments using an on-line modification of the single injection (indicator diffusion) method (Rippe & Stage 1978) and capillary filtration coefficients (CFC or LpS) were followed in all experiments. The capillary osmotic reflection coefficient was determined to 0.05 for NaCl, to 0.08 for sucrose, to 0.39 for inulin, to 0.57 for myoglobin and to 0.87 for albumin. These reflection coefficients were compatible with a 'small pore radius' of approximately 40 A (slit width (w) of approximately 50 A) according to modern hydrodynamic theories for the reflection coefficient and the parallel transcapillary pathway hypothesis. The best fit of the osmotic transient data to current theories for the reflection coefficient occurred if the major portion (86-87%) of the hydraulic conductivity (Lp) was accounted for by this paracellular 'small pore' (slit) pathway and if 3.0-4.1% of Lp could be ascribed to a transcellular pathway (sigma approximately I) while the remaining fraction (10%) of Lp was accounted for by a non-selective paracellular pathway (sigma approximately o); that is, by 'large pores'.
1. Mica sheets were made into membranes by a process of bombardment with fission fragments from a U235 source and subsequent etching with hydrofluoric acid. Pores formed by this process were essentially straight through the membrane, extremely uniform in size and elliptical in cross-section. On eight of these membranes, with pore radii ranging from 45 to 300 Å, air flow, water flow, and diffusion rates for a graded series of 7 solutes were measured. From measurements of the diffusion rate of mostly non-electrolytes, with radii between 2.5 and 22.5 Å, the true hindrance effect on diffusion within pores was determined.
1. The skeletal muscle capillary permeability to 131I− and [51Cr]EDTA was studied in the exercising human forearm of seventeen non-diabetics and seventeen long-term juvenile diabetics with clinical signs of diabetic angiopathy.
2. The permeability data was obtained using the indicator diffusion technique, which implies intra-arterial injection and collection of multiple venous samples during the first passage of the tracers through the organ. The capillary permeability was expressed as the capillary diffusion capacity, CDC, i.e. the maximum unidirectional flux of tracer over the capillary membrane of 100 g of muscle per unit concentration difference over the capillary.
3. The average values for both CDCI and CDCCr-EDTA were found to be significantly increased ( P < 0·001) in long-term diabetics as opposed to non-diabetics: CDC1 = 18·7 (SD, 3·2), 13·2 (SD, 1·0); CDCCr-EDTA = 6·74 (SD, 1·09), 3·73 (SD, 0·30)mol 100g−1 min−1 per mol ml−1 respectively.
4. The findings indicate an increased skeletal muscle capillary permeability in long-term diabetics which most probably is due to an increased permeability per unit capillary surface area.
A new method is described for the assessment of the dissolution behavior of solid dosage forms. The method, which is based on the mass transfer between solid and liquid phase in an exchange column, is shown to avoid some disadvantages of the commonly used beaker methods employing fixed liquid volumes. Its usefulness is demonstrated by results obtained with nondisintegrating and uniform granules of benzoic acid in water. The influence of various external parameters, such as liquid flow rate, cell cross-sectional area, amount of material, and particle diameter, is found to agree with theory and literature data. Because of its reproducibility and the absence of arbitrary external parameters, the method seems to be useful for a meaningful study of dissolution kinetics.
The in vitro release of medroxyprogesterone acetate from a silicone rubber matrix was studied. A nonlinear dependence of release rate upon medroxyprogesterone acetate concentration within the matrix was found. Based upon a model system, equations were derived to explain this behavior and to include other parameters which may influence the release rate. Since the model, in part, is dependent upon a receding medroxyprogesterone acetate layer within the matrix, a photograph depicting depletion zones as a function of time is presented. In contrast to the T. Higuchi model for drug release, this model includes the boundary diffusion layer. Comparison of the two models suggested that when the boundary layer was considered, a better fit of experimental data to theory was found. The applicability of the model to an in vivo system is discussed. This study has suggested that the partition coefficient, diffusion coefficients, medroxyprogesterone acetate concentration within the polymer, and agitation conditions play important roles in the release process.
1. Calculations of capillary permeability of perfused organs from indicator diffusion data are reviewed, and reconsidered in terms of two kinds of organ heterogeneity of capillary extraction.
3. In the presence of realistic back-diffusion and recirculation of the indicator, the foregoing expression for PS is converted into a secure lower bound of the organ PS. Using data from four previously published studies of the brain and of the heart as examples, it is shown that this lower bound approaches and sometimes exceeds the widely differing PS values estimated by four existing methods based on more restrictive assumptions. In particular, PS estimates from the initial extraction (Eo method) fall below the lower bound in three of the four cases.
4.The new results are valid without restriction on the magnitudes of the extraction fractions, so that they provide legitimate PS estimates even for highly diffusible indicators.
5. Quantitative hypotheses needed for more complete modelling of indicator diffusion are reviewed, and elaborated in terms of both deterministic and probabilistic models.
Permeability-surface area products (PS) for 51Cr-EDTA, [3H]mannitol, [14C]urea, and 22Na were measured in isolated, perfused, lower hindlimb muscles of anesthetized cats. The tracers were added stepwise to the arterial inflow, and Evans blue-labeled albumin was the reference indicator. At flow rates > 70 ml.min-1.100 g-1, the PS values (+/- SE) were 5.0 +/- 0.5, 7.6 +/- 1.2, 17.8 +/- 1.4, and 21.0 +/- 1.4 (n = 7, 4, 5, and 3 animals, respectively). The ratio of simultaneous PS measurements of mannitol and urea was 0.42 +/- 0.02 (n = 9), significantly less than the free diffusion coefficient ratio (0.49), indicating the presence of restricted diffusion. PS measurements were also made during osmotic flow (4.2 +/- 0.6 ml.min-1.100 g-1) induced by 20% NaCl. The data clearly showed that osmotic transients did not alter small solute permeability. Pore models were used to show that the PS data and previously reported reflection coefficient data were consistent with a single description of the capillary wall. This model contained a water-only pathway containing 60% of the hydraulic capacity and an extracellular route modeled by pores of 4 nm radius having 21,000 cm of area per unit membrane thickness (A/delta x).
The solvent-drag reflection coefficient (sigma f) was measured from plasma disappearance (integral-mass balance method) for native albumin and four fluorescent solutes of radii from 2 to 16 nm in the isolated, plasma-perfused cat hindlimb preparation. The data for the smallest solutes were measured > 2 h after tracer addition and at high filtration rates to avoid underestimation of sigma f due to tracer diffusion. A two-pore model was fit (small-pore and large-pore radii, approximately 3.5 and 23 nm, respectively, 84% of hydraulic capacity in small pores) to these data using an objective computer-based estimation procedure. In the model, membrane sigma f was determined by flow weighting the sigma f values for the two pathways. Also, the phenomenon of volume circulation among the pathways was included. In different limbs, the permeability-surface area (PS) product was measured for the smallest solute, alpha-lactalbumin, from its perfusate-disappearance transient and a linear diffusion model. The PS value estimated was 0.11 +/- 0.026 (95% confidence limits) ml.min-1 times 100 g muscle-1. These PS values were found to be coincident with those predicted using parameter sets derived from the multiparameter 95% confidence space consistent with the two-pore model fits. The two-pore model also closely predicted PS data for small solutes from other studies in skeletal muscle; however, it failed to adequately describe small-molecule transport data from osmotic transient studies. It was necessary to add a water-exclusive pathway (40% of total hydraulic capacity) to account for these latter data; however, the predictions with this addition were still consistent with the data measured in the present study. We conclude that pore models can describe both macromolecular and small solute reflection coefficient and PS data in skeletal muscle.
The goal of this investigation was to demonstrate whether the intrinsic flux of a drug diffusing across a membrane mounted in a flow-through diffusion cell may be accurately and easily determined by accounting for the accumulation in the receiver chamber.
Mathematical modeling, applied to transdermal diffusion, was used to calculate receiver concentration data for single layer and bilayer membranes. The data were interpreted using two apparent flux values, Japp1 and Japp2. Japp1 has been used extensively in the literature, but did not account for accumulation in the receiver. Japp2 did take the accumulation into consideration.
The results confirm that, generally, Japp1 values were not accurate estimates of the intrinsic flux. Japp2 values were significantly more accurate, especially prior to the maximum in receiver concentration.
Japp2 was an accurate measurement of intrinsic flux over the entire experimental time period, except at time zero. It was more accurate because it accounted for solute accumulation in the receiver compartment. The accuracy of the Japp2 approximation was practically independent of receiver volume, flow rate and donor volume. For very slowly permeating drugs, or a very small receiver volume combined with a high flow rate, the Japp1 estimate accurately reflected the intrinsic flux. Early time data were required to properly account for accumulation in the receiver cell. If such data were not available, the inverse Laplace method of determining intrinsic flux was preferable to the Japp2 calculation.
A method is described for studying transcapillary diffusion of K ⁴² in isolated perfused muscles of dogs. Blood flow and arteriovenous K ⁴² differences are measured and blood-tissue clearance calculated by the Fick principle. A theoretical relation between blood flow and blood-tissue clearance is developed for a uniform circulation characterized by a constant permeability—surface area product (PS). The experimental observations conform reasonably closely to prediction. However, systematic variation in measured PS product with changes in blood flow and vascular resistance indicate that the capillary circulation is not uniform.