[Show abstract][Hide abstract] ABSTRACT: BACKGROUND AND PURPOSE Although isoprenaline (ISO) is known to activate several ion currents in mammalian myocardium, little is known about the role of action potential morphology in the ISO-induced changes in ion currents. Therefore, the effects of ISO on action potential configuration, L-type Ca(2+) current (I(Ca) ), slow delayed rectifier K(+) current (I(Ks) ) and fast delayed rectifier K(+) current (I(Kr) ) were studied and compared in a frequency-dependent manner using canine isolated ventricular myocytes from various transmural locations. EXPERIMENTAL APPROACH Action potentials were recorded with conventional sharp microelectrodes; ion currents were measured using conventional and action potential voltage clamp techniques. KEY RESULTS In myocytes displaying a spike-and-dome action potential configuration (epicardial and midmyocardial cells), ISO caused reversible shortening of action potentials accompanied by elevation of the plateau. ISO-induced action potential shortening was absent in endocardial cells and in myocytes pretreated with 4-aminopyridine. Application of the I(Kr) blocker E-4031 failed to modify the ISO effect, while action potentials were lengthened by ISO in the presence of the I(Ks) blocker HMR-1556. Both action potential shortening and elevation of the plateau were prevented by pretreatment with the I(Ca) blocker nisoldipine. Action potential voltage clamp experiments revealed a prominent slowly inactivating I(Ca) followed by a rise in I(Ks) , both currents increased with increasing the cycle length. CONCLUSIONS AND IMPLICATIONS The effect of ISO in canine ventricular cells depends critically on action potential configuration, and the ISO-induced activation of I(Ks) - but not I(Kr) - may be responsible for the observed shortening of action potentials.
Full-text · Article · May 2012 · British Journal of Pharmacology
[Show abstract][Hide abstract] ABSTRACT: The aim of this work was to study antagonistic interactions between the effects of various types of Ca(2+) channel blockers and isoproterenol on the amplitude of L-type Ca(2+) current in canine ventricular cells.
Whole-cell version of the patch clamp technique was used to study the effect of isoproterenol on Ca(2+) current in the absence and presence of Ca(2+) channel-blocking agents, including nifedipine, nisoldipine, diltiazem, verapamil, CoCl(2) and MnCl(2) .
Five micromolar Nifedipine, 1 μM nisoldipine, 10 μM diltiazem, 5 μM verapamil, 3 mM CoCl(2) and 5 mM MnCl(2) evoked uniformly a 90-95% blockade of Ca(2+) current in the absence of isoproterenol. Isoproterenol (100 nM) alone increased the amplitude of Ca(2+) current from 6.8 ± 1.3 to 23.7 ± 2.2 pA/pF in a reversible manner. Isoproterenol caused a marked enhancement of Ca(2+) current even in the presence of nifedipine, nisoldipine, diltiazem and verapamil, but not in the presence of CoCl(2) or MnCl(2) .
The results indicate that the action of isoproterenol is different in the presence of organic and inorganic Ca(2+) channel blockers. CoCl(2) and MnCl(2) were able to fully prevent the effect of isoproterenol on Ca(2+) current, while the organic Ca(2+) channel blockers failed to do so. This has to be born in mind when the effects of organic Ca(2+) channel blockers are evaluated either experimentally or clinically under conditions of increased sympathetic tone.
[Show abstract][Hide abstract] ABSTRACT: Receptor-mediated changes in intracellular cyclic AMP concentration play critical role in the autonomic control of the heart, including regulation of a variety of ion channels via mechanisms involving protein kinase A, EPAC, or direct actions on cyclic nucleotide gated ion channels. In case of any ion channel, the actual signal transduction cascade can be identified by using properly modified cAMP derivatives with altered binding and activating properties. In this study we focus to structural modifications of cAMP resulting in specific activator and blocking effects on PKA or EPAC. Involvement of the cAMP-dependent signal transduction pathway in controlling rapid delayed rectifier K(+ ) current was studied in canine ventricular myocytes using these specific cAMP analogues. Adrenergic stimulation increased the density of I(Kr) in canine ventricular cells, which effect was mediated by a PKA-dependent but EPAC-independent pathway. It was also shown that intracellular application of large concentrations of cAMP failed to fully activate PKA comparing to the effect of isoproterenol, forskolin, or PDE-resistant cAMP derivatives. This difference was fully abolished following inhibition of phosphodiesterase by IBMX. These results are in line with the concept of compartmentalized release, action, and degradation of cAMP within signalosomes.
Full-text · Article · Aug 2011 · Current Medicinal Chemistry
[Show abstract][Hide abstract] ABSTRACT: Class 3 antiarrhythmic agents exhibit reverse rate-dependent lengthening of the action potential duration (APD), i.e. changes in APD are greater at longer than at shorter cycle lengths. In spite of the several theories developed to explain this reverse rate-dependency, its mechanism has been clarified only recently. The aim of the present study is to elucidate the mechanisms responsible for reverse rate-dependency in mammalian ventricular myocardium. Action potentials were recorded using conventional sharp microelectrodes from human, canine, rabbit, guinea pig, and rat ventricular myocardium in a rate-dependent manner. Rate-dependent drug-effects of various origin were studied using agents known to lengthen or shorten action potentials allowing thus to determine the drug-induced changes in APD as a function of the cycle length. Both drug-induced lengthening and shortening of action potentials displayed reverse rate-dependency in human, canine, and guinea pig preparations, but not in rabbit and rat myocardium. Similar results were obtained when repolarization was modified by injection of inward or outward current pulses in isolated canine cardiomyocytes. In contrast to reverse rate-dependence, drug-induced changes in APD well correlated with baseline APD values (i.e. that measured before the superfusion of drug or injection of current) in all of the preparations studied. Since the net membrane current (I(net)), determined from the action potential waveform at the middle of the plateau, was inversely proportional to APD, and consequently to cycle length, it is concluded that that reverse rate-dependency may simply reflect the inverse relationship linking I(net) to APD. In summary, reverse rate-dependency is an intrinsic property of drug action in the hearts of species showing positive APD - cycle length relationship, including humans. This implies that development of a pure K(+) channel blocking agent without reverse rate-dependent effects is not likely to be successful.
No preview · Article · Aug 2011 · Current Medicinal Chemistry
[Show abstract][Hide abstract] ABSTRACT: Background and objective: In spite of the widespread clinical use of articaine and ropivacaine there is little information available on the effects of these drugs on myocardial Ca2+ handling. In the present study, therefore, the concentration-dependent effects of articaine and ropivacaine on the components of intracellular Ca2+ handling were studied and compared in canine ventricular myocardium.
Methods: Contractility was measured in ventricular trabeculae, [Ca2+]i transients were recorded from electrically stimulated isolated ventricular myocytes loaded with the calcium-sensitive dye fura-2, L-type Ca2+ current was recorded under whole cell patch clamp conditions, and the release and reuptake of Ca2+ was monitored in sarcoplasmic reticulum vesicles.
Results: Articaine and ropivacaine caused a reversible and concentration-dependent decrease in amplitude of the [Ca2+]i transient (EC50 = 87.4 ± 12 and 99.3 ± 17 μmol l−1, respectively), which was congruent with the reduction obtained for contractility (EC50 = 73.7 ± 10 and 72.8 ± 14 μmol l−1, respectively). No significant change in diastolic [Ca2+]i was found. L-type Ca2+ current was significantly reduced by articaine and ropivacaine with EC50 values of 327 ± 56 and 263 ± 67 μmol l−1, respectively. Neither Ca2+ release and Ca2+ uptake nor the ATPase activity of the sarcoplasmic reticulum vesicles was altered by articaine or ropivacaine at concentrations less than 200 μmol l−1. In summary, articaine and ropivacaine caused no significant changes at the therapeutically relevant concentrations of the micromolar range. No significant differences between the effects of articaine and ropivacaine on contractility, [Ca2+]i transients, L-type Ca2+ current, and sarcoplasmic reticulum Ca2+ release and uptake were observed.
Conclusions: Under conditions of normal application both articaine and ropivacaine are free of cardiodepressant effects; however, a negative inotropic action can be anticipated in cases of accidental intravenous injection or overdose. The observed negative inotropic actions of articaine and ropivacaine are similar in magnitude, and can be mainly attributed to a reduction in net trans-sarcolemmal Ca2+ influx.
No preview · Article · Jan 2010 · European Journal of Anaesthesiology
[Show abstract][Hide abstract] ABSTRACT: Minimum fungicidal concentration (mFC) of caspofungin was determined against 16 Candida albicans and 16 C. krusei in Rpmi-1640 and antibiotic medium 3 (Am3). time-kill tests were performed on six C. albicans and four C. krusei strains at 0.06-16 mg/l caspofungin. mFC ranges after 48 h were 0.5-1 and 1-2 mg/l for C. albicans and C. krusei, respectively; one C. albicans and the C. krusei reference strain showed paradoxical growth (pG) in Rpmi-1640, respectively. in killing experiments, after 48 h caspofugin was fungicidal against two and four C. albicans in Rpmi-1640 (at 16 mg/l) and in Am3 (at >0.5 mg/l), respectively; pG was noted in three and two cases, respectively. Caspofungin at >2 and 0.5 mg/l was fungicidal against all tested C. krusei strains even after 24 h in Rpmi-1640 and Am3, respectively. Killing activity of caspofungin against C. albicans and C. krusei could be exactly measured only by killing curves.
No preview · Article · Feb 2009 · Journal of chemotherapy (Florence, Italy)
[Show abstract][Hide abstract] ABSTRACT: Despite the widespread clinical application of ropivacaine, there is little information on the cellular cardiac effects of the drug. In the current study, therefore, the concentration-dependent effects of ropivacaine on action potential morphology and the underlying ion currents were studied and compared with those of bupivacaine in isolated canine ventricular cardiomyocytes.
Action potentials were recorded from the enzymatically dispersed cells using sharp microelectrodes. Conventional patch clamp and action potential voltage clamp arrangements were used to study the effects of ropivacaine on transmembrane ion currents.
Ropivacaine induced concentration- and frequency-dependent changes in action potential configuration, including shortening of the action potentials, reduction of their amplitude and maximum velocity of depolarization, suppression of early repolarization, and depression of plateau. Reduction in maximum velocity of depolarization was characterized with an EC50 value of 81 +/- 7 microm at 1 Hz. Qualitatively similar results were obtained with bupivacaine (EC50 = 47 +/- 3 microm). Under voltage clamp conditions, a variety of ion currents were blocked by ropivacaine: L-type calcium current (EC50 = 263 +/- 67 microm), transient outward current (EC50 = 384 +/- 75 microm), inward rectifier potassium current (EC50 = 372 +/- 35 microm), rapid delayed rectifier potassium current (EC50 = 303 +/- 47 microm), and slow delayed rectifier potassium current (EC50 = 106 +/- 18 microm).
Ropivacaine, similarly to bupivacaine, can modify cardiac action potentials and the underlying ion currents at concentrations higher than the usual therapeutic range. However, in cases of overdose, cardiac complications may be anticipated both during and after anesthesia due to the blockade of various ion currents.
[Show abstract][Hide abstract] ABSTRACT: In spite of its widespread clinical application, there is little information on the cellular cardiac effects of articaine. In the present study, the concentration-dependent effects of articaine on action potential morphology and the underlying ion currents were studied in isolated canine ventricular cardiomyocytes.
Action potentials were recorded from the enzymatically dispersed myocytes using sharp microelectrodes (16 cells from 3 dogs). Conventional patch clamp and action potential voltage clamp arrangements were used to study the effects of articaine on transmembrane ion currents (37 cells from 14 dogs).
Articaine-induced concentration-dependent changes in action potential configuration including shortening of the action potentials, reduction of their amplitude and maximum velocity of depolarization (V(max)), suppression of early repolarization and depression of plateau. The EC50 value obtained for the V(max) block was 162 (sd 30) microM. Both the reduction of V(max) and action potential shortening were frequency dependent: the former was more prominent at shorter, while the latter at longer pacing cycle lengths. A rate dependent V(max) block, having rapid offset kinetics [tau = 91 (20) ms], was observed in addition to tonic block. Under voltage clamp conditions, a variety of ion currents were blocked by articaine: I(Ca) [EC50 = 471 (75) microM], I(to) [EC50 = 365 (62) microM], I(K1) [EC50 = 372 (46) microM], I(Kr) [EC50 = 278 (79) microM], and I(Ks) [EC50 = 326 (65) microM]. Hill coefficients were close to unity indicating a single binding site for articaine, except for I(K1).
Articaine can modify cardiac action potentials and ion currents at concentrations higher than the therapeutic range which can be achieved only by accidental venous injection. Since its suppressive effects on the inward and outward currents are relatively well balanced, the articaine-induced changes in action potential morphology may be moderate even in the case of overdose.
Full-text · Article · Dec 2007 · BJA British Journal of Anaesthesia