Niwanthi W Rajapakse

Baker IDI Heart and Diabetes Institute, Melbourne, Victoria, Australia

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Publications (28)96.65 Total impact

  • Niwanthi W Rajapakse · Florian Karim · Roger G Evans · David M Kaye · Geoffrey A Head
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    ABSTRACT: Augmenting endothelial specific transport of the nitric oxide precursor L-arginine via cationic amino acid transporter-1 (CAT1) can prevent obesity related hypertension. We tested the hypotheses that CAT1 overexpression prevents obesity-induced hypertension by buffering the influence of the sympathetic nervous system (SNS) on the maintenance of arterial pressure and by buffering pressor responses to stress. Wild type (WT; n=13) and CAT1 overexpressing mice (CAT+; n=13) were fed a normal or a high fat diet for 20 weeks. Mice fed a high fat diet were returned to the control diet before experiments commenced. Baseline mean arterial pressure (MAP) and effects of restraint-, shaker- and almond feeding-stress and ganglionic blockade (pentolinium; 5 mg/kg; i.p.) on MAP were determined in conscious mice. Fat feeding increased body weight to a similar extent in WT and CAT+ but MAP was greater only in WT compared to appropriate controls (by 29%). The depressor response to pentolinium was 65% greater in obese WT than lean WT (P < 0.001), but was similar in obese and lean CAT+ (P = 0.65). In lean WT and CAT+, pressor responses to shaker and feeding stress, but not restraint stress, were less in the latter genotype compared to the former (P ≤ 0.001). Pressor responses to shaker and feeding stress were less in obese WT than lean WT (P ≤ 0.001), but similar in obese and lean CAT+. The increase in MAP in response to restraint stress was less in obese WT (22 ± 2%), but greater in obese CAT+ (37 ± 2%), when compared to respective lean WT (31 ± 3%) and lean CAT+ controls (27 ± 2%; P ≤ 0.02). We conclude that CAT1 overexpression prevents obesity-induced hypertension by reducing the influence of the SNS on the maintenance of arterial pressure but not by buffering pressor responses to stress.
    PLoS ONE 07/2015; 10(7):e0131424. DOI:10.1371/journal.pone.0131424 · 3.23 Impact Factor
  • Niwanthi W Rajapakse · Shane Nanayakkara · David M. Kaye
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    ABSTRACT: A highly complex interplay exists between the heart and kidney in the setting of both normal and abnormal physiology. In the context of heart failure, a pathophysiological condition termed the cardiorenal syndrome (CRS) exists whereby dysfunction in the heart or kidney can accelerate pathology in the other organ. The mechanisms that underpin CRS are complex, and include neuro-hormonal activation, oxidative stress and endothelial dysfunction. The endothelium plays a central role in the regulation of both cardiac and renal function, and as such impairments in endothelial function can lead to dysfunction of both these organs. In particular, reduced bioavailability of nitric oxide (NO) is a key pathophysiologic component of endothelial dysfunction. The synthesis of NO by the endothelium is critically dependent on the plasmalemmal transport of its substrate, L-arginine, via the cationic amino acid transporter-1 (CAT1). Impaired L-arginine-NO pathway activity has been demonstrated individually in heart and renal failure. Recent findings also suggest abnormalities of the L-arginine-NO pathway also play a role in the pathogenesis of CRS and thus this pathway may represent a potential new target for the treatment of heart and renal failure. Copyright © 2015. Published by Elsevier Inc.
    Pharmacology [?] Therapeutics 05/2015; 154. DOI:10.1016/j.pharmthera.2015.05.011 · 9.72 Impact Factor
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    ABSTRACT: Endothelial dysfunction has been postulated to play a central role in the development of cardiac hypertrophy, likely due to reduced nitric oxide (NO) bioavailability. We tested the hypothesis that increased endothelial NO production, mediated by increased L-arginine transport, could attenuate pressure overload induced cardiac hypertrophy. Echocardiography and blood pressure measurements were performed 15 weeks after transverse aortic constriction (TAC) in WT mice (WT; n = 12) and in mice with endothelial-specific overexpression of the L-arginine transporter, CAT1, (CAT+; n = 12). TAC induced greater increases in heart weight to body weight ratio in WT (by 47%) than CAT+ mice (by 25%) compared to respective controls (P ≤ 0.05). Similarly, the increase LV wall thickness induced by TAC was significantly attenuated in CAT+ mice (P = 0.05). Cardiac collagen type I mRNA expression was greater in WT mice with TAC (by 22%; P = 0.03), but not CAT+ mice with TAC, compared to respective controls. TAC also induced lesser increases in β-MHC mRNA expression in CAT+ mice compared to WT (P ≤ 0.05). Left ventricular systolic pressure after TAC was 36 and 39% greater in WT and CAT+ mice respectively, compared to respective controls (P ≤ 0.001). TAC had little effect on left ventricular end diastolic pressure in both genotypes. Taken together, these data indicate that augmenting endothelial function by overexpression of L-arginine transport can attenuate pressure overload induced cardiac hypertrophy. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
    Experimental physiology 05/2015; 100(7). DOI:10.1113/EP085250 · 2.67 Impact Factor
  • Sanjaya Kuruppu · Niwanthi W Rajapakse · Rhys A Dunstan · A Ian Smith
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    ABSTRACT: This study examined the effect of nitric oxide on the production of soluble ECE-1. Activity of ECE-1 in media was measured using a quenched fluorescent substrate assay, and expressed as a percentage of control. Endothelial cells were incubated with the nitric oxide donor Diethylenetriamine NONOate (DETA; 250-800 µM), NOS substrate L-Arg (200-1,000 µM), a L-Arg transport inhibitor (L-Lys; 10 µM) and NOS inhibitors (L-Gln and N5-[imino(nitroamino)methyl]-L-ornithine, methyl ester, monohydrochloride (L-NAME); 10-100 µM). The effect of L-Arg (1,000 µM) was also tested in the presence of L-Lys (10 µM), L-Gln (100 µM) and L-NAME (10-100 µM). Ultracentrifugation (100,000×g, 4 °C, 1 h) completely removed ECE-1 activity from the supernatant. In addition, fractionation of concentrated media on a sucrose density gradient indicated that ECE-1 activity was localised to the mid portion of the gradient, thus suggesting the possible role of exosomes in ECE-1 release. Production of soluble ECE-1 by Ea.hy926 cells was inhibited significantly (P < 0.05, unpaired t test, n = 4) in the presence of DETA (75.31 ± 3.59; 800 µM) and L-Arg (60.97 ± 9.22; 1,000 µM). L-Arg-mediated reduction in the release of soluble ECE-1 was blocked by the inhibition of NOS using L-NAME (100 µM; 99.19 ± 0.58) and L-Gln (100 µM; 104.41 ± 0.65). In addition, the presence of L-Lys (10 µM) significantly blocked the L-Arg (1,000 µM)-induced reduction in soluble ECE-1 levels (122.38 ± 13.16). These treatments had no effect on the expression of ECE-1 on the cell surface. Our data provide evidence that NO can inhibit the production of soluble ECE-1 by endothelial cells.
    Molecular and Cellular Biochemistry 09/2014; 396(1-2). DOI:10.1007/s11010-014-2141-0 · 2.39 Impact Factor
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    ABSTRACT: AimHypertension is a major clinical complication of obesity. Our previous studies show that abnormal uptake of the nitric oxide precursor L-arginine, via the cationic amino acid transporter-1 (CAT1), contributes to endothelial dysfunction in cardiovascular disease.In the current study we tested the hypothesis that abnormal L-arginine transport may be a key mediator of obesity-induced hypertension.Methods Mean arterial pressure (MAP) was monitored by telemetry in conscious wild type mice (WT; n = 13) and transgenic mice with endothelial specific overexpression of CAT1 (CAT+; n=14) fed a normal or a high fat diet for 20 weeks. Renal angiotensin II (Ang II), CAT1 mRNA and plasma nitrate/nitrite levels were then quantified. In conjunction, plasma nitrate/nitrite levels were assessed in obese normotensive (n=15) and obese hypertensive subjects (n=15).ResultsBoth genotypes of mice developed obesity when fed a high fat diet (P ≤ 0.002). Fat fed WT had 13% greater MAP and 78% greater renal Ang II content, 42% lesser renal CAT1 mRNA levels and 42% lesser plasma nitrate/nitrite levels, than WT fed a normal fat diet (P ≤ 0.03). In contrast, none of these variables were significantly altered by high fat feeding in CAT+ mice (P ≥ 0.36). Plasma nitrate/nitrite levels were 17% less in obese hypertensives compared to obese normotensives (P = 0.02).Conclusion Collectively, these data indicate that obesity induced down-regulation of CAT1 expression and subsequent reduced bioavailability of nitric oxide may contribute to development of obesity-induced hypertension.This article is protected by copyright. All rights reserved.
    Acta Physiologica 07/2014; 212(1). DOI:10.1111/apha.12344 · 4.38 Impact Factor
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    Sanjaya Kuruppu · Niwanthi W Rajapakse · Dmitriy Minond · A Ian Smith
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    ABSTRACT: A non-membrane bound form of Neprilysin (NEP) with catalytic activity has the potential to cleave substrates throughout the circulation, thus leading to systemic effects of NEP. We used the endothelial cell line Ea.hy926 to identify the possible role of exosomes and A disintegrin & Metalloprotease-17 (ADAM-17) in the production of non-membrane bound NEP. Using a bradykinin based quenched fluorescent substrate (40μM) assay, we determined the activity of recombinant human NEP (rhNEP; 12ng), and NEP in the media of endothelial cells (10% v/v; after 24 hr incubation with cells) to be 9.35±0.70 and 6.54±0.41 μmols of substrate cleaved over 3 hrs respectively. The presence of NEP in the media was also confirmed by western blotting. At present there are no commercially available inhibitors specific for ADAM-17. We therefore synthesised two inhibitors TPI2155-14 and TPI2155-17, specific for ADAM-17 with IC50 values of 5.36 and 4.32 μM respectively. Treatment of cells with TPI2155-14 (15μM) and TPI2155-17 (4.3μM) resulted in a significant decrease in NEP activity in media (62.37 ± 1.43 and 38.30 ± 4.70 respectively as a% of control; P<0.0001), implicating a possible role for ADAM-17 in NEP release. However, centrifuging media (100,000g for 1 hr at 4°C) removed all NEP activity from the supernatant indicating the likely role of exosomes in the release of NEP. Our data therefore indicated for the first time that NEP is released from endothelial cells via exosomes, and that this process is dependent on ADAM-17.
    Biochemical and Biophysical Research Communications 04/2014; 446(2). DOI:10.1016/j.bbrc.2014.01.158 · 2.30 Impact Factor
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    ABSTRACT: Tissue hypoxia has been demonstrated, in both the renal cortex and medulla, during the acute phase of reperfusion after ischemia induced by occlusion of the aorta upstream from the kidney. But there are also recent clinical observations indicating relatively well preserved oxygenation in the non-functional transplanted kidney. To test whether severe acute kidney injury can occur in the absence of widespread renal tissue hypoxia, we measured cortical and inner medullary tissue PO2, and total renal oxygen delivery (DO2) and consumption (VO2) during the first 2 h of reperfusion after 60 min of occlusion of the renal artery in anesthetized rats. To perform this experiment, we employed a new method for measuring kidney DO2 and VO2 that relies on implantation of fluorescence optodes in the femoral artery and renal vein. We were unable to detect reductions in renal cortical or inner medullary tissue tissue oxygen tension (PO2) during reperfusion after ischemia localized to the kidney. This is likely explained by the observation that VO2 (-57%) was reduced by at least as much as DO2 (-45%), due to a large reduction in glomerular filtration (-94%). However, localised tissue hypoxia, as evidence by pimonidazole adduct immunohistochemistry, was detected in kidneys subjected to ischemia and reperfusion, particularly in, but not exclusive to, the outer medulla. Thus, cellular hypoxia, particularly in the outer medulla, may still be present during reperfusion even when reductions in tissue PO2 are not detected in the cortex or inner medulla.
    AJP Renal Physiology 03/2014; 306(9). DOI:10.1152/ajprenal.00281.2013 · 3.25 Impact Factor
  • Steven Fernandez · Niwanthi Rajapakse · David Kaye
    03/2014; 9(1):e177. DOI:10.1016/j.gheart.2014.03.1860
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    ABSTRACT: Oxidative stress may play an important role in the pathogenesis of hypertension. The aim of our study is to examine whether increased expression of the predominant endothelial l-arginine transporter, cationic amino acid transporter-1 (CAT1), can prevent oxidative stress-induced hypertension. Wild-type mice (WT; n = 9) and endothelial CAT1 overexpressing (CAT+) mice (n = 6) had telemetry probes implanted for the measurement of mean arterial pressure (MAP), heart rate (HR) and locomotor activity. Minipumps were implanted for infusion of the superoxide dismutase inhibitor diethyldithiocarbamic acid (DETCA; 30 mg kg(-1) day(-1) ; 14 days) or its saline vehicle. Baseline levels of MAP, HR and locomotor activity were determined before and during chronic DETCA administration. Mice were then killed, and their plasma and kidneys collected for analysis of F2 -isoprostane levels. Basal MAP was less in CAT+ (92 ± 2 mmHg; n = 6) than in WT (98 ± 2 mmHg; n = 9; P < 0.001). During DETCA infusion, MAP was increased in WT (by 4.2 ± 0.5%; P < 0.001) but not in CAT+, when compared to appropriate controls (PDETCA*genotype = 0.006). DETCA infusion increased total plasma F2 -isoprostane levels (by 67 ± 11%; P = 0.05) in WT but not in CAT+. Total renal F2 -isoprostane levels were greater during DETCA infusion in WT (by 72%; P < 0.001), but not in CAT+, compared to appropriate controls. Augmented endothelial l-arginine transport attenuated the prohypertensive effects of systemic and renal oxidative stress, suggesting that manipulation of endothelial CAT1 may provide a new therapeutic approach for the treatment of cardiovascular disease associated with oxidative stress.
    Acta Physiologica 01/2014; 210(4). DOI:10.1111/apha.12215 · 4.38 Impact Factor
  • Niwanthi Rajapakse · Florian Karim · Geoffery Head · David Kaye
    Heart, Lung and Circulation 12/2013; 22(1):S18. DOI:10.1016/j.hlc.2013.05.039 · 1.44 Impact Factor
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    Niwanthi W Rajapakse · Abigail L Chong · Wei-Zheng Zhang · David M Kaye
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    ABSTRACT: AIMSHYPOTHESIS: Impaired L-arginine transport has been reported in cardiovascular diseases, providing a possible mechanism for reduced nitric oxide (NO) production. Given that cardiovascular diseases are also associated with insulin resistance, and insulin is known to induce vasodilation via a NO-dependent pathway, we hypothesised that abnormal insulin modulation of L-arginine transport may contribute to vascular dysfunction in diabetes. Forearm blood flow (FBF) responses to insulin and sodium nitroprusside (SNP) were measured in control and type 2 diabetic volunteers using venous occlusion plethysmography. Effects of intra-arterial insulin on the forearm veno-arterial flux of arginine and related amino acids were determined by HPLC. The effect of locally delivered insulin on arginine transport was assessed during an intra-arterial infusion of [4,5-(3)H] L-arginine. In controls, intrabrachial infusion of 5 mUnits/min insulin lead to a progressive rise in FBF (p<0.001) while this was not evident in diabetics. In support of this observation, we observed a concomitant, significant increase in the flux of N-hydroxy-L-arginine (the NO precursor) in controls (baseline vs. 60 mins insulin: 16.2±12.2 vs. 33.0±13.1 nmol/100 ml tissue/min; p<0.01), whilst no increase was observed in diabetics. Moreover, insulin augmented the clearance of [(3)H]L-arginine from the forearm circulation in controls (baseline vs insulin: 123±22 vs. 150±28 ml/min; p<0.05) but not in diabetics. These findings suggest that insulin resistance may contribute substantially to the onset and development of cardiovascular disease in type 2 diabetics via abnormal insulin-mediated regulation of L-arginine transport.
    PLoS ONE 05/2013; 8(5):e61840. DOI:10.1371/journal.pone.0061840 · 3.23 Impact Factor
  • Sanjaya Kuruppu · Niwanthi W Rajapakse · A Ian Smith
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    ABSTRACT: Endothelin is one of the most potent peptide vasoconstrictors thus far characterised. It is produced by the cleavage of its precursor big endothelin-1 by endothelin-converting enzyme-1 (ECE-1). The endothelin system which includes endothelin-1 (ET-1), ET receptors and ECE-1 is well characterised in the kidney and is known to play a key role in the pathogenesis of end-stage renal disease (ESRD). Therefore, inhibition of ECE-1 and antagonism of ET receptors represent potential therapeutic approaches for the treatment of ESRD. Here, we review the current literature on the localisation of ECE-1 in the normal kidney and how ECE-1 expression is altered in pathological conditions leading to ESRD. We also discuss the roles of neutral endopeptidase (NEP) and chymase in mediating the production of ET-1 in the kidney in ESRD. As such, we also discuss that complete inhibition of ET-1 production in the kidney requires the inhibition of ECE-1, NEP and chymase.
    Pflügers Archiv - European Journal of Physiology 01/2013; 465(7). DOI:10.1007/s00424-013-1216-1 · 4.10 Impact Factor
  • Niwanthi W Rajapakse · David L Mattson
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    ABSTRACT: Purpose of review: L-Arginine (L-Arg) is the substrate for nitric oxide (NO) formation. Reduced NO bioavailability, particularly within the renal circulation, has been identified as a key factor in the pathogenesis of hypertension. This review focuses on the pathogenic role of abnormal L-Arg transport, particularly within the kidney, in hypertension. Recent findings: Most recent studies have attempted to restore NO bioavailability in cardiovascular diseases with the use of antioxidants to reduce NO inactivation, but this approach has failed to provide beneficial effects in the clinical setting. We argue that this may be due to reduced NO formation in hypertension, which has largely been overlooked as a means of restoring NO bioavailability in cardiovascular diseases. Recent data indicate that renal L-Arg transport plays an important role in regulating both renal perfusion and function and the long-term set point of arterial pressure in health. Perturbations in the renal L-Arg transport system can give rise to abnormal renal perfusion and function, initiating hypertension and related renal damage. Summary: Accordingly, we propose that L-Arg transporters are a new treatment target in hypertension and in disease states where renal NO bioavailability is disturbed.
    Current opinion in nephrology and hypertension 10/2012; 22(1). DOI:10.1097/MNH.0b013e32835a6ff7 · 3.96 Impact Factor
  • 33rd Annual Scientific Meeting of the; 08/2012
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    ABSTRACT: Low renal nitric oxide (NO) bioavailability contributes to the development and maintenance of chronic hypertension. We investigated whether impaired l-arginine transport contributes to low renal NO bioavailability in hypertension. Responses of renal medullary perfusion and NO concentration to renal arterial infusions of the l-arginine transport inhibitor l-lysine (10 μmol·kg(-1)·min(-1); 30 min) and subsequent superimposition of l-arginine (100 μmol·kg(-1)·min(-1); 30 min), the NO synthase inhibitor N(G)-nitro-l-arginine (2.4 mg/kg; iv bolus), and the NO donor sodium nitroprusside (0.24 μg·kg(-1)·min(-1)) were examined in Sprague-Dawley rats (SD) and spontaneously hypertensive rats (SHR). Renal medullary perfusion and NO concentration were measured by laser-Doppler flowmetry and polarographically, respectively, 5.5 mm below the kidney surface. Renal medullary NO concentration was less in SHR (53 ± 3 nM) compared with SD rats (108 ± 12 nM; P = 0.004). l-Lysine tended to reduce medullary perfusion (-15 ± 7%; P = 0.07) and reduced medullary NO concentration (-9 ± 3%; P = 0.03) while subsequent superimposition of l-arginine reversed these effects of l-lysine in SD rats. In SHR, l-lysine and subsequent superimposition of l-arginine did not significantly alter medullary perfusion or NO concentration. Collectively, these data suggest that renal l-arginine transport is impaired in SHR. Renal l-[(3)H]arginine transport was less in SHR compared with SD rats (P = 0.01). Accordingly, we conclude that impaired arginine transport contributes to low renal NO bioavailability observed in the SHR kidney.
    AJP Renal Physiology 03/2012; 302(12):F1554-62. DOI:10.1152/ajprenal.00084.2011 · 3.25 Impact Factor
  • N W Rajapakse · D L Mattson
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    ABSTRACT: To examine whether reduced renal arginine transport increases the responsiveness of the renal circulation to angiotensin II in salt sensitivity, renal perfusion responses to angiotensin II were examined in the presence of L-arginine transport inhibitor, L-lysine and subsequent L-arginine in Sprague Dawley (SD) and Dahl salt-sensitive (Dahl S) rats. Laser Doppler probes and a transonic flow probe were used to measure regional renal perfusion and total renal perfusion respectively. Renal perfusion responses to intravenous (i.v.) angiotensin II were sequentially examined under control conditions and during i.v. infusion of L-lysine, L-arginine or nitric oxide synthase inhibitor, N(G)-nitro-L-arginine. Angiotensin II (10 and 100 ng kg(-1) min(-1) , i.v.) reduced total renal (-10 ± 3 and -36 ± 5%) and cortical (-10 ± 2 and -28 ± 4%) but not medullary perfusion in SD rats. In these rats L-lysine enhanced the renal perfusion response (P = 0.003), whereas subsequent L-arginine reversed this effect (P = 0.04). Angiotensin II reduced total renal, cortical and medullary perfusion in Dahl S rats. In Dahl S rats fed high salt, L-lysine did not affect renal perfusion responses to angiotensin II, but subsequent L-arginine blunted the renal blood flow response (P = 0.01) and increased the medullary perfusion during angiotensin II infusion (P = 0.006). Intact renal L-arginine transport attenuates the vasoconstrictor effects of circulating angiotensin II in the renal cortex in SD rats. L-arginine also plays an important role in protecting the renal medullary circulation from the ischemic effects of angiotensin II in Dahl S rats.
    Acta Physiologica 06/2011; 203(3):391-400. DOI:10.1111/j.1748-1716.2011.02330.x · 4.38 Impact Factor
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    ABSTRACT: 1. Angiotensin (Ang) II has multiple actions in the renal medullary circulation. It can induce vasodilatation and blunt the response of medullary blood flow (MBF) to renal nerve activation through AT(1) receptor-mediated release of nitric oxide (NO) and/or vasodilator prostaglandins. These actions require high intravascular and/or intratubular AngII concentrations, so are not apparent under physiological conditions. 2. Nevertheless, these mechanisms blunt the responsiveness of MBF to AT(1) receptor-mediated vasoconstriction. When these protective mechanisms fail, as when oxidative stress reduces NO bioavailability in the medullary circulation, AngII reduces MBF. If sustained, reduced MBF leads to the development of hypertension. 3. Chronic activation of the renin-angiotensin system (RAS) induces oxidative stress in the kidney. Therefore, MBF may be reduced in models of hypertension associated with RAS activation both because AngII levels per se are increased and because of increased responsiveness of MBF to AngII-induced vasoconstriction. 4. Endogenous AngII enhances the responsiveness of MBF to renal nerve stimulation, whereas NO blunts it. Chronic RAS activation and/or oxidative stress should therefore be expected to enhance MBF responses to renal nerve stimulation. Consistent with this, reductions in MBF induced by renal nerve stimulation are enhanced in rabbits with AngII-induced hypertension, renovascular hypertension or after 9 weeks of fat feeding. 5. We conclude that the ability of endogenous AngII to reduce MBF and enhance the response of MBF to activation of the renal nerves could contribute to the development of hypertension under conditions of RAS activation, especially if accompanied by increased renal sympathetic nerve activity.
    Clinical and Experimental Pharmacology and Physiology 07/2009; 37(2):e58-69. DOI:10.1111/j.1440-1681.2009.05233.x · 2.37 Impact Factor
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    Niwanthi W Rajapakse · David L Mattson
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    ABSTRACT: 1. l-Arginine is the substrate for vascular nitric oxide (NO) formation. Under normal physiological conditions, intracellular l-arginine levels far exceed the K(m) of NO synthase for l-arginine. However, endogenous NO formation is dependent on extracellular l-arginine concentrations, giving rise to the concept of the 'l-arginine paradox'. 2. Nitric oxide production in epithelial and endothelial cells is closely coupled to cellular l-arginine uptake, indicating that l-arginine transport mechanisms play a major role in the regulation of NO-dependent function. 3. Consistent with the data in endothelial and epithelial cells are functional data indicating that exogenous l-arginine can increase renal vascular and tubular NO bioavailability and thereby influence kidney perfusion, function and arterial pressure. The integrated effect of increased cellular l-arginine transport is to lower arterial pressure. Therefore, the use of l-arginine in the treatment of hypertension warrants investigation. 4. Low NO bioavailability is central to the development and maintenance of hypertension and to related endothelial dysfunction and target organ damage. We propose that l-arginine can interrupt the vicious cycle that initiates and maintains low NO in hypertension by increasing the formation of NO.
    Clinical and Experimental Pharmacology and Physiology 12/2008; 36(3):249-55. DOI:10.1111/j.1440-1681.2008.05123.x · 2.37 Impact Factor
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    Niwanthi W Rajapakse · Carmen De Miguel · Satarupa Das · David L Mattson
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    ABSTRACT: Experiments were performed to determine whether exogenous L-arginine could ameliorate angiotensin II-induced hypertension and renal damage. Rats were instrumented with chronic indwelling femoral venous and arterial catheters for infusions of drugs and measurement of conscious arterial pressure. Arterial blood pressure significantly increased from 124+/-1 to 199+/-4 mm Hg, after 9 days of continuous infusion of angiotensin II (20 ng/kg per minute; IV; n=6 to 9). In contrast, the increase in arterial pressure after 9 days of angiotensin II infusion was significantly blunted by 45% (P=0.0003) in rats coadministered L-arginine (300 microg/kg per minute; IV; n=7 to 9). The glomerular injury index was significantly greater in rats administered angiotensin II in comparison with rats administered saline vehicle (P<0.001). Coinfusion of L-arginine significantly increased plasma nitrate/nitrite concentrations (P<0.001) and completely prevented angiotensin II-induced glomerular damage (P<0.001). Angiotensin II infusion alone and combined angiotensin II plus L-arginine infusion significantly increased urinary albumin excretion. Albuminuria in rats administered angiotensin II plus L-arginine is likely to be because of increased intraglomerular pressure. Our experiments demonstrate that L-arginine can blunt angiotensin II-induced hypertension and associated renal damage. This latter observation is most exciting because it indicates that increasing NO bioavailability, in addition to lowering arterial pressure, can greatly reduce hypertension-induced renal damage.
    Hypertension 12/2008; 52(6):1084-90. DOI:10.1161/HYPERTENSIONAHA.108.114298 · 6.48 Impact Factor
  • Niwanthi W Rajapakse · Gabriela A Eppel · Robert E Widdop · Roger G Evans
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    ABSTRACT: We tested the hypothesis that activation of angiotensin type 2 (AT(2)) receptors, by both exogenous and endogenous ANG II, modulates neurally mediated vasoconstriction in the renal cortical and medullary circulations. Under control conditions in pentobarbital-anesthetized rabbits, electrical stimulation of the renal nerves (RNS; 0.5-8 Hz) reduced renal blood flow (RBF; -88 +/- 3% at 8 Hz) and cortical perfusion (CBF; -92 +/- 2% at 8 Hz) more than medullary perfusion (MBF; -67 +/- 6% at 8 Hz). Renal arterial infusion of ANG II, at a dose titrated to reduce RBF by approximately 40-50% (5-50 blunted responses of MBF to RNS, without significantly affecting responses of RBF or CBF. Subsequent administration of PD123319 (1 mg/kg plus 1 during continued renal arterial infusion of ANG II did not significantly affect responses of RBF or CBF to RNS but enhanced responses of MBF, so that they were similar to those observed under control conditions. In contrast, administration of PD123319 alone blunted responses of CBF and MBF to RNS. Subsequent renal arterial infusion of ANG II in PD123319-pretreated rabbits restored CBF responses to RNS back to control levels. In contrast, ANG II infusion in PD123319-pretreated rabbits did not alter MBF responses to RNS. These data indicate that exogenous ANG II can blunt neurally mediated vasoconstriction in the medullary circulation through activation of AT(2) receptors. However, AT(2)-receptor activation by endogenous ANG II appears to enhance neurally mediated vasoconstriction in both the cortical and medullary circulations.
    AJP Regulatory Integrative and Comparative Physiology 01/2007; 291(6):R1669-76. DOI:10.1152/ajpregu.00183.2006 · 3.11 Impact Factor

Publication Stats

229 Citations
96.65 Total Impact Points


  • 2013–2015
    • Baker IDI Heart and Diabetes Institute
      • Diabetic Complications Division
      Melbourne, Victoria, Australia
  • 2002–2014
    • Monash University (Australia)
      • Department of Physiology
      Melbourne, Victoria, Australia
    • University of Auckland
      • Department of Physiology
      Auckland, Auckland, New Zealand
  • 2012
    • University of Vic
      Vic, Catalonia, Spain
  • 2008–2011
    • Medical College of Wisconsin
      • Department of Physiology
      Milwaukee, Wisconsin, United States
  • 2003
    • University of Texas Southwestern Medical Center
      Dallas, Texas, United States