Increase In Renal Vascular Resistance Research Articles

Effects of perfusion pressure on Ca2+ sparks in rat renal afferent arterioles

Mechanical force can be transduced into freshly isolated renal vascular smooth muscle cells (VSMCs) to trigger Ca2+ sparks. Sparks can trigger Spontaneous Transient Inward Currents (STICs), and regulate myogenic tone via membrane depolarization. Electrophysiological studies indicate the presence of STICs in isolated renal VSMCs. Hence, we sought to determine whether the occurrence of Ca2+ sparks is increased during pressure induced myogenic constriction. Line scan images were collected from perfused rat afferent arterioles at 500 Hz using laser scanning confocal microscopy. We detected spontaneous Ca2+ sparks in 40% of the VSMCs at a perfusion pressure of 80 mm Hg. These sparks were suppressed by 50 μM ryanodine and were augmented by 0.5 mM caffeine. The mean spontaneous Ca2+ spark frequencies were 0.14±0.04 sparks/s (n=23 cells) and 0.24±0.11 sparks/s (n=13 cells) at 80 mm Hg and 120 mm Hg, respectively. These suggest that the increase in frequency of sparks is probably related to pressure induced myogenic constriction. Intriguingly, the frequency of Ca2+ sparks observed in SHR afferent arteriole was double compared to that of normotensive rats at the same perfusion pressure. These observations implicate that the enhanced Ca2+ sparks frequency may contribute to the increase of renal vascular resistance in SHR.

Caracterización histológica y hemodinámica del injerto con isquemia caliente según el método de preservación

Objective:The non-heart-beating donor has been proposed as a solution to donor shortage for renal transplantation. Because the nature of such donors, the kidneys so derived have been damaged by primary warm ischemia (WI), and so potentially they may never function. Minimizing graft injury is especially important in case of transplantation form marginal donors because of a high rate of delayed graft function or primary nonfunction. The aim of this experimental study is to assess the structural and hemodynamic consequences of hypothermic perfusion (HP) versus cold storage (CS), in renal allograft after a period of WI. Material and Methods:We used 20 mini-pigs. WI was achieved by vascular pedicle occlusion during 45 min. We divided organs in 4 groups: A (n=5), kidneys with WI and then transplanted; group B (n=5), grafts with WI and implanted after HP with Belzer solution in our computerized perfusion system. Group C-control, (n=5) transplanted without WI and D (n=5) with WI and 60 min of CS in UW-Viaspan solution.All the procedure was recorded by a computerized data system. Renal vascular resistance (RVR) and renal vascular flow (RVF) were automatically calculated by means of mathematical formulas after renal transplantation. Subsequently histological study was completed in all cases. Results: We observed two patterns after transplantation:1. Initial increase of RVR with posterior decrease and increase of vascular flow: in organs with WI and HP prior to transplantation (group B) // organs transplanted without WI (group C-control). Electronic and conventional microscopy showed integrity of endothelial and tubule structure.2. Initial decrease with posterior increase of RVR. Organs with WI (group A) // organs with WI and CS (group D). Structural study showed endothelial and tubule disruption. Conclusión:In our experimental model machine perfusion preserves endothelial and tubule structure of kidneys with WI. After transplantation the hemodynamic pattern of grafts with WI and HP is similar to the control group (without WI and direct transplantation).

Relationships between Endogenous Sex Hormone Concentrations and Vascular Function in Postmenopausal Women

Differences in postmenopausal endogenous sex hormone concentrations are associated with varying risks of a growing number of common diseases. The relationships between postmenopausal endogenous sex hormone concentrations and vascular function are not well understood. We examined in postmenopausal women the relationships between endogenous sex hormone concentrations and both blood pressure (BP) and renal vascular resistance (RVR), at baseline and in response to infused angiotensin II (AngII). SUBJECTS, INTERVENTIONS, AND MAIN OUTCOME MEASURES: A total of 34 hypertensive, postmenopausal women were studied in low-sodium and/or high-sodium balance. Serum estradiol, serum progesterone, BP, and RVR were measured at baseline. BP and RVR were remeasured after AngII infusion. In low-sodium balance, the increases in systolic and diastolic BP in response to infused AngII were blunted with increased serum progesterone concentrations (P < 0.05). The increase in RVR in response to infused AngII was also blunted with increased serum progesterone concentrations (P < 0.005). The relationships between progesterone concentration and vascular response to AngII were independent of age, body mass index, and estradiol concentration. There were no significant correlations between estradiol concentration and BP or RVR response to AngII. There were no significant correlations between sex hormone concentrations and baseline BP or RVR. In high-sodium balance, there were no significant associations between sex hormone concentrations and vascular measures. In postmenopausal women in low-sodium balance, the pressor and renovascular responses to AngII are blunted with increased endogenous progesterone concentrations. Our findings suggest a role for endogenous progesterone in modulating vascular function, even within the low postmenopausal range.

Vasodilatation of afferent arterioles and paradoxical increase of renal vascular resistance by furosemide in mice

Loop diuretics like furosemide have been shown to cause renal vasodilatation in dogs and humans, an effect thought to result from both a direct vascular dilator effect and from inhibition of tubuloglomerular feedback. In isolated perfused afferent arterioles preconstricted with angiotensin II or N(G)-nitro-L-arginine methyl ester, furosemide caused a dose-dependent increase of vascular diameter, but it was without effect in vessels from NKCC1-/- mice suggesting that inhibition of NKCC1 mediates dilatation in afferent arterioles. In the intact kidney, however, furosemide (2 mg/kg iv) caused a 50.5 +/- 3% reduction of total renal blood flow (RBF) and a 27% reduction of superficial blood flow (SBF) accompanied by a marked and immediate increase of tubular pressure and volume. At 10 mg/kg, furosemide reduced RBF by 60.4 +/- 2%. Similarly, NKCC1-/- mice responded to furosemide with a 45.4% decrease of RBF and a 29% decrease of SBF. Decreases in RBF and SBF and increases of tubular pressure by furosemide were ameliorated by renal decapsulation. In addition, pretreatment with candesartan (2 mg/kg) or indomethacin (5 mg/kg) attenuated the reduction of RBF and peak urine flows caused by furosemide. Our data indicate that furosemide, despite its direct vasodilator potential in isolated afferent arterioles, causes a marked increase in flow resistance of the vascular bed of the intact mouse kidney. We suggest that generation of angiotensin II and/or a vasoconstrictor prostaglandin combined with compression of peritubular capillaries by the expanding tubular compartment are responsible for the reduction of RBF in vivo.

Effects of indomethacin on systemic and renal haemodynamics in conscious lambs

Both prostaglandins (PGs) PGE(2) and PGI(2) can act as renal vasodilators, these effects being exacerbated when the renin-angiotensin system is activated. Therefore, we hypothesized that PGs would play a more predominant role in modulating renal haemodynamics in the newborn period, when the renin-angiotensin system is activated. To this end, the role of endogenously produced PGs in modulating systemic and renal haemodynamics was investigated in two groups of conscious lambs aged approximately 1 and approximately 6 weeks. Arterial pressure, venous pressure and renal blood flow were measured for 5 min before (control) and for 20 min after intravenous injection of vehicle (experiment 1). Twenty-four hours later, this protocol was repeated with intravenous injection of the non-selective cyclo-oxygenase inhibitor indomethacin (1 mg kg(-1), experiment 2). Heart rate was calculated from the systolic peak of the arterial pressure waveform, and renal vascular resistance (RVR) was calculated from the measured variables. In response to indomethacin but not vehicle, in both age groups of lambs there was an increase in mean arterial pressure and pulse interval, as well as a marked increase in RVR. These responses to indomethacin were, however, transient, with baseline levels being resumed within minutes. Although the hypothesis that PGs play a greater role in modulating renal haemodynamics early in life is not supported, these data do provide evidence that endogenously produced PGs modulate systemic and renal haemodynamics during postnatal maturation. It is apparent, however, that other vasoactive factors must be rapidly recruited in order to buffer the circulatory responses to removal of vasodilatory PGs in the developing newborn.

Modeling Preeclampsia: The True Model for the Uniquely Human Disease Preeclampsia Is the Human Female Not the Pregnant Rat

To the Editor: Gadonski et al1 describe a series of elegant experiments in which they used a rodent (rat) model to show that chronic reduction in uterine perfusion pressure was associated with an elevation of arterial pressure and that this elevation in AP was associated with a rise in serum levels of interleukin-6. They further showed that this rise in arterial pressure occurs after chronic elevation of interleukin-6 in pregnant but not in nonpregnant rats and is associated with increase in renal vascular resistance and reduction in renal plasma flow, as well as glomerular filtration …

Severe hypoxia and renal sympathetic nerve activity in the preterm fetus

Low systemic blood flow occurs in the first day of life in one third of infants born before 30 weeks gestation, and is highly associated with impaired renal function. Although the mechanisms are poorly understood it is suggested that impaired perfusion is mediated by increased vascular resistance, and is associated with hypoxia. In preterm fetal sheep (103 days; term 147 days) we examined the hypothesis that increased renal sympathetic nerve activity (RSNA) is the primary mediator of increased renal vascular resistance (RVR) during and after severe hypoxia. During hypoxia, the initial increase in RVR was maximal after 4 min, then failed. In parallel with RVR changes, RSNA recorded using an adapted telemetry system (Telemetry Research Ltd), initially increased then became suppressed. This fall in RSNA corresponded closely with the start of a rapid fall in blood pressure, strongly suggesting fetal compromise. During recovery from hypoxia, there was a delayed increase in RVR. This increase was tightly correlated with a secondary increase in RSNA. These findings indicate, for the first time, that failure of RSNA is likely to mediate the fall in RVR during severe hypoxia, and that after hypoxia increased RSNA is likely to mediate renal hypoperfusion. These findings improve our understanding of the mechanisms which may underlie renal impairment in the preterm neonate. Supported by the Health Research Council of New Zealand.

Renal Function Alterations in Ex‐Vivo Perfusion in Diabetic Rats

Diabetic nephropathy is a major complication associated with poor diabetic control and a leading cause of end stage renal failure. We have developed a model to study early events of this pathology by using Wistar male control (Ct, n=6) and diabetic rats (n=5) induced by streptozotocin (STZ, 50mg/kg, iv, 21 days). The animals were anaesthetized with sodium pentobarbital (50 mg/kg), the right kidney was removed without interruption of the flow and placed in the perfusion system containing Krebs-Henseleit solution plus 6% bovine serum albumin. Inulin clearance (GFR), renal vascular resistance (RVR), sodium tubular transport (%TNa+), potassium tubular transport (%TK+) and chloride tubular transport (%TCl¯) were determined every 10 min. In STZ group we observed an increase in RVR, with peak values at 60 min perfusion time (STZ: 28±9 vs Ct: 5±0.03 mmHg/mL/g/min), associated with an extremely reduced in GFR (STZ: 0.2±0.04 vs Ct: 0,8±0.06 mL/g/min). The TNa+ (STZ: 33±2 vs Ct: 88±1 %), TK+ (STZ: 33±7 vs Ct: 67±2 %), and TCl¯ (STZ: 34±7 vs Ct: 86±0.1 %), were also reduced in STZ rats. Conclusion: These preliminary findings demonstrate that a major defect has been observed in STZ kidneys that is reflected on RVR, GFR and ionic dynamics measured during renal perfusion. This model could be a very important tool in order to evaluate early changes observed in experimental diabetes.

Early Renal Injury Induced by Caffeine Consumption in Obese, Diabetic ZSF1 Rats

Our previous studies indicate that prolonged caffeine consumption exacerbates renal failure in nephropathy associated with the metabolic syndrome. Reduced activity of the antioxidant defense system and beneficial effects of antioxidant therapy have been reported in diabetic rats and humans. The purpose of this study was to examine the early renal effects of caffeine consumption and the effects of concomitant antioxidant therapy in young obese, diabetic ZSF1 rats. Eleven-week-old male ZSF1 rats were randomized to drink tap water, caffeine (0.1%), tempol (1 mmol/L), or a solution containing caffeine and tempol for nine weeks. Caffeine significantly reduced body weight and glycosuria (weeks 2–9), improved glucose tolerance (week 9), had no effect on elevated plasma triglycerides, plasma cholesterol (week 9) and blood pressure (week 9), and significantly increased plasma cholesterol level (weeks 5 and 9). Yet, as early as after two weeks, caffeine greatly augmented proteinuria and increased renal vascular resistance (RVR) and heart rate (HR: week 9). Tempol had no effects on metabolic status and development of proteinuria, did not alter caffeine-induced metabolic changes and early proteinuria, and attenuated caffeine-induced increase in HR and RVR. Immunohistochemical analysis revealed significant glomerular and interstitial inflammation, proliferation, and fibrosis in control animals. Caffeine augmented the influx of glomerular and interstitial macrophages (ED1+ cells) influx, glomerular and tubular proliferative response, and glomerular collagen IV content. Tempol abolished the exacerbation of renal inflammation, proliferation, and fibrosis induced by caffeine. In conclusion, in nephropathy associated with the metabolic syndrome, caffeine—most likely through the interaction with adenosine receptors and interference with anti-inflammatory and/or glomerular hemodynamic effects of adenosine—augments proteinuria and stimulates some of the key proliferative mechanisms involved in glomerular remodeling and sclerosis. Tempol does not prevent early renal injury (i.e., proteinuria) induced by caffeine, yet abolishes late renal inflammatory, proliferative, and fibrotic change induced by chronic caffeine consumption in obese ZSF1 rats.

The Dual Endothelin Receptor Antagonist Bosentan Prevents the Acute Sickle Cell-Related Hypoxic Lung and Kidney Injury in Transgenic SAD Mice.

Vaso-occlusive (VOC) crisis is a major manifestation of sickle cell disease (SCD) leading to acute organ damages. Increased levels of endothelin-1 (ET-1), a potent vasoconstrictor peptide, have been reported in SCD patients. In addition, ETB receptor has been identified in red blood cells (RBCs) membrane and in vitro, ET-1 increases the activity of Gardos channel in sickle RBCs, which is important in generation of dehydrated sickle RBCs. First, we evaluated the effects of the dual-endothelin-receptor (ETR) antagonist bosentan in vitro on mouse RBCs from transgenic SAD mice and in vivo on hypoxic model of acute sickle VOCs. ET-1 (500 nM) significantly increased Gardos channel activity in SAD mouse RBCs, this was prevented by bosentan (10 μM). We next evaluated the in vivo effects of bosentan (100 mg/kg/d by gavage for 14 days) in both control and SAD mice. Under normoxic condition, bosentan did not modify either RBCs density profiles or RBCs K+ content in SAD mice, suggesting that ET-1 may not contribute to sickle cell dehydration in the steady state. The bosentan-treated mice were then exposed to hypoxia (8%, 18 hrs). We first evaluated the cardiac output and renal artery blood flow (RBF) by ultrasound color imaging. In SAD mice only, hypoxia induced a marked reduction in RBF (− 40%; p<0.01, n=10) and in the time-average mean blood velocities (p<0.01) compared to basal values. Such changes were not explained by changes in cardiac output. Infusion of bosentan reversed the increase in renal vascular resistance by 50% (p<0.01) within 5 minutes, suggesting an ET-1-dependent vasoconstriction on the SCD renal vasculature. In a third set of experiments, we evaluated the effect of bosentan on experimental lung and kidney VOC-induced damages. Normal and SAD mice were exposed to 46 hrs hypoxia (8%) followed by 2 hrs recovery under normoxia. Mice were treated with either vehicle or bosentan (100 mg/kg/d, starting one week before hypoxia). We evaluated RBF and cardiac output, complete blood counts, RBCs density profiles and cation content, bronchoalveolar fluid leukocyte counts, kidney and lung histopathology. In SAD mice exposed to hypoxia, bosentan amelioratedRBCs dehydration;the reduction in RBF;congestion of the renal microcirculation and injury;lung injury and pulmonary vessel constriction;renal and pulmonary leukocytic infiltrates.In conclusion, ETR blockade was beneficial by preventing RBC dehydration in vivo with protection from kidney and lung injury. In addition, we provide the first evidence that VOC is due not only to cell entrapment within the microvasculature but also to marked ET-dependent vasoconstriction. These data provide new insights into the possible use of bosentan in the treatment of acute VOCs in SCD.

Dissecting the Causes of Oxidative Stress in an In Vivo Model of Hypertension

The article “Role of extracellular superoxide dismutase in the mouse angiotensin slow pressor response” describes studies in EC-SOD knockout and wild-type mice by Welch et al.1 This study is a continuation of a series of investigations by these researchers who have advanced our understanding of the role of reactive oxygen species (ROS) in the pathogenesis of hypertension. Of particular note, this group combines sophisticated physiological measurements (renal hemodynamics and conscious blood pressure [BP] by telemetry) with in vitro measures of protein and mRNA expression, as well as markers of oxidative stress. There is clear evidence that increased oxidative stress is both a cause and a consequence of hypertension.2,3 Oxidative stress means that an imbalance develops between the pro-oxidant and antioxidant systems, with increased ROS generation and/or decreased antioxidant capacity. There are many oxidases responsible for ROS production and many antioxidant defenses, in both intracellular and extracellular locations. Dissecting the role of individual players is quite difficult, because, for example, there are 3 species of SOD, including EC-SOD, but no selective EC-SOD inhibitors are available. This study exploits the availability of the EC-SOD knockout mice to investigate the importance of this enzyme in BP control. One novel observation in this study is the demonstration that young adult EC-SOD knockout mice exhibit an elevated baseline BP of ≈10 mm Hg, as well as an increase in renal vascular resistance and …

Androgens potentiate renal vascular responses to angiotensin II via amplification of the Rho kinase signaling pathway

This study assessed whether the Rho kinase signaling pathway contributes to androgenic amplification of angiotensin II (Ang II) induced pressor and renal constrictor responses. Mean arterial pressure (MAP) responses to angiotensin II receptor 1 (AT1) inhibition were measured in conscious male New Zealand genetically hypertensive rats (NZGH) subjected to sham operation, castration or castration+testosterone replacement. MAP and renal vascular resistance (RVR) responses to Ang II were recorded with and without a Rho kinase inhibitor, fasudil, in anesthetized NZGH. Western blot was used to analyze target protein expression in the kidney. MAP responses to AT1 receptor inhibition and exogenous Ang II were attenuated in castrated NZGH. The increase in RVR (mm Hg/ml/min/g kidney) at the maximum dose of Ang II was significantly lower in castrated NZGH than in sham operated NZGH. Testosterone replacement restored RVR responses to Ang II in castrated rats. Fasudil treatment reduced both MAP and RVR responses to Ang II in each group. In addition, the differential MAP and RVR responses to Ang II amongst the three groups were significantly attenuated by Rho kinase inhibition. Western blot showed that Rho kinase protein expression was reduced by castration, while testosterone replacement restored the Rho kinase protein levels in castrated rats. The phosphorylation of myosin phosphatase target subunit 1 (MYPT1), a downstream target of Rho kinase, was also increased by androgens. Collectively, these results indicate that androgens potentiate Ang II-induced renal vascular responses, an effect mediated at least partly via up-regulation of the Rho kinase signaling pathway.

Renal endothelial dysfunction and impaired autoregulation after ischemia-reperfusion injury result from excess nitric oxide

Endothelial dysfunction in ischemic acute renal failure (IARF) has been attributed to both direct endothelial injury and to altered endothelial nitric oxide synthase (eNOS) activity, with either maximal upregulation of eNOS or inhibition of eNOS by excess nitric oxide (NO) derived from iNOS. We investigated renal endothelial dysfunction in kidneys from Sprague-Dawley rats by assessing autoregulation and endothelium-dependent vasorelaxation 24 h after unilateral (U) or bilateral (B) renal artery occlusion for 30 (U30, B30) or 60 min (U60, B60) and in sham-operated controls. Although renal failure was induced in all degrees of ischemia, neither endothelial dysfunction nor altered facilitation of autoregulation by 75 pM angiotensin II was detected in U30, U60, or B30 kidneys. Baseline and angiotensin II-facilitated autoregulation were impaired, methacholine EC(50) was increased, and endothelium-derived hyperpolarizing factor (EDHF) activity was preserved in B60 kidneys. Increasing angiotensin II concentration restored autoregulation and increased renal vascular resistance (RVR) in B60 kidneys; this facilitated autoregulation, and the increase in RVR was abolished by 100 microM furosemide. Autoregulation was enhanced by N(omega)-nitro-l-arginine methyl ester. Peri-ischemic inhibition of inducible NOS ameliorated renal failure but did not prevent endothelial dysfunction or impaired autoregulation. There was no significant structural injury to the afferent arterioles with ischemia. These results suggest that tubuloglomerular feedback is preserved in IARF but that excess NO and probably EDHF produce endothelial dysfunction and antagonize autoregulation. The threshold for injury-producing, detectable endothelial dysfunction was higher than for the loss of glomerular filtration rate. Arteriolar endothelial dysfunction after prolonged IARF is predominantly functional rather than structural.

Long-term effects of percutaneous nephrolithotomy on renal morphology and arterial vascular resistance as evaluated by color Doppler ultrasonography: preliminary report

We evaluated the long-term effects of percutaneous nephrolithotomy (PNL) on renal morphology and vascular resistance. Parenchyma thickness, echogenicity and resistive index (RI) of upper, middle and lower poles of operated and contralateral kidneys of 41 patients with 82 renal units who underwent unilateral PNL with single pole access between 2000 and 2002 were examined separately by color Doppler ultrasonography. Mean patient age and duration between PNL and evaluation time were 38.29+/-11.53 years and 46.44+/-10.9 months, respectively. In operated kidney, mean RI, parenchyma thickness and echogenicity of the access pole were not statistically different than those of the adjacent two poles (0.608+/-0.053 vs. 0.608+/-0.052 for RI, P=0.895; 11.46+/-2.58 vs. 11.41+/-2.68 mm for parenchyma thickness, P=0.838; 0.049+/-0.31 vs. 0.073+/-0.33 for parenchyma echogenicity, P=0.160, respectively). Although mean RI and parenchyma thickness of access pole were statistically significantly different than the mean values of contralateral kidney (0.562+/-0.032 and 14.31+/-1.37 mm, respectively), no statistical difference was found between mean parenchyma echogenicities of both of them (echogenicity of contralateral kidney was 0, P=0.317). No significant difference was found between the average echogenicities of the three poles of the operated and contralateral kidneys (0.063+/-0.32 vs. 0, P=0.080). In 14 patients RI decreased from 0.694+/-0.058 to 0.602+/-0.056 in operated kidney (P=0.001) and from 0.604+/-0.06 to 0.559+/-0.031 in contralateral kidney (P=0.018) following PNL. It seems that PNL does not cause renal scarring, renal parenchymal loss or increase in renal vascular resistance in the long term. However, prospective studies must be performed for more definitive conclusions.

EFFECTS OF N‐ACETYL‐l‐CYSTEINE ON RENAL HAEMODYNAMICS AND FUNCTION IN EARLY ISCHAEMIA–REPERFUSION INJURY IN RATS

1. Renal ischaemia-reperfusion (IR) severely compromises kidney function and has been shown to cause persistent abnormalities in intrarenal blood flow. The aim of the present study was to examine whether N-acetyl-L-cysteine (NAC), a thiol-containing anti-oxidant, improves renal haemodynamics and function during early reperfusion in rats subjected to renal IR. 2. Male Sprague-Dawley rats were divided into groups receiving either isotonic saline (IR-Saline; n = 8) or NAC (IR-NAC; n = 8) prior to (200 mg/kg, i.p., 24 and 12 h before acute experimentation) and during acute renal clearance experiments (bolus 150 mg/kg followed by a continuous infusion of 43 mg/kg per h, i.v.). During acute experimentation, thiobutabarbital-anaesthetized rats were subjected to a right-sided nephrectomy, followed by left kidney IR (40 min renal artery occlusion). Left kidney function and blood flow and intrarenal cortical and outer medullary perfusion measured by laser-Doppler flowmetry was analysed at baseline, during ischaemia and for 80 min of reperfusion. 3. Renal IR produced an approximate 85% reduction in glomerular filtration rate (GFR) and a pronounced increase in fractional urinary sodium excretion, throughout reperfusion, with no statistically significant differences between groups. 4. During reperfusion, total renal blood flow and cortical and outer medullary perfusion rapidly returned to levels not significantly different from baseline in both groups. The relative increase in renal vascular resistance in response to IR was more pronounced in NAC-treated rats compared with saline-treated animals (P < 0.05). 5. In conclusion, treatment with NAC did not improve kidney function during the first 80 min after renal IR. In addition, the marked reduction in GFR following reperfusion was not associated with any detectable abnormalities in intrarenal perfusion.

Modulation by NO of renal myogenic autoregulation: TGF dependency

Blood flow to the kidney is unique in that it is regulated to serve the urine-producing function of the kidney and is far in excess of any conceivable metabolic demand. As renal blood flow (RBF) is so high, and resistance thus so low, the capillaries in the glomerulus are potentially exposed to high transmural pressures. The need to stabilize glomerular capillary pressure plus the need for stable tubular loads to permit regulation of salt excretion has focused attention on the mechanisms by which RBF is stabilized when blood pressure fluctuates. Renal autoregulation, i.e. regulation by the kidney of its own blood flow, effectively stabilizes RBF over a wide range of blood pressure. In turn, autoregulation is the product of two mechanisms: a myogenic mechanism located entirely within the vascular smooth muscle of the preglomerular microcirculation, and tubuloglomerular feedback (TGF) by which information about the distal tubular load is transmitted through the macula densa and the juxtaglomerular apparatus to the afferent arteriole. Both these mechanisms are extensively modulated by a variety of paracrine and endocrine agents. In the 15 years since Tolins et al. (1990) showed that the nitro-arginine derivative Nω-monomethyl-l-arginine (l-NMMA) causes profound renal vasoconstriction, a great number of studies have been performed to characterize and understand the contribution of nitric oxide to the regulation of RBF. These studies have revealed that the renal circulation is perhaps the most sensitive in the body to non-selective inhibition of nitric oxide synthase (NOS) with complete inhibition causing at least a twofold increase in renal vascular resistance that is independent of the pronounced pressor response seen after systemic NOS inhibition. Early studies did not detect any effect of NOS inhibition on renal autoregulation. More recent studies using the classic steady-state, pressure ramp approach have shown that the lower pressure limit of autoregulation is extended to still lower pressures. At the same time, time series analysis has shown distinct augmentation of the myogenic mechanism after non-selective NOS inhibition. These effects have largely been attributed to inhibition of endothelial NOS although the kidney also contains neuronal NOS (nNOS), located largely at and around the macula densa. While selective inhibition of nNOS has been problematic, a number of studies have combined to demonstrate that inhibition of nNOS at the macula densa causes a stronger TGF response (e.g. Ichihara et al. 1998 and other references found in Just & Arendshorst, 2005). In this issue of The Journal of Physiology, Just & Arendshorst (2005) report studies that bear on two aspects of how NO affects renal autoregulation. As non-selective NOS inhibition causes such strong renal vasoconstriction, it is often considered that the enhancement of autoregulation is a consequence of the vasoconstriction. However, blood pressure fluctuates both upward and downward and NOS inhibition improves the stabilization of RBF. The major point made by Just and Arendshorst is the demonstration, which was logically necessary, that non-selective NOS inhibition augments both pressure-induced vasoconstriction and pressure-induced vasodilatation in the kidney. This finding is supported by the demonstration that equivalent vasoconstriction caused by Ang II does not duplicate the autoregulatory result, showing that the vasoconstriction is not sufficient to explain the change in autoregulation. In addition, they show in the same rats that non-selective NOS inhibition does not alter myogenic autoregulation in the hindquarters, indicating regional specificity. The authors further show that the augmentation of renal myogenic autoregulation induced by non-selective NOS inhibition is abrogated in the presence of furosemide. Furosemide of course is a loop diuretic which, at the dose used, blocks NaCl reabsorption from the thick ascending limb and effectively clamps TGF off. The strength of this interaction is somewhat surprising, given that both sources of nitric oxide within the renal cortex were inhibited, only one of which is under TGF control, and that nitric oxide is highly diffusive. However, this is undoubtedly a very interesting result that indicates regulation of the myogenic mechanism by tubular elements. Thus, a control system, TGF, that plays an important role linking renal inputs (RBF and GFR) to output (tightly regulated salt excretion) has now been linked mechanistically to the other autoregulatory system that had been presumed to be independent of tubular function.

Effects of Calcium Channel Blockade on Angiotensin II-Induced Peritubular Ischemia in Rats

Recent studies have indicated that derangement of peritubular capillary (PTC) circulation with consequent tubulointerstitial hypoxia plays a pivotal role in the pathogenesis of renal injury. The present study was performed to determine whether azelnidipine, a new dihydropyridine calcium channel blocker, attenuates angiotensin II (AngII)-induced peritubular ischemia in anesthetized rats. The superficial PTCs were visualized directly using an intravital fluorescence videomicroscope system, and the PTC blood flow was evaluated by analyzing the velocity of fluorescein isothiocyanate-labeled erythrocytes. Intravenous infusion of AngII (50 ng/kg/min, 10 min) significantly increased mean arterial pressure (MAP) and renal vascular resistance (RVR) (by 35 +/- 3% and 110 +/- 32%, respectively), and decreased total renal blood flow (RBF) and PTC erythrocyte velocity (by -34 +/- 4 and -37 +/- 1%, respectively). Treatment with azelnidipine (5 microg/kg/min i.v., 10 min) had no effect on basal MAP, RBF, RVR, or PTC erythrocyte velocity. However, azelnidipine markedly attenuated the AngII-induced increases in MAP (7 +/- 3%) and RVR (40 +/- 4%) and decreases in RBF (-24 +/- 1%) and PTC erythrocyte velocity (-22 +/- 1%). Similar attenuation in the AngII-induced responses of MAP, RBF, RVR, and PTC erythrocyte velocity were observed in rats treated with a higher dose of azelnidipine (20 microg/kg/min i.v., 10 min), which significantly decreased basal MAP and RVR and increased RBF and PTC erythrocyte velocity. These data suggest that calcium channel blockade attenuates AngII-induced peritubular ischemia, which may be involved in its beneficial effects on renal injury.

Renal 20-HETE inhibition attenuates changes in renal hemodynamics induced byl-NAME treatment in pregnant rats

We previously reported that inhibition of nitric oxide (NO) synthesis by N-nitro-L-arginine methyl ester (L-NAME) during late pregnancy leads to increased production of renal vascular 20-hydroxyeicosatetraenoic acid (20-HETE), a cytochrome P-450 (CYP) 4A-derived vasoconstrictor, in pregnant rats. However, the effect of upregulation of vascular 20-HETE production on renal function after NO inhibition is not known. To test the hypothesis that increased gestational vascular 20-HETE synthesis after NO inhibition is involved in mediating blood pressure and renal functional changes, we first determined the IC(50) value of the effect of nitroprusside (SNP), a NO donor, on renal 20-HETE production in cortical microsomes. We then divided pregnant rats and age-matched virgin rats into a vehicle control group, an L-NAME treatment group (0.25 mg/ml in drinking water), and a group treated with L-NAME plus N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS; CYP4A-selective inhibitor, 10 mg.kg(-1).day(-1) iv). After 4 days of treatment, we measured blood pressure, renal blood flow (RBF), renal vascular resistance (RVR), and glomerular filtration rate (GFR) in each group. The addition of SNP (IC(50) = 22 microM) decreased renal cortical 20-HETE production. In pregnant rats, L-NAME treatment led to significantly higher mean arterial pressure (MAP) and RVR, and lower RBF and GFR. Combined treatment with DDMS and L-NAME significantly attenuated the increases in MAP and RVR and the decrease in GFR, but not the reduction in RBF induced by L-NAME treatment. L-NAME and L-NAME plus DDMS had no significant impact on renal hemodynamics in virgin rats. In addition, chronic treatment with DDMS selectively inhibited cortical 20-HETE production without a significant effect on CYP4A expression in L-NAME-treated pregnant rats. In conclusion, NO effectively inhibits renal cortical microsomal 20-HETE production in female rats. In pregnant rats, the augmentation of renal 20-HETE production after NO inhibition is associated with increased MAP and RVR, whereas decreased GFR is negated by treatment of a selective and competitive CYP4A inhibitor. These results demonstrate that the interaction between renal 20-HETE and NO is important in the regulation of renal function and blood pressure in pregnant rats.

Renal haemodynamics and function in weanling rats treated with enalapril from birth

1. Inhibition of the renin-angiotensin system (RAS) during kidney development produces chronic alterations in renal morphology and function that have been characterized in detail in adult animals. The aim of the present study was to determine the consequences of neonatal angiotensin-converting enzyme (ACE) inhibition on renal haemodynamics and function in rats at a much earlier age, namely 3-4 weeks. 2. Male Wistar pups received daily intraperitoneal injections of enalapril (10 mg/kg) or isotonic saline from birth until 24-28 days of age, when renal haemodynamics and function were assessed using clearance techniques under pentobarbital anaesthesia. 3. Enalapril-treated rats showed significant reductions in glomerular filtration rate (GFR; -44 +/- 6%; P < 0.05), effective renal plasma flow (ERPF; -33 +/- 6%; P < 0.05) and filtration fraction (-16 +/- 3%; P < 0.05) compared with saline-treated controls. Although mean arterial pressure tended to be lower in enalapril-treated rats, this group demonstrated a significant increase in renal vascular resistance compared with control rats (RVR; 46 +/- 6 vs 32 +/- 3 mmHg/mL per.min per g.kidney weight, respectively; P < 0.05). In enalapril-treated rats, urine osmolality was reduced (-59 +/- 5%; P < 0.05) and urine flow rate and fractional urinary excretion rates of sodium and potassium were markedly elevated compared with controls (P < 0.05). Enalapril-treated rats showed severe renal histological abnormalities, including wall thickening of cortical arterioles, papillary atrophy and tubulointerstitial alterations, mimicking those described previously in similarly treated rats examined in adulthood. 4. In conclusion, neonatal ACE inhibition in rats induces pronounced alterations in renal haemodynamics and function, characterized by reductions in ERPF and GFR, increased RVR and impaired tubular sodium and water reabsorption, which are evident at weaning.

Nitric oxide modulates renal vasoconstrictor effect of endothelin-1 in conscious lambs

To test the hypothesis that nitric oxide (NO) buffers the renal vasoconstrictor effects of endothelin-1 (ET-1) early in life, renal haemodynamic responses to ET-1 were measured in the presence and absence of endogenously produced NO in conscious lambs. Renal haemodynamic effects of ET-1 were measured for 5 min before (control) and 20 min after intraarterial injection of ET-1 before and after pretreatment with 20 mg/kg of the L-arginine analogue N(G)-nitro-L-arginine methyl ester (L-NAME), (experiment 1) and its inactive isomer D-NAME (experiment 2) in conscious lambs aged approximately 1 week (N=7) and approximately 6 weeks (N=6). The two experiments were carried out in random order at intervals of 24-48 h. In lambs aged approximately 6 weeks, a marked increase in renal vascular resistance (RVR) was elicited by ET-1 administration; this response was enhanced twofold following pretreatment with L-NAME. In 1-week-old lambs, however, an increase in RVR in response to ET-1 occurred only after pretreatment with L-NAME. Therefore, we accept our hypothesis and conclude that NO buffers the renal vasoconstrictor effects of ET-1 early in life.