Renal Cortical Blood Flow Research Articles

Renal cortical and medullary blood flow responses to altered NO availability in humans

The objective of this study was to quantify regional renal blood flow in humans. In nine young volunteers on a controlled diet, the lower abdomen was CT-scanned, and regional renal blood flow was determined by positron emission tomography (PET) scanning using H(2)(15)O as tracer. Measurements were performed at baseline, during constant intravenous infusion of nitric oxide (NO) donor glyceryl nitrate and after intravenous injection of NO synthase inhibitor N(ω)-monomethyl-L-arginine (L-NMMA). Using the CT image, the kidney pole areas were delineated as volumes of interest (VOI). In the data analysis, tissue layers with a thickness of one voxel were eliminated stepwise from the external surface of the VOI (voxel peeling), and the blood flow subsequently was determined in each new, reduced VOI. Blood flow in the shrinking VOIs decreased as the number of cycles of voxel peeling increased. After 4-5 cycles, blood flow was not reduced further by additional voxel peeling. This volume-insensitive flow was measured to be 2.30 ± 0.17 ml·g tissue(-1)·min(-1) during the control period; it increased during infusion of glyceryl nitrate to 2.97 ± 0.18 ml·g tissue(-1)·min(-1) (P < 0.05) and decreased after L-NMMA injection to 1.57 ± 0.17 ml·g tissue(-1)·min(-1) (P < 0.05). Cortical blood flow was 4.67 ± 0.31 ml·g tissue(-1)·min(-1) during control, unchanged by glyceryl nitrate, and decreased after L-NMMA [3.48 ± 0.23 ml·(g·min)(-1), P < 0.05]. PET/CT scanning allows identification of a renal medullary region in which the measured blood flow is 1) low, 2) independent of reduction in the VOI, and 3) reactive to changes in systemic NO supply. The technique seems to provide indices of renal medullary blood flow in humans.

Blockade of T‐type calcium channel with R(−) efonidipine ameliorates the renal ischemia induced by angiotensin II in Sprague‐Dawley rats

Blockade of T‐type calcium channel (TCC) has been shown to protect from renal injury. We have previously demonstrated ‐that blockade of TCC with R(−) efonidipine (R(−)EFO), a specific TCC blocker, could selectively increase renal medullary blood flow (MBF) in Sprague‐Dawley (SD) rats. Present study was designed to determine whether blockade of TCC with R(−)EFO would ameliorate the renal medullary ischemia induced by angiotensin II (AngII) in SD rats. Renal cortical blood flow (CBF) and MBF were simultaneously monitored by laser‐Doppler flowmetry in anesthetized SD rats. Following 30 min. baseline, AngII was intravenously infused for 30 min (50 ng/kg/min), and then R(−)EFO (0.25mg/h, intravenously) was infused together for 90 min and compared to the vehicle group. Urine was collected for hydrogen peroxide excretion (UH2O2V) measurement. Infusion of AngII significantly reduced renal CBF and MBF by 22±3.4% and 28±1.7% (P&lt;0.05), respectively. However, the reduction of MBF was significantly attenuated by R(−)EFO (P&lt;0.05). No significant change was observed in CBF between the two groups. Moreover, R(−)EFO significantly attenuated the increase in UH2O2V induced by AngII (P&lt;0.05). We conclude that blockade of TCC with R(−)EFO would attenuate AngII‐induced reductions in renal MBF by altering AngII‐induced renal oxidative stress. Supported by the Grants‐in‐Aid from the Min. of ECSST of Japan (No. 17590181)

Natural history and progression of atherosclerotic renal vascular stenosis

Date written: December 2008Final submission: October 2009

Exogenous L‐Arginine (L‐Arg) Reverses Angiotensin II (AngII)‐Mediated Renal Cortical and Medullary Vasoconstriction and Improves Pressure Diuresis and Natriuresis

We previously demonstrated that exogenous L‐Arg restores the blunted pressure diuresis and natriuresis relationship and normalizes arterial blood pressure in AngII‐infused rats. The present study determined the role of L‐Arg to modulate total renal blood flow and renal cortical and medullary blood flow in anesthetized, AngII‐infused, Sprague‐Dawley rats (n&gt;6/group). Renal blood flow was measured by ultrasonic flowmetry and regional renal blood flow was evaluated with the use of implanted optical fibers for laser‐Doppler flowmetry. Aortic clamps were utilized to maintain renal perfusion pressure at 110 mmHg throughout the experiments. Intravenous infusion of AngII (20 ng/kg/min) resulted in a significant decrease in total renal blood flow from 7.2 ± 0.5 ml/min/gkwt to 5.3 ± 0.3 ml/min/gkwt. The AngII infusion did not significantly alter superficial cortical blood flow but significantly decreased medullary blood flow by 19 ± 4%. Co‐infusion of L‐Arg (300 μg/kg/min) reversed the effects of AngII to decrease renal blood flow and renal medullary perfusion. The present data demonstrate that AngII‐mediated vasoconstriction of the renal cortical and medullary vasculature is reversed by exogenous L‐Arg; these experiments indicate an important role for nitric oxide to blunt the renal vasoconstrictor, anti‐diuretic, and pro‐hypertensive effects of AngII. Supported by HL29587 and DK062803.

Renal vasoconstrictor and pressor responses to angiotensin IV in mice are AT1a-receptor mediated

Angiotensin (Ang) IV was reported to induce renal vasoconstriction or vasodilation in rats via AT1 or AT4 receptors, respectively, whereby the latter one has been identified to be the insulin-regulated aminopeptidase (IRAP). We investigated the effects of Ang IV on mean arterial pressure (MAP) and renal cortical blood flow (CBF) in AT1a, AT1b, AT2 receptor and IRAP knockout (-/-) mice and their corresponding wild-type littermates. Ang II, known as a renal vasoconstrictor in mice, was used as a reference. MAP was recorded via a femoral catheter and CBF was measured using a light amplification by stimulated emission of radiation (LASER) Doppler probe; cortical vascular resistance (CVR) was calculated as MAP divided by CBF. Baseline MAP, CBF and CVR in AT1a (-/-) mice were significantly lower than wild-type mice. AT2 (-/-) mice had a significantly higher baseline MAP, but similar CBF. In wild-type mice, Ang IV and Ang II induced dose-dependent pressor and renal vasoconstrictor responses, which were antagonized by the AT1 receptor blocker candesartan. These responses were almost completely absent in AT1a (-/-) mice, but were enhanced in AT2 (-/-) mice; responses in AT1b (-/-) and IRAP (-/-) mice were comparable to those in corresponding wild-type mice. Ang IV mediates pressure and renal vasoconstrictor effects in mice via AT1a receptors, whereas IRAP/AT4 is not involved.

Blood Pressure, Blood Flow, and Oxygenation in the Clipped Kidney of Chronic 2-Kidney, 1-Clip Rats

Angiotensin II maintains renal cortical blood flow and renal oxygenation in the clipped kidney of early 2-kidney, 1-clip Goldblatt hypertensive (2K,1C) rats. The involvement of Ang II is believed to decline, whereas oxidative stress increases during the progression of 2K,1C hypertension. We investigated the hypothesis that the acute administration of drugs to inhibit reactive oxygen species (Tempol), angiotensin II type 1 receptors (candesartan), or angiotensin-converting enzyme (enalaprilat) lowers mean arterial pressure and increases kidney blood flow and oxygenation in the clipped kidney of chronic 2K,1C rats in contrast to sham controls. Twelve months after left renal artery clipping or sham, mean arterial pressure, renal cortical blood flow, and renal cortical and medullary oxygen tension were measured after acute administration of Tempol followed by enalaprilat or candesartan followed by enalaprilat. The mean arterial pressure of the 2K,1C rat was reduced by candesartan (−9%) and, more effectively, by Tempol (−35%). All of the applied treatments had similar blood pressure-lowering effects in sham rats (average: −21%). Only Tempol increased cortical blood flow (+35%) and cortical and medullary oxygen tensions (+17% and +94%, respectively) in clipped kidneys of 2K,1C rats. Administration of enalaprilat had no additional effect, except for a modest reduction in cortical blood flow in the clipped kidney of 2K,1C rats when coadministered with candesartan (−10%). In conclusion, acute administration of Tempol is more effective than candesartan in reducing the mean arterial blood pressure and improving renal blood perfusion and oxygenation in the clipped kidney of chronic 2K,1C rats.

Estrogen Is Renoprotective via a Nonreceptor-dependent Mechanism after Cardiac Arrest In Vivo

Severe ischemia induces renal injury less frequently in women than men. In this study, cardiac arrest and cardiopulmonary resuscitation were used to assess whether estradiol is renoprotective via an estrogen receptor (ER)-dependent mechanism. Male and female C57BL/6 and ER gene-deleted mice underwent 10 min of cardiac arrest followed by cardiopulmonary resuscitation. Serum chemistries and renal stereology were measured 24 h after arrest. Estrogen did not affect mean arterial pressure, regional renal cortical blood flow, and arterial blood gases. Hence, female kidneys were protected (mean +/- SEM: blood urea nitrogen, 65+/- 21 vs.149+/- 27 mg/dl, P = 0.04; creatinine, 0.14 +/- 0.05 vs. 0.73 +/- 0.16 mg/dl, P = 0.01; volume of necrotic tubules, 7 +/- 1% vs. 10 +/- 0%, P = 0.04). Estrogen also reduced renal injury. In intact females (n = 5), ovariectomized/vehicle-treated (n = 8), and ovariectomized/estrogen-treated (n = 8) animals, blood urea nitrogen was 65 +/- 21, 166 +/- 28, and 50 +/- 14 mg/dl (P = 0.002); creatinine was 0.14 +/- 0.05, 0.74 +/- 0.26, and 0.23 +/- 0.27 mg/dl (P = 0.014); necrotic tubules were 2.5 +/- 0.25%, 12.0 +/- 1.9%, and 5.0 +/- 1.6% (P = 0.004), respectively. In ER-[alpha] and ER-[beta] gene-deleted mice and controls estradiol-reduced functional injury (blood urea nitrogen: estradiol 117 +/- 71, vehicle 167 +/- 56, P = 0.007; creatinine: estradiol 0.5 +/- 0.5, vehicle 1.0 +/- 0.4, P = 0.013), but the effect of estradiol was not different between ER-[alpha] or ER-[beta] gene-deleted mice. Adding ICI 182,780 to estradiol did not alter injury. In women, kidneys were protected from cardiac arrest through estrogen. Estradiol-mediated renoprotection was not affected by ER deletion or blockade. Estradiol is renoprotective after cardiac arrest. The results indicate that estradiol renoprotection is ER-[alpha] and ER-[beta] independent.

Proinsulin C-peptide reduces diabetes-induced glomerular hyperfiltration via efferent arteriole dilation and inhibition of tubular sodium reabsorption

C-peptide reduces diabetes-induced glomerular hyperfiltration in diabetic patients and experimental animal models. However, the mechanisms mediating the beneficial effect of C-peptide remain unclear. We investigated whether altered renal afferent-efferent arteriole tonus or alterations in tubular Na+ transport (T(Na)) in response to C-peptide administration mediate the reduction of diabetes-induced glomerular hyperfiltration. Glomerular filtration rate, filtration fraction, total and cortical renal blood flow, total kidney O2 consumption (QO2), T(Na), fractional Na+ and Li+ excretions, and tubular free-flow and stop-flow pressures were measured in anesthetized adult male normoglycemic and streptozotocin-diabetic Sprague-Dawley rats. The specific effect of C-peptide on transport-dependent QO2 was investigated in vitro in freshly isolated proximal tubular cells. C-peptide reduced glomerular filtration rate (-24%), stop-flow pressure (-8%), and filtration fraction (-17%) exclusively in diabetic rats without altering renal blood flow. Diabetic rats had higher baseline T(Na) (+40%), which was reduced by C-peptide. Similarly, C-peptide increased fractional Na+ (+80%) and Li+ (+47%) excretions only in the diabetic rats. None of these parameters was affected by vehicle treatments in either group. Baseline QO2 was 37% higher in proximal tubular cells from diabetic rats than controls and was normalized by C-peptide. C-peptide had no effect on ouabain-pretreated diabetic cells from diabetic rats. C-peptide reduced diabetes-induced hyperfiltration via a net dilation of the efferent arteriole and inhibition of tubular Na+ reabsorption, both potent regulators of the glomerular net filtration pressure. These findings provide new mechanistic insight into the beneficial effects of C-peptide on diabetic kidney function.

Recovery from renal ischemia-reperfusion injury is associated with altered renal hemodynamics, blunted pressure natriuresis, and sodium-sensitive hypertension.

The present studies evaluated intrarenal hemodynamics, pressure natriuresis, and arterial blood pressure in rats following recovery from renal ischemia-reperfusion (I/R) injury. Acute I/R injury, induced by 40 min of bilateral renal arterial occlusion, resulted in an increase in plasma creatinine that resolved within a week. Following 5 wk of recovery on a 0.4% NaCl diet, the pressure-natriuresis response was assessed in anesthetized rats in which the kidney was denervated and extrarenal hormones were administered intravenously. Increasing renal perfusion pressure (RPP) from 107 to 141 mmHg resulted in a fourfold increase in urine flow and sodium excretion in sham control rats. In comparison, pressure diuresis and natriuresis were significantly attenuated in post-I/R rats. In sham rats, glomerular filtration rate (GFR) averaged 1.6 +/- 0.2 mlxmin(-1)xg kidney weight(-1) and renal blood flow (RBF) averaged 7.8 +/- 0.7 mlxmin(-1)xg kidney weight(-1) at RPP of 129 mmHg. Renal cortical blood flow, measured by laser-Doppler flowmetry, was well autoregulated whereas medullary blood flow and renal interstitial hydrostatic pressure increased directly with elevated RPP in sham rats. In contrast, GFR and RBF were significantly reduced whereas medullary perfusion and interstitial pressure demonstrated an attenuated response to RPP in post-I/R rats. Further experiments demonstrated that conscious I/R rats develop hypertension when sodium intake is increased. The present data indicate that the pressure-natriuretic-diuretic response in I/R rats is blunted because of a decrease in GFR and RBF and the depressed pressure-dependent increase in medullary blood flow and interstitial pressure.

Roles of Insulin Receptor Substrates (IRS) in renal function and renal hemodynamics.

We have reported previously that renal hemodynamic abnormalities exist in the prediabetic stage of type II diabetic rats. At this stage these rats have insulin resistance. It is well known that insulin resistance is frequently associated with renal abnormalities, but the mechanism underlying this association has remained speculative. Although insulin is known to modify renal hemodynamics, little is known about the roles of insulin receptor substrates (IRS1, IRS2) in the renal actions of insulin. To address this issue, we investigated in wild type (WT), IRS1‐deficient (IRS1‐/‐), and IRS2‐deficient (IRS2‐/‐) mice. IRS2‐/‐ mice had elevated blood pressure and glucose level as expected. 24‐h urine collections revealed that creatinine clearance did not significantly differ between these groups. Fractional excretion of sodium was significantly lower in IRS1‐/‐ and IRS2‐/‐ compared to WT mice. Albuminuria was found in IRS1‐/‐ and IRS2‐/‐ groups. We assessed autoregulatory responses of total renal blood flow (RBF), and of superficial (SBF) and deep renal cortical blood flow (DBF) to stepwise reductions of renal perfusion pressure (RPP) induced by a manual clamp on the abdominal aorta. RBF, SBF and DBF fell significantly more in IRS1‐/‐ and IRS2‐/‐ than in WT mice. These results indicate that IRS plays a major role in the stimulation of renal functions and renal hemodynamics in type II diabetes.

Role of Acid‐Sensing Ion Channels in hemodynamic effects following nitric oxide inhibition in the rat

Acid‐sensing ion channels (ASICs), a member of degenerin/ENaC/ASIC family, plays ubiquitous roles in the cardiovascular (CV) system as they contribute to exercise pressor reflex and potentiate myogenic tone. As nitric oxide (NO) is a ubiquitous player in CV regulation, this study evaluated ASICs role and the mechanisms of their involvement regulating systemic and renal hemodynamic changes following NO blockade. In the anesthetized rat, amiloride (AM; 2.5 mg/kg) or benzamil (BENZ; 0.7 mg/kg), ENaC/ASIC inhibitor, increased basal mean blood pressure (MBP) by 25±5 and 55±7 mmHg, respectively, but reduced renal cortical blood flow (RCBF) by 111±7 or 99±6 PU, respectively, suggesting that ASICs helps in maintaining BP and renal perfusion. AM or BENZ enhanced L‐NAME‐induced increase in MBP by 44±6 or 55±7%, respectively, but blunted the decrease in RCBF. Verapamil (1 mg/kg), a voltage‐operated calcium channel (VOCC) blocker, reversed the effects of AM and BENZ on basal and L‐NAME‐induced increase in MBP suggesting the contribution of IC Ca2+ to the effects of ASICs. KB‐R7943 5 mg/kg), a Na+/Ca2+ exchange (NCX) inhibitor, also reversed AM or BENZ effects on basal MBP but enhanced the effects on RCBF suggesting an effect of ionic movement via NCX. We suggest that ASICs regulate systemic and renal hemodynamics and the pressor responses due to NO withdrawal by mechanisms involving ionic movement across VOC and NCX.

IRAP and AT1 receptor mediated effects of angiotensin IV

IRAP and AT₁ receptor mediated effects of angiotensin IV

Assessment of a New Mathematical Model for the Computation of Numerical Parameters Related to Renal Cortical Blood Flow and Fractional Blood Volume by Contrast-Enhanced Ultrasound

We analyzed the value of a new mathematical model for the quantification of renal cortical blood flow and fractional blood volume by contrast-enhanced ultrasound after the injection of sulfur hexafluoride–filled microbubbles. A vessel-mimicking phantom experiment was preliminarily performed which showed that the effect of microbubble diffusion is negligible compared with the effect of liquid drag. Twelve healthy volunteers (7 male, 5 female; 27 to 48 years [ n = 6; group 1], and 61 to 80 years [ n = 6; group 2], respectively), with normal renal and cardiac function and not undergoing any pharmacologic treatment, were examined. In each volunteer, both kidneys were scanned after intravenous injection of sulfur hexafluoride–filled microbubbles at a slow rate (4.8 mL at a flow of 4.0 mL/min), and the refill kinetics of the renal cortex after microbubble destruction was evaluated by echo-signal intensity quantification. The progressive replenishment of the renal vessels was approximated both by standard negative exponential function and by the piecewise linear function resulting from our mathematical model. A better dataset approximation was provided by piecewise linear versus standard negative exponential function (overall mean square error: 0.44 vs. 0.51; p < 0.05, Wilcoxon test). The piecewise linear function provided a curve composed of four linear tracts ( n = 3 volunteers; 2 from group 1 and 1 from group 2), three linear tracts ( n = 6 volunteers; 3 from group 1 and 3 from group 2) or two linear tracts ( n = 3 volunteers; 1 from group 1 and 2 from group 2). The piecewise linear function versus standard negative exponential function improved data approximation for the computation of numerical values related to renal cortical blood flow velocity and fractional blood volume. (E-mail: quaia@univ.trieste.it)

Abnormal autoregulation and tubuloglomerular feedback in prediabetic and diabetic OLETF rats

The mechanisms underlying the development and prevention of diabetic nephropathy are still not fully understood. In the present study in the Otsuka Long-Evans Tokushima Fatty (OLETF) model of type 2 diabetic rats, we investigated whether renal hemodynamic abnormalities exist and whether they precede the onset of diabetes. Using OLETF rats in both prediabetic and diabetic stages, we assessed autoregulatory responses of total renal blood flow (RBF) and of superficial (SBF) and deep renal cortical (DBF) blood flow to stepwise reductions of renal perfusion pressure (RPP) induced by a manual clamp on the abdominal aorta. During clamp-induced reductions of RPP by 10 or 20 mmHg, RBF fell significantly more in OLETF rats than in lean control [Long-Evans Tokushima Otsuka (LETO)] rats. Whereas SBF showed no significant changes in either OLETF rats or LETO rats during mild clamping, DBF decreased significantly more in OLETF rats than LETO rats. Reduced autoregulatory efficiency in OLETF rats was observed in both prediabetic and diabetic stages. Micropuncture studies showed that tubuloglomerular feedback (TGF) responses of stop flow pressure are reduced in prediabetic (-7.3 vs. -25.7%) as well as in diabetic OLETF rats compared with LETO control rats (-4.4 vs. -18.8%). Renal corticotomy was performed to measure glomerular capillary pressure (Pgc) directly. Pgc of deep cortical glomeruli was higher than superficial glomerular Pgc in both strains of rats, but the difference was especially pronounced in OLETF rats (deep 78 +/- 2 vs. superficial 57 +/- 4 mmHg). This study demonstrates reduced autoregulatory adjustments and impaired TGF efficiency in prediabetic OLETF rats. Thus abnormal RBF regulation precedes the onset of diabetes and is especially pronounced in the deep cortical region.

Thermal oscillations in rat kidneys: an infrared imaging study

A high-resolution infrared (IR) camera was used to assess rhythmicity in localized renal blood flow, including the extent of regions containing nephrons with spontaneous oscillations in their individual blood flow. The IR imaging was able to follow changes in rat renal perfusion during baseline conditions, during occlusion of the main renal artery and during the administration of either saline or papaverine. Concurrent recordings were made of tubular pressure in superficial nephrons. Spontaneous vascular oscillations centred around 0.02–0.05 Hz and approximately 0.01 Hz could be detected reproducibly by IR imaging. Their spectral characteristics and their response to papaverine were in line with tubular pressure measurements. The intensity of and synchrony between thermal signals from different local areas of the kidney may allow, after surgical exposure, non-invasive imaging of functional clusters involved in renal cortical blood flow. Through visualization of the spatial extent of thermal oscillations, IR imaging holds promise in assessing kidney autoregulatory mechanisms.

Brain and peripheral angiotensin II type 1 receptors mediate renal vasoconstrictor and blood pressure responses to angiotensin IV in the rat

Angiotensin (Ang) IV was reported to increase renal cortical blood flow (CBF) via putative angiotensin IV receptor (AT4) stimulation but reduce total renal blood flow (RBF) via angiotensin II type 1 (AT1) receptors. We investigated the effect of Ang IV on simultaneously measured mean arterial pressure (MAP), RBF, and CBF. The possible involvement of AT1 or AT4 receptors, the possible natriuretic effect, and responses to central administration were also explored. Intravenous injections of Ang IV dose dependently increased MAP and decreased CBF and RBF; these effects were abolished by AT1 receptor blockade. These reductions in CBF and RBF highly correlated as did renal vascular responses to Ang II and fenoldopam. Ang IV did not induce renal vasodilation even following AT1 receptor blockade. Intrarenal Ang IV infusion reduced CBF and RBF but had no natriuretic effect. Central Ang IV administration induced an AT1-mediated immediate increase in MAP and renal vascular resistance and a secondary increase in RBF. AT4 selective ligands, LVV-hemorphin-7 and AT4-16 (intravenous, intrarenal or intracerebroventricular), had no effects on MAP, RBF or urinary sodium excretion. Additional in-vitro experiments indicated that the majority of the Ang IV-sensitive aminopeptidase activity in kidney membranes is attributed to aminopeptidase-N. Insulin-regulated aminopeptidase (IRAP)/AT4 receptors are involved in neither the regulation of RBF or CBF nor in the handling of renal sodium. Ang IV increases MAP and induces renal vasoconstriction via stimulation of brain and peripheral AT1 receptors and may be involved in the regulation of renal blood flow and blood pressure.

Peroxisome proliferator-activated receptor α activation attenuated angiotensin type 1-mediated but enhanced angiotensin type 2-mediated hemodynamic effects to angiotensin II in the rat

Peroxisome proliferator-activated receptors (PPARs) are ligand-dependent nuclear transcription factors that regulate beta-oxidation of fatty acids in various tissues. PPARalpha ligands also protect against pathological damage especially resulting from angiotensin II hypertension. The modulating effect of PPARalpha on hemodynamic effects elicited by angiotensin II under normal conditions, however, is not fully known. We therefore evaluated renal and systemic hemodynamic effects of angiotensin II in normal animals treated with PPARalpha ligands. PPARalpha ligands clofibrate (250 mg/kg), fenofibrate (100 mg/kg), or pirixinic acid (WY14643; 45 mg/kg) each elicited an increase in renal peroxisomal beta-oxidation, accompanied by increased renal nitric oxide production. Clofibrate blunted the angiotensin II (3-100 ng/kg)-induced increase in mean arterial blood pressure (P < 0.05) but attenuated the reduction in renal cortical blood flow (laser Doppler flowmetry; P < 0.05). N(omega)-nitro-L-arginine methyl ester (L-NAME) but not D-NAME (100 mg/l) blunted clofibrate-induced inhibition of angiotensin II responses. In the presence of the angiotensin type 1 (AT1)-antagonist losartan (3 mg/kg), clofibrate uncovered a hypotensive effect of angiotensin II and further blunted the residual renal vasoconstriction. L-NAME or the angiotensin type 2 (AT2)-antagonist (S-[+]-1-[(4-dimethylamino]-3-methylphenyl)methyl]-5-[diphenylacetyl]-4,5,6,7-tetrahydro-1H-imidazol[4,5-c]pyridine-6-carboxilic acid; PD123319), but not D-NAME, blunted the effects of losartan and blocked the hypotensive effects of angiotensin II in losartan-treated rats. Except in rats treated for 7 days with WY14643, AT1-receptor expression was downregulated (P < 0.05) while AT2-receptor expression was upregulated (P < 0.05) in renal cortical homogenates from rats treated with clofibrate or WY14643. These data suggest that PPARalpha activation counters AT1-mediated pressor and vasoconstrictor effects and that, during AT1 receptor blockade, PPARalpha activation leads to hypotension coupled to AT2-receptor activation by a mechanism probably involving nitric oxide production.

Tempol improves renal hemodynamics and pressure natriuresis in hyperthyroid rats

Hyperthyroidism in rats is associated with increased oxidative stress. These animals also show abnormal renal hemodynamics and an attenuated pressure-diuresis-natriuresis (PDN) response. We analyzed the role of oxidative stress as a mediator of these alterations by examining acute effects of tempol, a superoxide dismutase mimetic. The effects of increasing bolus doses of tempol (25-150 micromol/kg) on mean arterial pressure (MAP), renal vascular resistance (RVR), and cortical (CBF) and medullary (MBF) blood flow were studied in control and thyroxine (T4)-treated rats. In another experiment, tempol was infused at 150 micromol.kg(-1).h(-1) to analyze its effects on the glomerular filtration rate (GFR) and on PDN response in these animals. Tempol dose dependently decreased MAP and RVR and increased CBF and MBF in control and T4-treated rats, but the T4 group showed a greater responsiveness to tempol in all of these variables. The highest dose of tempol decreased RVR by 13.5 +/- 2.1 and 5.5 +/- 1.2 mmHg.ml(-1).min(-1) in hyperthyroid (P < 0.01) and control rats, respectively. GFR was not changed by tempol in controls but was significantly increased in the hyperthyroid group. Tempol did not change the absolute or fractional PDN responses of controls but significantly improved those of hyperthyroid rats, although without attaining normal values. Tempol increased the slopes of the relationship between renal perfusion pressure and natriuresis (T4+tempol: 0.17 +/- 0.05; T4: 0.09 +/- 0.03 microeq.min(-1).g(-1).mmHg(-1); P < 0.05) and reduced 8-isoprostane excretion in hyperthyroid rats. These results show that antioxidant treatment with tempol improves renal hemodynamic variables and PDN response in hyperthyroid rats, indicating the participation of an increased oxidative stress in these mechanisms.

Angiotensin II Type 2 Receptors and Nitric Oxide Sustain Oxygenation in the Clipped Kidney of Early Goldblatt Hypertensive Rats

Angiotensin-converting enzyme inhibitors (ACEIs) decrease the glomerular filtration rate and renal blood flow in the clipped kidneys of early 2-kidney, 1-clip Goldblatt hypertensive rats, but the consequences for oxygenation are unclear. We investigated the hypothesis that angiotensin II type 1 or angiotensin II type 2 receptors or NO synthase mediate renal oxygenation responses to ACEI. Three weeks after left renal artery clipping, kidney function, oxygen (O(2)) use, renal blood flow, renal cortical blood flow, and renal cortical oxygen tension (Po(2)) were measured after acute administration of an ACEI (enalaprilat) and after acute administration of ACEI following acute administration of an angiotensin II type 1 or angiotensin II type 2 receptor blocker (candesartan or PD-123,319) or an NO synthase blocker (N(G)-nitro-L-arginine methyl ester with control of renal perfusion pressure) and compared with mechanical reduction in renal perfusion pressure to the levels after ACEI. The basal renal cortical Po(2) of clipped kidneys was significantly lower than contralateral kidneys (35+/-1 versus 51+/-1 mm Hg; n=40 each). ACEI lowered renal venous Po(2), cortical Po(2), renal blood flow, glomerular filtration rate, and cortical blood flow and increased the renal vascular resistance in the clipped kidney, whereas mechanical reduction in renal perfusion pressure was ineffective. PD-123,319 and N(G)-nitro-L-arginine methyl ester, but not candesartan, reduced the Po(2) of clipped kidneys and blocked the fall in Po(2) with acute ACEI administration. In conclusion, oxygen availability in the clipped kidney is maintained by angiotensin II generation, angiotensin II type 2 receptors, and NO synthase. This discloses a novel mechanism whereby angiotensin can prevent hypoxia in a kidney challenged with a reduced perfusion pressure.

Elimination of Organic Anions in Response to an Early Stage of Renal Ischemia-Reperfusion in the Rat: Role of Basolateral Plasma Membrane Transporters and Cortical Renal Blood Flow

Background/Aims:The knowledge of molecular mechanisms determining drug pharmacokinetics in pathological states is relevant for the development of new therapeutic approaches. This study was undertaken to evaluate the cortical renal blood flow (cRBF) and the renal protein expression of the organic anion transporters (OAT1 and OAT3) in association with the elimination of organic anions in an early stage of renal ischemia-reperfusion. Methods:Ischemic acute renal failure (ARF) was induced in adult male Wistar rats by occlusion of both renal pedicles during 60 min, followed by 60 min of reperfusion (ARF group). Pair-fed sham-operated rats served as controls. The renal protein expression of OAT1 and OAT3 was evaluated by immunohistochemistry techniques and by Western blotting in renal cortex homogenates and in basolateral plasma membranes. A pharmacokinetic study of p-aminohippurate (PAH, a prototypical organic anion) was performed. cRBF was determined using fluorescent microspheres. Results: ARF rats displayed a significant decrease in systemic clearance and in renal excretion of PAH. OAT1 and OAT3 protein abundance showed a statistically significant reduction both in homogenates and in basolateral plasma membranes from ARF rats. Immunohistochemical studies confirmed the changes in the cortical renal expression of these transporters. ARF animals also showed a decrease in cRBF. Conclusions: The decrease in PAH elimination observed in an early stage of renal ischemia-reperfusion in male Wistar rats might be explained by the sum of the lower OAT1 and OAT3 expression in renal basolateral plasma membranes plus the decrease in cRBF. These findings might have significant implications in the development of novel pharmacological strategies to be applied in the initial stages of ischemic ARF.