Nitric oxide-angiotensin II interactions in angiotensin II-dependent hypertension

Renal vascular responses to angiotensin II infusion in two kidneys-one clip hypertensive rats under partial ischemia/reperfusion with and without ischemia preconditioning: the roles of AT1R blockade and co-blockades of AT1R and MasR

Background and purpose

The renin-angiotensin system activation, partial ischemia/reperfusion (IR) injury, and hypertension contribute to the development of acute kidney injury. The study aims to look at the vascular responses of angiotensin II (Ang II) during Ang II type 1 receptor (AT1R) blockade (losartan) or co-blockades of AT1R and Mas receptor (A779) in two kidneys one clip (2K1C) hypertensive rats which subjected to partial IR injury with and without ischemia preconditioning (IPC).

Experimental approach

Thirty-three 2K1C male Wistar rats with systolic blood pressure ≥ 150 mmHg were divided into three groups of sham, IR, and IPC + IR divided into two sub-groups receiving losartan or losartan + A779. The IR group had 45 min partial kidney ischemia, while the IPC + IR group had two 5 min cycles of partial ischemia followed by 10 min of reperfusion and then 45 min of partial kidney ischemia followed by reperfusion. The sham group was subjected to similar surgical procedures except for IR or IPC.

Findings/Results

Ang II increased mean arterial pressure in all the groups, but there were no significant differences between the sub-groups. A significant difference was observed in the renal blood flow response to Ang II between two sub-groups of sham and IR groups treated with AT1R blockade alone or co-blockades of AT1R + A779.

Conclusion and implications

These findings demonstrated the significance of AT1R and Mas receptor following partial renal IR in the renal blood flow responses to Ang II in 2K1C hypertensive rats.

Tempol, a superoxide dismutase-mimetic drug, prevents chronic ischemic renal injury in two-kidney, one-clip hypertensive rats

Tempol, a superoxide dismutase-mimetic drug, has been shown to attenuate radical-induced damage, exerting beneficial effects in the animal models of oxidative stress and hypertension. This study evaluated the effect of Tempol on renal structural and functional alterations in two-Kidney, one-Clip hypertensive rats. In this study, young male Wistar rats had the left kidney clipped (2K1C), and sham-operated animals (Sham) were used as controls. Animals received Tempol (1mmol/L in drinking water) or vehicle for 5 weeks. Systolic blood pressure was evaluated once a week. At the end of the experimental protocol, the animals were placed in metabolic cages to collect urine (24h) and then anesthetized with thiopental (70mg/kg i.p.) to collect blood by puncturing the descending aorta for biochemical analysis, and the clipped kidney for morphological and immunohistochemical analyses. The vasodilator effect of Tempol was evaluated in mesenteric arterial bed (MAB) isolated from adult Wistar rats. The chronic treatment with Tempol prevented the development of hypertension and the increased plasma levels of urea, creatinine, and 8-isoprostane in 2K1C animals. Tempol also improved both glomeruli number and kidney volume to normal levels in the 2K1C+Tempol group. In addition, the treatment prevented the increased collagen deposition and immunostaining for renin, caspase-3, and 8-isoprostane in the stenotic kidney of 2K1C animals. Moreover, Tempol induced a dose-dependent vasodilator response in MAB from Wistar rats. These results suggest that Tempol protects the stenotic kidney against chronic ischemic renal injury and prevents renal dysfunction in the 2K1C model, probably through its antioxidant, vasodilator and antihypertensive actions.

Collecting Duct Nitric Oxide Synthase 1ß Activation Maintains Sodium Homeostasis During High Sodium Intake Through Suppression of Aldosterone and Renal Angiotensin II Pathways

BACKGROUND

During high sodium intake, the renin-angiotensin-aldosterone system is downregulated and nitric oxide signaling is upregulated in order to remain in sodium balance. Recently, we showed that collecting duct nitric oxide synthase 1β is critical for fluid-electrolyte balance and subsequently blood pressure regulation during high sodium feeding. The current study tested the hypothesis that high sodium activation of the collecting duct nitric oxide synthase 1β pathway is critical for maintaining sodium homeostasis and for the downregulation of the renin-angiotensin-aldosterone system-epithelial sodium channel axis.

METHODS AND RESULTS

Male control and collecting duct nitric oxide synthase 1β knockout (CDNOS1KO) mice were placed on low, normal, and high sodium diets for 1 week. In response to the high sodium diet, plasma sodium was significantly increased in control mice and to a significantly greater level in CDNOS1KO mice. CDNOS1KO mice did not suppress plasma aldosterone in response to the high sodium diet, which may be partially explained by increased adrenal AT1R expression. Plasma renin concentration was appropriately suppressed in both genotypes. Furthermore, CDNOS1KO mice had significantly higher intrarenal angiotensin II with high sodium diet, although intrarenal angiotensinogen levels and angiotensin-converting enzyme activity were similar between knockout mice and controls. In agreement with inappropriate renin-angiotensin-aldosterone system activation in the CDNOS1KO mice on a high sodium diet, epithelial sodium channel activity and sodium transporter abundance were significantly higher compared with controls.

CONCLUSIONS

These data demonstrate that high sodium activation of collecting duct nitric oxide synthase 1β signaling induces suppression of systemic and intrarenal renin-angiotensin-aldosterone system, thereby modulating epithelial sodium channel and other sodium transporter abundance and activity to maintain sodium homeostasis.

Epoxyeicosatrienoic acid analog attenuates the development of malignant hypertension, but does not reverse it once established: a study in Cyp1a1-Ren-2 transgenic rats

OBJECTIVE

We evaluated the therapeutic effectiveness of a new, orally active epoxyeicosatrienoic acid analog (EET-A) in rats with angiotensin II (ANG II)-dependent malignant hypertension.

METHODS

Malignant hypertension was induced in Cyp1a1-Ren-2 transgenic rats by activation of the renin gene using indole-3-carbinol (I3C), a natural xenobiotic. EET-A treatment was started either simultaneously with I3C induction process (early treatment) or 10 days later during established hypertension (late treatment). Blood pressure (BP) (radiotelemetry), indices of renal and cardiac injury, and plasma and kidney levels of the components of the renin-angiotensin system (RAS) were determined.

RESULTS

In I3C-induced hypertensive rats, early EET-A treatment attenuated BP increase (to 175 ± 3 versus 193 ± 4 mmHg, P < 0.05, on day 13), reduced albuminuria (15 ± 1 versus 28 ± 2 mg/24 h, P < 0.05), and cardiac hypertrophy as compared with untreated I3C-induced rats. This was associated with suppression of plasma and kidney ANG II levels (48 ± 6 versus 106 ± 9 and 122 ± 19 versus 346 ± 11 fmol ml or g, respectively, P < 0.05) and increases in plasma and kidney angiotensin (1-7) concentrations (84 ± 9 versus 37 ± 6 and 199 ± 12 versus 68 ± 9 fmol/ml or g, respectively, P < 0.05). Remarkably, late EET-A treatment did not lower BP or improve renal and cardiac injury; indices of RAS activity were not affected.

CONCLUSION

The new, orally active EET-A attenuated the development of experimental ANG II-dependent malignant hypertension, likely via suppression of the hypertensiogenic axis and augmentation of the vasodilatory/natriuretic axis of RAS.

Myoglobin facilitates angiotensin II-induced constriction of renal afferent arterioles

Vasoconstriction plays an important role in the development of acute kidney injury in rhabdomyolysis. We hypothesized that myoglobin enhances the angiotensin II (ANG II) response in afferent arterioles by increasing superoxide and reducing nitric oxide (NO) bioavailability. Afferent arterioles of C57Bl6 mice were isolated perfused, and vasoreactivity was analyzed using video microscopy. NO bioavailability, superoxide concentration in the vessel wall, and changes in cytosolic calcium were measured using fluorescence techniques. Myoglobin treatment (10−5 M) did not change the basal arteriolar diameter during a 20-min period compared with control conditions. NG-nitro-l-arginine methyl ester (l-NAME, 10−4 M) and l-NAME + myoglobin reduced diameters to 94.7 and 97.9% of the initial diameter, respectively. Myoglobin or l-NAME enhanced the ANG II-induced constriction of arterioles compared with control (36.6 and 34.2%, respectively, vs. 65.9%). Norepinephrine responses were not influenced by myoglobin. Combined application of myoglobin and l-NAME further facilitated the ANG II response (7.0%). Myoglobin or l-NAME decreased the NO-related fluorescence in arterioles similarly. Myoglobin enhanced the superoxide-related fluorescence, and tempol prevented this enhancement. Tempol also partly prevented the myoglobin effect on the ANG II response. Myoglobin increased the fura 2 fluorescence ratio (cytosolic calcium) during ANG II application (10−12 to 10−6 M). The results suggest that the enhanced afferent arteriolar reactivity to ANG II is mainly due to a myoglobin-induced increase in superoxide and associated reduction in the NO bioavailability. Signaling pathways for the augmented ANG II response include enhanced cytosolic calcium transients. In conclusion, myoglobin may contribute to the afferent arteriolar vasoconstriction in this rhabdomyolysis model.

NADPH Oxidase in the Renal Microvasculature Is a Primary Target for Blood Pressure–Lowering Effects by Inorganic Nitrate and Nitrite

Renal oxidative stress and nitric oxide (NO) deficiency are key events in hypertension. Stimulation of a nitrate–nitrite–NO pathway with dietary nitrate reduces blood pressure, but the mechanisms or target organ are not clear. We investigated the hypothesis that inorganic nitrate and nitrite attenuate reactivity of renal microcirculation and blood pressure responses to angiotensin II (ANG II) by modulating nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and NO bioavailability. Nitrite in the physiological range (10 −7 –10 −5 mol/L) dilated isolated perfused renal afferent arterioles, which were associated with increased NO. Contractions to ANG II (34%) and simultaneous NO synthase inhibition (56%) were attenuated by nitrite (18% and 26%). In a model of oxidative stress (superoxide dismutase-1 knockouts), abnormal ANG II–mediated arteriolar contractions (90%) were normalized by nitrite (44%). Mechanistically, effects of nitrite were abolished by NO scavenger and xanthine oxidase inhibitor, but only partially attenuated by inhibiting soluble guanylyl cyclase. Inhibition of NADPH oxidase with apocynin attenuated ANG II–induced contractility (35%) similar to that of nitrite. In the presence of nitrite, no further effect of apocynin was observed, suggesting NADPH oxidase as a possible target. In preglomerular vascular smooth muscle cells and kidney cortex, nitrite reduced both basal and ANG II–induced NADPH oxidase activity. These effects of nitrite were also abolished by xanthine oxidase inhibition. Moreover, supplementation with dietary nitrate (10 −2 mol/L) reduced renal NADPH oxidase activity and attenuated ANG II–mediated arteriolar contractions and hypertension (99±2–146±2 mm Hg) compared with placebo (100±3–168±3 mm Hg). In conclusion, these novel findings position NADPH oxidase in the renal microvasculature as a prime target for blood pressure–lowering effects of inorganic nitrate and nitrite.

Exogenous L-arginine attenuates the effects of angiotensin II on renal hemodynamics and the pressure natriuresis-diuresis relationship

Administration of exogenous L-arginine (L-Arg) attenuates angiotensin-II (AngII)-mediated hypertension and kidney disease in rats. The present study assessed renal hemodynamics and pressure diuresis-natriuresis in anaesthetized rats infused with vehicle, AngII (20 ng/kg per min i.v.) or AngII + L-Arg (300 μg/kg per min i.v.). Experiments in isolated aortic rings were carried out to assess L-Arg effects on the vasculature. Increasing renal perfusion pressure (RPP) from ~100 to 140 mmHg resulted in a nine- to tenfold increase in urine flow and sodium excretion rate in control animals. In comparison, AngII infusion significantly reduced renal blood flow (RBF) and glomerular filtration rate (GFR) by 40-42%, and blunted the pressure-dependent increase in urine flow and sodium excretion rate by 54-58% at elevated RPP. Supplementation of L-Arg reversed the vasoconstrictor effects of AngII and restored pressure-dependent diuresis to levels not significantly different from control rats. Dose-dependent contraction to AngII (10(-10) mol/L to 10(-7) mol/L) was observed with a maximal force equal to 27 ± 3% of the response to 10(-5) mol/L phenylephrine. Contraction to 10(-7) mol/L AngII was blunted by 75 ± 3% with 10(-4) mol/L L-Arg. The influence of L-Arg to blunt AngII-mediated contraction was eliminated by endothelial denudation or incubation with nitric oxide synthase inhibitors. Furthermore, the addition of 10(-3) mol/L cationic or neutral amino acids, which compete with L-Arg for cellular uptake, blocked the effect of L-Arg. Anionic amino acids did not influence the effects of L-Arg on AngII-mediated contraction. These studies show that L-Arg blunts AngII-mediated vascular contraction by an endothelial- and nitric oxide synthase-dependent mechanism involving cellular uptake of L-Arg.

Angiotensin II-induced hypertension blunts thick ascending limb NO production by reducing NO synthase 3 expression and enhancing threonine 495 phosphorylation

Thick ascending limbs reabsorb 30% of the filtered NaCl load. Nitric oxide (NO) produced by NO synthase 3 (NOS3) inhibits NaCl transport by this segment. In contrast, chronic angiotensin II (ANG II) infusion increases net thick ascending limb transport. NOS3 activity is regulated by changes in expression and phosphorylation at threonine 495 (T495) and serine 1177 (S1177), inhibitory and stimulatory sites, respectively. We hypothesized that NO production by thick ascending limbs is impaired by chronic ANG II infusion, due to reduced NOS3 expression, increased phosphorylation of T495, and decreased phosphorylation of S1177. Rats were infused with 200 ng·kg(-1)·min(-1) ANG II or vehicle for 1 and 5 days. ANG II infusion for 5 days decreased NOS3 expression by 40 ± 12% (P < 0.007; n = 6) and increased T495 phosphorylation by 147 ± 26% (P < 0.008; n = 6). One-day ANG II infusion had no significant effect. NO production in response to endothelin-1 was blunted in thick ascending limbs from ANG II-infused animals [ANG II -0.01 ± 0.06 arbitrary fluorescence units (AFU)/min vs. 0.17 ± 0.02 AFU/min in controls; P < 0.01]. This was not due to reduced endothelin-1 receptor expression. Phosphatidylinositol 3,4,5-triphosphate (PIP3)-induced NO production was also reduced in ANG II-infused rats (ANG II -0.07 ± 0.06 vs. 0.13 ± 0.04 AFU/min in controls; P < 0.03), and this correlated with an impaired ability of PIP3 to increase S1177 phosphorylation. We conclude that in ANG II-induced hypertension NO production by thick ascending limbs is impaired due to decreased NOS3 expression and altered phosphorylation.

Antihypertensive and renoprotective actions of soluble epoxide hydrolase inhibition in ANG II-dependent malignant hypertension are abolished by pretreatment with L-NAME

OBJECTIVE

The present study was performed to investigate in a model of malignant hypertension if the antihypertensive actions of soluble epoxide hydrolase (sEH) inhibition are nitric oxide (NO)-dependent.

METHODS

ANG II-dependent malignant hypertension was induced through dietary administration for 3 days of the natural xenobiotic indole-3-carbinol (I3C) in Cyp1a1-Ren-2 transgenic rats. Blood pressure (BP) was monitored by radiotelemetry and treatment with the sEH inhibitor [cis-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyl-oxy]-benzoic acid (c-AUCB)] was started 48 h before administration of the diet containing I3C. In separate groups of rats, combined administration of the sEH inhibitor and the nonspecific NO synthase inhibitor [Nω-nitro-L-arginine methyl ester (L-NAME)] on the course of BP in I3C-induced and noninduced rats were evaluated. In addition, combined blockade of renin-angiotensin system (RAS) was superimposed on L-NAME administration in separate groups of rats. After 3 days of experimental protocols, the rats were prepared for renal functional studies and renal concentrations of epoxyeicosatrienoic acids (EETs) and their inactive metabolites dihydroxyeicosatrienoic acids (DHETEs) were measured.

RESULTS

Treatment with c-AUCB increased the renal EETs/DHETEs ratio, attenuated the increases in BP, and prevented the decreases in renal function and the development of renal damage in I3C-induced Cyp1a1-Ren-2 rats. The BP lowering and renoprotective actions of the treatment with the sEH inhibitor c-AUCB were completely abolished by concomitant administration of L-NAME and not fully rescued by double RAS blockade without altering the increased EETs/DHETEs ratio.

CONCLUSION

Our current findings indicate that the antihypertensive actions of sEH inhibition in this ANG II-dependent malignant form of hypertension are dependent on the interactions of endogenous bioavailability of EETs and NO.

Soluble epoxide hydrolase inhibition exhibits antihypertensive actions independently of nitric oxide in mice with renovascular hypertension

OBJECTIVE

The present study was performed to examine whether the blood pressure (BP)-lowering effects of soluble epoxide hydrolase (sEH) inhibition in two-kidney, one-clip (2K1C) Goldblatt hypertension are nitric oxide (NO) dependent.

METHODS

Mice lacking the endothelial NO synthase (eNOS) gene (eNOS-/-) and their wild-type controls (eNOS+/+) underwent clipping of one renal artery. BP was monitored by radiotelemetry and the treatment with the sEH inhibitor cis-4-[4-(3-adamantan-1-yl-ureido)cyclohex-yloxy]-benzoic acid (c-AUCB) was initiated on day 25 after clipping and lasted for 14 days. Renal concentrations of epoxyeicosatrienoic acids (EETs) and their inactive metabolite dihydroxyeicosatrienoic acids (DHETs) were measured in the nonclipped kidney. Renal NO synthase (NOS) activity was determined by measuring the rate of formation of L-[(14)C]citruline from L-[(14)C]arginine.

RESULTS

Treatment with the sEH inhibitor elicited similar BP decreases that were associated with increases in daily sodium excretion in 2K1C eNOS+/+ as well as 2K1C eNOS-/- mice. In addition, treatment with the sEH inhibitor increased the ratio of EETs/DHETs in the nonclipped kidney of 2K1C eNOS+/+ as well as 2K1C eNOS-/- mice. Treatment with the sEH inhibitor did not alter renal NOS activity in any of the experimental groups.

CONCLUSIONS

Collectively, our present data suggest that the BP-lowering effects of chronic sEH inhibition in 2K1C mice are mainly associated with normalization of the reduced availability of biologically active EETs in the nonclipped kidney and their direct natriuretic actions.

The role of oxidative stress in renovascular hypertension

1. There is mounting evidence that increased oxidative stress and sympathetic nerve activity play important roles in renovascular hypertension. In the present review, we focus on the importance of oxidative stress in two distinct populations of neurons involved with cardiovascular regulation: those of the rostral ventrolateral medulla (RVLM) and those of the paraventricular nucleus of the hypothalamus (PVN) in the maintenance of sympathoexcitation and hypertension in two kidney-one clip (2K1C) hypertensive rats. Furthermore, the role of oxidative stress in the clipped kidney is also discussed. 2. In the studies reviewed in this article, it was found that hypertension and renal sympathoexcitation in 2K1C rats were associated with an increase in Angiotensin II type one receptor (AT(1) ) expression and in oxidative markers within the RVLM, PVN and in the clipped kidneys of 2K1C rats. Furthermore, acute or chronic anti-oxidant treatment decreased blood pressure and sympathetic activity, and improved the baroreflex control of heart rate and renal sympathetic nerve activity in 2K1C rats. Tempol or vitamin C administration in the RVLM, PVN or systemically all reduced blood pressure and renal sympathetic activity. Cardiovascular improvement in response to chronic anti-oxidant treatment was associated with a downregulation of AT(1) receptors, as well as oxidative markers in the central nuclei and clipped kidney. 3. The data discussed in the present review support the idea that an increase in oxidative stress within the RVLM, PVN and in the ischaemic kidney plays a major role in the maintenance of sympathoexcitation and hypertension in 2K1C rats.

Endogenous angiotensin II modulates nNOS expression in renovascular hypertension

Nitric oxide (NO) influences renal blood flow mainly as a result of neuronal nitric oxide synthase (nNOS). Nevertheless, it is unclear how nNOS expression is modulated by endogenous angiotensin II, an inhibitor of NO function. We tested the hypothesis that the angiotensin II AT1 receptor and oxidative stress mediated by NADPH oxidase contribute to the modulation of renal nNOS expression in two-kidney, one-clip (2K1C) hypertensive rats. Experiments were performed on male Wistar rats (150 to 170 g body weight) divided into 2K1C (N = 19) and sham-operated (N = 19) groups. nNOS expression in kidneys of 2K1C hypertensive rats (N = 9) was compared by Western blotting to that of 2K1C rats treated with low doses of the AT1 antagonist losartan [10 mg x kg(-1) x day(-1); N = 5] or the superoxide scavenger tempol [0.2 mmol x kg(-1) x day(-1); N = 5], which still remain hypertensive. After 28 days, nNOS expression was significantly increased by 1.7-fold in the clipped kidneys of 2K1C rats and by 3-fold in the non-clipped kidneys of 2K1C rats compared with sham rats, but was normalized by losartan. With tempol treatment, nNOS expression increased 2-fold in the clipped kidneys and 1.4-fold in the non-clipped kidneys compared with sham rats. The changes in nNOS expression were not followed by changes in the enzyme activity, as measured indirectly by the cGMP method. In conclusion, AT1 receptors and oxidative stress seem to be primary stimuli for increased nNOS expression, but this up-regulation does not result in higher enzyme activity.

Blood pressure, blood flow, and oxygenation in the clipped kidney of chronic 2-kidney, 1-clip rats: effects of tempol and Angiotensin blockade

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.

Protective actions of estrogen on angiotensin II-induced hypertension: role of central nitric oxide

The present study tested the hypotheses that 1) nitric oxide (NO) is involved in attenuated responses to ANG II in female mice, and 2) there is differential expression of neuronal NO synthase (nNOS) in the subfornical organ (SFO) and paraventricular nucleus (PVN) in response to systemic infusions of ANG II in males vs. females. Aortic blood pressure (BP) was measured in conscious mice with telemetry implants. N(G)-nitro-l-arginine methyl ester (l-NAME; 100 microg x kg(.-1)day(-1)), an inhibitor of NOS, was administrated into the lateral cerebral ventricle for 14 days before and during ANG II pump implantation. Central infusion of l-NAME augmented the pressor effects of systemic ANG II in females (Delta21.5 + or - 2.2 vs. Delta9.2 + or - 1.5 mmHg) but not in males (Delta29.4 + or - 2.5 vs. Delta30.1 + or - 2.5 mmHg). Central administration of N(5)-(1-imino-3-butenyl)-l-ornithine (l-VNIO), a selective nNOS inhibitor, also significantly potentiated the increase in BP induced by ANG II in females (Delta17.5 + or - 3.2 vs. Delta9.2 + or - 1.5 mmHg). In gonadectomized mice, central l-NAME infusion did not affect the pressor response to ANG II in either males or females. Ganglionic blockade after ANG II infusion resulted in a greater reduction in BP in central l-NAME- or l-VNIO-treated females compared with control females. Western blot analysis of nNOS protein expression indicated that levels were approximately 12-fold higher in both the SFO and PVN of intact females compared with those in intact males. Seven days of ANG II treatment resulted in a further increase in nNOS protein expression only in intact females (PVN, to approximately 51-fold). Immunohistochemical studies revealed colocalization of nNOS and estrogen receptors in the SFO and PVN. These results suggest that NO attenuates the increase in BP induced by ANG II through reduced sympathetic outflow in females and that increased nNOS protein expression associated with the presence of female sex hormones plays a protective role against ANG II-induced hypertension in female mice.

Blockade of AT1 receptor partially restores vasoreactivity, NOS expression, and superoxide levels in cerebral and carotid arteries of hindlimb unweighting rats

Previous studies have demonstrated activation of the local renin-angiotensin system in hindlimb unweighting (HU) rat vasculature. The present study intended to identify the effects of blockade of angiotensin II (ANG II) type 1 (AT1) receptors with losartan on vascular reactivity, nitric oxide synthase (NOS) expression, and superoxide anion (O2•−) levels in 3-wk HU rat cerebral and carotid arteries. Three weeks later, vasoconstriction, vasodilatation, endothelial NOS (eNOS) and inducible NOS (iNOS) protein, as well as O2•− levels in rat cerebral and carotid arteries were examined. We found that HU enhanced maximal response to KCl/5-hydroxytryptamine ( P < 0.01) in basilar arteries and KCl/phenylephrine ( P < 0.05) in common carotid arteries from HU rats. Acetylcholine induced concentration-dependent vasodilatation in all the artery rings, but with significantly smaller amplitude in basilar ( P < 0.01) and common carotid ( P < 0.05) arteries from HU rats than those from control rats. Chronic treatment with losartan partially restored response to vasoconstrictors and acetylcholine-induced vasodilatation in basilar ( P < 0.01) and common carotid ( P < 0.05) arteries from losartan-treated HU rats. Furthermore, iNOS content in cerebral arteries and eNOS/iNOS content in carotid arteries were significantly ( P < 0.01) increased in HU rats. Meanwhile, HU increased O2•− levels in all the layers of these arteries. However, losartan restored NOS content and O2•− levels toward normal. These results suggested that the HU-induced enhancement of vasoconstriction and reduction in endothelium-dependent relaxation involved alterations in O2•− and NOS content through an ANG II/AT1 receptor signaling pathway.

Exogenous L-arginine ameliorates angiotensin II-induced hypertension and renal damage in rats

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.

Pivotal role of angiotensin II receptor subtype 1A in the development of two-kidney, one-clip hypertension: study in angiotensin II receptor subtype 1A knockout mice

OBJECTIVE

The present study was performed to examine in two-kidney, one-clip (2K1C) Goldblatt hypertensive mice: first, the relative contribution of angiotensin II receptor subtypes 1A (AT(1A)) and 1B (AT(1B)); second, the role of angiotensin II type 2 (AT(2)) receptors in the development of hypertension in wild-type (AT(1A)+/+) and AT(1A) receptor knockout (AT(1A)-/-) mice; and third, the role of increased nitric oxide synthase activity in counteracting the hypertensinogenic action of angiotensin II in this model.

METHODS

AT(1A)+/+ and AT(1A)-/- mice underwent clipping of one renal artery and were infused with either saline vehicle or selective AT(2) receptor agonist CGP-42112A (CGP). Blood pressure was monitored by radiotelemetry. Blood pressure responses to the nitric oxide synthase inhibitor nitro-L-arginine-methyl-ester were evaluated.

RESULTS

AT(1A)+/+ mice responded to clipping by a rise in blood pressure that was not modified by CGP infusion. Clip placement caused a slight increase in blood pressure in AT(1A)-/- mice that remained significantly lower than in AT(1A)+/+ mice. Acute nitric oxide synthase inhibition caused greater increase in blood pressure in 2K1C/AT(1A)+/+ than in AT(1A)+/+ mice.

CONCLUSION

The present data support the critical role of AT(1A) receptors in the development of 2K1C hypertension, whereas AT(1B) receptors play only a minor role in blood pressure regulation in this model of angiotensin II-dependent hypertension. Activation of AT(2) receptors does not play an antagonistic role in the AT(1) receptor-mediated hypertensinogenic actions of angiotensin II in this model. Finally, enhanced nitric oxide synthase activity plays a protective role by counteracting the vasoconstrictor influences of angiotensin II in 2K1C hypertensive mice.

Superoxide and its interaction with nitric oxide modulates renal function in prehypertensive Ren-2 transgenic rats

OBJECTIVE

The present study was performed to examine the role of superoxide (O2*) and its interaction with nitric oxide (NO) in the regulation of renal function in prehypertensive heterozygous Ren-2 transgenic rats (TGR).

METHODS

Renal responses to the O2* scavenger, tempol (150 microg/min per 100 g), and/or the NO synthase inhibitor, nitro-L-arginine methylester (L-NAME; 5 microg/min per 100 g), infused alone or in combination directly into the renal artery were evaluated in anesthetized heterozygous male TGR and aged-matched Hanover Sprague-Dawley rats (HanSD).

RESULTS

There were no differences in arterial pressure (122 +/- 3 versus 115 +/- 2 mmHg), renal plasma flow (RPF; 2.09 +/- 0.1 versus 2.07 +/- 0.1 ml/min per g), glomerular filtration rate (GFR; 0.73 +/- 0.1 versus 0.74 +/- 0.1 ml/min per g) or sodium excretion (0.63 +/- 0.13 versus 0.67 +/- 0.16 micromol/min per g) between TGR and HanSD. Tempol alone caused significant increases in RPF and GFR (10 +/- 4% and 12 +/- 2%, respectively) in TGR but not in HanSD. Tempol also caused greater sodium excretory responses in TGR compared to HanSD (112 +/- 16% versus 43 +/- 7%; P < 0.05). 8-Isoprostane excretion was significantly higher in TGR than in HanSD (10.2 +/- 0.8 versus 6.5 +/- 0.7 pg/min per g), which was attenuated by tempol. L-NAME caused greater decreases in RPF and GFR in TGR (-34 +/- 4% and -22 +/- 4%, respectively) than in HanSD (-19 +/- 3% and -10 +/- 4%, respectively). Co-infusion of tempol partially attenuated the renal hemodynamic and excretory responses to L-NAME in TGR.

CONCLUSIONS

These data suggest that the enhanced O2* activity and its interaction with NO during the prehypertensive phase in TGR modulates renal hemodynamic and excretory function, which may contribute to the development of hypertension in this transgenic rat model.

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.

Renoprotective effects of neuronal NOS-derived nitric oxide and cyclooxygenase-2 metabolites in transgenic rats with inducible malignant hypertension

The present study was performed to determine the effects of neuronal nitric oxide synthase (nNOS) and cyclooxygenase-2 (COX-2) inhibition on blood pressure and renal hemodynamics in transgenic rats with inducible ANG II-dependent malignant hypertension [strain name: TGR(Cyp1a1Ren2)]. Male Cyp1a1-Ren2 rats (n = 7) were fed a normal diet containing indole-3-carbinol (I3C; 0.3%) for 6-9 days to induce malignant hypertension. Mean arterial pressure (MAP) and renal hemodynamics were assessed in pentobarbital sodium-anesthetized Cyp1a1-Ren2 rats before and during intravenous infusion of the nNOS inhibitor S-methyl-l-thiocitrulline (l-SMTC; 1 mg/h). In hypertensive Cyp1a1-Ren2 rats, l-SMTC increased MAP from 169 +/- 3 to 188 +/- 4 mmHg (P < 0.01), which was a smaller increase than in noninduced rats (124 +/- 9 to 149 +/- 9 mmHg, P < 0.01, n = 5). Additionally, l-SMTC decreased renal plasma flow (RPF) to a similar extent (-34 +/- 13 vs. -35 +/- 12%) in the hypertensive and normotensive rats (4.1 +/- 0.2 to 2.7 +/- 0.5 and 3.1 +/- 0.3 to 2.0 +/- 0.3 ml x min(-1) x g(-1), respectively, P < 0.01) but did not alter glomerular filtration rate (GFR) in either group. In additional experiments, administration of the COX-2 inhibitor, nimesulide (3 mg/kg i.v.), during simultaneous infusion of l-SMTC decreased MAP in both hypertensive and noninduced rats (182 +/- 2 to 170 +/- 3 mmHg and 153 +/- 3 to 140 +/- 3 mmHg, respectively, P < 0.01). Nimesulide also decreased RPF (1.9 +/- 0.2 to 0.8 +/- 0.1 ml x min(-1) x g(-1), P < 0.01) and GFR (0.9 +/- 0.1 to 0.4 +/- 0.1 ml x min(-1) x g(-1), P < 0.01) in hypertensive rats but did not alter RPF or GFR in noninduced rats. The present findings demonstrate that both nNOS-derived NO and COX-2 metabolites exert pronounced renal vasodilator influences in hypertensive Cyp1a1-Ren2 rats. The data also indicate that the renal vasodilator effects of COX-2-derived prostanoids in hypertensive Cyp1a1-Ren2 rats are not dependent on nNOS activity.

LACK OF BLOOD PRESSURE SALT-SENSITIVITY SUPPORTS A PREGLOMERULAR SITE OF ACTION OF NITRIC OXIDE IN TYPE I DIABETIC RATS

1. The relationship between sodium intake and blood pressure is affected differently by changes in angiotensin (Ang) II and preglomerular resistance, and this study measured that relationship to evaluate the link between nitric oxide and blood pressure early in diabetes. 2. Rats were chronically instrumented, placed on high-sodium (HS = 12 mEq/d) or low-sodium (LS = 0.07 mEq/d) intake diets and assigned to either vehicle- (V) or Nomega-nitro-L-arginine methyl ester- (L-NAME; L) treated groups. Mean arterial pressure (MAP) was measured 18 h/day for a 6-day control and 14-day streptozotocin diabetic period in each animal. 3. The MAP of the control period averaged 95 +/- 1 and 94 +/- 1 mmHg in the LSV and HSV rats and 116 +/- 2 and 124 +/- 1 mmHg in the LSL and HSL rats, respectively (LSL vs HSL was significant at P < 0.05). Diabetes increased MAP only in the LSL and HSL rats to 141 +/- 2 mmHg and 152 +/- 2, respectively, similar to our previous reports, and those respective 25 and 28 mmHg increases were a parallel shift in the pressure natriuresis relationship. However, the apparent difference between the LSL and HSL groups when compared was a parallel of the control MAP difference. Plasma renin activity (PRA) in the control period averaged 1.5 +/- 0.5 and 8.1 +/- 1.8 ng AI/mL per h in the HSV and LSV rats, and 0.8 +/- 0.2 and 2.8 +/- 0.5 ng AI/mL per h in the HSL and LSL rats, respectively, and increased similarly by 4.6-fold in the HSL and 4.8-fold in the LSL rats during diabetes. Glomerular filtration rate (GFR) increased in the vehicle but not the L-NAME-treated groups, consistent with our previous reports. 4. Thus, the hypertension caused by the onset of diabetes in L-NAME-treated rats was not salt-sensitive. The normal modulation of PRA by salt intake and the failure of GFR to increase are consistent with our hypothesis that nitric oxide may protect against hypertension early in diabetes by preventing preglomerular vasoconstriction by AngII.

Interaction of Endothelial Nitric Oxide and Angiotensin in the Circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.

Enhanced superoxide generation modulates renal function in ANG II-induced hypertensive rats

This study was performed to examine the role of superoxide formation in the regulation of renal hemodynamic and excretory function and to assess its contribution in the pathogenesis of ANG II-dependent hypertension. Renal responses to acute intra-arterial infusion of the O2− scavenger tempol (50 μg·min−1·100 g body wt−1) with or without catalase (1,500 U·min−1·100 g−1; both native and polyethylene glycol-catalase), which reduces H2O2, were evaluated in anesthetized male Sprague-Dawley rats treated chronically with ANG II (65 ng/min) for 2 wk and compared with nontreated control rats. In ANG II-treated hypertensive rats, tempol caused increases in medullary (13 ± 2%), cortical (5 ± 2%), and total renal blood flow (9 ± 2%) without altering systemic arterial pressure. There were also increases in glomerular filtration rate (9 ± 2%), urine flow (17 ± 4%), and sodium excretion (26 ± 5%). However, tempol infusion in nontreated normotensive rats did not cause significant changes in any of these renal parameters. Coinfusion of catalase with tempol did not alter the responses observed with tempol alone, indicating that the observed renal responses to tempol in ANG II-treated rats were attributed to its O2− scavenging effects without the involvement of H2O2. Tempol infusion also significantly decreased 8-isoprostane excretion in ANG II-treated rats (39 ± 6%) without changes in H2O2 excretion. However, coinfusion of catalase reduced H2O2 excretion in both ANG II-treated (41 ± 6%) and nontreated rats (28 ± 5%). These data demonstrate that enhanced generation of O2− modulates renal hemodynamic and tubular reabsoptive function, possibly leading to sodium retention and thus contributing to the pathogenesis of ANG II-induced hypertension.

Saline-induced natriuresis and renal blood flow in conscious dogs: effects of sodium infusion rate and concentration

AIM

This study focused on static and dynamic changes in total renal blood flow (RBF) during volume expansion and tested whether a change in RBF characteristics is a necessary effector mechanism in saline-induced natriuresis.

METHODS

The aortic flow subtraction technique was used to measure RBF continuously. Identical amounts of NaCl (2.4 mmol kg(-1)) were given as slow isotonic (Iso, 120 min), slow hypertonic (Hyper, 120 min), and rapid isotonic loads (IsoRapid, 30 min).

RESULTS

During Iso and IsoRapid, arterial blood pressure increased slightly (6-7 mmHg), and during Hyper it remained unchanged. Iso and Hyper increased sodium excretion (4 +/- 1 to 57 +/- 27 and 10 +/- 4 to 79 +/- 28 micromol min(-1), respectively) and decreased plasma renin activity (by 38% and 29%), angiotensin II (by 56% and 58%) and aldosterone (by 47% and 65%), while RBF remained unchanged. IsoRapid caused a similar increase in sodium excretion (to 72 +/- 19 micromol min(-1)), a similar decrease in renin system activity, but a 15% elevation of RBF (282 +/- 22 to 324 +/- 35 mL min(-1)). Selected frequency domain parameters of RBF autoregulation did not change in response to any load.

CONCLUSIONS

In response to slow saline loading simulating daily sodium intake, the rate of sodium excretion may increase 10-20-fold without any change in mean arterial blood pressure or in RBF. Regulatory responses to changes in total body NaCl levels appears, therefore, to be mediated primarily by neurohumoral mechanisms and may occur independent of changes in arterial pressure or RBF.

Interaction of central Ang II and NO on the cardiac sympathetic afferent reflex in dogs

The aim of this study was to test the hypothesis that the central angiotensin II (Ang II) and nitric oxide (NO) systems interact to modulate the cardiac sympathetic afferent reflex (CSAR). All dogs were anesthetized with alpha-chloralose (100 mg/kg, iv). They were sino-aortic baroreceptor denervated and vagotomized throughout the experiment renal sympathetic nerve activity responses to cardiac sympathetic afferent stimulation and the central gain of the CSAR were measured. Three protocols were performed: (1) intracerebroventricular injection (icv, 3 microg/h or 6 microg/h) of Ang II with and without N(omega)-nitro-L-arginine methyl ester (L-NAME) (icv, 1 mg/kg), (2) L-NAME (icv) with and without Ang II (icv, 6 microg/h), and (3) administration of the specific neural NO synthase (nNOS) inhibitor, S-Methyl-L-thiocitrulline (MeTC) (icv, 0.1 or 1 mM, 0.5 ml in 5 min) with and without pretreatment with the angiotensin type 1 receptor antagonist, losartan (icv, 0.125 mg/kg). The primary findings were (1) Ang II alone did not significantly affect the central sensitivity of the CSAR. However, Ang II with L-NAME enhanced this reflex, (2) even though L-NAME alone augmented the CSAR, this excitatory effect was further potentiated in the presence of Ang II and (3) MeTC significantly enhanced the central sensitivity of the CSAR. However, this enhancement did not occur after pretreatment with losartan. These data suggest that Ang II interacts with NO in the brain to modulate the CSAR and that inhibition of NO is required for facilitation of the CSAR by Ang II.

Angiotensin type 2 receptor-mediated phosphorylation of eNOS in the aortas of mice with 2-kidney, 1-clip hypertension

To evaluate the role of vascular angiotensin II (Ang II) type 2 (AT2) receptor in renovascular hypertension, we investigated expressions of AT2 receptor and endothelial nitric oxide synthase (eNOS) in thoracic aortas of mice with 2-kidney, 1-clip (2K1C) hypertension. The mRNA levels of AT2 receptor in aortas, but not those of AT1 and bradykinin B2 receptors, increased 14 days but not 42 days after clipping. The contractile response to Ang II (>0.1 micromol/L) was attenuated in aortic rings excised 14 days after clipping and was restored to that of rings from sham mice by antagonists of AT2 receptor (PD123319) and B2 receptor (icatibant). The aortic levels of total eNOS, phosphorylated eNOS at Ser1177 (p-eNOS), total Akt, and phosphorylated Akt at Ser473 (p-Akt) were increased in 2K1C mice on day 14, whereas only eNOS levels were increased on day 42. The aortic cGMP levels were 20-fold greater in 2K1C mice on day 14 compared with sham mice. Administration of nicardipine for 4 days before the excision of aortas 14 days after clipping not only reduced blood pressure but also decreased the aortic levels of eNOS, p-eNOS, Akt, p-Akt, and cGMP to sham levels, whereas the administration of PD123319 or icatibant to 2K1C mice decreased p-eNOS and cGMP to sham levels without affecting blood pressure and the levels of eNOS, Akt and p-Akt. These results suggest that vascular NO production is enhanced by increased eNOS phosphorylation via the activation of AT2 receptors in the course of 2K1C hypertension.

Roles of Nitric Oxide and Oxidative Stress in the Regulation of Blood Pressure and Renal Function in Prehypertensive Ren-2 Transgenic Rats

AIMS

The present study was performed to evaluate the role of nitric oxide (NO) and its interaction with superoxide anion (O2-) in the regulation of blood pressure (BP) and renal function during the developmental phase of hypertension in Ren-2 transgenic rats (TGR). The first aim was to compare BP and renal functional responses to acute NO synthase (NOS) inhibition achieved by intravenous (i.v.) infusion of Nomega-nitro-L-arginine-methyl ester (L-NAME) in prehypertensive heterozygous TGR and in transgene-negative Hannover Sprague-Dawley (HanSD) rats. The second aim was to evaluate whether scavenging of O2- by infusion of the superoxide dismutase mimetic tempol increases NO bioavailability which therefore should augment BP and renal functional responses to L-NAME.

METHODS

Rats were anesthetized, prepared for clearance experiments and BP and renal functional responses were evaluated in response to i.v. L-NAME administration (20 microg.100 g(-1).min(-1)) without or with tempol pretreatment (i.v., 300 microg.100 g(-1).min(-1)). In renal cortical tissue, nitrotyrosine protein expression was assessed by immunoblotting as marker of O2- production and urinary 8-epi-PGF(2alpha) excretion as marker of intrarenal oxidative stress was assessed by enzyme immunoassay.

RESULTS

BP, glomerular filtration rate (GFR), renal plasma flow (RPF) and sodium excretion were similar in TGR and HanSD. L-NAME infusion induced greater increases in BP in TGR than in HanSD (+42 +/- 4 vs. +25 +/- 3 mmHg, p < 0.05). In the absence of a significant change in GFR, L-NAME caused similar decreases in RPF (-32 +/- 6 and -25 +/- 4%, p < 0.05) in TGR and HanSD. Despite significantly higher renocortical expression of nitrotyrosine and urinary 8-epi-PGF2alpha excretion in TGR than in HanSD, pretreatment with tempol did not augment the rise in BP and the decrease in RPF induced by L-NAME.

CONCLUSIONS

The greater BP response to L-NAME in TGR suggests that prehypertensive TGR exhibit an enhanced NO activity in the systemic vasculature as compared with HanSD. Despite increased intrarenal oxidative stress in TGR, the dependency of the intrarenal vascular tone on NO appears to be similar in TGR and HanSD. The lack of a compensatory increase in renal NO activity may partially account for the enhanced renal vascular response to ANG II present in TGR.

Differential effects of 20-hydroxyeicosatetraenoic acid on intrarenal blood flow in the rat

We recently demonstrated that endothelin-1-induced medullary vasodilation despite a potent cortical vasoconstriction in the rat kidney may be accounted for by 20-hydroxyeicosatetraenoic acid (20-HETE) production. This study characterized the effects of 20-HETE and its metabolites, 20-hydroxy prostaglandin E(2) (20-OH PGE(2)) and 20-hydroxy prostaglandin F(2alpha) (20-OH PGF(2alpha)), and the contribution of nitric oxide (NO) and prostanoids to the changes evoked in cortical blood flow (CBF) and medullary blood flow (MBF). We tested the hypothesis that 20-HETE produces qualitatively different regional hemodynamic effects in the kidney with 20-OH PGF(2alpha) or 20-OH PGE(2), accounting for the vasoconstriction or vasodilation, respectively, in the cortex and medulla. Renal intra-arterial infusion of 1, 2.5, 5, and 10 ng/min 20-HETE decreased CBF by 10 +/- 3, 24 +/- 4, 40 +/- 7, and 58 +/- 9 perfusion units (PU), respectively, but increased MBF by 4 +/- 2, 16 +/- 4, 27 +/- 3, and 41 +/- 10 PU, respectively. 20-OH PGF(2alpha) mimics the effects of 20-HETE, as did PGF(2alpha). However, 20-OH PGE(2) increased both CBF and MBF, as did PGE(2). Indomethacin (5 mg/kg) blunted the effects of 20-HETE but not that of 20-OH PGE(2) and 20-OH PGF(2alpha). However, SQ29548 ([1S-[1alpha,2alpha(Z),3alpha,4alpha]]-7-[3[[2-[(phenylamino)carbonyl[hydrazino]methyl]-7-oxabicyclo]2.2.1]hept-2-yl]-5-heptenoic acid) (0.1 mg/kg), a prostaglandin H(2)/thromboxane A(2) receptor antagonist, blunted the cortical and medullary hemodynamic effects elicited by 20-HETE, 20-OH PGE(2), 20-OH PGF(2alpha), and PGF(2alpha) but not PGE(2). N(omega)-L-nitro arginine methyl ester (5 mg/kg), the inhibitor of NO synthase, exacerbated the cortical constrictor effects of 20-HETE and 20-OH PGF(2alpha) without affecting the medullary perfusion produced by 20-HETE or its metabolites. These findings suggest that 20-HETE, through its hydroxyl metabolites, produced differential effects in the kidney. The medullary perfusion appears to be independent of NO.

TP receptors regulate renal hemodynamics during angiotensin II slow pressor response

We investigated the hypothesis that thromboxane A2(TxA2)-prostaglandin H2receptors (TP-Rs) mediate the hemodynamic responses and increase in reactive oxygen species (ROS) to ANG II (400 ng·kg−1·min−1sc for 14 days) using TP-R knockout (TP −/−) and wild-type (+/+) mice. TP −/− had normal basal mean arterial blood pressure (MAP) and glomerular filtration rate but reduced renal blood flow and increased filtration fraction (FF) and renal vascular resistance (RVR) and markers of ROS (thiobarbituric acid-reactive substances and 8-isoprostane PGF2α) and nitric oxide (NOx). Infusion of ANG II into TP +/+ increased ROS and thromboxane B2(TxB2) and increased RVR and FF. ANG II infusion into TP −/− mice reduced ANG I and increased aldosterone but caused a blunted increase in MAP (TP −/−: +6 ± 2 vs. TP +/+: +15 ± 3 mmHg) and failed to increase FF, ROS, or TxB2but increased NOx and paradoxically decreased RVR (−2.1 ± 1.7 vs. +2.6 ± 0.8 mmHg·ml−1·min−1·g−1). Blockade of AT1receptor of TP −/− mice infused with ANG II reduced MAP (−8 mmHg) and aldosterone but did not change the RVR or ROS. In conclusion, during an ANG II slow pressor response, AT1receptors activate TP-Rs that generate ROS and prostaglandins but inhibit NO. TP-Rs mediate all of the increase in RVR and FF, part of the increase in MAP, but are not implicated in the suppression of ANG I or increase in aldosterone. TP −/− mice have a basal increase in RVR and FF associated with ROS.

Src kinase mediates angiotensin II-dependent increase in pulmonary endothelial nitric oxide synthase

We have previously demonstrated that angiotensin II (Ang II) stimulates nitric oxide (NO) production in bovine pulmonary artery endothelial cells (BPAECs) by increasing NO synthase (NOS) expression via the type 2 receptor. The purpose of this study was to identify the Ang II-dependent signaling pathway that mediates this increase in endothelial NOS (eNOS). The Ang II-dependent increase in eNOS expression is prevented when BPAECs are pretreated with the tyrosine kinase inhibitors, herbimycin A and 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-D]pyrimidine, which also blocked Ang II-dependent mitogen-activated protein kinase (MAPK) kinase/extracellular-regulated protein kinase (MEK)-1 and MAPK phosphorylation, suggesting that Src is upstream of MAPK in this pathway. Transfection of BPAECs with an Src dominant negative mutant cDNA prevented the Ang II-dependent Src activation and increase in eNOS protein expression. PD98059, a MEK-1 inhibitor, prevented the Ang II-dependent phosphorylation of extracellular-regulated protein kinases 1 and 2 and increase in eNOS expression. Neither AG1478, an epidermal growth factor receptor kinase inhibitor, nor AG1295, a platelet derived growth factor receptor kinase inhibitor, had any effect on Ang II-stimulated Src activity, MAPK activation, or eNOS expression. Pertussis toxin prevented the Ang II-dependent increase in Src activity, MAPK activation, and eNOS expression. These data suggest that Ang II stimulates Src tyrosine kinase via a pertussis toxin-sensitive pathway, which in turn activates the MAPK pathway, resulting in increased eNOS protein expression in BPAECs.

Contributions of nitric oxide and prostanoids and their signaling pathways to the renal medullary vasodilator effect of U46619 (9-11-dideoxy-11 alpha,9a-epoxymethano-prostaglandin F(2a)) in the rat

We recently demonstrated that U46619 (9-11-dideoxy-11 alpha,9a-epoxymethano-prostaglandin F(2a)) evoked a medullary vasodilation and a reduction in blood pressure despite a potent cortical vasoconstriction in the anesthetized rat. The present study tested the hypothesis that nitric oxide (NO) and prostanoids contribute to U46619-induced increase in medullary blood flow (MBF). U46619 at 1, 3, and 5 microg/kg increased MBF (above basal values) by 16 +/- 3, 45 +/- 10, and 58 +/- 8 perfusion units, respectively, and increased NO current in the medulla by 17 +/- 4, 34 +/- 7, and 60 +/- 12 pA, respectively. N(omega)-L-Nitro-arginine methyl ester (5 mg/kg), the inhibitor of NO production, attenuated the increase in MBF (75 +/- 8%, p < 0.05) as did indomethacin (10 mg/kg), the inhibitor of cyclooxygenase (38 +/- 5%, p < 0.05), suggesting the involvement of NO and dilator prostanoids. H-Arg-Lys-Arg-Ala-Arg-Lys-Glu-OH, a synthetic peptide and selective inhibitor of cGMP-dependent protein kinase, attenuated U46619-induced medullary perfusion (52 +/- 6%, p < 0.05), but H-89 ((N-[2-((p-bromocinnamyl)aminoethyl)]-5-isoquinolinesulfonamide hydrochloride), a cell-permeable, selective, and potent inhibitor of cAMP-dependent protein kinase A, was without effect. Glybenclamide, a K(ATP) channel blocker, also blunted the increase by U46619 in MBF (58 +/- 7%, p < 0.05). These data suggest that NO and prostanoids contribute to U46619-induced medullary perfusion and that the effects of these mediators are coupled to activation of protein kinase G and K(ATP) channels but not protein kinase A.

Essential role of AT1A receptor in the development of 2K1C hypertension

The aims of this study were to delineate the relative contribution of angiotensin II (ANG II) subtype 1A (AT1A) and 1B (AT1B) receptors to the development of two-kidney, one-clip (2K1C) Goldblatt hypertension in mice, to examine if increased nitric oxide synthase (NOS) activity counteracts the vasoconstrictor influences of ANG II in 2K1C hypertensive mice, and to determine the role of ANG II type 2 (AT2) receptors in 2K1C hypertension in mice. AT(1A) ANG II receptor knockout (AT1A-/-) and wild-type (AT1A+/+) mice underwent clipping of the right renal artery. Systolic blood pressure (SBP) was significantly lower in AT1A-/- compared with AT1A+/+ mice, and neither clip placement nor AT2 receptor blockade with PD 123319 (PD) altered SBP in AT1A-/- mice. A significant and sustained rise in SBP from 119+/-5 to 163+/-6 mm Hg was observed in the 2K1C AT1A+/+ mice from day 10 to day 26. Chronic PD infusion did not alter the course of hypertension in 2K1C/AT1A+/+. Acute PD infusion did not alter mean arterial pressure (MAP) in AT1A+/+, PD/AT1A+/+, 2K1C/AT1A+/+, PD/2K1C/AT1A+/+, AT1A-/-, PD/AT1A-/-, and PD/2K1C/AT1A-/- mice compared with basal levels. In contrast, acute PD infusion caused significant increases in MAP in 2K1C/AT1A-/- mice. The subsequent acute NOS inhibition caused greater increases in MAP in 2K1C/AT1A+/+ and PD/2K1C/AT1A+/+ mice than in AT1A+/+ and PD/AT1A+/+ mice. These results support the essential role of AT1A receptors in mediating 2K1C hypertension and support the hypothesis that augmented NO production serves as a counteracting system in this model of hypertension.

Physiological mechanisms regulating the expression of endothelial-type NO synthase

Although endothelial nitric oxide synthase (eNOS) is a constitutively expressed enzyme, its expression is regulated by a number of biophysical, biochemical, and hormonal stimuli, both under physiological conditions and in pathology. This review summarizes the recent findings in this field. Shear stress, growth factors (such as transforming growth factor-beta, fibroblast growth factor, vascular endothelial growth factor, and platelet-derived growth factor), hormones (such as estrogens, insulin, angiotensin II, and endothelin 1), and other compounds (such as lysophosphatidylcholine) upregulate eNOS expression. On the other hand, the cytokine tumor necrosis factor-alpha and bacterial lipopolysaccharide downregulate the expression of this enzyme. The growth status of cells, the actin cytoskeleton, and NO itself are also important regulators of eNOS expression. Both transcriptional and posttranscriptional mechanisms are involved in the expressional regulation of eNOS. Different signaling pathways are involved in the regulation of eNOS promoter activity and eNOS mRNA stability. Changes in eNOS expression and activity under pathophysiological conditions and the pharmacological modulation of eNOS expression are subject of a subsequent brief review (part 2) to be published in the next issue of this journal.

Effect of 1-week losartan administration on bile duct-ligated cirrhotic rats with portal hypertension

BACKGROUND/AIMS

Nitric oxide and angiotensin play important roles in the pathogenesis of the hemodynamic derangement in cirrhosis and portal hypertension. The hemodynamic effects of losartan, an angiotensin II type 1 receptor antagonist, in cirrhotic patients with portal hypertension are conflicting. This study was undertaken to explore the possible mechanism of action of losartan on portal hypertension in cirrhotic rats produced by bile duct ligation (CBL).

METHODS

Three weeks after surgery, CBL and sham-operated rats randomly received vehicle or losartan (3 mg/kg per 12 h by gavage) for 1 week. Hemodynamic values, hormone levels, and aortic eNOS protein expression were measured after drug administration.

RESULTS

In CBL rats, 1-week losartan treatment decreased portal pressure and ameliorated hyperdynamic circulation associated with a blunted vascular response to N(omega)-nitro-L-arginine methyl ester infusion. The hematocrit increased and the plasma volume, aldosterone, plasma renin activity, norepinephrine, and nitrate and nitrite levels decreased. The eNOS protein expression was reduced in CBL rats receiving losartan compared with those receiving vehicle.

CONCLUSIONS

One-week losartan treatment in CBL rats decreased portal pressure and ameliorated hyperdynamic circulation. In addition to the suppression of renin-angiotensin axis, the reduced aortic eNOS protein expression may play a partial role for the mechanism of action of losartan in CBL rats.

Soluble epoxide hydrolase inhibition lowers arterial blood pressure in angiotensin II hypertension

Epoxyeicosatrienoic acids (EETs) have antihypertensive properties and play a part in the maintenance of renal microvascular function. A novel approach to increase EET levels is to inhibit epoxide hydrolase enzymes that are responsible for conversion of biologically active EETs to dihydroxyeicosatrienoic acids (DHETs) that are void of effects on the preglomerular vasculature. We hypothesized that inhibition of soluble epoxide hydrolase (sEH) would lower blood pressure in angiotensin II (Ang II) hypertension. Rat renal cortical tissue was harvested and urine collected 2 weeks following implantation of an osmotic minipump containing Ang II (60 ng/min). Renal cortical sEH protein expression was significantly higher in Ang II hypertension compared with normotensive animals. Likewise, urinary 14,15-DHET levels were significantly increased in hypertensive compared with normotensive animals and averaged 8.1 +/- 1.3 and 2.7 +/- 1.1 ng/d; respectively. In additional experiments, the sEH inhibitor N-cyclohexyl-N-dodecyl urea (NCND; 3 mg/d) or vehicle (corn oil, 0.5 mL) was administered daily by intraperitoneal injection starting on day 10. Administration of NCND for 4 days lowered systolic blood pressure by 30 mm Hg in Ang II hypertensive animals, whereas the corn oil vehicle had no effect on blood pressure in normotensive or Ang II hypertensive animals. Measurement of blood pressure by indwelling arterial catheters in conscious animals with free movement in their cages confirmed that NCND had antihypertensive properties. Arterial blood pressure averaged 119 +/- 5 mm Hg in normotensive, 170 +/- 3 mm Hg in hypertensive and 149 +/- 10 mm Hg in NCND-treated, Ang II-infused animals. Administration of the potential metabolite of NCND, N-cyclohexylformamide to Ang II hypertensive rats did not lower the systolic blood pressure. These studies demonstrate that increased sEH expression in the Ang II hypertensive kidney leads to increased EET hydration. Moreover, sEH plays a role in the regulation of blood pressure, and inhibition of sEH during Ang II hypertension is antihypertensive.

Sustained influence of the renal nerves to attenuate sodium retention in angiotensin hypertension

Recent studies indicate that baroreflex suppression of renal sympathetic nerve activity is sustained for up to 5 days of ANG II infusion; however, steady-state conditions are not associated with ANG II hypertension of this short duration. Thus the major goal of this study was to determine whether neurally induced increments in renal excretory function during chronic intravenous infusion of ANG II are sustained under more chronic conditions when hypertension is stable and sodium balance is achieved. Experiments were conducted in five conscious dogs subjected to unilateral renal denervation and surgical division of the urinary bladder into hemibladders to allow separate 24-h urine collection from denervated (Den) and innervated (Inn) kidneys. ANG II was infused after control measurements for 10 days at a rate of 5 ng. kg(-1). min(-1). Twenty-four-hour control values for mean arterial pressure (MAP) and the ratio for urinary sodium excretion from Den and Inn kidneys (Den/Inn) were 92 +/- 4 mmHg and 0.99 +/- 0.05, respectively. On days 8-10 of ANG II infusion, MAP was stable (+30 +/- 3 mmHg) and sodium balance was achieved. Whereas equal amounts of sodium were excreted from the kidneys during the control period, throughout ANG II infusion there was a greater rate of sodium excretion from Inn vs. Den kidneys (day 10 Den/Inn sodium = 0.56 +/- 0.05), indicating chronic suppression of renal sympathetic nerve activity. The greater rate of sodium excretion in Inn vs. Den kidneys during renal sympathoinhibition also revealed a latent impairment in sodium excretion from Den kidneys. Although the Den/Inn for sodium and the major metabolites of nitric oxide (NO) decreased in parallel during ANG II hypertension, the Den/Inn for cGMP, a second messenger of NO, remained at control levels throughout this study. This disparity fails to support the notion that a deficiency in NO production and action in Den kidneys accounts for the impaired sodium excretion. Most importantly, these results support the contention that baroreflex suppression of renal sympathetic nerve activity is sustained during chronic ANG II hypertension, a response that may play an important role in attenuating the rise in arterial pressure.

Role of nNOS in regulation of renal function in angiotensin II-induced hypertension

Previous studies have indicated that in normotensive rats, NO produced by neuronal NO synthase (nNOS) plays an important role in modulating tubuloglomerular feedback (TGF)-mediated afferent arteriolar constriction. It has also been shown that in angiotensin (Ang) II-infused hypertensive rats, there is a reduced ability of nNOS-derived NO to counteract this vasoconstriction. The present study was performed to (1) assess in vivo renal functional responses to intrarenal nNOS inhibition in control and Ang II-infused rats and (2) determine whether changes in renal function following nNOS inhibition are mediated by unopposed stimulation of Ang II receptor subtype 1 (AT(1)). Wistar rats were infused with either saline (SAL) or Ang II (80 ng/min) by osmotic minipumps implanted subcutaneously. Mean arterial blood pressure of SAL- and Ang II-infused rats on day 13 after implantation averaged 121+/-4 (n=28) and 151+/-5 (n=30), respectively (P<0.05). There were no differences in glomerular filtration rate (GFR) (0.68+/-0.09 versus 0.59+/-0.09 mL. min(-1). g(-1)), renal plasma flow (RPF) (2.66+/-0.31 versus 2.34+/-0.39 mL. min(-1). g(-1)), and absolute sodium excretion (0.37+/-0.07 versus 0.42+/-0.09 micromol. min(-1). g(-1)). Intrarenal infusion of SAL did not change GFR, RPF, and sodium excretion in either SAL-infused (n=7) or Ang II-infused rats (n=8). Acute intrarenal administration of the nNOS inhibitor S-methyl-L-thiocitrulline (L-SMTC; 0.3 mg/h) decreased GFR, RPF, and sodium excretion in SAL-infused rats (n=9) by 29+/-4%, 38+/-4%, and 70+/-4% compared with control values (P<0.05). The pretreatment by the AT(1) receptor antagonist candesartan (750 ng IR) in SAL-infused rats (n=7) effectively prevented the decrease in RPF (-3+/-3%) elicited by nNOS inhibition and resulted in an increase in GFR (+25+/-12, P<0.05) and a concomitant greater increase in sodium excretion (84+/-12%, P<0.05) compared with control values. In contrast, in Ang II-infused rats (n=10) intrarenal inhibition of nNOS by L-SMTC did not cause significant decreases in GFR, RPF and sodium excretion (-2+/-2%, -15+/-10%, and -14+/-10%, respectively). These results suggest that in normotensive rats nNOS-derived NO counteracts Ang II-mediated vasoconstriction in the pre- and postglomerular microcirculation. Furthermore, Ang II-infused rats exhibit an impaired ability to release NO by nNOS. Decreased nNOS activity is likely to account at least partially for the enhanced TGF responsiveness in Ang II-infused rats and thus may contribute to the maintenance of hypertension in this model.

Role of nitric oxide in regional blood flow in angiotensin II-induced hypertensive rats

The present study was designed to evaluate the contribution of nitric oxide (NO) to regional hemodynamics during the early phase of angiotensin II (Ang II)-induced hypertension. The responses of regional blood flow to chronic NO synthase inhibition with N(G)-nitro-L-arginine methyl ester (L-NAME) were assessed using radioactive microspheres in conscious Ang II-infused hypertensive rats. Ang II-infused rats (270 ng/kg/min, subcutaneously for 12 days: n=11) showed higher mean arterial pressure (MAP: 153+/-4 mmHg) and total peripheral resistance (TPR: 1.61+/-0.06 mmHg/min/ml), and lower cardiac output (CO: 102+/-3 ml/min) than vehicle-infused normotensive rats (115+/-2 mmHg, 0.96+/-0.05 mmHg/min/ml and 130+/-7 ml/min, n=11, respectively). The blood flow rates in the brain, spleen, large intestine and skin were significantly reduced in Ang III-infused rats compared with vehicle-infused rats, while those in the lung, heart, liver, kidney, adrenal gland, small intestine, and skeletal muscle were similar. Treating Ang II-infused rats with L-NAME (75 mg/l in drinking water for 10 days, n=11) resulted in higher MAP (166+/-6 mmHg) and TPR (1.89+/-0.18 mmHg/min/ml) and lower CO (87+/-7 m/min) than untreated Ang II-infused rats. L-NAME-treated Ang II-infused rats showed widespread increases in regional vascular resistance and reduced blood flow rates in the kidney (3.81+/-0.27 ml/min/g) and skeletal muscle (0.20+/-0.03 ml/min/g) compared with untreated Ang II-infused rats (6.88+/-0.27 and 0.33+/-0.04 ml/min/g, respectively). However, there were no significant differences in the flow rates of other organs investigated between these animals. An NO donor, (+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (FK409: 30 microg/kg/min, i.v.), significantly decreased MAP (110+/-6 mmHg) and TPR (1.23+/-0.18 mmHg/min/ml) without significant changes in CO (89+/-9 ml/min) in L-NAME-treated Ang II-infused rats. Furthermore, FK409 partially reversed blood flow rates in the kidney (4.72+/-0.40 ml/min/g) and skeletal muscle (0.25+/-0.02 ml/min/g)in these animals. These results suggest that NO counteracts, at least in part, the vasoconstrictor effects of elevated Ang II levels in renal and skeletal muscle vascular beds, and is an important modulator in the regulation of blood flow to these organs during the development of Ang II-induced hypertension.

Interaction of Angiotensin II and Nitric Oxide in Isolated Perfused Afferent Arterioles of Mice

Abstract. The present study was performed to evaluate angiotensin II (Ang II)—nitric oxide (NO) interaction in afferent arterioles (Af) of wild-type mice and mice that are homozygous (-/-) for disruption of the endothelial NO synthase (eNOS) gene. Af were microperfused, and the dose responses were assessed for the NO precursor L-arginine (n= 4), NO inhibitor NG-nitro-L-arginine methyl ester (L-NAME,n= 5), L-NAME after pretreatment with L-arginine (n= 5), Ang II (n= 8), and Ang II after pretreatment with L-NAME (n= 7). Acute administration of L-arginine and L-NAME (both in doses from 10-6to 10-3mol/L) did not change arteriolar diameter. Moreover, pretreatment with L-arginine did not change the response to L-NAME. However, Ang II, applied in doses of 10-12, 10-10, 10-8, and 10-6mol/L, significantly reduced the lumen to 66.5 ± 7.0% and 62.2 ± 8.0% at 10-8and 10-6mol/L Ang II, respectively. The contraction was augmented after L-NAME pretreatment (19.5 ± 13.6% and 25.5 ± 10.2% at 10-8and 10-6mol/L Ang II, respectively). In eNOS (-/-) mice (n= 8), the response to Ang II also was enhanced (9.1 ± 6.0% and 11.2 ± 8.2% at 10-8and 10-6mol/L Ang II, respectively). Female mice did not differ from male mice in their reactivity to Ang II (n= 9) and Ang II + L-NAME pretreatment (n= 11). The study shows that (1) it is feasible to microperfuse mouse Af, (2) the basal production of endothelial NO is very low and not inducible by L-arginine in Af of mice, and (3) a counteracting effect of NO is initiated by Ang II. High Ang II sensitivity in eNOS (-/-) mice underscores the considerable role of endothelial-derived NO to balance Ang II vasoconstriction in Af.

Enhanced renal microvascular reactivity to angiotensin II in hypertension is ameliorated by the sulfonimide analog of 11,12-epoxyeicosatrienoic acid

OBJECTIVES

Epoxygenase metabolites produced by the kidney affect renal blood flow and tubular transport function and 11,12-epoxyeicosatrienoic acid (11,12-EET) has been putatively identified as an endothelium-derived hyperpolarizing factor. The current studies were performed to determine the influence of 11,12-EET on the regulation of afferent arteriolar diameter in angiotensin II-infused hypertensive rats.

MATERIALS AND METHODS

Male Sprague-Dawley rats received angiotensin II (60 ng/min) or vehicle via an osmotic minipump. Angiotensin II-infused hypertensive and vehicle-infused normotensive rats were studied for 2 weeks following implantation of the minipump. Renal microvascular responses to the sulfonimide analog of 11,12-EET (11,12-EET-SI) and angiotensin II were observed utilizing the in-vitro juxtamedullary nephron preparation. Renal cortical epoxygenase enzyme protein levels were quantified by Western blot analysis. Renal microvessels were also isolated and epoxygenase metabolite levels measured by negative ion chemical ionization (NICI)/gas chromatography-mass spectroscopy.

RESULTS

Systolic blood pressure averaged 118 +/- 2 mmHg prior to pump implantation and increased to 185 +/- 7 mmHg in rats infused with angiotensin II for 2 weeks. Afferent arteriolar diameters of 2-week normotensive animals averaged 22 +/- 1 microm. Diameters of the afferent arterioles were 17% smaller in hypertensive rats (P< 0.05); however, arterioles from both groups responded to 11,12-EET-SI (100 nmol) with similar 15-17% increases in diameter. As we previously demonstrated, the afferent arteriolar reactivity to angiotensin II was enhanced in angiotensin II-infused animals. Interestingly, elevation of 11,12-EET-SI levels to 100 nmol reversed the enhanced vascular reactivity to angiotensin II associated with angiotensin II hypertension. Renal microvascular EET levels were not different between groups and averaged 81 +/- 9 and 87 +/- 13 pg/mg per 30 min in normotensive and hypertensive animals, respectively. Renal cortical microsomal levels of the epoxygenase CYP2C23 and CYP2C11 proteins were also similar in normotensive and angiotensin II hypertensive rats.

CONCLUSIONS

Taken together, these data support the concept that renal microvascular 11,12-EET activity and levels may not properly offset the enhanced angiotensin II renal vasoconstriction during angiotensin II hypertension.

Effect of nitric oxide inhibition on kidney function in experimental renovascular hypertension

We examined the effect of acute systemic blockade of nitric oxide (NO) synthesis on blood pressure and renal function in rats with angiotensin II dependent two-kidney, one-clip Goldblatt hypertension. Hypertensive animals had significantly higher blood pressures, plasma NO metabolite concentrations and urinary NO metabolite excretion rates than control rats. Intravenous administration of N(G)-nitro-L-arginine methylester (L-NAME) (10 mg/kg) increased mean arterial pressure in both hypertensive and control animals with the magnitude of increase being greater in hypertensive than control rats (32 +/- 3 vs. 20 +/- 2 mmHg, p < 0.05). L-NAME did not affect glomerular filtration rates of normal and clipped kidneys but significantly decreased non-clipped kidney glomerular filtration rate (1.1 +/- 0.1 vs. 0.7 +/- 0.1 ml/min per g kidney wt, p < 0.05). Blood flow to normal and non-clipped kidneys fell in response to L-NAME. Percent reduction in renal blood flow produced by L-NAME was significantly greater in non-clipped than normal kidneys (38 +/- 3 vs. 24 +/- 2%, p < 0.05). In contrast, clipped kidney blood flow increased after L-NAME (3.3 +/- 0.2 vs. 4.0 +/- 0.2 ml/min per g kidney wt, p < 0.05). An identical improvement in clipped kidney blood flow occurred when arterial pressure was raised with aortic constriction indicating that the systemic pressor effect of L-NAME was responsible for this finding. These results indicate that NO plays an important role in systemic and non-clipped kidney hemodynamics in renovascular hypertension. Because NO has little influence on stenotic kidney function, the stimulus for increased NO system activity in this disease appears to be vascular shear stress rather than elevated circulating or intrarenal angiotensin II concentrations.

Superoxide inhibits neuronal nitric oxide synthase influences on afferent arterioles in spontaneously hypertensive rats

This study was designed to determine the influence of increased superoxide anion in neuronal nitric oxide synthase (nNOS)-dependent regulation of afferent arterioles in spontaneously hypertensive rats (SHR). Afferent arteriolar diameters of male Wistar-Kyoto rats (WKY) and SHR were assessed in vitro with the blood-perfused juxtamedullary nephron technique and averaged 21.6+/-1.6 (n=6) and 18.8+/-1.2 (n=7) micrometer, respectively. The superoxide dismutase mimetic Tempol (1, 10, and 100 micromol/L) did not influence afferent arterioles of WKY but significantly increased afferent arteriolar diameters of SHR by 20.6+/-5.5%, 25.2+/-5.4%, and 23.3+/-4.9%, respectively. In WKY (n=6), the nNOS inhibitor S-methyl-L-thiocitrulline (L-SMTC; 10 micromol/L) and the NOS inhibitor N(omega)-nitro-L-arginine (L-NNA; 100 micromol/L) significantly decreased afferent arteriolar diameters (19.6+/-1.6 micrometer) by 11.9+/-3.1% and 21.0+/-3.9%, respectively. In SHR (n=7), L-SMTC did not influence afferent arteriolar diameters (21.0+/-1.5 micrometer), but L-NNA exerted an afferent arteriolar constriction (14.8+/-3.2%) that was similar to the response observed in WKY. Experiments were also performed in the presence of 100 micromol/L Tempol. In afferent arterioles of WKY (n=6), Tempol treatment did not modulate the basal diameters (21.5+/-1.2 micrometer) or the constrictor response to L-SMTC (10.6+/-2.1%) or L-NNA (19.3+/-3.3%). In SHR (n=8), Tempol significantly increased afferent arteriolar diameters by 22.5+/-4.3% and enhanced afferent arteriolar constrictor responses to L-SMTC (18.4+/-2.7%) and L-NNA (31.9+/-2.6%). However, the nitric oxide donor S-nitroso-N-acetylpenicillamine (10 micromol/L), which similarly increased afferent arteriolar diameters (17.2+/-2.3%, n=6), did not affect afferent arteriolar responses to L-SMTC (1.5+/-2.7%) or L-NNA (18.6+/-2.3%). These suggest that superoxide anion inhibits the control of afferent arteriolar diameters by nNOS in SHR.