Effects of angiotensin II on isolated rabbit afferent arterioles

Reactivity of renal and mesenteric resistance vessels to angiotensin II is mediated by NOXA1/NOX1 and superoxide signaling

Activation of NADPH oxidase (NOX) enzymes and the generation of reactive oxygen species and oxidative stress regulate vascular and renal function and contribute to the pathogenesis of hypertension. The present study examined the role of NOXA1/NOX1 function in vascular reactivity of renal and mesenteric resistance arteries/arterioles of wild-type and Noxa1-/- mice. A major finding was that renal blood flow is less sensitive to acute stimulation by angiotensin II (ANG II) in Noxa1-/- mice compared with wild-type mice, with a direct action on resistance arterioles independent of nitric oxide (NO) bioavailability. These functional results were reinforced by immunofluorescence evidence of NOXA1/NOX1 protein presence in renal arteries, afferent arterioles, and glomeruli as well as their upregulation by ANG II. In contrast, the renal vascular response to the thromboxane mimetic U46619 was effectively blunted by NO and was similar in both mouse genotypes and thus independent of NOXA1/NOX1 signaling. However, phenylephrine- and ANG II-induced contraction of isolated mesenteric arteries was less pronounced and buffering of vasoconstriction after acetylcholine and nitroprusside stimulation was reduced in Noxa1-/- mice, suggesting endothelial NO-dependent mechanisms. An involvement of NOXA1/NOX1/O2•- signaling in response to ANG II was demonstrated with the specific NOXA1/NOX1 assembly inhibitor C25 and the nonspecific NOX inhibitor diphenyleneiodonium chloride in cultured vascular smooth muscle cells and isolated mesenteric resistance arteries. Collectively, our data indicate that the NOX1/NOXA1/O2•- pathway contributes to acute vasoconstriction induced by ANG II in renal and mesenteric vascular beds and may contribute to ANG II-induced hypertension.NEW & NOTEWORTHY Renal reactivity to angiotensin II (ANG II) is mediated by superoxide signaling produced by NADPH oxidase (NOX)A1/NOX1. Acute vasoconstriction of renal arteries by ANG was blunted in Noxa1-/- compared with wild-type mice. NOXA1/NOX1/O2•- signaling was also observed in ANG II stimulation of vascular smooth muscle cells and isolated mesenteric resistance arteries, indicating that it contributes to ANG II-induced hypertension. A NOXA1/NOX1 assembly inhibitor (C25) has been characterized that inhibits superoxide production and ameliorates the effects of ANG II.

Prostaglandin E2 mediates connecting tubule glomerular feedback

Connecting tubule glomerular feedback (CTGF) is a mechanism in which Na reabsorption in the connecting tubule (CNT) causes afferent arteriole (Af-Art) dilation. CTGF is mediated by eicosanoids, including prostaglandins and epoxyeicosatrienoic acids; however, their exact nature and source remain unknown. We hypothesized that during CTGF, the CNT releases prostaglandin E2, which binds its type 4 receptor (EP4) and dilates the Af-Art. Rabbit Af-Arts with the adherent CNT intact were microdissected, perfused, and preconstricted with norepinephrine. CTGF was elicited by increasing luminal NaCl in the CNT from 10 to 80 mmol/L. We induced CTGF with or without the EP4 receptor blocker ONO-AE3-208 added to the bath in the presence of the epoxyeicosatrienoic acid synthesis inhibitor MS-PPOH. ONO-AE3-208 abolished CTGF (control, 9.4 ± 0.5; MS-PPOH+ONO-AE3-208, -0.6 ± 0.2 μm; P<0.001; n=6). To confirm these results, we used a different, specific EP4 blocker, L161982 (10(-5) mol/L), that also abolished CTGF (control, 8.5 ± 0.9; MS-PPOH+L161982, 0.8 ± 0.4 μm; P<0.001; n=6). To confirm that the eicosanoids that mediate CTGF are released from the CNT rather than the Af-Art, we first disrupted the Af-Art endothelium with an antibody and complement. Endothelial disruption did not affect CTGF (7.9 ± 0.9 versus 8.6 ± 0.6 μm; P=NS; n=7). We then added arachidonic acid to the lumen of the CNT while maintaining zero NaCl in the perfusate. Arachidonic acid caused dose-dependent dilation of the attached Af-Art (from 8.6 ± 1.2 to 15.3 ± 0.7 μm; P<0.001; n=6), and this effect was blocked by ONO-AE3-208 (10(-7) mol/L). We conclude that during CTGF, the CNT releases prostaglandin E2, which acts on EP4 on the Af-Art inducing endothelium-independent dilation.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Nitrite-derived nitric oxide formation following ischemia-reperfusion injury in kidney

Nitric oxide (NO) is synthesized from l-arginine by nitric oxide synthase (NOS), and nitrite and nitrate are believed to be waste forms of NO. We previously reported an enzyme-independent pathway of NO generation from nitrite in acidic conditions. In this study, we show nitrite-derived NO formation in renal ischemia-reperfusion injury using electron paramagnetic resonance (EPR) spectroscopy. In this experiment, we utilized a stable isotope of [(15)N]nitrite as a source of nitrite to distinguish l-arginine-derived NO from [(15)N]nitrite-derived (15)NO. Intravenous infusion of a stable isotope of [(15)N]nitrite ((15)NO(2)(-)) facilitated the formation of Hb(15)NO during renal ischemia, which demonstrated that the origin of NO was nitrite. The EPR signal of Hb(15)NO in kidney appeared after 40 min of renal ischemia, and renal reperfusion decreased the Hb(15)NO level in the kidney and increased it in blood by contrast. In addition, the amount of HbNO was nitrite concentration dependent, and this formation was NOS independent. Our findings suggest that nitrite can be an alternative source of NO in ischemic kidney and that it binds with hemoglobin and then is spread by the circulation after reperfusion.

Eicosanoid regulation of the renal vasculature

Even though it has been recognized that arachidonic acid metabolites, eicosanoids, play an important role in the control of renal blood flow and glomerular filtration, several key observations have been made in the past decade. One major finding was that two distinct cyclooxygenase (COX-1 and COX-2) enzymes exist in the kidney. A renewed interest in the contribution of cyclooxygenase metabolites in tubuloglomerular feedback responses has been sparked by the observation that COX-2 is constitutively expressed in the macula densa area. Arachidonic acid metabolites of the lipoxygenase pathway appear to be significant factors in renal hemodynamic changes that occur during disease states. In particular, 12(S)- hydroxyeicosatetraenoic acid may be important for the full expression of the renal hemodynamic actions in response to angiotensin II. Cytochrome P-450 metabolites have been demonstrated to possess vasoactive properties, act as paracrine modulators, and be a critical component in renal blood flow autoregulatory responses. Last, peroxidation of arachidonic acid metabolites to isoprostanes appears to be involved in renal oxidative stress responses. The recent developments of specific enzymatic inhibitors, stable analogs, and gene-disrupted mice and in antisense technology are enabling investigators to understand the complex interplay by which eicosanoids control renal blood flow.

EP(1) and EP(4) receptors mediate prostaglandin E(2) actions in the microcirculation of rat kidney

Vasodilator prostaglandin PGE(2) protects the kidney from excessive vasoconstriction during contraction of extracellular fluid volume and pathophysiological states. However, it is not yet clear which of the four known E-prostanoid (EP) receptors is localized to resistance vessels and mediates net vasodilation. In the present study, we assessed the presence, signal transduction, and actions of EP receptor subtypes in preglomerular arterioles of Sprague-Dawley rat kidneys. RNA encoding EP(1), an EP(1)-variant, and EP(4) receptors was identified by RT-PCR in freshly isolated preglomerular microvessels; cultured preglomerular vascular smooth muscle cells (VSMC) had EP(1)-variant and EP(4) RNA but lacked EP(1). EP(2) and EP(3) receptors were undetectable in both vascular preparations. In studies of cell signaling, stimulation of cAMP by various receptor agonists is consistent with primary actions of PGE(2) on the EP(4) receptor, with no inhibition of cAMP by EP(1) receptors. Studies of cytosolic calcium concentration in cultured renal VSMC support an inhibitory role of EP(4) during ANG II stimulation. In vivo renal blood flow (RBF) studies indicate that the EP(4) receptor is the primary receptor mediating sustained renal vasodilation produced by PGE(2), whereas the EP(1) receptor elicits transient vasoconstriction. The EP(1)-variant receptor does not appear to possess any cAMP or cytosolic calcium signaling capable of affecting RBF. Collectively, these studies demonstrate that the EP(4) receptor is the major receptor in preglomerular VSMC. EP(4) mediates PGE(2)-induced vasodilation in the rat kidney and signals through G(s) proteins to stimulate cAMP and inhibit cytosolic calcium concentration.

Prostaglandins buffer ANG II-mediated increases in cytosolic calcium in preglomerular VSMC

In order to exert an appropriate biological effect, the action of the vasoconstrictive hormone angiotensin II (ANG II) is modulated by vasoactive factors such as prostaglandins PGE2 and PGI2. The present study investigates whether prostaglandins alter ANG II-mediated increases in cytosolic calcium concentration ([Ca2+]i) in vascular smooth muscle cells (VSMC) isolated from rat renal preglomerular arterioles. [Ca2+]i was assessed using the calcium-sensitive dye fura 2 and a microscope-based photometer system. ANG II (10(-7) M) caused a biphasic, time-dependent [Ca2+]i response: an initial peak increase from 52 +/- 7 to 264 +/- 25 nM, followed by a sustained plateau of 95 +/- 9 nM in cultured VSMC. Coadministration of PGE2 or PGI2 or synthetic mimetics caused dose-dependent decreases in the peak [Ca2+]i response to ANG II, with attenuation of 40-50%. This degree of inhibition was even more pronounced in individual freshly isolated preglomerular VSMC. Increasing cAMP levels in cultured VSMC, by using either a cell-permeable analog or inhibiting phosphodiesterase activity, mirrored the antagonistic effects of prostaglandins on ANG II-stimulated increases in [Ca2+]i. Radioimmunoassays demonstrate that ANG II (10(-7) M) stimulates production of PGI2 and PGE2; the stable prostacyclin metabolite 6-keto-PGF(1alpha) was released in 10-fold greater concentrations than PGE(2.) Indomethacin blockade of prostaglandin production potentiated both the peak (264 to 337 +/- 26 nM) and sustained [Ca2+]i responses (95 to 181 +/- 22 nM) to ANG II. When prostaglandin analogs were added during indomethacin treatment, the ANG II response was restored to the typical pattern. In conclusion, we demonstrate that modulation of intracellular calcium levels is one mechanism by which prostaglandins can buffer ANG II-mediated constriction in renal preglomerular VSMC. PGI2 is more potent than PGE2 in this regard.

Carbenoxolone damages endothelium and enhances vasoconstrictor action in aortic rings

Carbenoxolone causes hypertension indirectly by inhibition of 11beta-hydroxysteroid dehydrogenase and consequent elevation of intracellular glucocorticoid levels and enhancement of vasoconstrictor action. We performed the present study to determine whether carbenoxolone also enhances vascular tone directly by mechanisms independent of glucocorticoids and other systemic influences. Exposure of rat aortic rings to 10 to 100 micromol/L carbenoxolone in aerated Krebs-Henseleit buffer for 24 hours resulted in concentration-dependent increases in angiotensin II (Ang II) (100 nmol/L)-stimulated contractions and significant shifting of the phenylephrine cumulative contraction curve to the left but not increases in KCI (120 mmol/L)-stimulated contractions. Maximal enhancement of Ang II contraction was 39 percent. In contrast, brief (15-minute) exposure to 100 micromol/L carbenoxolone did not alter Ang II contractions. Mechanical denudation of the endothelium obviated enhancement of Ang II contractions by carbenoxolone, suggesting interaction of carbenoxolone with the endothelium. Endothelium-dependent relaxation of precontracted rings to acetylcholine or ATP was reduced by more than 90 percent by 24-hour pretreatment with 100 micromol/L carbenoxolone but not with 100 micromol/L deoxycorticosterone acetate (a mineralocorticoid) or 100 mu mol/L glycyrrhizic acid (a natural 11beta-hydroxysteroid dehydrogenase inhibitor). Vascular smooth muscle relaxation with sodium nitroprusside was not inhibited by carbenoxolone. Incubation of cultured endothelial cells with 100 mu mol/L carbenoxolone for 24 hours did not inhibit nitric oxide synthase activity, as measured by conversion of [3H]L-arginine to [3H]L-citrulline. Electron micrography demonstrated that endothelial cell ultrastructure but not vascular smooth muscle cell ultrastructure was abnormal after incubation of rings for 24 hours with 100 micromol/L carbenoxolone. These studies suggest that carbenoxolone concentrations higher than 10 micromol/L enhance vasoconstrictor action via selective toxicity to the endothelium and elimination of endothelium-dependent relaxation.

Vasodilation induced by vasopressin V2 receptor stimulation in afferent arterioles

We have previously reported that vasopressin (AVP) V2 receptor stimulation increased renal blood flow in dogs anesthetized with pentobarbital. In this study, we examined the direct effects of AVP on afferent arterioles to clarify the role played by V2 receptors in regulating afferent arteriolar tone. We microdissected a superficial afferent arteriole with glomerulus from the kidney of a New Zealand White rabbit. Each afferent arteriole was cannulated with a pipette system and microperfused in vitro at 60 mm Hg. The effects of vasoactive substances were evaluated by changes in the lumen diameter of afferent arterioles. We found that AVP decreased the lumen diameter of microperfused afferent arterioles dose-dependently and that a V1 antagonist, OPC21268, inhibited the vasoconstrictor action of AVP. However, AVP 10(-8) M increased the lumen diameter of norepinephrine (NE)-constricted afferent arterioles pretreated with OPC21268 (OPC + NE, 8.2 +/- 0.7 microns; OPC + NE + AVP, 9.9 +/- 0.9 microns*; *P < 0.05, N = 13). This vasodilatory effect of AVP was abolished by pretreatment with a V2 antagonist, OPC31260. Desmopressin (dDAVP), a V2 agonist, increased the lumen diameter of the NE-constricted afferent arterioles (NE, 7.4 +/- 0.9 microns; NE + dDAVP, 10.1 +/- 0.7 microns*; *P < 0.05, N = 9). These results suggest that AVP V2 receptors are present in rabbit afferent arterioles and that V2 receptor stimulation induces vasodilation in rabbit afferent arterioles.