ANG II AT₂receptor modulates AT₁receptor-mediated descending vasa recta endothelial Ca²⁺signaling

Abstract

kidney; vasoconstriction; vasodilation; medulla; blood flow BLOOD FLOW TO THE MEDULLA of the kidney is maintained through the tonic and stimulated release of nitric oxide (NO). Intrarenal infusion of vasoconstrictors such as norepinephrine and angiotensin II (ANG II) cause vasoconstriction and a reduction of renal blood flow. In response to vasoconstrictor infusions, compensatory release of NO helps to preserve the small fraction of the total renal blood flow that reaches the medulla of the kidney (38,43,44). An important physiological role for the production of NO in the control of medullary blood flow has also been proposed based on the observation that NO synthase (NOS) inhibition leads to reduction of medullary blood flow, sodium retention, and hypertension (16,17,23). Calcium sensitive isoforms of NOS are expressed in a variety of structures within the renal medulla including the medullary thick ascending limb (mTAL), outer medullary collecting duct (OMCD), and descending vasa recta (DVR) endothelia (17,23). ANG II causes a compensatory increase in medullary NO generation (44) but reduces DVR endothelial intracellular calcium concentration ([Ca 2ϩ ] i ) and inhibits increases in [Ca 2ϩ ] i by bradykinin (BK) (25,31). Previous studies have shown that ANG II type 1 (AT 1 ) and 2 (AT 2 ) receptors are expressed in outer medullary vascular bundles (19,39). Motivated by this, we sought to determine which receptor subtype mediates the unusual suppressive effect of ANG II on DVR endothelial calcium signaling. We found that the AT 1 receptor blocker losartan inhibits the ANG II effect on DVR endothelia, but a high concentration of losartan is required. Interestingly, the ability of ANG II to reduce endothelial [Ca 2ϩ ] i elevation by BK and acetylcholine (ACh) is markedly enhanced when AT 2 receptors are blocked with PD-123319, implying an important role for AT 2 receptor activation to facilitate endothelial [Ca 2ϩ ] i signaling by vasodilators. We also examined the [Ca 2ϩ ] i transients induced by ANG II in the cytoplasm of isolated DVR smooth muscle/pericytes. In contrast to effects on endothelia, ANG II-induced pericyte [Ca 2ϩ ] i transients exhibited classical peak and plateau responses that were blocked by losartan but not PD-123319. METHODSIn vitro isolation of DVR. Studies were performed in 267 vessels from Sprague-Dawley rats (80-150 g body wt, Harlan) anesthetized before nephrectomy by an intraperitoneal bolus of thiopental (50 mg/kg body wt). DVR were microdissected as previously described from outer medullary vascular bundles and mounted on glass pipettes using a standard apparatus (ITM; San Antonio, TX) (26). The following buffer

Microvascular Effects of Aldosterone and Angiotensin Type 2 Receptors

“…In that preparation, AT2R activation increases NO bioavailability by inhibiting generation of reactive oxygen species. 17,19 Taken together, the finding by Gonzales et al 1 that AT2R activation is vasodilatory largely supports previous observations.…”

“…Systemic AT2R activation through infusion of angiotensin II plus AT1 receptor blockers lowers blood pressure 9 and AT2-null mice have mild hypertension with an exaggerated blood pressure response to deoxycorticosterone acetate plus salt. 10,11 Carotid arteries, 12 coronary arterioles, 13 renal afferent arterioles, 14 -16 and descending vasa recta 17 are dilated by AT2R activation. The participation of bradykinin B2 receptor activation and NO generation in AT2R-mediated vasodilation is a common finding but it is not uniform in all experimental preparations.…”

Disparate Roles of AT 2 Receptors in the Renal Cortical and Medullary Circulations of Anesthetized Rabbits

“…38 Our data regarding the effects of PD123319 therefore suggest that AT 2 receptor activation can blunt AT 1 -mediated release of vasodilator factors that act on vascular elements controlling MBF. At face value, this conclusion seems at odds with recent in vitro findings by Rhinehart et al 19 in isolated, outer medullary, descending vasa recta, vascular elements that are probably important in the control of MBF. They showed that AT 2 receptor activation blunted vasoconstrictor responses to angiotensin II in these vessels and also opposed angiotensin …”

“…Consistent with this notion, AT 2 receptor activation opposes AT 1 -mediated vasoconstriction and endothelial calcium signaling in isolated, descending vasa recta in vitro. 19 Therefore, we tested the effects of candesartan and PD123319 on the responses of regional kidney perfusion and renal excretory function to the renal-arterial administration (infusion and bolus) of angiotensin peptides in anesthetized rabbits. We tested the responses to angiotensin II itself, angiotensin III (moderate selectivity for AT 2 receptors 20,21 ), the highly selective AT 2 agonist CGP42112A, 22 and angiotensin(1-7), because the latter has both a high affinity for AT 2 receptors 21 and vasodilator effects that are possibly mediated independently of AT 1 and AT 2 receptors.…”

High-Salt Diet Inhibits Expression of Angiotensin Type 2 Receptor in Resistance Arteries

“…AT2R-dependent activation of phosphatases could modulate the phosphorylation of contractile proteins and their sensitivity to increases in [Ca 2ϩ ]i, leading to vasodilation without a marked reduction of intracellular calcium. 45,46 However, considering the technical limitations of the present study, this and other potential mechanisms implicated 6 will need further investigation.…”