Renal Vascular Reactivity in Hypertension

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ABSTRACT CD38 ADP ribosyl (ADPR) cyclase is a membrane-bound enzyme that produces metabolites known to promote Ca2+ mobilization mediated by ryanodine receptors (RyR) in arteriolar smooth muscle cells. We posit that renal CD38 is central to the development of angiotensin II (Ang II)-induced hypertension (AIH) and that CD38-deficient mice exhibit less pronounced renal vasoconstriction, Na+ retention and AIH than do wild-type (WT) mice. AIM 1 tests the hypothesis that CD38 ADPR cyclase participates in the development of AIH such that Ang II produces less pronounced hypertension in CD38-/- (global genetic deficiency) vs. WT mice. Less severe hypertension is also predicted in WT mice with targeted, renal-specific partial knockdown of CD38 induced by siRNA. The severity of AIH is determined by measuring 24-hr arterial pressure (telemetry) in conscious, unrestrained mice before and during chronic Ang II infusion. Also measured are 24 hr urinary excretion of nitrite/nitrate and 8-iso-PGF2¿ to assess nitric oxide (NO) production and oxidative stress. CD38 mRNA and ADPR cyclase activity will be quantified in preglomerular vessels. AIM 2 assesses the contribution of CD38 ADPR cyclase to renal vasoconstriction and the rightward shift in the pressure-natriuresis relation in AIH. We predict that the kidneys of CD38-deficient mice excrete Na+ more rapidly in response to an acute salt load than WT mice during development of AIH, with excretion rates becoming similar in established AIH. Conscious and anesthetized mice with global knockout and renal-specific knockdown of CD38 will be evaluated in both phases. AIM 3 evaluates the hypothesis that CD38 is the major ADPR cyclase mediating G- protein coupled receptor-elicited Ca2+ signaling involving RyR and Ca2+-induced Ca2+ release in isolated afferent arterioles and renal vasoconstriction in vivo. We predict that Ca2+ signaling and vascular responsiveness to vasoconstrictor agents (Ang II, ET-1, TxA2) are attenuated in CD38-deficient vs. WT mice during control and AIH conditions. Scatchard analysis of radioligand binding will characterize Ang II, ET-1, and TP receptor affinity and/or density in renal microvessels. Our goal is to identify a sequence of events that precede or occur early during the development of hypertension and thus are more likely to be causative than secondary, pressure-dependent consequences. Combining gene-targeted deletion of CD38, global and renal- specific, with pharmacological inhibition of ADPR cyclase and RyR-mediated Ca2+ release will provide important new information that CD38 is the primary cyclase family member mediating Ca2+ signaling in the renal microcirculation and its functional significance in long-term regulation of renal vasoconstriction, Na+ retention and the development of AIH. Successful completion of our novel studies of this underappreciated pathway will significantly advance our understanding of cellular/molecular mechanisms of Ca2+ signaling in the renal microcirculation and regulation of renal vascular reactivity in health and disease, making a major impact on the field.