Exaggerated Ca2+ signaling in preglomerular arteriolar smooth muscle cellsof genetically hypertensive rats

Experiments were conducted to gain insight into mechanisms responsible for exaggerated renal vascular reactivity to ANG II and vasopressin (AVP) in sp ontaneously hypertensive rats (SHR) during the development of hypertension. Cytosolic calcium concentration ([Ca2+](i)) was measured by ratiometric fu ra 2 fluorescence and a microscope-based photometer. Vascular smooth muscle cells (SMC) from preglomerular arterioles were isolated and dispersed usin g an iron oxide-sieving method plus collagenase treatment. ANG II and AVP p roduced rapid and sustained increases in [Ca2+](i). ANG II elicited similar dose-dependent increases in [Ca2+](i) in SMC from SHR and Wistar-Kyoto rat s(WKY). In contrast, AVP caused almost twofold larger responses in afferent arteriolar SMC from SHR. ANG II effects were inhibited by the AT(1) recept or antagonist losartan. AVP action was blocked by the V-1 receptor antagoni st [d(CH2)(5),Tyr(NH2)(9)]AVP. In SMC pre-treated with nifedipine, neither ANG II nor AVP elicited [Ca2+](i) responses. Poststimulation nifedipine rev ersed elevated [Ca2+](i) basal levels. Short-term reductions in external [C a2+](i) (EGTA) mimicked the nifedipine effects. Our study shows that AT(1) and V-1 receptors stimulate [Ca2+](i) by a common mechanism characterized b y preferential action on voltage-gated L-type channels sensitive to dihydro pyridines. Calcium signaling elicited by AT(1) receptors does not differ be tween SHR and WKY; thus the in vivo exaggerated reactivity may be dependent on interactions with other cell types, e.g., endothelium. In contrast, AVP produced larger changes in [Ca2+](i) in arteriolar SMC from SHR, and such direct effects can account for the exaggerated renal blood flow responses.