The aim of the present work was to characterize angiotensin II (ANG II) receptors and their effect on intracellular free Ca2+ concentration ([Ca2+]i) in proliferating aortic smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Independently from the proliferating state of cultures, apparent affinities of ligands (ANG II > losartan > > CGP-42112A) were consistent with the presence of AT1 receptors in primary cells from SHR and WKY. In proliferating cultures, increases in [Ca2+]i elicited by ANG II (100 nM) were dramatically attenuated or abolished in VSMCs from both strains compared with confluent and postconfluent cultures. Ca2+ releases induced by ionomycin and by ANG II in the absence of extracellular Ca2+ were also impaired in proliferating cultures. In addition, no significant strain difference was found in proliferating cultures with respect to ANG II receptor density, basal [Ca2+]i, and ANG II-induced increases in [Ca2+]i. However, ANG II receptor density significantly increased in SHR, but not in WKY VSMCs at postconfluence. Furthermore, basal [Ca2+]i was elevated in confluent and postconfluent cultures from SHR but not WKY. In confluent cultures, ANG II- and ionomycin-induced Ca2+ releases were enhanced in SHR VSMCs compared with WKY VSMCs. These results show that ANG II-induced Ca2+ release and ionomycin-sensitive Ca2+ stores are enhanced in SHR VSMCs but dramatically decreased in proliferating VSMC cultures from both strains. Mechanisms underlying these alterations remain to be defined. However, the results suggest that alterations in ANG II AT1 receptor density and in intracellular Ca2+ handling in confluent and postconfluent cultures are not associated with the proliferative phenotype of SHR VSMCs. In addition, no evidence for any change in ANG II receptor subtype associated with proliferation of VSMCs was found in either strain.