The aim of this study was to investigate ANG II-induced Ca2+ signaling in freshly isolated afferent arterioles (AA) from two-kidney, one-clip hypertensive (2K1C) rats, which have an elevated plasma and renal ANG II level, and different perfusion pressure and vascular tone in the clipped and nonclipped kidney. The Ca2+ responses in vessels from 2K1C and control rats were similar in all groups (P>0.1). The intracellular Ca2+ (Cai2+) response in the afferent arteriole after 10(-8) M ANG II stimulation was 0.57+/-0.10, 0.50+/-0.07, 0.48+/-0.04, and 0.36+/-0.05 in the control, sham, nonclipped, and clipped kidney, respectively. These data were consistent with the finding of unchanged AT(1a)R mRNA levels in AAs from all groups. Although the absolute values were similar, the dose-response curves to ANG II were different. In the control, sham, and nonclipped kidney from 2K1C, the dose-response curve leveled off between 10(-8) and 10(-6) M ANG II. In the clipped kidney, the dose-response curve was linear, with a significantly increased response at 10(-6) M compared with 10(-8) M ANG II (P<0.05). Inhibition of cyclooxygenase-1 (COX-1) with indomethacin enhanced the ANG II response in the nonclipped (Delta0.30+/-0.09) and clipped (Delta0.30+/-0.09) kidneys from 2K1C (P<0.005), but not in control rats (Delta-0.02+/-0.11, P>0.8). Conclusively, the ANG II-induced Cai2+ response was reduced by COX-1-derived prostaglandins in 2K1C, in contrast to control animals, where the COX-1 inhibition had no effect. COX-2 inhibition with NS-398 did not increase the ANG II-mediated Cai2+ response in any of the groups.