Calcium entry and mobilization signaling pathways in ANG II-induced renal vasoconstriction in vivo

The purpose of this study was to determine the relative importance of calcium signaling pathways in angiotensin II (ANG II)-induced renal vasoconstriction in vivo. Nifedipine was used to antagonize dihydropyridine-sensitive voltage-dependent calcium channels; BAY K 8644 was employed to activate these calcium channels. Intracellular calcium mobilization was evaluated using TMB-8 or heparin to inhibit calcium release from sarcoplasmic reticulum. Renal blood flow was measured by electromagnetic flowmetry in anesthetized euvolemic Wistar-Kyoto rats. The animals were pretreated with indomethacin to avoid interactions with prostaglandins. ANG II (2 ng) or BAY K 8644 (1 microgram) was injected into the renal artery to produce a transient 30-50% decrease in renal blood flow without affecting arterial pressure. Coadministration of nifedipine with BAY K 8644 produced dose-dependent inhibition of the maximum renal vasoconstriction elicited by BAY K 8644. The calcium-channel antagonist had similar effects on ANG II-induced renal vasoconstriction. Nifedipine exerted maximum inhibition by blocking 50% of the peak ANG II response. To evaluate intracellular calcium mobilization, TMB-8 or heparin was coadministered with ANG II. Each agent produced dose-dependent inhibition of up to 50% of the maximum renal vasoconstriction produced by ANG II. The inhibitory effects of nifedipine and TMB-8 were additive; neither agent had an effect when ANG II AT1 receptors were antagonized with losartan. These observations indicate that one-half of the ANG II-induced constriction of renal resistance vessels is mediated by voltage-dependent L-type calcium channels responsive to the dihydropyridine nifedipine. The remaining 50% of the renal vasoconstriction elicited by ANG II is mediated by inositol 1,4,5-trisphosphate-mediated calcium mobilization from intracellular sources. The additive nature of the inhibitory effects indicates distinct mechanisms involving calcium mobilization and calcium entry signaling pathways that are of equal importance in ANG II activation of AT1 receptors to trigger constriction of renal resistance vessels under basal conditions.