Endothelin-A and -B receptors, superoxide, and Ca2+ signaling in afferent arterioles.

It is unknown if endothelin-A and -B receptors (ET(A)R and ET(B)R) activate the production of superoxide via NAD(P)H oxidase and subsequently stimulate the formation of cyclic adenine diphosphate ribose (cADPR) in afferent arterioles. Vessels were isolated from rat kidney and loaded with fura 2. Endothelin-1 (ET-1) rapidly increased cytosolic Ca(2+) concentration (Ca(2+)) by 303 nM. The superoxide dismutase mimetic tempol, the NAD(P)H oxidase inhibitor apocynin, and nicotinamide, an inhibitor of ADPR cyclase, diminished the response by approximately 60%. The ET(B)R agonist sarafotoxin 6c (S6c) increased peak Ca(2+) by 117 nM. Subsequent addition of ET-1 in the continued presence of S6c caused an additional Ca(2+) peak of 225 nM. Neither nicotinamide or 8-bromo- (8-Br) cADPR nor apocynin decreased the Ca(2+) response to S6c, but inhibited the subsequent Ca(2+) response to ET-1. The ET(B)R blockers BQ-788 and A-192621 prevented the S6c Ca(2+) peak and reduced the ET-1 response by more than one-half, suggesting an ET(B)R/ET(A)R interaction. In contrast, the ET(A)R blocker BQ-123 had no effect on the S6c Ca(2+) peak and obliterated the subsequent ET-1 response. ET-1 immediately stimulated superoxide formation (measured with TEMPO-9-AC, 68 arbitrary units) that was inhibited 95% by apocynin or diphenyl iodonium. S6c or IRL-1620 increased superoxide by 8% of that caused by subsequent ET-1 addition. We conclude that ET(A)R activation of afferent arterioles increases the formation of superoxide that accounts for approximately 60% of subsequent Ca(2+) signaling. ET(B)R activation appears to result in only minor increases in superoxide production. Nicotinamide and 8-Br-cADPR results suggest that ET-1 (and primarily ET(A)R) causes the activation of vascular smooth muscle cell-ADPR cyclase.