Angiotensin II, reactive oxygen species, and Ca2+ signaling in afferent arterioles

In afferent arteriolar vascular smooth muscle cells, ANG II induces a rise in cytosolic Ca2+ ([Ca2+](i)) via inositol trisphosphate receptor (IP3R) stimulation and by activation of the adenine diphosphate ribose (ADPR) cyclase to form cyclic ADPR, which sensitizes the ryanodine receptor (RyR) to Ca2+. We hypothesize that ANG II stimulation of NAD(P) H oxidases leads to the formation of superoxide anion (O-2(-center dot)), which, in turn, activates ADPR cyclase. Afferent arterioles were isolated from rat kidney with the magnetized microsphere and sieving technique and loaded with fura-2 to measure [Ca2+](i). ANG II rapidly increased [Ca2+](i) by 124 +/- 12 nM. In the presence of apocynin, a specific inhibitor of NAD(P) H oxidase assembly, the [Ca2+](i) response was reduced to 35 +/- 5 nM (P < 0.01). Tempol, a superoxide dismutase mimetic, did not alter the [Ca2+](i) response to ANG II at a concentration of 10(-4) M (99 +/- 12 nM), but 10(-3) M tempol reduced the response to 32 +/- 3 nM (P < 0.01). The addition of nicotinamide, an inhibitor of ADPR cyclase, to apocynin or tempol (10(-3) M) resulted in no further inhibition. Measurement of superoxide production with the fluorescent probe tempo 9-AC showed that ANG II caused an increase of 48 +/- 20 arbitrary units; apocynin or diphenyl iodonium (an inhibitor of flavoprotein oxidases) inhibited the response by 94%. Hydrogen peroxide (H2O2) was studied at physiological (10(-7) M) and higher concentrations. In the presence of H2O2 (10(-7) M), neither baseline [Ca2+](i) nor the response to ANG II was altered (125 +/- 15 nM), whereas H2O2 (10(-6) and 10(-5) M) inhibited the [Ca2+](i) response to ANG II by 35 and 46%, respectively. We conclude that ANG II rapidly activates NAD( P) H oxidases of afferent arterioles, leading to the formation of O-2(-center dot), which then stimulates ADPR cyclase to form cADPR. cADPR, by sensitizing the RyR to Ca2+, augments the Ca2+ response (calcium-induced calcium release) initiated by activation of the IP3R.