Voltage-gated Ca2+ entry and ryanodine receptor Ca2+ induced Ca2+ release in preglomerular arterioles

We have previously shown that in afferent arterioles, angiotensin II ( ANG II) involves activation of the inositol trisphosphate receptor ( IP3R), activation of adenine diphosphoribose ( ADPR) cyclase, and amplification of the initial IP3R- stimulated release of cytosolic Ca2+ ([ Ca2+](i)) from the sarcoplasmic reticulum ( SR) ( Fellner SK, Arendshorst WJ. Am J Physiol Renal Physiol 288: F785 - F791, 2004). The response of the ryanodine receptor ( RyR) to local increases in [ Ca2+](i) is defined as calcium- induced calcium release ( CICR). To investigate whether Ca2+ entry via voltage-gated channels ( VGCC) can stimulate CICR, we treated fura 2-loaded, freshly isolated afferent arterioles with KCl ( 40 mM; high KCl). In control arterioles, peak [ Ca2+](i) increased by 165 +/- 10 nM. Locking the RyR in the closed position with ryanodine ( 100 mu M) inhibited the [ Ca2+](i) response by 59% ( P < 0.01). 8-Br cADPR, a specific blocker of the ability of cyclic ADPR ( cADPR) to sensitize the RyR to Ca2+, caused a 43% inhibition. We suggest that the lower inhibition by 8- Br cADPR ( P = 0.02, ryanodine vs. 8- Br cADPR) represents endogenously active ADPR cyclase. Depletion of SR Ca2+ stores by inhibiting the SR Ca2+- ATPase with cyclopiazonic acid or thapsigargin blocked the [ Ca2+](i) responses to KCl by 51% ( P not significant vs. ryanodine or 8- Br cADPR). These data suggest that about half of the increase in [Ca2+](i) induced by high KCl is accomplished by activation of CICR through the ability of entered Ca2+ to expose the RyR to high local concentrations of Ca2+ and that endogenous cADPR contributes to the process.