Cyclic ADP ribose (cADPR) is a potent Ca2+-releasing agent, and putative second messenger, the endogenous levels of which are tightly regulated by synthetic (ADP-ribosyl cyclases) and degradative (cADPR hydrolase) enzymes. These enzymes have been characterized in a number of mammalian and invertebrate tissues and their activities are often found on a single polypeptide. β-NAD+, cGMP and nitric oxide (NO) have been reported to mobilize Ca2+ in the sea urchin egg via the cADPR-mediated pathway. We now report that in sea urchin egg homogenates, nicotinamide inhibits the Ca2+-mobilizing action of β-NAD+, cGMP and NO, but has no effect on cADPR-induced Ca2+ release. Moreover, nicotinamide inhibits cGMP-induced regenerative Ca2+ waves in the intact sea urchin egg. By successfully separating the cADPR-metabolizing machinery from that which releases Ca2+, we have shown that nicotinamide inhibits cADPR-mediated Ca2+ signalling at the level of cADPR generation. Importantly, nicotinamide had no effect upon the hydrolysis of cADPR, and its selective action on cyclase activity was supported by its inhibition of purified Aplysia ADP-ribosyl cyclase, which does not exhibit detectable hydrolytic activity. The action of nicotinamide in blocking Ca2+ release by β-NAD+, cGMP and NO strongly suggests that these agents act as modulators of cADPR synthesis rather than to sensitize calcium release channels to cADPR.

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