Nicotinamide inhibits cyclic ADP-ribose-mediated calcium signalling in sea urchin eggs

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.