NICOTINAMIDE INHIBITS CYCLIC ADP-RIBOSE-MEDIATED CALCIUM SIGNALING INSEA-URCHIN EGGS

Cyclic ADP ribose (cADPR) is a potent Ca2+-releasing agent, and putati ve second messenger, the endogenous levels of which are tightly regula ted by synthetic (ADP-ribosyl cyclases) and degradative (cADPR hydrola se) enzymes. These enzymes have been characterized in a number of mamm alian and invertebrate tissues and their activities are often found on a single polypeptide. beta-NAD(+), cGMP and nitric oxide (NO) have be en reported to mobilize Ca2+ in the sea urchin egg via the cADPR-media ted pathway. We now report that in sea urchin egg homogenates, nicotin amide inhibits the Ca2+-mobilizing action of beta-NAD(+), cGMP and NO, but has no effect on cADPR-induced Ca2+ release. Moreover, nicotinami de inhibits cGMP-induced regenerative Ca2+ waves in the intact sea urc hin egg. By successfully separating the cADPR-metabolizing machinery f rom that which releases Ca2+, we have shown that nicotinamide inhibits cADPR-mediated Ca2+ signalling at the level of cADPR generation. Impo rtantly, nicotinamide had no effect upon the hydrolysis of cADPR, and its selective action on cyclase activity was supported by its inhibiti on of purified Aplysia ADP-ribosyl cyclase, which does not exhibit det ectable hydrolytic activity. The action of nicotinamide in blocking Ca 2+ release by beta-NAD(+), cGMP and NO strongly suggests that these ag ents act as modulators of cADPR synthesis rather than to sensitize cal cium release channels to cADPR.