Kinetic properties of nicotinic acid adenine dinucleotide phosphate-induced Ca2+ release

Three endogenous molecules have now been shown to release Ca2+ in the sea urchin egg: inositol trisphosphate (InsP3), cyclic adenosine 5'-diphosphate ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP), a derivative of NADP. While the mechanism through which the first two molecules are able to release Ca2+ is established and well characterized with InsP3 and cADPR-activating InsP3 and ryanodine receptors, respectively, the newly described NAADP has been shown to release Ca2+ via an entirely different mechanism. The most striking feature of this novel Ca2+ release mechanism is its inactivation, since subthreshold concentrations of NAADP are able to fully and irreversibly desensitize the channel. In the present study we have investigated the fast kinetics of activation and inactivation of NAADP-induced Ca2+ release. NAADP was found to release Ca2+ in a biphasic manner, and such release was preceded by a pronounced latent period, which was inversely dependent on concentration. Moreover, the kinetic features of NAADP-induced Ca2+ release were not altered by pretreatment with low concentrations of NAADP, although the extent of Ca2+ release was greatly affected. Our data suggest that the inactivation of NAADP-induced Ca2+ release is an all-or-none phenomenon, and while some receptors have been fully inactivated, those that remain sensitive to NAADP do so without any change in kinetic features.