Nicotinic acid adenine dinucleotide phosphate (NAADP) is present at micromolar concentrations in sea urchin spermatozoa

Nicotinic acid adenine dinucleotide phosphate (NAADP) has been shown to induce Ca2+ release in numerous cellular models, ranging from marine invertebrates to mammals. However, endogenous levels of this pyridine dinucleotide have yet to be demonstrated. In the sea urchin egg, NAADP receptors are abundant but have the peculiarity of being inactivated at low concentrations (picomolar) and activated at higher concentrations (nanomolar) which apparently rules out any possibility of the receptor being activated by concentration rises induced by a slow enzymatic formation in the cytosol. One of the most important events of fertilization is a Ca2+ transient in the egg, which leads to egg activation. The mechanisms which underlie the transient are still unclear and several theories persist including the existence of a sperm receptor and that soluble factors may pass from the sperm to the egg cytosol. We have investigated the possibility that NAADP might be present in sperm. Indeed, we found that sea water-activated spermatozoa are able to synthesize NAADP and that sperm extracts contain micromolar concentrations of the messenger. Although it is unlikely that NAADP alone mediates the fertilization wave, our data suggest that transfer of NAADP from spermatozoa to egg may play a role in this phenomenon.