NAADP triggers the fertilization potential in starfish oocytes

In invertebrates oocytes or eggs, the fertilization or activation potential establishes the fast electrical block to polyspermy and, in some species, provides the Ca²⁺ influx which contributes to the following intracellular Ca²⁺ wave. In echinoderms, the molecule triggering the activation potential is still unknown. The aim of this study was to assess whether nicotinic acid-adenine dinucleotide phosphate (NAADP) elicited the fertilization potential in starfish oocytes. The changes in membrane potential induced by the sperm were measured in oocytes held at a low resting potential, so that the Ca²⁺-action potential was inactivated and only the initial slower depolarization caused by the sperm could be studied. Decreasing extracellular Na⁺ concentration did not prevent the onset of the fertilization potential, while removal of external Ca²⁺ abolished it. The pre-incubation with SK&F 96365 and verapamil and the pre-injection of BAPTA inhibited the fertilization potential, while the injection of heparin only reduced its duration. The biophysical and pharmacological properties of the sperm-elicited depolarization were similar to those displayed by the NAADP-activated Ca²⁺-mediated current recently described in starfish oocytes. Indeed, the desensitization of NAADP-receptors prevented the onset of the fertilization potential. Taken together, these data suggest that NAADP could trigger the fertilization potential in starfish oocytes.