Development of Ca²⁺-release mechanisms during oocyte maturation of the starfish Asterina pectinifera

An important step for successful fertilization and further development is the increase in intracellular Ca²⁺ in the activated oocyte. It has been known that starfish oocytes become increasingly sensitive to inositol 1,4,5-trisphosphate (IP3) during meiotic maturation to exhibit highly efficient IP3-induced Ca²⁺ release (IICR) by the time of germinal vesicle breakdown (GVBD). However, we noted that the peak level of intracellular Ca²⁺ increase after insemination is already high in the maturing oocytes before GVBD. Using maturing oocytes before GVBD, we investigated Ca²⁺ release mechanisms other than IICR. We report here that Ca²⁺-release mechanisms dependent on nicotinic acid adenine dinucleotide phosphate (NAADP) and nicotinamide adenine dinucleotide (NADP), the precursor of NAADP, became functional prior to the development of IICR mechanisms. As with IP3, but unlike NAADP, the Ca²⁺ stores responsive to NADP are sensitized during the meiotic maturation induced by 1-methyladenine (1-MA). This suggests that the process may represent a physiological response to the maturation hormone. NADP-dependent Ca²⁺ release in immature oocytes, however, did not induce oocyte maturation by itself, but was enhanced by the conditions mimicking the increases of intracellular Ca²⁺ and pH that take place in the maturing oocytes of starfish.