Role of dihydropyridine-sensitive calcium channels in meiosis and fertilization in the bivalve molluscs Ruditapes philippinarum and Crassostrea gigas

Prophase-arrested oocytes of Ruditapes philippinarum can not be fertilized or stimulated by a depolarizing agent such as an excess of KCI, in contrast to the situation found in Crassostrea gigas. We have performed a comparati ve study between the two situations found in these species. In vitro, both of these oocytes can be triggered to reinitiate meiosis following a treatme nt by serotonin which promotes an intracellular calcium surge. Ruditapes an d Crassostrea oocytes further arrest in metaphase I, at which stage they ca n be either activated by sperm or by excess KCI. These treatments trigger a n intracellular calcium increase. This suggests that functional voltage-ope rated Ca2+ channels are expressed in Ruditapes during the course of maturat ion between prophase and metaphase I. Results obtained using pharmacologica l tools and direct binding of specific dihydropyridines, strongly suggest t hat these channels are dihydropyridine-sensitive calcium channels. In Rudit apes they become functional after 5-HT stimulation, their number increasing before GVBD. In Crassostrea the dihydropyridine-sensitive Ca2+ channels ar e already present at prophase stage and their density is constant from prop hase to metaphase I. Moreover, we have shown for Ruditapes and Crassostrea that: 1) the addition of 10 mu M of S(-)BayK8644, an agonist of dihydropyri dine-sensitive calcium channels to metaphase-arrested oocytes releases them from metaphase block; and 2) incubating these oocytes with nicardipine, a potent blocker of dihydropyridine-sensitive Ca2+ channels, inhibits both th eir activation by excess KCl or fertilization. Taken together these data su ggest that the absence of dihydropyridine-sensitive Ca2+ channels in the me mbrane of prophase-arrested oocytes of Ruditapes may account for their inab ility to be fertilized at this stage, while the presence of dihydropyridine -sensitive Ca2+ channels in prophase-arrested oocytes of Crassostrea may ex plain their fertilizability at this stage. (C) 2000 Editions scientifiques et medicales Elsevier SAS.