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

Abstract

Prophase-arrested oocytes of Ruditapes philippinarum can not be fertilized or stimulated by a depolarizing agent such as an excess of KCl, in contrast to the situation found in Crassostrea gigas. We have performed a comparative study between the two situations found in these species. In vitro, both of these oocytes can be triggered to reinitiate meiosis following a treatment by serotonin which promotes an intracellular calcium surge. Ruditapes and Crassostrea oocytes further arrest in metaphase I, at which stage they can be either activated by sperm or by excess KCl. These treatments trigger an intracellular calcium increase. This suggests that functional voltage-operated Ca2+ channels are expressed in Ruditapes during the course of maturation between prophase and metaphase I. Results obtained using pharmacological tools and direct binding of specific dihydropyridines, strongly suggest that these channels are dihydropyridine-sensitive calcium channels. In Ruditapes they become functional after 5-HT stimulation, their number increasing before GVBD. In Crassostrea the dihydropyridine-sensitive Ca2+ channels are already present at prophase stage and their density is constant from prophase to metaphase I. Moreover, we have shown for Ruditapes and Crassostrea that: 1) the addition of 10 microM of S(-)BayK8644, an agonist of dihydropyridine-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 their activation by excess KCl or fertilization. Taken together these data suggest that the absence of dihydropyridine-sensitive Ca2+ channels in the membrane of prophase-arrested oocytes of Ruditapes may account for their inability to be fertilized at this stage, while the presence of dihydropyridine-sensitive Ca2+ channels in prophase-arrested oocytes of Crassostrea may explain their fertilizability at this stage.

Ion currents modulating oocyte maturation in animals

“…In Spisula, nuclear maturation requires external calcium influx through voltage-gated channels in order to undergo GVBD. This is also the case for the oocytes of the surf clam, Ruditapes and Crassostrea [Dubè 1992;Leclerc et al 2000]. The present hypothesis is that an early calcium influx is a prerequisite to allow nuclear maturation in either PI- [Colas and Dubè 1998] or MI-arrested oocytes of bivalves [Moreau et al 1996].…”

Calcium signals and oocyte maturation in marine invertebrates

“…In this type of bivalve (e.g. Ruditapes, Hiatella, Patinopecten, Mercenaria, and Crassostrea), as well as in Spisula, prophase-arrested oocytes that are removed from the ovary can be triggered to resume meiosis in vitro by micromolar concentrations of 5HT (Osanai and Kuraishi, 1988;Guerrier et al, 1993;Deguchi and Osanai, 1995;Fong et al, 1997;Colas and Dubé, 1998;Leclerc et al, 2000;Zhang et al, 2009;Yuan et al, 2012). Accordingly, because of its bioactivity and localization within the gonad, 5HT is believed to function in vivo as a physiologically relevant neurohormone that generally triggers intraovarian meiotic reinitiation from prophase I, spawning of maturing oocytes, and a subsequent metaphase I arrest in bivalve oocytes.…”

Roles of extracellular ions and pH in 5-HT-induced sperm motility in marine bivalve

“…These data strongly suggest that initiation of sperm motility induced by 5-HT could be related to 5-HT-dependent [Ca 2C ] i rise and oscillations and confirms the essential roles of [Ca 2C ] e stores. Similarly, the Ca 2C rise required for GVBD induction has been suppressed in the oocytes of a marine bivalve (Hiatella flaccida), Pacific oyster, and Manila clam following incubation in Ca 2C -free ASW or 5-HT (Deguchi & Osanai 1995, Leclerc et al 2000, Tanabe et al 2006. [Ca 2C ] i plays key role in the regulation of flagellar beating (Darszon et al 2011).…”