Meiosis reinitiation in Ruditapes philippinarum (Mollusca): involvement of L-calcium channels in the release of metaphase I block.

Prophase-arrested oocytes of Ruditapes philippinarum cannot be fertilised or stimulated by excess KCl, in contrast to the situation found in other bivalve species such as Barnea and Spisula. However, these oocytes can be triggered to reinitiate meiosis following treatment by serotonin or several pharmacological agents (calcium ionophores, thapsigargin, weak bases) which promote an intracellular calcium surge. Ruditapes oocytes further arrest in metaphase I, at which stage they can be activated either by sperm or by excess KCl. This suggests that functional voltage-operated calcium channels are expressed in this species during the course of maturation. Using pharmacological tools and direct binding of specific dihydropyridines, we demonstrate that these channels are L-type calcium channels which become functional after serotonin stimulation, their number increasing before germinal vesicle breakdown. Moreover we establish that: (1) the addition of 20 microM S(-)BayK8644, an agonist of L-type calcium channels, to metaphase-arrested oocytes releases them from metaphase block; (2) incubating these oocytes with PN200-110, a potent blocker of L-type calcium channels, inhibits their activation by both excess KCl and fertilisation. Taken together these data suggest that the absence of L-type calcium channels in the membrane of prophase-arrested oocytes of Ruditapes may account for their inability to be fertilised.