4-AMINOPYRIDINE ACTS AS A WEAK BASE AND A CA2-PHILIPPINARUM( MOBILIZING AGENT IN TRIGGERING OOCYTE MEIOSIS REINITIATION AND ACTIVATION IN THE JAPANESE CLAM RUDITAPES)

Ovarian oocytes of the prosobranch mollusc Patella vulgata and the pel ecypod Ruditapes philippinarum are arrested during prophase of the fir st maturation division. Release from this blockade, which is revealed by germinal vesicle breakdown, drives these oocytes to a second arrest in metaphase I, at which time the oocytes become fertilizable. The re spective roles of Ca2+ and H+ ion movements during this early step in meiosis reinitiation has not been fully established yet. In this work we reveal the presence of acidic vesicles and report that bafilomycin A(1) and N,N'-dicyclohexylcarbodiimide, two inhibitors of the vacuolar -type H+-ATPase, applied to Ruditapes oocytes, produce a significant i nhibition of their response to the natural neurohormone serotonin. Sin ce sodium deprivation did not affect this response, this suggests that a v-type ATPase pump, possibly located in the membrane of these acidi c vesicles, may play a subtle role in the cascade of events that relea ses oocytes from their prophase block. We then describe how 4-aminopyr idine, a drug reputed to be a K+ channel antagonist, triggers both mei osis reinitiation and activation of Patella and Ruditapes oocytes. Thi s agent acts as a weak base, its effect depending on external pH. More over, using the fluorescent probes BCECF and Fluo-3/AM, we observe tha t this drug both alkalinizes the endoplasm and promotes an intracellul ar Ca2+ surge. This dual effect may explain why Ruditapes oocytes no l onger stop in metaphase under these conditions and behave like other b ivalve species which are directly fertilizable at the germinal vesicle stage.