THE SODIUM REQUIREMENT FOR ACTIVATION OF SURF CLAM OOCYTES

We have investigated the external Na+ requirement for the activation of surf clam oocytes. Resumption of meiosis, evidenced by germinal vesicle breakdown (GVBD), may be triggered artificially by elevating the external K+ concentration in the virtual absence of external Na+ (< 5 μM). However, the kinetics of GVBD is slowed down considerably in the absence of Na+. The oocytes normally release acid upon activation through an Na+-dependent mechanism thought to cause an intracellular pH (pHi) rise. Addition of the weak base ammonia, which raises directly pHi, allows oocytes activated by K+ in Na+-free medium to undergo GVBD with normal kinetics. We suggest that a rise of pHi is not essential for GVBD in surf clam oocytes but it normally contributes to accelerate the rate of reactions leading to activation. © 1990.