The relationships between early ionic events, the pattern of protein synthesis, and oocyte activation in the surf clam, Spisula solidissima

Abstract

The ionic events linked to activation of surf clam ( Spisula solidissima) oocytes include a transient increased Ca 2+ influx and an acid release. The aim of the present work was to further elucidate the respective roles of these two ionic events and to clarify the possible role of protein kinase C in the sequence of events leading to oocyte activation. K +-enriched seawater, ammonium chloride, and the phorbol ester 12- O-tetradecanoyl-13-phorbol acetate (TPA), a protein kinase C activator, were tested for their ability to promote germinal vesicle breakdown (GVBD), an acid release, increased 45Ca 2+ uptake, and a shift in the pattern of protein synthesis. Oocytes activated by addition of K + ions release an amount of H + similar to that induced by fertilization, with the same time course, show an increased, verapamil-sensitive, 45Ca 2+ uptake that is proportional to the amount of added K +, and undergo a shift in their pattern of protein synthesis, which requires the presence of external Ca 2+. Ammonium chloride, at concentrations causing a higher production of acid than that induced by K + ions or fertilization, does not trigger GVBD nor any increased 45Ca 2+ uptake or any detectable shift in the pattern of protein synthesis. Combined additions of ammonium chloride with subthreshold concentrations of K + ions allow GVBD to occur, thus revealing a synergistic effect of ammonia and K + ions. TPA slowly induces GVBD, an Na +-dependent acid release, and a shift in the pattern of protein synthesis, in the absence of increased 45Ca 2+ uptake. Our results lead us to propose the following sequence of events for the activation of Spisula oocytes: an increased Ca 2+ influx contributes to activate protein kinase C which causes a Na +-dependent acid release leading to a rise of pH i. This rise of pH i, although insufficient by itself, may set the pH i in a permissive range for activation to occur through the action of other protein kinase C-sensitive events leading to the production of meiosis-inducing proteins.