Increased intracellular pH is not necessary for ribosomal protein S6 phosphorylation, increased protein synthesis, or germinal vesicle breakdown in Xenopus oocytes

An increase in intracellular pH (pH i) and ribosomal protein S6 phosphorylation during Xenopus oocyte maturation has been reported by several laboratories. In this paper, the question of whether the pH i increase is necessary to induce S6 phosphorylation, an increase in protein synthesis, or germinal vesicle breakdown (GVBD) was assessed using sodium-free medium and the putative Na H exchange blocker amiloride. Sodium-free medium decreased basal pH i by 0.3 unit and prevented increases in pH i in response to both insulin and progesterone, but S6 phosphorylation occurred normally with both hormones. GVBD occurred normally in sodium-free medium in response to progesterone, but the effect of insulin was reduced by 60%. In sodium-containing medium, amiloride inhibited GVBD and prevented insulin or progesterone-induced increases in pH i but the hormone-induced increase in S6 phosphorylation was unaffected. In the absence of sodium, amiloride inhibited GVBD but did not affect pH i, indicating that amiloride inhibits GVBD by a pH i-independent mechanism. Both progesterone and insulin increased protein synthesis in oocytes by 35%, and amiloride inhibited basal protein synthesis but not the increase with hormone. In the presence of cholera toxin, protein synthesis increases with insulin were inhibited but increased S6 phosphorylation was unaffected. Priming of animals with pregnant mare's serum gonadotropin prior to oocyte isolation reduced the time required for progesterone-induced GVBD, and increased the synchrony of GVBD of the oocyte population. Priming also increased oocyte basal pH i and basal protein synthesis as well as the magnitude of the increase in protein synthesis with progesterone but had no effect on S6 phosphorylation. The results indicate that in Xenopus oocytes increased pH i is not necessary for increased S6 phosphorylation, increased protein synthesis, or GVBD in response to insulin or progesterone nor is increased S6 phosphorylation sufficient for GVBD or increased protein synthesis.