Intracellular pH plays a role in regulating protein synthesis in Xenopus oocytes

Full-grown Xenopus oocytes undergo meiotic maturation in response to progesterone stimulation. Using [14C]dimethyloxazolidine dione (DMO), we have measured a cytoplasmic alkalization in these oocytes starting at pH 7.14 +/- 0.17 during the germinal vesicle (GV) stage, and increasing to 7.56 +/- 0.14 at the time of germinal vesicle breakdown (GVBD). During this period, the rate of protein synthesis increases 2-fold from 18.9 +/- 3.1 to 37.7 +/- 8.8 ng/hr/oocyte. Artificial alkalization of GV stage oocytes to pHi 7.68 +/- 0.16, by exposure to the weak bases trimethylamine, methylamine, procaine, or imidazole, led to a 1.8-fold increase in the synthetic rate. Intracellular acidification from 7.5 back to 7.0 had no apparent effect on the elevated rate of protein synthesis following GVBD. Therefore, a cytoplasmic alkalization in the range of 7.5 to 7.6 seems to be one of the events that is necessary for initiating the increase in protein synthesis in maturing Xenopus oocytes; however, it does not appear that an elevated pHi is necessary to maintain the increased synthetic rate following GVBD.