Intracellular pH change does not accompany egg activation in the mouse

In the sea urchin, some other marine invertebrates, and the frog, Xenopus, egg activation at fertilization is accompanied by an increase in intracellular pH (pH(i)). We measured pH(i) in germinal vesicle (GV)-intact mouse oocytes, ovulated eggs, and in vivo fertilized zygotes using the pH indicator dye, SNARF-1. The mean pH, was 6.96 +/- 0.004 (+/- SEM) in GV-intact oocytes, 7.00 +/- 0.01 in ovulated, unfertilized eggs, and 7.02 +/- 0.01 in fertilized zygotes, indicating no sustained changes in pH(i) after germinal vesicle breakdown (GVBD) or fertilization. To examine whether transient changes in pH(i) occur shortly after egg activation, mouse eggs were parthenogenetically activated by 7% ethanol in phosphate buffered saline (PBS); no significant change in pH(i) followed ethanol activation. Since increased Na+/H+ antiporter activity is responsible for pH(i) increase in the sea urchin, pH(i) was measured in the absence of added bicarbonate or COP (a condition under which the antiporter would be the only major pH(i) regulatory mechanism able to operate, since the others were bicarbonate-dependent) in GV-intact oocytes, ovulated eggs, and in vivo fertilized zygotes to determine whether a Na+/H+ antiporter was activated. There was no physiologically significant difference in pH(i) after GVBD or fertilization, when pH(i) was measured in bicarbonate-free medium, nor any change upon parthenogenetic activation. Thus, a change in pH(i) is not a feature of egg activation in the mouse. (C) 1996 Wiley-Liss, Inc.