A comparative study of the membrane potential from before fertilization through early cleavage in two frogs, Rana pipiens and Xenopus laevis.

The membrane potential of Rana pipiens eggs (-55.0 mV +/- 11.2(16)) was more likely to recover from impalement and was always more negative than that of eggs of Xenopus laevis (-19.3 mV +/- 4.2(68)). It was also much more negative than previously reported. Essentially similar membrane resistance changes were measured in the two frog species through fertilization and cleavage. Small transient depolarizations only associated with the onset of the fertilization potential in Xenopus could be prevented by hyperpolarizing the egg membrane prior to fertilization. Repolarization was variable and longer in Rana and often accompanied by large transient spontaneous depolarizations. Insemination time, the time between fertilization and cleavage and the first cleavage division cycle, were all about twice as long in Rana. Xenopus egg cleavage was invariably accompanied by pronounced transient hyperpolarizations that were essentially absent in Rana.