Internal pH of Xenopus oocytes: a study of the mechanism and role of pH changes during meiotic maturation.

The internal pH (pHi) of Xenopus laevis oocytes, as measured by the DMO method, covered a broad range of values from 7.06 +/- 0.01 to 7.93 +/- 0.01, with a mean value of 7.43 +/- 0.03. The pHi measured by DMO and microelectrodes was nearly identical in control and maturing oocytes from the same batch. The oocytes from most females elevated their pHi in response to progesterone, reaching a maximum elevation of 0.30 +/- 0.03 pH units above control values at 100% germinal vesicle breakdown (GVBD). However, some females were found to contain oocytes that already had an elevated pHi of 7.71 +/- 0.03 which did not significantly increase during maturation. Human chorionic gonadotrophin (hcG)-stimulated females had oocytes with slightly higher control pHi values than oocytes from nonstimulated females but still showed the same elevation in response to progesterone. Thus, the "stimulated" state of oocyte physiology as induced by hcG did not account for the variation in control pHi and responsiveness to progesterone. Other aspects of this variability are discussed. Elevating or lowering the external pH is shown to elevate and lower pHi, respectively, in a stable and predictable manner. Using this approach to change pHi we have found no effect of changes in pHi on the rate of protein synthesis in control and maturing oocytes. Similarly, pHi had only a slight facilitating effect on the rate of GVBD. A pH indicator gel was used to demonstrate that the pHi increase during oocyte maturation involved an acid efflux. We conclude that an elevated pHi is not necessary for oocyte maturation, yet the mechanism of the pHi elevation is discussed as a possible lead to events that are necessary.