Changes in membrane potential, membrane resistance, and intracellular H, K, Na, and Cl activities during the progesterone-induced maturation of urodele amphibian oocytes

Changes in intracellular activities of H⁺, K⁺, Na⁺, and Cl⁻ ions were recorded with ion-selective microelectrodes during progesterone-induced maturation of full-grown oocytes of the urodele amphibians Ambystoma mexicanum and Pleurodeles waltlii. The membrane potential (E_m) and electrical resistance (R_m) were also determined. During the first hours after initiation of maturation, the oocytes slowly depolarized and R_m gradually increased. By the end of maturation of Pleurodeles oocytes E_m had stabilized at about −10 mV and R_m had increased from 410 to 1760 kΩ. The same initial pattern was observed for Ambystoma, but in most oocytes a rapid transition occurred at about the time of germinal vesicle breakdown (GVBD): E_m spontaneously shifted from about −15 to about +30 mV; simultaneously R_m dropped from 1230 down to 100 kΩ (i.e., less than the initial 270 kΩ resistance). The internal K⁺ activity did not show any important variation during maturation of Ambystoma and Pleurodeles oocytes. Na⁺ activity increased slightly at the onset of GVBD in Ambystoma; a further marked increase of Na⁺, accompanied by an increase in Cl⁻ activity, was observed as soon as E_m shifted to a positive value. In Pleurodeles sodium activity was also more elevated in matured than in immature oocytes. The average pH of Ambystoma immature oocytes was 7.48 ± 0.05 (external pH 7.5). A transient alkalinization to 7.64 ± 0.04 took place during the first 4–6 hr postprogesterone. Cytoplasmic pH was restored to 7.50 ± 0.07 between 10 and 12 hr postprogesterone, before the onset of GVBD and the shift of E_m. The difference between the measured oocyte pH and the calculated equilibrium pH decreases during the course of maturation, due partly to the depolarization of E_m.