| Abstract: | Electropotential differences between the cytoplasm and external medium have been compared in the mature R. pipiens occyte and the ovulated unfertilized egg as a function of Na]o, K]o, Ca]o and Cl]o. In solutions containing 1.0 mM Ca++ the oocyte behaved as though it were predominantly permeable to K+ and Cl?, i.e., like a KCl electrode. However, the steady potential decreased with decreasing Ca]o and in 5 × 10?4 mM Ca]o the oocyte membrane behaved like a NaCl electrode. Studies on the steady potential as a function of Na]o, K]o and Cl]o in 1.0 mM Ca++ or Ca-free solutions suggest that Ca++ controls the passive permeability of the oocyte membrane to Na+ and Cl?. In the ovulated unfertilized egg the K+ selectivity of the cell membrane disappeared and the system behaved like a NaCl electrode. No effect of external Ca++ or K+ concentration changes on the steady potential was observed. These results indicate that the ion permeability properties of the ovulated egg are similar to that of the ovarian oocyte in Ca-deficient medium, and suggests that the mechanism of ovulation may involve the removal of Ca++ regulation of ion permeability of the egg cell membrane. |
