AbstractElectropotential differences between the cytoplasm and external medium have been compared in the matureR. pipiensoccyte and the ovulated unfertilized egg as a function of Nao, Ko, Caoand Clo. 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 Caoand in 5 × 10−4mM Caothe oocyte membrane behaved like a NaCl electrode. Studies on the steady potential as a function of Nao, Koand Cloin 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 cel
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