The lonic permeability of the Amphibian oocyte in the presence or absence of external calcium

The membrane potential of the Rana oocyte, as determined by intracellular microelectrodes, ranges from approximately −40 to −70 mV (inside negative). Upon removal of external Ca, a large, rapid depolarization in membrane potential occurs. The depolarization is reversible by both Ca and Mg. In the presence of Ca, the magnitude of the membrane potential is primarily dependent on the K distribution across the oocyte membrane with a lesser but significant Na dependence. Upon Ca removal, the Na dependence is greatly increased, indicating an increased Na permeability. Direct measurements of Na, K and Cl permeability by tracer‐flux analysis indicate the following. In the presence of Ca the permeabilities are approximately: Na (1.0 × 10⁻⁶ cm/sec); K (5.6 × 10⁻⁶ cm/sec); Cl (1.4 × 10⁻⁶ cm/sec). Upon Ca removal the K and Cl permeabilities are not significantly altered. However, there is an approximately four‐fold increase in Na permeability (4.5 × 10⁻⁶ cm/sec). This increase explains the observed depolarization in membrane potential upon Ca removal. It has been suggested previously that Ca may regulate the in vivo depolarization observed during transition from oocyte to ovulated egg in the amphibian. The present experiments indicate that, if Ca does play a role, its role is not analogous to the in vitro effect on the oocyte, since the ovulated egg has reduced Na as well as K permeability.