Permeation of divalent and monovalent cations through the ovarian oocyte membrane of the mouse.

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Ovarian oocytes were isolated from adult mice and intracellular recording was performed using single glass micro-electrodes. The resting potential was - 7.0 +/- 1.8 mV in standard solution, and the oocyte showed a regenerative response at the cessation of hyperpolarizing current pulse. Ca spikes were observed under Na+-free conditions. The overshoot of the spike increased 28 mV for a 10-fold increase in [Ca2+]o and showed saturation as [Ca2+]o was elevated. The spike was insensitive to tetrodotoxin (TTX) and was blocked by polyvalent cations such as Co2+, Cd2+, Mn2+ and La3+. Sr2+ or Ba2+ substituted for Ca2+ in generating action potentials. Na spikes were observed under Ca2+-free conditions. The overshoot of the spike showed the slope of 39 mV for a 10-fold increase in [Na+]o and a saturation was detected when [Na+]o was raised. The spike was resistant to TTX and was blocked by Ca antagonists such as Co2+, Cd2+, Mn2+ or La3+. Li+ substituted for Na+ in producing spikes, while Rb+ did not. The overshoot and maximum rate of rise of the Na spike became smaller when Ca2+ was present in the bathing solution, indicating a competition between Na+ and Ca2+. Mn2+ acted not only as a Ca blocker but also as a charge carrier during excitation. Mn spikes were detected in Na+-, Ca2+-free solutions and were blocked by Ca antagonists. The resting membrane is permeable to not only Na+ but also to some extent to K+. It is suggested that the ovarian oocyte membrane of the mouse has voltage-dependent Ca channels, and both divalent (Ca2+, Sr2+, Ba2+, Mn2+) and monovalent (Na+, Li+) cations can pass through the Ca channels to generate action potentials.

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