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

Abstract

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.