Action potentials dependent on monovalent cations in developing mouse embryos.

Action potentials were examined using intracellular recording techniques to study the ionic mechanisms of excitability in oocytes and embryos of the mouse from the 1-cell through to the 16-cell stages of development. At all stages examined, action potentials dependent on monovalent cations (Na+ or Li+) were observed under Ca2+-free conditions, and the maximum rate of rise (MRR) of the Na action potential was larger than that of the Li action potential at a given concentration of monovalent cations. Both the Na and Li action potentials were insensitive to tetrodotoxin, and they were blocked by inorganic (Co2+, Cd2+, Mn2+, La3+) and organic (diltiazem) Ca antagonists. These properties were exactly the same as those of the Ca channels present in the membranes of the mouse embryos. In addition, competition was observed between permeant monovalent and divalent cations: the overshoot and MRR of the Na or Li action potentials were reduced in the presence of Ca2+. These results suggest that Na+ or Li+ go through the Ca channels when the external Ca2+ concentration was very low, and that the Ca channels are more permeable to Na+ than to Li+. Separate Na channels could not be detected or induced at any stages of development.