A role for action potentials in maturing Rana pipiens oocytes

Intracellular electrical recording and voltage-clamp techniques were used to investigate possible roles for the action potentials (ap's) of Rana pipiens oocytes. The peak of each spontaneous or evoked ap is at or near the equilibrium potential for Na; therefore, the internal Na+ activity (aiNa) can be calculated from the Nernst equation. During maturation from metaphase I to metaphase II, aiNa increases from about 5 to 23 mM. By using three methods to prevent ap's from firing (voltage clamping, increasing the mechanical damage, adding 5 mM CoCl2 to the Ringer's) it was shown that the increase in aiNa required the existence of ap's. External Co2+ appeared to prevent the ap's by blocking only the Na+ current. Calculations showed that the Na+ influx during an ap could account for the observed increase in aiNa if a gradient or nonuniform distribution of Na+ exists within the oocyte. Preventing the ap's also delayed the onset of shock activatibility, a criterion of maturity. I propose that ap's load the oocyte with Na+ which may regulate the rate of maturation either directly or indirectly.