Novel postfertilization inward Ca2+ current in ascidian eggs ensuring a calcium entry throughout meiosis

The conductance change after fertilization in the oocyte of the ascidian Ciona intestinalis has been followed by the whole cell patch-clamp technique. Two new inward currents, which are absent in unfertilized eggs, are elicited by hyperpolarization from a holding potential of +20 mV, which is the resting potential soon after fertilization. These currents reach their maximum level during the first meiotic division cycle, and then decrease in intensity, becoming almost undetectable at the 2-cell stage. These currents are most easily seen at high concentrations of barium. At least one, and likely both, of these currents appears to be carried by Ca ions. One of the currents is blocked by low concentrations of gadolinium; the other one is blocked by higher concentrations, although gadolinium at these levels does not block fertilization and the associated early depolarizing jump of the eggs. Thus these currents are not carried by channels that mediate the fertilization current. However, gadolinium blocks normal transition to 2-cell stage and blocks current oscillations synchronous to free calcium oscillations that occur normally in eggs around meiosis II. The electrical signature of calcium-release activated currents, taken together with these findings, suggests that these inward currents ensure a calcium entry pathway throughout meiosis. A plausible function of these currents may be to refill the Ca stores that are depleted after fertilization and that are required to progress into mitotic cell division. This interpretation is reinforced by experiments on unfertilized eggs with intracellular Ca stores depleted by thapsigargin, where both the newly described currents are observed.