NOVEL POSTFERTILIZATION INWARD CA2-ENTRY THROUGHOUT MEIOSIS( CURRENT IN ASCIDIAN EGGS ENSURING A CALCIUM)

The conductance change after fertilization in the oocyte of the ascidi an Ciona intestinalis has been followed by the whole cell patch-clamp technique. Two new inward currents, which are absent in unfertilized e ggs, are elicited by hyperpolarization from a holding potential of +20 mV, which is the resting potential soon after fertilization. These cu rrents reach their maximum level during the first meiotic division cyc le, and then decrease in intensity, becoming almost undetectable at th e 2-cell stage. These currents are most easily seen at high concentrat ions of barium. At least one, and likely both, of these currents appea rs to be carried by Ca ions. One of the currents is blocked by low con centrations of gadolinium; the other one is blocked by higher concentr ations, although gadolinium at these levels does not block fertilizati on and the associated early depolarizing jump of the eggs. Thus these currents are not carried by channels that mediate the fertilization cu rrent. However, gadolinium blocks normal transition to 2-cell stage an d blocks current oscillations synchronous to free calcium oscillations that occur normally in eggs around meiosis II. The electrical signatu re of calcium-release activated currents, taken together with these fi ndings, suggests that these inward currents ensure a calcium entry pat hway throughout meiosis. A plausible function of these currents may be to refill the Ca stores that are depleted after fertilization and tha t are required to progress into mitotic cell division. This interpreta tion is reinforced by experiments on unfertilized eggs with intracellu lar Ca stores depleted by thapsigargin, where both the newly described currents are observed. (C) 1996 Academic Press, Inc.