Transmembrane regulation of intracellular calcium by a plasma membrane sodium/calcium exchanger in mouse ova

Regulation of cytoplasmic free calcium concentration ([Ca2+)](i)) is a key factor for maintenance of viability of cells, including oocytes. Indeed, du ring fertilization of an ovum, [Ca2+](i) is known to undergo oscillations, but it is unknown how basal [Ca2+](i) or calcium oscillations ape regulated . In the present study we investigated the role of the plasma membrane in r egulating [Ca2+](i) of metaphase II-arrested mouse oocytes (ova). Ova were collected from B6C3F1 mice treated with eCG (10 IU) and hCG (5 IU), and int racellular calcium was determined by means of fura-2. Extracellular calcium flux across the zona pellucida was detected noninvasively by a calcium ion -selective, self-referencing microelectrode that was positioned by a comput er-controlled micromanipulator. Under basal conditions ova exhibited a calc ium net efflux of 20.6 +/- 5.2 fmol/cm(2) per sec (n = 69). Treatment of ov a with ethanol (7%) or thapsigargin (25 nM-2.5 mu M) transiently increased intracellular calcium and stimulated calcium efflux that paralleled levels of [Ca2+](i). The presence of a Na+/Ca2+ exchanger was indicated by experim ents employing both bepridil, an inhibitor of Na+/Ca2+ exchange, and sodium -depleted media. In the presence of bepridil, a net influx of calcium was r evealed across the zona pellucida, which was reflected by an increase in th e [Ca2+](i.) In addition, replenishment of extracellular sodium to ova that had been incubated in sodium-depleted media induced a large calcium efflux , consistent with the actions of Na+/Ca2+ exchange. Sodium/calcium exchange in mouse ova may be an important mechanism that regulates [Ca2+](i).