Na +Ca 2+ antiporter activity of rat hepatocytes. Effect of adrenalectomy on Ca 2+ uptake and release from plasma membrane vesicles

Abstract

The presence and mode of Na +Ca 2+ antiporter activity were studied in hepatocytes isolated from sham-operated or adrenalectomized rats and in inside-out plasma membrane vesicles isolated from rat liver. Decreasing extracellular Na + (Na o +) immediately increased cytosolic free calcium (Ca i 2+). The rise in Ca i 2+ was proportional to the reduction in Na o + and was caused by an increased calcium influx, presumably on the Na +Ca 2+ antiporter operating in the reverse mode. Perfusing the cells with Ca 2+-free media stimulated Ca 2+ efflux and decreased Ca i 2+, an effect dependent on Na o +. This suggests an activation of the forward mode of Na +Ca 2+ exchange. There was little difference in these parameters between sham and adx groups. In contrast, steady-state calcium uptake by inside-out plasma membrane vesicles was inhibited 40% after adrenalectomy. The decreased calcium uptake was not caused by a deficiency in the ATP-dependent Ca 2+ pump, whose K m and V max were unaffected by adrenalectomy, but by an Na +-dependent leak from the vesicles. Ca 2+ efflux was proportional to the extravesicular Na + concentration, suggesting that the calcium leak may take place on a Na +Ca 2+ antiporter. This Na +-dependent calcium efflux was significantly increased in vesicles prepared from adx rat livers. These results suggest that hepatocytes have functional Na +Ca 2+ antiporters that can operate in both forward and reverse modes. Under normal conditions, the Na +Ca 2+ a antiporter apparently operates in the reverse mode as a Ca 2+ influx pathway. The increase in Na +-dependent Ca 2+ efflux evoked by adrenalectomy in plasma membrane vesicles could explain the recent results we obtained in hepatocytes isolated from adx rats, showing increased calcium influx, increased Ca i 2+, increased intracellular calcium sequestration, and increased plasmalemmal calcium cycling.