NA+-CA2+ ANTIPORTER ACTIVITY OF RAT HEPATOCYTES - EFFECT OF ADRENALECTOMY ON CA2+ UPTAKE AND RELEASE FROM PLASMA-MEMBRANE VESICLES

The presence and mode of Na+-Ca2+ 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+-Ca2+ antiporter operating in the reverse mode. Perfusing the cells with Ca2+-free media stimulated Ca2+ efflux and decreased Ca(i)2+, an effect dependent on Na(o)+. This suggests an activation of the forward mode of Na+-Ca2+ 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 Ca2+ pump, whose K(m) and V(max) were unaffected by adrenalectomy, but by an Na+-dependent leak from the vesicles. Ca2+ efflux was proportional to the extravesicular Na+ concentration, suggesting that the calcium leak may take place on a Na+-Ca2+ antiporter. This Na+-dependent calcium efflux was significantly increased in vesicles prepared from adx rat livers. These results suggest that hepatocytes have functional Na+-Ca2+ antiporters that can operate in both forward and reverse modes. Under normal conditions, the Na+-Ca2+ antiporter apparently operates in the reverse mode as a Ca2+ influx pathway. The increase in Na+-dependent Ca2+ 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.