The Na+/Ca2+ exchange system of the liver cell.

The efflux of 45Ca (Caeff) from rat liver slices incubated in a Krebs Ringer bicarbonate buffer showed two components with first order rate constants (kCa) of 0.0625 min-1 and 0.0101 min-1 respectively. The slow component corresponds to Caeff from the intracellular medium. Ouabain decreased kCa from the slow component (ksCa) by about 30%. External Na+ or Ca2+ did not affect ksCa. However, the inhibitory effect of ouabain was not observed in the absence of either Na+ or Ca2+ from the incubation medium. Concomitant to a decrease in ksCa, ouabain increased 45Ca uptake (Caup) by about 30%. Sodium ionophores also increased Caup, an effect which was not observed in the absence of external Na+. 22Na efflux was inhibited 50% by ouabain. In the presence of ouabain, external Ca2+ was able to significantly increase ksNa from 0.0475 min-1 to 0.0506 min-1. External Na+ dit not affect ksCa from preloaded slices containing high calcium and low sodium when incubated in a Ca2+-free medium. These results would indicate that a Na+/Ca2+ exchange system in operative in the plasma membrane of the hepatocyte. It is estimated that about 60% of the calcium entering the hepatocyte moves in through this exchanger. The evidence obtained also suggests that the Na+/Ca2+ exchanger operates unidirectionally, extruding internal Na+ in exchange for external Ca2+.