Liver plasma membrane calcium transport. Evidence for a Na+-dependent Ca2+ flux.

Plasma membrane vesicles isolated from rat liver exhibited an azide-insensitive Mg2+-ATP-dependent Ca2+ pump which accumulated Ca2+ at a rate of 5.1 +/- 0.5 nmol of calcium/mg of protein/min and reached a total accumulation of 33.2 +/- 2.6 nmol of calcium/mg of protein in 20 microM Ca2+ at 37 degrees C. Equiosmotic addition of 50 mM Na+ resulted in a loss of accumulated calcium. Measurement of Mg2+-ATP-dependent Ca2+ uptake in the presence of 50 mM Na+ revealed no effect of Na+ on the initial rate of Ca2+ uptake, but a decrease in the total accumulation. The half-maximal effect of Na+ on Ca2+ accumulation was achieved at 14 mM. The Ca2+ efflux rate constant in the absence of Na+ was 0.16 +/- 0.01 min-1, whereas the efflux rate constant in the presence of 50 mM Na+ was 0.25 +/- 0.02 min-1. Liver homogenate sedimentation fractions from 1,500 to 105,000 X g were assayed for azide-insensitive Mg2+-ATP-dependent Ca2+ accumulation. Na+-sensitive Ca2+ uptake activity was found to specifically co-sediment with the plasma membrane-associated enzymes, 5'-nucleotidase and Na+/K+-ATPase, whereas Na+-insensitive Ca2+ uptake was found to co-sediment with the endoplasmic reticulum-associated enzyme, glucose-6-phosphatase. The plasma membrane Ca2+ pump was also distinguished from the endoplasmic reticulum Ca2+ pump by its sensitivity to inhibition by vanadate. Half-maximal inhibition of plasma membrane Ca2+ uptake occurred at 0.8 microM VO4(3-), whereas half-maximal inhibition of microsomal Ca2+ uptake occurred at 40 microM.