Calcium Uptake in Isolated Hepatic Plasma‐Membrane Vesicles

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Abstract
A liver plasma‐membrane fraction capable of Ca2+ uptake was isolated. The fraction exhibited high Na+, K+‐ATPase, low glucose‐6 phosphatase activity, and transported alanine in a Na+‐dependent fashion. The uptake of Ca2+ was ATP‐dependent; UTP, GTP, or CTP did not substitute for ATP. The presence of oxalate did not significantly alter the rate of uptake. The pH optimum of the reaction was basic (no uptake was visible at pH 6.8). These properties are at variance with those of the endoplasmic reticulum Ca2+ uptake system, which is oxalate‐dependent, and has an acid pH optimum. The ATP‐dependent Ca2+ uptake has a Km(Ca2+) of 1.4 × 10–8 M and a Vmax of transport of 30 nmol × mg protein–1× min–1. No conclusive results were obtained on the calmodulin‐sensitivity of the process: addition of calmodulin to the vesicles did not stimulate uptake, and the anti‐calmodulin drug trifluoperazine had no inhibitory effect. However, another anti‐calmodulin drug (R 24571) had a limited, but statistically significant, inhibitory action. A partial release of the accumulated Ca2+ from the vesicles could be induced by the addition of Na+, and incubation of the vesicles in a high Na+ medium (as compared to high K+ medium) resulted in lower (about 25%) calcium uptake. Partial release of the accumulated Ca2+ could be induced also by the addition of H+. The releasing effect of H+, taken together with the absence of Ca2+ uptake at acid pH, suggests the possibility of a H+/Ca2+ exchange.