Calcium transport in plasma membrane subfractions of rat liver

The selective demonstration of ATP dependent calcium uptake and calcium dependent phosphorylation of a 135 kD intermediate in isolated basolateral liver plasma membrane vesicles suggest that the basolateral plasma membrane of rat hepatocytes contains an ATP dependent calcium transport system with an apparent molecular weight of 135,000. The failure to demonstrate ATP dependent calcium uptake in simultaneously isolated canalicular plasma membrane vesicles suggests the absence of a similar ATP dependent calcium transport system on the canalicular domain. Additional phosphoproteins of 200 and 110 kD could be assigned to microsomal contamination of the isolated plasma membrane subfractions. A canalicular specific 70 kD phosphorylated intermediate presumably does not represent a Ca++ ATPase. Calcium dependent phosphorylation of the basolateral 135 kD intermediate is maximal at physiologic pH (7.0, 7.5), when the calcium pump is active but not at pH 8.0. The 135 kD phosphoprotein has also been shown for erythrocyte Ca++ ATPase and for kidney basolateral Ca++ ATPase (De Smedt et al., 1984; Niggli et al., 1979).