Evidence for High-Affinity Ca2+-ATPase Activity and ATP-Driven Ca2+-Transport in Membrane Preparations of the Gill Epithelium of the Cichlid Fish Oreochromis Mossambicus

Abstract

A high-affinity Ca2+-ATPase activity was demonstrated among the phosphatase activities in plasma membranes of tilapia branchial epithelium: its characteristics (K0.5 = 0.063 μmol l−1 Ca2+, Vmax = 6.02 -mol P1h−;1 mg−1 protein at 37°C) resemble those of Ca2+-translocating enzymes present in mammalian erythrocytes or enterocytes. The ratio of this Ca2+-ATPase activity to Na+/K+-ATPase activity was 1:20.4. Radioimmunoassayable calmodulin was demonstrated in the Ca2+-ATPase-containing membrane fraction. ATP-dependent Ca2+-transport was demonstrated in tight-vesicle preparations of the branchial cell membranes; 30 % of the vesicles in the preparation were inside-out, 44 % were right-side-out and 26 % were leaky. The characteristics of the active Ca2+-transport activity are consistent with a Ca2+-extrusion mechanism involving high-affinity Ca2+-ATPase activity. The branchial Ca2+-transport activity per fish, as calculated on the basis of the transport activity determined for the vesicle preparation, is of the order of the branchial Ca2+-influx rates observed in vivo. The data provide the first biochemical evidence for active Ca2+ -transport in plasma membranes of branchial epithelium. A model is presented for the mechanism of active transepithelial Ca2+-transport in fish gills.