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Journal of Experimental Biology 119,335-347 (1985)
Published by Company of Biologists 1985

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

GERT FLIK ¹, JEANNE H. VAN RIJS ¹, and SJOERD E. WENDELAAR BONGA ¹

¹ Department of Zoology II, University of NijmegenToernooiveld 25, 6525 ED Nijmegen, The Netherlands

A high-affinity Ca²⁺-ATPase activity was demonstrated among the phosphatase activities in plasma membranes of tilapia branchial epithelium: its characteristics (K_0.5 = 0.063 µmol l^-1 Ca²⁺, V_max = 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.

Key words: Freshwater teleost fish, gills, high-affinity Ca²⁺-ATPase, ATP-driven Ca²⁺-transport

Accepted on June 4, 1985