CELLULAR CALCIUM-TRANSPORT IN FISH - UNIQUE AND UNIVERSAL MECHANISMS

The tilapia, Oreochromis mossambicus, is a truly euryhaline species in that it lives, grows, and reproduces in freshwater as well as in full -strength seawater. The gills, intestine, and kidneys show ionoregulat ory adaptations fundamental for the calcium balance of this fish in th ese vastly different ionic media. This review focuses on Calcium flows in these ionoregulatory organs and the changes that occur in the Ca2-transporting mechanisms in the basolateral plasma membrane compartmen t of the cells that make up their ion-transporting epithelia. Influx o f Ca2+ via the gills is comparable in freshwater and seawater; also, t he Ca2+-ATPase and Na+/Ca2+ exchanger in branchial epithelial plasma m embranes have comparable activities in fish adapted to freshwater and in those adapted to seawater Ussing chamber experiments with isolated opercular membranes (a flat epithelium with chloride cells) suggest th at chloride-cell-mediated, inward Ca2+ transport is largely dependent on Na+-dependent mechanisms. The Ca2+-ATPase is thought to play a ''ho usekeeping'' role in cellular Ca2+ regulation. Intestinal epithelial C a2+ flux is lower In seawater fish than in freshwater fish, and this m ay reflect adaptation to the imminent overload of calcium in seawater, wherefor the up take of water the fish drinks a 10-millimolar Ca solu tion. Intestinal Ca2+ transport is fully dependent on serosal Na+. Acc ordingly a powerful Na+/Ca2+ exchanger operates in the basolateral pla sma membrane of the enterocyte, and in particular the capacity of this transporter decreases in seawater fish. The kidney of freshwater fish es produces a typical dilute and hypocalcic urine; in seawater, urine production decreases and the urine calcium concentration is always hig her than that of the plasma. Exchange activity of Na+ and Ca2+ is unde tectably, low, or absent in renal tissue plasma membranes. However,a h igh-affinity, high-capacity Ca2+-ATPase activity appears to correlate with Ca2+ reabsorption, as its activity significantly decreases after transfer of the fish to seawater. It thus appears that ATP- and Na+-de pendent Ca2+ pumps are differentially expressed in gills, intestine, a nd kidney. Their activity may explain an ATP- or Na+ -dependence of Ca 2+ flow in the pertinent epithelium.