COVARIATION IN REGULATION OF AFFINITY FOR BRANCHIAL ZINC AND CALCIUM-UPTAKE IN FRESH-WATER RAINBOW-TROUT

The possible coupling between regulation of the affinities for branchi al Zn and Ca influx was investigated in juvenile rainbow trout Oncorhy nchus mykiss acclimated to relatively hard fresh water ([Ca]=1.0 mmol l(-1)). The K-m for branchial Ca influx was manipulated experimentally by exposing the fish to 2.3 mu mol l(-1) waterborne Zn for a total of 28 days. This procedure resulted in rapidly increased K-m values for both Ca and Zn influx, an effect that remained through the experimenta l period. There was a significant linear correlation (r = 0.88, P < 0. 02) between K-m values for Ca and Zn measured at the same time points, Zn exposure caused progressively increasing maximum rate of transport , J(max), values for Zn relative to the control value, but there was l ittle, if any, effect on Jmax for Ca. These results support the idea o f a shared transport site for Zn and Ca at the apical membrane of the gill epithelium and suggest that there is a certain degree of coregula tion of branchial Zn and Ca uptake in rainbow trout. Removal of Ca fro m the water resulted in a large (six- to 24-fold) increase in affinity (decreased K-m) for Zn influx and a modest (1.1- to 1.8-fold) increas e in J(max) for Zn, Thus, Ca is a competitive inhibitor of Zn influx. In water lacking Ca, the K-m for Zn in Zn-acclimated fish was no diffe rent from that of the control fish, suggesting that the Ca2+/Zn2+ tran sporter was regulated to improve Ca uptake.