EFFECTS OF ZINC ON THE KINETICS OF BRANCHIAL CALCIUM-UPTAKE IN FRESH-WATER RAINBOW-TROUT DURING ADAPTATION TO WATERBORNE ZINC

The effects of sublethal waterborne Zn2+ (150 mu g 1(-1)=2.3 mu mol 1( -1)) on the kinetics of unidirectional Ca2+ influx were studied in juv enile freshwater rainbow trout during chronic exposure (60 days) at a water [Ca2+] of 1.0 mmol 1(-1). An unexposed group held under identica l conditions served as control. The presence of Zn2+ in the water incr eased the apparent K-m for Ca2+ influx by up to 300 % with only a smal l inhibitory effect (35 % at most) on the maximum rate of uptake (J(ma x)) These results, in combination with earlier data showing that Ca2competitively inhibits Zn2+ uptake, suggest that Zn2+ and Ca2+ compete for the same uptake sites. Acute withdrawal of Zn2+ after 3 h of expo sure resulted in a 23-fold reduction in K-m for Ca2+, but a persistent small depression of J(max) During prolonged exposure to Zn2+, the app arent K-m for Ca2+ remained greatly elevated and J(max) remained sligh tly depressed. The actual Ca2+ influx in hard water ([Ca2+]=1.0 mmol 1 (-1)) decreased marginally and paralleled the small changes in J(max). The increases in apparent K-m had a negligible influence on the actua l Ca2+ influx because K-m values (38-230 mu mol 1(-1)), even when elev ated by Zn2+, remained below the water [Ca2+] (1000 mu mol 1(-1)). Rai nbow trout exposed to Zn2+ exhibited a slower rate of protein synthesi s in the gills (measured on day 23) and an increased tolerance to Zn2 challenge (measured on both days 27 and 50). Unidirectional Zn2+ infl ux, measured at the end of the exposure period, was significantly redu ced in the Zn2+-exposed fish. There were no changes in hepatic or bran chial Zn2+, Cu2+ or metallothionein concentrations. We hypothesize tha t, during exposure to sublethal [Zn2+] in hard water, the fish may cha nge the K-m for a mutual Ca2+/Zn2+ carrier so as to reduce markedly Zn 2+ influx without greatly altering Ca2+ influx. This reduced Zn2+ infl ux, rather than metallothionein induction, may be the basis of adaptat ion to elevated concentrations of waterborne Zn2+.