CA2+ VERSUS ZN2+ TRANSPORT IN THE GILLS OF FRESH-WATER RAINBOW-TROUT AND THE COST OF ADAPTATION TO WATERBORNE ZN2+

Previous work suggested that Ca2+ and Zn2+ share a common uptake pathway in rainbow trout gills, We here report on relationships between the kinetic variables for unidirectional Ca2+ influx and unidirectional Zn2+ influx during a 1 month exposure of freshwater rainbow trout to Zn2+ (150 mu g l(-1)=2.3 mu mol l(-1) as total zinc, Zn), Initial exposure to Zn2+ caused a large competitive inhibition of Ca2+ influx, as indicated by a threefold increase in apparent K-m for Ca2+ (measured in the presence of Zn2+), There was also a smaller non-competitive inhibition (50% decrease in J(max)) of the Ca2+ transport system, which was abolished after 1-2 weeks of exposure, The K-m, measured in the absence of Zn2+, decreased dramatically (i.e. elevated affinity) on days 1-4 but increased thereafter; both true and apparent K-m finally stabilized significantly above control levels, However, the K-m values for Ca2+ (<200 mu mol l(-1)) were low relative to the Ca2+ level in the water (1000 mu mol l(-1)), and therefore the changes did not influence the actual Ca2+ influx of the fish, which tracked J(max), In contrast, water [Zn2+] (2.3 mu mol l(-1) as total Zn) was close to the reported apparent K-m (3.71 mu mol l(-1)) for Zn2+ influx in the presence of 1000 mu mol l(-1) Ca2+, Unidirectional Zn2+ influx increased during the first week of exposure to waterborne Zn2+, followed by a persistent reduction to about 50% of control levels, effects that may be largely explained by the observed changes in true K-m for Ca2+, We speculate that the initial response of the fish to elevated [Zn2+] is to compensate for a reduced availability of Ca2+ by markedly increasing the affinity of a dual Ca2+/Zn2+ transporter, Once the Ca2+ influx is 'corrected' by restoration of functional transport sites (J(max)), the system is tuned to limit the influx of Zn2+ by a persistent reduction in the affinities for both ions. The changes in influx characteristics for Ca2+ and Zn2+ were correlated with internal physiological alterations indicative of adaptation to Zn2+ and increased metabolic cost, Depressed plasma [Ca] was corrected within 1 week, and there were no effects on whole-body [Ca] or [Zn]. A slight accumulation of Zn in the gills was associated with increased branchial metallothionein levels, Rates of protein synthesis and degradation in the gills were initially increased and whole-body growth was transiently impaired, effects which were reversed after 18 days of exposure, Sublethal challenge with Zn2+ (at 450 mu g l(-1)=6.9 mu mol l(-1) as total Zn) always depressed plasma [Ca] in control fish, but by 1 month of exposure to Zn2+ at 150 mu g l(-1) (as total Zn), experimental fish were resistant to challenge, However, the fish did not acquire increased survival tolerance (LT(50)) to a lethal concentration of Zn2+ (4 mg l(-1)=61 mu mol l(-1) as total Zn).