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

Previous work suggested that Ca2+ and Zn2+ share a common uptake pathw ay in rainbow trout gills, We here report on relationships between the kinetic variables for unidirectional Ca2+ influx and unidirectional Z n2+ influx during a 1 month exposure of freshwater rainbow trout to Zn 2+ (150 mu g l(-1)=2.3 mu mol l(-1) as total zinc, Zn), Initial exposu re to Zn2+ caused a large competitive inhibition of Ca2+ influx, as in dicated by a threefold increase in apparent K-m for Ca2+ (measured in the presence of Zn2+), There was also a smaller non-competitive inhibi tion (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 absenc e of Zn2+, decreased dramatically (i.e. elevated affinity) on days 1-4 but increased thereafter; both true and apparent K-m finally stabiliz ed 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, wat er [Zn2+] (2.3 mu mol l(-1) as total Zn) was close to the reported app arent 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 fir st week of exposure to waterborne Zn2+, followed by a persistent reduc tion to about 50% of control levels, effects that may be largely expla ined by the observed changes in true K-m for Ca2+, We speculate that t he initial response of the fish to elevated [Zn2+] is to compensate fo r a reduced availability of Ca2+ by markedly increasing the affinity o f a dual Ca2+/Zn2+ transporter, Once the Ca2+ influx is 'corrected' by restoration of functional transport sites (J(max)), the system is tun ed to limit the influx of Zn2+ by a persistent reduction in the affini ties for both ions. The changes in influx characteristics for Ca2+ and Zn2+ were correlated with internal physiological alterations indicati ve of adaptation to Zn2+ and increased metabolic cost, Depressed plasm a [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 assoc iated with increased branchial metallothionein levels, Rates of protei n synthesis and degradation in the gills were initially increased and whole-body growth was transiently impaired, effects which were reverse d 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, th e fish did not acquire increased survival tolerance (LT(50)) to a leth al concentration of Zn2+ (4 mg l(-1)=61 mu mol l(-1) as total Zn).