SUMMARY
The mechanism of branchial lead uptake and interplay with Ca2+transport was investigated in the freshwater rainbow trout Oncorhynchus mykiss. Lead significantly reduced Ca2+ influx by approximately 40% and 30% after exposure to 2.3±0.1 and 1.4±0.2μmol l-1 dissolved lead, respectively, for 0-48 h. Acute inhibition of Ca2+ influx by lead exhibited typical Michaelis-Menten kinetics with an approximate 16-fold increase in Km, whereas Jmax values did not significantly change, yielding an inhibitor constant(Ki,Pb) of 0.48 μmol l-1. Alternative analyses suggest the possibility of a mixed competitive/non-competitive interaction at the highest lead concentration tested (4.8 μmol l-1). Branchial lead accumulation was reduced with increasing waterborne Ca2+ concentrations, suggesting a protective effect of Ca2+ against lead uptake at the gill. The apical entries of Ca2+ and lead were both inhibited (55% and 77%, respectively) by the addition of lanthanum (1 μmol l-1) to the exposure water. The use of cadmium (1 μmol l-1) and zinc (100 μmol l-1) as voltage-independent calcium channel competitors also reduced branchial lead uptake by approximately 56% and 47%, respectively. Nifedipine and verapamil (up to 100 μmol l-1), both voltage-dependent calcium channel blockers, had no effect on gill lead accumulation. CaCl2 injection reduced both Ca2+ and lead uptake by the gills. This suggests transport of lead through apical voltage-independent calcium channels, similar to the entry of Ca2+. High-affinity Ca2+-ATPase activity was not acutely affected by lead, but a significant 80% reduction in activity occurred during exposure for 96 h to 5.5±0.4 μmol l-1 dissolved lead, indicating a possible non-competitive component to lead-induced Ca2+ disruption. The effect of lead on Ca2+ efflux was investigated and found to be insignificant. We conclude that uptake of lead occurs, at least in part, by the same mechanism as Ca2+, which results in disruption of Ca2+ influx and ultimately Ca2+ homeostasis.
