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First published online January 27, 2004
Journal of Experimental Biology 207, 813-825 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00826

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Characterization of branchial lead-calcium interaction in the freshwater rainbow trout Oncorhynchus mykiss

Joseph T. Rogers^* and Chris M. Wood

Department of Biology, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4K1, Canada

^* Author for correspondence (e-mail: joerog{at}mcmaster.ca)

Accepted 8 December 2003

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±0.2 µmol 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 K_m, whereas J_max values did not significantly change, yielding an inhibitor constant (K_i,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.

Key words: waterborne lead, calcium, competition, rainbow trout, Oncorhynchus mykiss, branchial uptake

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