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WATER AND ELECTROLYTE HOMEOSTASIS

Mechanisms behind Pb-induced disruption of Na⁺ and Cl^– balance in rainbow trout (Oncorhynchus mykiss)

¹Department of Biology, McMaster University, Hamilton, Ontario; and ²Biology Department, University of Ottawa, Ottawa, Ontario, Canada

Submitted 1 June 2004 ; accepted in final form 12 April 2005

The mechanism of Pb-induced disruption of Na⁺ and Cl^– balance was investigated in the freshwater rainbow trout (Oncorhynchus mykiss). Na⁺ and Cl^– influx rates were reduced immediately in the presence of 2.40 ± 0.24 and 1.25 ± 0.14 µM Pb, with a small increase in efflux rates occurring after 24-h exposure. Waterborne Pb caused a significant decrease in the maximal rate of Na⁺ influx without a change in transporter affinity, suggesting a noncompetitive disruption of Na⁺ uptake by Pb. Phenamil and bafilomycin markedly reduced Na⁺ influx rate but did not affect Pb accumulation at the gill. Time-course analysis in rainbow trout exposed to 0, 0.48, 2.4, and 4.8 µM Pb revealed time- and concentration-dependent branchial Pb accumulation. Na⁺-K⁺-ATPase activity was significantly reduced, with 4.8 µM exposure resulting in immediate enzyme inhibition and 0.48 and 2.4 µM exposures inhibiting activity by 24 h. Reduced activity was weakly correlated with gill Pb accumulation after 3- and 8-h exposures; this relationship strengthened by 24 h. Reduced Na⁺ uptake was correlated with gill Pb burden after exposures of 3, 8, and 24 h. Immediate inhibition of branchial carbonic anhydrase activity occurred after 3-h exposure to 0.82 ± 0.05 or 4.30 ± 0.05 µM Pb and continued for up to 24 h. We conclude that Pb-induced disruption of Na⁺ and Cl^– homeostasis is in part a result of rapid inhibition of carbonic anhydrase activity and of binding of Pb with Na⁺-K⁺-ATPase, causing noncompetitive inhibition of Na⁺ and Cl^– influx.

waterborne lead; Na⁺-K⁺-ATPase; carbonic anhydrase; ionoregulation