Mechanisms behind Pb-induced disruption of [Na.sup.+] and [Cl.sup.-] balance in rainbow trout (Oncorhynchus mykiss)

The mechanism of Pb-induced disruption of [Na.sup.+] and [Cl.sup.-] balance was investigated in the freshwater rainbow trout (Oncorhynchus mykiss). [Na.sup.+] and [Cl.sup.-] influx rates were reduced immediately in the presence of 2.40 [+ or -] 0.24 and 1.25 [+ or -] 0.14 [micro]M 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.sup.+] influx without a change in transporter affinity, suggesting a noncompetitive disruption of [Na.sup.+] uptake by Pb. Phenamil and bafilomycin markedly reduced [Na.sup.+] 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 [micro]M Pb revealed time- and concentrationdependent branchial Pb accumulation. [Na.sup.+]-[K.sup.+]-ATPase activity was significantly reduced, with 4.8 [micro]M exposure resulting in immediate enzyme inhibition and 0.48 and 2.4 [micro]M 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.sup.+] 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 [+ or -] 0.05 or 4.30 [+ or -] 0.05 [micro]M Pb and continued for up to 24 h. We conclude that Pb-induced disruption of [Na.sup.+] and [Cl.sup.-] homeostasis is in part a result of rapid inhibition of carbonic anhydrase activity and of binding of Pb with [Na.sup.+]-[K.sup.+]-ATPase, causing noncompetitive inhibition of [Na.sup.+] and [Cl.sup.-] influx. waterborne lead; [Na.sup.+]-[K.sup.+]-ATPase; carbonic anhydrase; ionoregulation