A pharmacological examination of Na+ and Cl- transport in two species of freshwater fish.

We examined branchial Na(+) and Cl(-) uptake in two species of stenohaline, freshwater fish (goldfish and the Amazonian neon tetra). Kinetic analysis revealed that the two species had similar uptake capacities and affinities for Na(+) and Cl(-). However, while uptakes of Na(+) and Cl(-) (JNain and JClin, respectively) by goldfish were completely inhibited at pH 4.5 and below, uptake in tetras was unaffected by pH down to 3.25. Examination of Cl(-) transport with blockers indicated that goldfish and neon tetras utilize Cl(-)/HCO-3 exchange; SITS and SCN(-) inhibited Cl(-) uptake in both species. In contrast, large differences in Na(+) transport were indicated between the species. In goldfish, exposure to four Na(+)/H(+) exchange blockers, as well as the Na(+) channel blocker phenamil, strongly inhibited JNain. Further, Na(+) and Cl(-) uptake were strongly inhibited by the Na(+)/K(+)/Cl(-) cotransport inhibitor furosemide, as was JNain in "Cl(-)-free" water and JClin in "Na(+)-free" water. This suggests the presence of multiple transporters and possibly even a direct linkage between the transport of Na(+) and Cl(-) in goldfish. In contrast, none of these drugs strongly reduced Na(+) transport in neon tetras, which raises the possibility of a significantly different Na(+) transport mechanism in this acid-tolerant species.