Na+:K+- and HCO3−-stimulated ATPase activities in the gills and kidneys of thermally acclimated rainbow trout, Salmo gairdneri

Gill and kidney Mg2+-dependent, Na+:K+- and HCO3−-stimulated ATPase (EC 3.6.1.3) activities were estimated at 25 °C and at acclimation temperature in rainbow trout, Salmo gairdneri, acclimated to 2, 10, and 18 °C, as were plasma levels of sodium, potassium, and chloride. Sodium and chloride exhibited no significant variation between 2 and 18 °C; potassium levels were elevated at 18 °C. When assayed at 25 °C Mg2+-dependent and HCO3−-stimulated ATPase activities did not vary consistently or significantly in relation to acclimation temperature. Under comparable assay conditions both gill and kidney Na+:K+-stimulated activities declined at higher acclimation temperatures. Significant increases in all activities were encountered when preparations were incubated at the appropriate acclimation temperature. The data suggest that the branchial Na+:K+–ATPase system serves primarily as a high-temperature amplifier of sodium uptake, and may contribute little to the maintenance of sodium balance in the cold-adapted animal. No evidence of a critical involvement of HCO3−-stimulated ATPase in ionic regulations was obtained.