Sodium or potassium ions activate different kinetics of gill Na, K-ATPase in three seawater-and freshwater-acclimated euryhaline teleosts

Abstract

The effects of [Na+] or [K+] on Na, K-ATPase activity of FW-acclimated and SW-acclimated tilapia, puffer and milkfish were examined in gill homogenates. [Na+] or [K+] stimulated Na, K-ATPase hydrolyzing ATP in all experimental groups. ATP hydrolysis stimulated by [Na+] or [K+] followed Michaelian-Menten kinetics. Km values for [K+] (i.e., Km(K)), were lower in SW- than FW-acclimated tilapia and puffer fishes (tilapia: 8.69 +/- 0.22 vs. 11.93 +/- 1.17 mM; puffer: 13.51 +/- 1.39 vs. 30.52 +/- 2.66 mM). Km values for [Na+] (i.e., KMNa), were lower in FW- than SW-acclimated milkfish (3.76 +/- 0.54 vs. 7.55 +/- 1.08 mM). These data suggest that [K+] stimulates ATP hydrolysis to rates higher in SW- than FW-acclimated tilapia and puffer fishes, while [Na+] stimulated ATP hydrolysis at rates higher in FW- than SW-acclimated milkfish. This is the first demonstration that Na, K-ATPase activity of euryhaline tilapia, puffer, and milkfish modulated by [Na+] or [K+] have different effects between FW- and SW-acclimated groups. Such responses as changes in properties of branchial Na, K-ATPase may contribute to improve the osmoregulatory capacity of tilapia, puffer and milkfish to acclimate in seawater and fresh water. (C) 2005 Wiley-Liss, Inc.