Regulatory Functions of Na⁺+K⁺-ATPase in Marine and Estuarine Animals

Summary

Despite many studies which suggest a link between blood Na⁺ regulation and the action of Na⁺+K⁺-ATPase in aquatic animals, the precise role of the ATPase remains unclear. The restricted distribution of Na⁺+K⁺-ATPase in basolateral membranes of gill cells would promote sodium uptake from the medium, but precludes direct participation of the enzyme in salt secretion. Following alteration of external salt concentration, some euryhaline osmoregulators exhibit adaptive changes in Na⁺+K⁺-ATPase activity, but others do not. In those animals which do exhibit such changes, short-term modulation of enzymatic activity may be superimposed upon long-term changes in enzyme abundance. In assays of Na⁺+K⁺-ATPase activity, NH₄ ⁺ substitutes effectively for K⁺, but it is unclear whether this biochemical property is related to Na⁺/NH₄ ⁺” exchange in intact gill. Multiple functions of the enzyme, possibly including cell volume regulation and generation of Na⁺ gradients capable of driving Na⁺-coupled transport systems, may obscure its role in transepithelial Na⁺ movement. The recent demonstration of ATP-dependent Na⁺ transport by basolateral membrane vesicles from crustacean gills may allow an improved estimation of the significance of Na⁺+K⁺-ATPase in blood Na⁺ regulation.