The Na+-plus-K+-dependent adenosine triphosphatase

(Na++K+-ATPase) is a transmembrane protein composed of three subunits ( a, b, g) responsible for pumping three Na+ out of animal cells in exchange for two K+ or NH4+ per ATP hydrolyzed. Situated in the basolateral membrane of most epithelial cells, the Na++K+-ATPase not only mediates electrogenic transfer of Na+ from the cytosol to the extracellular fluid and K+ or NH4+ from the extracellular fluid to the cytosol but also establishes electrochemical gradients used by apical and basolateral transporters such as the Na+/H+ exchanger, Na+/K+/2Cl- cotransporter and various ion channels. Originally described enzymatically in homogenates of nerves from the shore crab Carcinus maenas (Skou, 1957), the Na++K+-ATPase a- and b-subunits were first cloned and sequenced from mammalian kidney (Shull et al., 1985, 1986). The a-subunit provides the catalytic function of the Na++K+-ATPase, binding and hydrolyzing ATP and itself becoming phosphorylated during the transport cycle. The a-subunit also binds ouabain, a highly specific inhibitor. The b-subunit is thought to participate in anchoring the complex in the basolateral membrane and the g-subunit, not always present, appears to serve a regulatory role (for a review, see Therien and Blostein, 2000). Many studies have shown that the enzymatic activity of the Na++K+-ATPase in homogenates of ion-transporting gill tissue is enhanced when euryhaline crustaceans are subjected to osmoregulatory stress following transfer from sea water to dilute salinities (for reviews, see Towle, 1990; Péqueux, 1995). For example, the specific Na++K+-ATPase activity in homogenates or microsomal fractions of posterior gills o