Characterization of transport Na+-ATPases in gills of freshwater tilapia

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Abstract
Branchial plasma membranes from the freshwater cichlid teleostOreochromis mossambicus (tilapia) contain two Na+-dependent ATPases: Na+/K+ ATPase, and an amiloride-sensitive ATPase which is postulated to operate as a Na+/H+ (−NH4 +) ATPase. It is suggested that both enzyme activities are located in the basolateral membrane system of the chloride cells. K+ has opposing effects on the two enzymes: it stimulates Na+/K+ ATPase and inhibits Na+/H+ (−NH4 +) ATPase activity. Na+/H+ ATPase appears more sensitive to NH4 + at low concentrations than Na+/K+ ATPase and the stimulatory effect by NH4 + ions on the first enzyme could be important in facilitating NH4 + excretion by tilapia gills under physiological conditions.In vitro maximum stimulation by NH4 + is similar for the two enzymes (200%). In contrast to Na+/K+ ATPase, Na+/H+ ATPase activity is inhibited by supra-physiological (>20 mM) concentrations of NH4 +.