Characterization of transport Na(+)-ATPases in gills of freshwater tilapia : Evidence for branchial Na(+)/H (+) (-NH4 (+)), ATPase activity in fish gills.

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 (+).