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In vivo and in vitro experiments elicited different responses to ammonia nitrogen (ammonia-N) of gill and mantle Na,K-ATPase and ouabain-insensitive Na-ATPase activities in the Philippine clam Tapes philippinarum. Short-term (120 h) exposed clams to sublethal ammonia-N (NH(3)+NH (4) (+) ) concentrations (1.5 and 3.0 mg/L ammonia-N) showed enhanced gill and mantle ouabain-insensitive ATPase activity and decreased mantle Na,K-ATPase activity with respect to unexposed clams, while gill Na,K-ATPase was unaffected. In vitro experiments showed that NH (4) (+) could efficiently replace Na(+) in ouabain-insensitive ATPase activation and K(+), but not Na(+), in Na, K-ATPase activation. Simple saturation kinetics was constantly followed with similar K (0.5) values to that of the substituted cation. The same maximal ouabain-insensitive ATPase activation was obtained at 80 mM Na(+) or NH (4) (+) in the gills and at 50 mM Na(+) or NH (4) (+ ) in the mantle and that of Na,K-ATPase at 10 mM K(+) or NH (4) (+) in the presence of 100 mM Na(+) in both tissues. The two coexistent ATPase activities maintained their typical response to ouabain also when stimulated by NH (4) (+) : when activated by Na(+)+K(+) or by Na(+)+NH (4) (+) the ATPase activity was completely suppressed by 10(-3 )M ouabain, whereas the Na(+)- or NH (4) (+) -stimulated ATPase activity was unaffected by up to 10(-2 )M ouabain. The whole of the data suggests a possible involvement of the two ATPase activities in NH (4) (+) transmembrane transport.

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