Response of Na⁺-dependent ATPase Activities to the Contaminant Ammonia Nitrogen in Tapes philippinarum: Possible ATPase Involvement in Ammonium Transport

Abstract

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 (NH₃+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.