Modulation of Gill Na+,K+-ATPase Activity by Ammonium Ions: Putative Coupling of Nitrogen Excretion and Ion Uptake in the Freshwater Shrimp Macrobrachium olfersii

Abstract

The effect of NH4+ ions on (Na+ ,K +)-ATPase hydrolytic activity was examined in a gill microsomal fraction from M. olfersii. In the absence of NH4+ ions, K+ ions stimulated ATP hydrolysis, exhibiting cooperative kinetics (nH=0.8), to a maximal specific activity of V=556.1±22.2 nmol.min-1.mg-1 with K0.5=2.4±0.1 mmol.L-1. No further stimulation by K+ ions was observed in the presence of 50 mmol.L-1 NH4+ ions. ATP hydrolysis was also stimulated by NH4+ ions obeying Michaelian kinetics to a maximal specific activity of V=744.8±22.3 nmol.min-1.mg-1 and KM=8.4±0.2 mmol.L-1. In the presence of 10 mmol.L-1 K+ ions, ATP hydrolysis was synergistically stimulated by NH4+ ions to V=689.8±13.8 nmol.min-1.mg-1 and K0.5=6.6±0.1 mmol.L-1, suggesting that NH 4+ ions bind to different sites than K+ ions. PNPP hydrolysis was also stimulated cooperatively by K+ or NH 4+ ions to maximal values of V= 235.5± 11.8 nmol.min-1.mg-1 and V=234.8±7.0 nmol.min -1.mg-1, respectively. In contrast to ATP hydrolysis, K+ -phosphatase activity was not synergistically stimulated by NH4+ and K+ ions. These data suggest that at high NH4+ ion concentrations, the (Na+, K +)-ATPase exposes a new site; the subsequent binding of NH 4+ ions stimulates ATP hydrolysis to rates higher than those for K+ ions alone. This is the first demonstration that (Na+, K+)-ATPase activity in a freshwater shrimp gill is modulated by ammonium ions, independently of K+ ions, an effect that may constitute a fine-tuning mechanism of physiological relevance to osmoregulatory and excretory processes in palaemonid shrimps. © 2004 Wiley-Liss, Inc.