The linkage between Na+ uptake and ammonia excretion in rainbow trout: Kinetic analysis, the effects of (NH4)(2)SO4 and NH4HCO3 infusion and the influence of gill boundary layer pH

The nature of the linkage between between branchial ammonia excretion (J(Am m)) and unidirectional Na+ influx (J(in)(Na)) was studied in the freshwater rainbow trout (Oncorhynchus mykiss). Arterial plasma total [ammonia], P-NH 3 and J(Amm) were all elevated approximately threefold by intravascular inf usion for 24 h with either 70 mmol l(-1) (NH4)(2)SO4 or 140 mmol l(-1) NH4H CO3 at a rate of approximately 400 mu mol kg-l h-l. Both treatments markedl y stimulated J(in)(Na). NH4HCO3 induced metabolic alkalosis in the blood pl asma, whereas (NH4)(2)SO4 caused a slight metabolic acidosis. Experiments w ith Hepes-buffered water (5 mmol l(-1)) under control conditions demonstrat ed that increases in gill boundary layer pH were associated with decreases in both J(in)(Na) and J(Amm) Thus, the stimulation of J(in)(Na) caused by a mmonium loading was not simply a consequence of a Na+-coupled H+ extrusion mechanism activated by internal acidosis or by alkalosis in the gill bounda ry layer, Indeed, there was no stimulation of net acidic equivalent excreti on accompanying NH4HCO3 infusion. Michaelis-Menten kinetic analysis by acut e variation of water [Na+] demonstrated that both infusions caused an almos t twofold increase in J(max)(Na) but no significant change in K-m, indicati ve of an increase in transporter number or internal counterion availability without an alteration in transporter affinity for external Na+. The increa se in J(in)(Na),a was larger with (NH4)(2)SO4 than with NH4HCO3 infusion an d in both cases lower than the increase in J(Amm). Additional evidence of q uantitative uncoupling was seen in the kinetics experiments, in which acute changes in J(in)(Na) of up to threefold had negligible effects on J(Amm) u nder either control or ammonium-loaded conditions. In vitro measurements of branchial Na+/K+-ATPase activity demonstrated no effect of NH4+ concentrat ion over the concentration range observed in,vivo in infused fish. Overall, these results are consistent with a dominant role for NH3 diffusion as the normal mechanism of ammonia excretion, but indicate that ammonium loading directly stimulates J(in)(Na), perhaps by activation of a non-obligatory Na +/NH4+ exchange rather than by an indirect effect (e.g, Na+-coupled H+ excr etion) mediated by altered internal or external acid-base status.