Effect of low pH exposure on Na(+) regulation in two cichlid fish species of the Amazon.

We evaluated the effects of acute exposure to low pH on Na(+) regulation in two Amazon cichlids collected from natural ion-poor "blackwaters", angelfish (Pterophyllum scalare) and discus (Symphysodon discus). Na(+) uptake kinetic parameters, unidirectional Na(+) fluxes, and net Cl(-) fluxes were determined at pH6.0 and 3.6. At pH6.0, both species presented low unidirectional Na(+) flux rates, with kinetics showing a relatively low affinity for Na(+) (angelfish Km=79, discus Km=268μmolL(-1)), with similar maximum transport capacities (Jmax~535nmolg(-1)h(-1)). Overall, there appeared to be high sensitivity to inhibition by low pH, yet low intrinsic branchial permeability limiting diffusive ion effluxes, resulting in relatively low net loss rates of Na(+), the same strategy as seen previously in other blackwater cichlids, and very different from the strategy of blackwater characids. At low pH, Na(+) uptake in angelfish was inhibited competitively (increased Km=166μmolL(-1)) and non-competitively (decreased Jmax=106nmolg(-1)h(-1)), whereas in discus, only a decrease in Jmax (112nmolg(-1)h(-1)) was statistically significant. An acute reduction in H(+)-ATPase activity, but not in Na(+)/K(+)-ATPase activity, in the gills of angelfish suggests a possible mechanism for this non-competitive inhibition at low pH. Discus fish were more tolerant to low pH than angelfish, as seen by lesser effects of exposure to pH3.6 on unidirectional Na(+) uptake and efflux rates and net Na(+) and Cl(-) loss rates. Overall, discus are better than angelfish in maintaining ionic balance under acidic, ion-poor conditions.