RESUMO
We tested the hypothesis that water Ca2+ is involved in control of branchial Na+ permeability in low pH tolerant convict cichlids and black neon tetras. We measured Na+ efflux in water with different Ca2+ concentrations during exposure to low pH, silver, and copper, at levels which are known to stimulate Na+ efflux. For convict cichlids at pH 7.5 exposure to 0 µmol L-1 Ca2+caused Na+ efflux to rise 2.5 times above controls at 100 µmol L-1 Ca2+. However, raising [Ca2+] to 500 µmol L-1 had no effect. Upon exposure to pH 3.5 (control [Ca2+]) Na+ efflux rose almost 5× and increasing the [Ca2+] 5-fold did not reduce the magnitude of stimulation. Exposure to 1 µmol L-1 silver and 25 µmol L-1 copper stimulated Na+ efflux 7×, and 2×, respectively. Raising [Ca2+] concentration during metal exposure halved the stimulation of Na+ efflux caused by silver, and eliminated the stimulation elicited by copper. For black neon tetras raising or lowering water [Ca2+] had no effect on Na+ efflux at pH 7.5. Exposure to pH 3.5 caused Na+ efflux to rise 2.5× but changing [Ca2+] had no effect. Exposure to 1 µmol L-1 silver, or 25 µmol L-1 copper caused Na+ efflux of tetras to rise 4-fold and 3-fold, respectively. Raising [Ca2+] during silver exposure reduced the stimulation of Na+ efflux by about 50%, but during copper exposure increased [Ca2+] had no effect on stimulation of Na+ efflux. These results suggest water Ca2+ plays a role in control of branchial Na+ permeability in cichlids, but perhaps not tetras. In addition, the silver and copper concentrations required to inhibit Na+ uptake and stimulate Na+ efflux were higher than the concentrations used on non-characids and non-cichlids, which indicates that our fish are much more tolerant of these metals.
