Effect of water calcium, copper, and silver on branchial Na permeability in a characid and cichlid fish.

We tested the hypothesis that water Ca 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 Ca 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 Cacaused Na efflux to rise 2.5 times above controls at 100 μmol L Ca. However, raising [Ca] to 500 μmol L had no effect. Upon exposure to pH 3.5 (control [Ca]) Na efflux rose almost 5× and increasing the [Ca] 5-fold did not reduce the magnitude of stimulation. Exposure to 1 μmol L silver and 25 μmol L copper stimulated Na efflux 7×, and 2×, respectively. Raising [Ca] 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 [Ca] had no effect on Na efflux at pH 7.5. Exposure to pH 3.5 caused Na efflux to rise 2.5× but changing [Ca] had no effect. Exposure to 1 μmol L silver, or 25 μmol L copper caused Na efflux of tetras to rise 4-fold and 3-fold, respectively. Raising [Ca] during silver exposure reduced the stimulation of Na efflux by about 50%, but during copper exposure increased [Ca] had no effect on stimulation of Na efflux. These results suggest water Ca 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.