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

Abstract

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.

Keywords: Calcium; Fish; Gills; Ionoregulation.