Sodium-dependent copper uptake across epithelia: a review of rationale with experimental evidence from gill and intestine

AbstractThe paper reviews the evidence for apparent sodium-dependent copper (Cu) uptake across epithelia such as frog skin, fish gills and vertebrate intestine. Potential interactions between Na+ and Cu during transfer through epithelial cells is rationalized into the major steps of solute transfer: (i) adsorption on to the apical/mucosal membrane, (ii) import in to the cell (iii) intracellular trafficking, and (iv) export from the cell to the blood. Interactions between Na+ and Cu transport are most likely during steps (i) and (ii). These ions have similar mobilities (λ) in solution (λ, Na+, 50.1; Cu2+, 53.6 cm2 Int. ohms−1 equiv−1); consequently, Cu2+ may compete equally with Na+ for diffusion to membrane surfaces. We present new data on the Na+ binding characteristics of the gill surface (gill microenvironment) of rainbow trout. The binding characteristics of Na+ and Cu2+ to the external surface of trout gills are similar with saturation of ligands at nanomolar concentrations of solutes. At the mucosal/apical membrane of several epithelia (fish gills, frog skin, vertebrate intestine), there is evidence for both a Cu-specific channel (CTR1 homologues) and Cu leak through epithelial Na+ channels (ENaC). Cu2+ slows the amiloride-sensitive short circuit current (Isc) in frog skin, suggesting Cu2+ binding to the amiloride-binding site of ENaC. We present examples of data from the isolated perfused catfish intestine showing that Cu uptake across the whole intestine was reduced by 50% in the presence of 2 mM luminal amiloride, with 75% of the overall inhibition attributed to an amiloride-sensitive region in the middle intestine. Removal of luminal Na+ produced more variable results, but also reduced Cu uptake in catfish intestine. These data together support Cu2+ modulation of ENaC, but not competitive entry of Cu2+ through ENaC. However, in situations where external Na+ is only a few millimoles (fish gills, frogs in freshwater), Cu2+ leak through ENaC is possible. CTR1 is a likely route of Cu2+ entry when external Na+ is higher (e.g. intestinal epithelia). Interactions between Na+ and Cu ions during intracellular trafficking or export from the cell are unlikely. However, effects of intracellular chloride on the Cu-ATPase or ENaC indicate that Na+ might indirectly alter Cu flux. Conversely, Cu ions inhibit basolateral Na+K+-ATPase and may increase [Na+]i