Sodium-dependent copper uptake across epithelia: a review of rationale with experimental evidence from gill and intestine.
The 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 (lambda) in solution (lambda, Na(+), 50.1; Cu(2+), 53.6 cm(2) Int. ohms(-1) equiv(-1)); consequently, Cu(2+) 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 Cu(2+) 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). Cu(2+) slows the amiloride-sensitive short circuit current (I(sc)) in frog skin, suggesting Cu(2+) 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 Cu(2+) modulation of ENaC, but not competitive entry of Cu(2+) through ENaC. However, in situations where external Na(+) is only a few millimoles (fish gills, frogs in freshwater), Cu(2+) leak through ENaC is possible. CTR1 is a likely route of Cu(2+) 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).
Handy
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RD
Department of Biological Sciences, The University of Plymouth, Drake Circus, Plymouth, UK. rhandy@plymouth.ac.uk
Eddy
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FB
Baines
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H
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001001:Anura; D001667:Binding, Competitive; D002397:Catfishes; D027682:Cation Transport Proteins; D002462:Cell Membrane / Q000378:metabolism; D003300:Copper / Q000097:blood / Q000378:metabolism*; D000079923:Copper Transporter 1; D015536:Down-Regulation; D004848:Epithelium / Q000378:metabolism*; D005880:Gills / Q000378:metabolism*; D007408:Intestinal Absorption / Q000187:drug effects; D007413:Intestinal Mucosa / Q000378:metabolism*; D017136:Ion Transport / Q000187:drug effects; D008564:Membrane Potentials; D008565:Membrane Proteins / Q000378:metabolism; D017686:Oncorhynchus mykiss; D012964:Sodium / Q000032:analysis / Q000097:blood / Q000378:metabolism*; D015222:Sodium Channels / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors
D027682:Cation Transport Proteins; D000079923:Copper Transporter 1; D008565:Membrane Proteins; D015222:Sodium Channels; D003300:Copper; D000584:Amiloride; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase