Biology of the 2Na(+)/1H(+) antiporter in invertebrates

The functional expression of membrane transport proteins that are responsib le for exchanging sodium and protons is a ubiquitous phenomenon. Among vert ebrates the Na+/H+ antiporter occurs in plasma membranes of polarized epith elial cells and non-polarized cells such as red blood cells, muscle cells, and neurons, and in each cell type the transporter exchanges one sodium for one hydrogen ion, is inhibited by amiloride, and regulates intracellular p H and sodium concentration within tight limitations. In polarized epithelia l cells this transporter occurs in two isoforms, each of which is restricte d to either the brush border or basolateral cell membrane, and perform some swhat different tasks in the two locations. In prokaryotic cells, sodium/pr oton exchange occurs by an electrogenic 1Na(+)/2H(+) antiporter that is cou pled to a primary active proton pump and together these two proteins are ca pable of tightly regulating the intracellular concentrations of these catio ns in cells that may occur in environments of 4 M NaCl or pH 10-12; Inverte brate epithelial cells from the gills, gut, and kidney also exhibit electro genic sodium/proton exchange, but in this instance the transport stoichiome try is 2Na(+)/1H(+). As with vertebrate electroneutral Na+/H+ exchange, the invertebrate transporter is inhibited by amiloride, but because of the occ urrence of two external monovalent cation binding sites, divalent cations a re able to replace external sodium and also be transported by this system. As a result, both calcium and divalent heavy metals, such as zinc and cadmi um, are transported across epithelial brush border membranes in these anima ls and subsequently undergo a variety of biological activities once accumul ated within these cells. Absorbed epithelial calcium in the crustacean hepa topancreas may participate in organismic calcium balance during the molt cy cle and accumulated heavy metals may undergo complexation reactions with in tracellular anions as a detoxification mechanism. Therefore, while the basi c process of sodium/proton exchange may occur in invertebrate cells, the pr esence of the electrogenic 2Na(+)/1H(+) antiporter in these cells allows th em to perform a wide array of functions without the need to develop and exp ress additional specialized transport proteins. (C) 2001 Wiley-Liss, Inc.