Epithelial pH and Ion Transport Regulation by Proton Pumps and Exchangers

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Abstract
This study reports on the interaction between transepithelial Na+ transport and H+ secretory and intracellular pH (pHi) regulating mechanisms in the model 'tight' epithelium of frog skin. We have used 22Na isotope fluxes and fixed end-point titration to measure undirectional Na+ fluxes, net Na absorption (J(net)Na) and proton secretion (J(net)H), and electrophysiological techniques (double-barrelled ion-sensitive microelectrodes and cell membrane current--voltage relations) to determine intracellular activities of Na+, Cl- and H+ and the conductance of apical membranes to Na+ (gNa) and of basolateral membranes to K+ (gK). In dilute mucosal solutions or in the absence of a permeant anion (Cl-) or counter-current (open-circuit conditions) to accompany Na+ uptake, the J(net)Na is electrically coupled to J(net)H via an electrogenic apical H+-ATPase (located in mitochondria-rich cells). Both fluxes proceed via mitochondria-rich cells and are inhibited by blockers of carbonic anhydrase and H+-ATPase and stimulated by aldosterone and acid load. In high NaCl-containing mucosal solutions or in short-circuit conditions, the J(net)Na becomes uncoupled from J(net)H and proceeds mainly via the principal cells in the epithelium, in which pHi is regulated by basolateral Na+/H+ and Cl-/HCO3- exchangers. Under these conditions, J(net)Na, gNa and gK vary directly and in parallel with pHi, when pHi is changed by permeable weak acids or bases. There is also co-variance between gNa and pHi accompanying spontaneous variations in J(net)Na and when Na+ transport is stimulated by aldosterone or inhibited with ouabain. We conclude that the level of intracellular H+, modulated by H+ pump and Na+/H+ and Cl-/HCO3- exchangers provides an intrinsic regulation of epithelial Na+ transport.