title: Intracellular pH and Na fluxes in barnacle muscle with evidence for reversal of the ionic mechanism of intracellular pH regulation.
abstract: The ion transport mechanism that regulates intracellular pH (pHi) in giant barnacle muscle fibers was studied by measuring pHi and unidirectional Na+ fluxes in internally dialyzed fibers. The overall process normally results in a net acid extrusion from the cell, presumably by a membrane transport mechanism that exchanges external Na+ and HCO-3 for internal Cl- and possibly H+. However, we found that net transport can be reversed either by lowering [HCO-3]o and pHo or by reducing [Na+]o. This reversal (acid uptake) required external Cl-, was stimulated by raising [Na+]i, and was blocked by SITS. When the transporter was operating in the net forward direction (acid extrusion), we found a unidirectional Na+ influx of approximately 60 pmol . cm-2 . s-1, which required external HCO-3 and internal Cl- and was stimulated by cyclic AMP and blocked by SITS or DIDS. These properties of the Na+ influx are all shared with the net acid extrusion process. We also found that under conditions of net forward transport, the pHi-regulating system mediated a unidirectional Na+ efflux, which was significantly smaller than the simultaneous Na+ influx. These data are consistent with a reversible transport mechanism which, even when operating in the net forward direction, mediates a small amount of reversed transport. We also found that the ouabain-sensitive Na+ efflux was sharply inhibited by acidic pHi, being totally absent at pHi values below approximately 6.8.
mesh_terms: D000136:Acid-Base Equilibrium; D000818:Animals; D002254:Carbonates / Q000494:pharmacology; D002712:Chlorides / Q000502:physiology; D000242:Cyclic AMP / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D009132:Muscles / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*; D001468:Thoracica
chemical_list: D002254:Carbonates; D002712:Chlorides; D010042:Ouabain; D012964:Sodium; D000242:Cyclic AMP