| 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. |
