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University Department of Pharmacology, Oxford, UK.

1. HCO3(-)-dependent mechanisms involved in the regulation of intracellular pH (pHi) were characterized using double-barrelled pH-sensitive microelectrodes in smooth muscle cells of the isolated guinea-pig ureter. 2. Removal of external Cl- in the presence of CO2-HCO3- caused a transient alkalosis, consistent with the presence of Cl(-)-HCO3- exchange, before pHi slowly recovered. Recovery from acidosis in the presence of CO2-HCO3- was not affected, at a time when intracellular Cl- would have been maximally depleted, indicating that a counter transport of Cl- and HCO3- was not involved. The recovery was also not affected by amiloride, indicating that Na(+)-H+ exchange was not involved. 3. A transient hyperpolarization was associated with the recovery from acidosis in the presence of CO2-HCO3-, consistent with rheogenic coupling of Na(+)-HCO3- cotransport. However, depolarization caused by elevation of the extracellular potassium (K+o) concentration, which should favour inward transport by the rheogenic mechanism, caused a fall in pHi and decreased the rate of recovery from acidosis. Furthermore, ouabain abolished the transient hyperpolarization without affecting the recovery of pHi. It is concluded that Na(+)-HCO3- cotransport in the ureter is electroneutral. 4. Recovery from acidosis in the presence of CO2-HCO3- was insensitive to DIDS even after prolonged pre-equilibriation and extreme acidosis. The results suggest that Na(+)-HCO3- cotransport in the ureter is insensitive to DIDS and that Cl(-)-HCO3- exchange does not reverse to contribute to the extrusion of acid equivalents. A HCO3- conductance may account for the Na(+)-independent, HCO3(-)-dependent recovery from extreme acidosis. 5. Recovery from experimentally induced alkalosis was inhibited by Cl(-)-free conditions and by DIDS, indicating that Cl(-)-HCO3- exchange was involved. 6. It is concluded that pHi in the smooth muscle of guinea-pig ureter is controlled by three transport mechanisms. By far the most important is an electroneutral Na(+)-HCO3- cotransporter. Na(+)-H+ exchange appears to play little role in the presence of the physiological buffer. Both of these mechanisms extrude acid equivalents and so protect the cell against its fairly substantial intrinsic intracellular acid loading. Cl(-)-HCO3- exchange, on the other hand, is stimulated by intracellular alkalosis to transport acid equivalents into the cell and so restore a more normal pHi.

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