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University Department of Pharmacology, Oxford, UK.

1. Intracellular pH (pHi) of smooth muscle cells in isolated strips of guinea-pig femoral artery was measured using double-barrelled pH-sensitive microelectrodes. 2. In modified Krebs solution equilibrated with 5% CO2, pHi was 7.26 +/- 0.14 (n = 36; mean +/- S.D. of an observation) and the membrane potential (Em) was -60.5 +/- 5.5 mV. Removal of CO2 from the superfusing solution caused an immediate transient alkalosis before pHi stabilized at much the same value (7.28 +/- 0.14; n = 16) as in the presence of CO2. 3. The rate of recovery of pHi from experimentally induced acidosis was not measurably affected by the presence or nominal absence of CO2-HCO3-. 4. Application of amiloride (100 microM) blocked recovery from acidosis in the nominal absence of CO2-HCO3- and caused a progressive fall in pHi. In the presence of CO2-HCO3-, application of amiloride allowed a slow recovery to pHi 6.7-7.0, but completely prevented full recovery to the normal pHi. 5. Removal of extracellular Na+ (Na+o) caused a dramatic, progressive fall in pHi in both the presence and nominal absence of CO2-HCO3-. 6. The amiloride-insensitive extrusion of acid equivalents observed in the presence of CO2-HCO3- to pHi 6.7-7.0 was inhibited by removal of Na+o but was not affected by preequilibration with DIDS (see Methods). 7. It is concluded that Na(+)-H+ exchange is largely responsible for the effective extrusion of acid equivalents in these arterial cells, at least from relatively small perturbations. A DIDS-insensitive, Na(+)- and HCO3(-)-dependent mechanism provides some recovery from acidosis to a relatively low pHi. 8. Comparison with data obtained in exactly the same manner in smooth muscle cells of the guinea-pig ureter indicates that there are significant differences in the regulation of pHi in different smooth muscles.