REGULATION OF INTRACELLULAR PH IN THE SMOOTH-MUSCLE OF GUINEA-PIG URETER - NA+ DEPENDENCE

1. Mechanisms involved in the regulation of intracellular pH (pH(i)) in smooth muscle cells of guinea-pig ureter have been investigated using double-barrelled pH-sensitive microelectrodes in isolated strips of tissue. 2. Removal of CO2-HCO3- from the superfusing solution caused a fall in the steady-state pH(i) except in a few cells which had been excised from the animal for many hours (usually >24 h). The pH(i) value was 7.22 +/- 0.09 (n = 89; mean +/- S.D. of an observation) in solution buffered With 5% CO2-21 mM HCO3-, compared with 6.92 +/- 0.24 (n = 67) in the nominal absence of CO2-HCO3-, Recovery from experimentally induced acidosis was faster in the presence, rather than nominal absence, of CO2-HCO3- (mean half-times of 2.7 +/- 0.7 min, n = 41, and 4.6 +/- 1.3 min, n = 12, respectively). These results suggest the presence of both HCO3- -dependent and -independent mechanisms for the effective extrusion of acid equivalents. 3. Recovery from acidosis was dependent on external Na+ (Na-o(+)) in both the presence and nominal absence of CO2-HCO3-, with an apparent half-maximal activation at approximately 4 and 20 mM Na-o(+) respectively. Removal of Na-o(+) in the steady state caused a fall in pH(i) which proceeded at a faster rate in the presence rather than in the nominal absence of CO2-HCO3- 4. Amiloride (100 mu M-1 mM) reversibly inhibited the recovery from acidosis and caused a fall in the steady-state pH(i) when applied in the nominal absence of CO2-HCO3-, but had no measurable effect on either the recovery from acidosis or steady-state pH(i) in the presence of CO2-HCO3-. These results suggest that Na+-H+ exchange was responsible for extrusion of acid equivalents in the nominal absence of CO2 and HCO3-, but that it played little part under more physiological conditions. 5. Although Na+-H+ exchange appeared to be activated below a pH(i) of about 7.2, it was incapable of maintaining a 'normal' pH(i) in the nominal absence of CO2-HCO3- in freshly excised cells, where values between 6.06 and 6.89 were recorded. Only in aged preparations, in which the intrinsic intracellular acid loading was substantially reduced (as judged from the rate of acidification on application of amiloride in the nominal absence of CO2-HCO3-) did the steady-state value approximate to that observed in the presence of CO2-HCO3-, at about 7.2.