Movement of Acid Equivalents Across the Mammalian Smooth Muscle Cell Membrane

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Abstract
Factors affecting intracellular pH (pHi) in the smooth muscle of guinea pig ureter have been investigated using pH-sensitive microelectrodes. Associated acids and bases appear to have free passage across the cell membrane but results suggest very low permeability to charged acid equivalents, thus implicating carrier-mediated movements in many of the observed pHi transients. Recovery from acidosis in the nominal absence of CO2 was inhibited by removal of Na+ and by the presence of amiloride, indicating that Na+/H+ exchange was responsible. The presence of CO2 resulted in a faster recovery from acidosis but, since intracellular buffering power was not increased, not a substantially faster effective extrusion of protons. Surprisingly, amiloride no longer caused discernable inhibition. Recovery from moderate acidosis remained Na+ dependent but was not inhibited by DIDS or acetazolamide or by the absence of Cl−, suggesting a dominant Na+-, and HCO3−-dependent mechanism unlike any hitherto described. Recovery from alkalosis was inhibited by DIDS and Cl−-free conditions, indicating that Cl−/HCO3− exchange was involved. Results suggest reversal of this mechanism on extreme acidosis. Experiments in vascular smooth muscle with fluorescent indicators confirm the presence of Na+/H+ exchange but provide conflicting evidence about the presence and properties of the HCO3−-dependent mechanism.

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