Changes of intracellular pH in rat mesenteric vascular smooth muscle with high-K+ depolarization

1. In mesenteric vascular smooth muscle cells changes in extracellular pH are rapidly transmitted to the cytoplasm. The mechanism involved is unknown, but it may be due to a high proton permeability of the surface membrane, in which case changes in membrane potential would alter the driving force for proton entry. We have therefore examined the voltage sensitivity of intracellular pH (pHi) in these cells. 2. Strips of mesenteric resistance vessels were loaded with SNARF-1 to monitor pHi and tension was simultaneously measured. Tissues were superfused with oxygenated solutions at 37 degrees C and pH 7.4. Isosmotic substitution of K+ for Na+ was used to depolarize the preparations. 3. pHi was found to be sensitive to alteration of [K+]. Depolarization of the tissue with K+ caused contraction and produced transient increases in pHi. When pHi regulation was blocked there was no significant change in the size of the alkalinization induced by high K+, thus it is unlikely that the results can be explained by voltage sensitivity of pHi regulating mechanisms. 4. In nominally Ca(2+)-free solution, the tissue does not contract and the alkalinization with high K+ was significantly greater than that occurring in 3 mM Ca2+. 5. There was a rapid acidification, when pHi regulation was blocked, which is consistent with a high proton permeability. 6. The effects of membrane potential on pHi have been modelled and show that they can be accounted for by effects of voltage on H+ influx through a proton channel. The effects of changing external pH on H+ influx also fit the model. Estimation of the proton permeability gave a high value (0.4 cm s-1). 7. The results presented demonstrate (i) a voltage sensitivity of pHi in mesenteric vascular smooth muscle cells and (ii) a particularly high permeability of the membrane to protons. The physiological significance of these findings is discussed.