Increased pH induces tension development in rat anococcygeus muscle by intracellular calcium release

The relationship between extracellular pH (pHe) alterations and muscle tension was studied in rat anococcygeus muscle. Increased cytosolic calcium levels induced smooth muscle contraction and increased tension. Extracellular alkalinization (pH 8.2) with 20 mM NH4Cl produced a sustained increase in tension of the same magnitude as phenylephrine (PHE)-stimulated contraction (NH4Cl = 22.0 +/- 2.8 mm; PHE = 21.7 +/- 3.1 mm). The muscle relaxed when the pH returned to pH 7.4. This increase in tension seems to be independent of extracellular calcium influx because it was not inhibited in Ca(2+)-free EGTA-PSS. Extracellular acidification with 10 mM sodium acetate, pH 6.8, produced no changes in tension or PHE-stimulated contractile response. The data suggest that pH changes lead to a release of stored intracellular calcium, with a consequent increase in tension.