The effects of changes in extracellular pH (pHo) on intracellular Ca2+ concentration ([Ca2+]i) or intracellular pH (pHi) were measured simultaneously with muscle tension in canine tracheal smooth muscle strips. [Ca2+]i and pHi were measured using the fluorescent dyes fura-2 and BCECF, respectively. During high K+-induced contractions (24.2, 36.4 or 72.7 mM) at pH 7.4, pH0 was changed to 7.8 or 7.0 with NaOH or HCl, respectively. Induced changes in pHi were equal to ~ 50 % of the changes in pH0. Alkalinization significantly increased [Ca2+]i and enhanced muscle contraction at all concentrations of K+ but did not alter the relationship between muscle tension and [Ca2+]i. Acidification significantly decreased [Ca2+]i without changing muscle tone; hence, the muscle tension - [Ca2+]i relationship was shifted to the left. These results suggest that changes in pHo can alter airway smooth muscle tone by changing [Ca2+]i and pHi.
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