The effects of extracellular pH on intracellular pH, Ca²⁺ and tension of canine tracheal smooth muscle strips

The effects of changes in extracellular pH (pH_o) on intracellular Ca²⁺ concentration ([Ca²⁺]_i) or intracellular pH (pH_i) were measured simultaneously with muscle tension in canine tracheal smooth muscle strips. [Ca²⁺]_i and pH_i 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 pH_i 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 pH_o can alter airway smooth muscle tone by changing [Ca²⁺]_i and pH_i.