Effects of PCO2, pH and extracellular calcium on contraction of airway smooth muscle from rats.

The effect of changes in PCO2 on airway smooth muscle was studied in acetylcholine-induced contractions of isolated rat trachea. Elevation of superfusate PCO2 from control PCO2, 38 mm Hg (pH 7.49), to 168 mm Hg (pH 6.74) decreased tension to 68% of control tension; reduction of PCO2 to 19 mm Hg (pH 7.84) increased tension to 104%. Similar effects on tension occurred when pH was altered by varying superfusate bicarbonate concentration at constant PCO2. Modification of the response to changes in PCO2 by varying extracellular calcium (Ca2+) concentration and also by verapamil indicated that changes in PCO2 and pH may alter Ca2+ uptake by the smooth muscle. Calcium uptake was measured by 45Ca2+ and the lanthanum method. At control pH 7.49, net Ca2+ uptake was 5.34 mmol Ca2+/kg trachea 60 min after the onset of contraction; this decreased to 4.26 at pH 6.88, and increased to 6.58 at pH 7.85. The results suggest that the mechanism whereby changes in PCO2 affect airway smooth muscle contraction is a pH-dependent alteration of Ca2+ uptake.