ROLE OF INTRACELLULAR PH IN RELAXATION OF PORCINE TRACHEAL SMOOTH-MUSCLE BY RESPIRATORY GASES

Hypercapnia and hypoxia both relax airway smooth muscle, but the mechanisms responsible are poorly understood. Because hypercapnia and hypoxia can each decrease intracellular pH (pH(i)) and acidosis can inhibit Ca2+ channels, we hypothesized that decreased pH(i) mediates relaxation of trachealis muscle by each of these respiratory gases. To examine the relationship between pH(i) and tone, we measured isometric tension, bath pH, and fluorescence intensity (540 nm) in porcine tracheal smooth muscle strips loaded with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein and excited alternately with 440- and 500-nm light. Strips equilibrated in Krebs-Henseleit solution bubbled with 95% O-2-5% CO2 were contracted with carbachol and then relaxed with either 95% N-2-5% CO2 or 93% O-2-7% CO2. The ratio of fluorescence intensity at 500 nm to 440 nm was calibrated vs. pH(i) with use of nigericin. Baseline pH(i) was 7.19 +/- 0.03 (n = 13). Hypoxia decreased active tension by similar to 60% but did not change pH(i). Hypercapnia induced decreases in tension that were associated with substantial decreases in pH(i). Thus, decreased pH(i) does not mediate hypoxic relaxation, but the relaxation during physiologically relevant increases in CO2 concentration is associated with significant cellular acidification.