Topographical differences in the physiological response of canine airway smooth muscle

The resting membrane potential (Em), electrical and contractile response were studied in three anatomically separate areas of trachealis muscle and also in bronchial muscle (2nd generation) isolated from mongrel dogs. These responses were studied before and after administration of a single dose of histamine, serotonin, carbachol and prostaglandin F2 alpha. A cross-sectional area of airway smooth muscle in each preparation was directly assessed from the histological sections. These values were used to evaluate isometric tension (in g/mm2). It was found that (1) cross-sectional area of airway smooth muscle varies with location; (2) resting membrane potential of airway smooth muscle from upper, middle and lower portions of dog trachea was similar, i.e., -60.4 +/- 0.8, -60.2 +/- 0.4 and -60.8 +/- 0.9 mV, respectively; (3) resting membrane potential of bronchial airway smooth muscle (2nd generation) was -63.4 +/- 0.5 significantly higher (P less than 0.01) than the membrane potential observed in the trachealis muscle; (4) both contractile and electrical changes after single doses of histamine and serotonin administration showed a vertical gradient response. The largest contractile and electrical responses to these two agents were found in preparations from the lower portion of trachea and the smallest response was observed in upper tracheal preparations; (5) contractile responses after a single dose of carbachol and PGF2 alpha, when expressed as isometric tension also showed vertical gradient. In contrast, membrane potential changes after carbachol and prostaglandin F2 alpha were similar in all three types of tracheal segments. Airway smooth muscle of the 2nd generation of bronchi showed large responses after single doses of histamine, carbachol and prostaglandin F2 alpha but the response after serotonin was smaller than that seen in the lower trachea.