Adrenoceptor-mediated mechanical responses of canine tracheal smooth muscle

The effect of tone on responses of canine tracheal smooth muscle (TSM) to norepinephrine (NE) was studied to elucidate the role of sympathetic innervation and adrenoceptors in the control of the airways. Electrical field stimulation produced contraction of TSM in vitro which was augmented by eserine, depressed by phentolamine, potentiated by propranolol in the presence of K+ (14 mM) and almost eliminated by tetrodotoxin or atropine. Resting TSM did not contract in response to NE (10(-8) to 10(-4) M) in the presence or absence of propranolol (10(-5)M). The addition of NE (10(-8) to 10(-6) M) at the plateau of contraction produced by K+ (22.8 mM), histamine (10(-6) M) or acetylcholine (5 X 10(-8) M) produced a further phentolamine-sensitive contraction which was potentiated by beta adrenoceptor blockade with propranolol (10(-5) M). The addition of tyramine (10(-5) to 10(-4) M) at the plateau of contraction produced by K+ (22.8 mM) produced a further contraction which was potentiated by propranolol (10(-5) M) and reduced by phentolamine (10(-5) M). Although the response to NE in the presence of elevated tone was contractile at low concentrations of NE (10(-8) to 10(-6) M), a propranolol-sensitive relaxant response was elicited at higher NE concentrations (10(-5) and 10(-4) M). Maximum contractions to NE in the absence or presence of beta-blockade were dependent on the tone of the muscle. These findings suggest a functional adrenergic innervation of canine TSM and the presence of alpha and beta adrenoceptors which mediate contractile and relaxant responses, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)