Contraction of dog trachealis muscle in vivo: role of alpha-adrenergic receptors.

Contraction, relaxation, and longitudinal tension were recorded by isometric strain gauge arches attached to cervical tracheal muscle (CTM) in 60 spontaneously breathing dogs anesthetized with pentobarbital. Intravenous norepinephrine (NE) (3 X 10(-9), 6 X 10(-9), 1.2 X 10(-8), and 2.4 x 10(-8) mol/kg) increased spontaneous mechanical activities (SMA) and caused dose related contraction of CTM in all dogs even though there was no pretreatment with beta-blockers. These activities were first potentiated by propranolol and then prevented by phentolamine. NE briefly decreased SMA and induced CTM relaxation prior to the onset of contraction in one-third of dogs. Propranolol prevented this initial relaxation. CTM responses induced by NE were 1) not significantly altered by atropine, tripelennamine, bilateral vagotomy, curarization, and complete tracheal transection below transducer sites; 2) unrelated to passive constriction of cervical trachea associated with airway elongation; and 3) independent of reflexes initiated by elevations of systemic arterial pressure. The moles per kilogram doses of acetylcholine were found to exceed those of NE when their intravenous administration caused equal CTM contractions in the same dog. These findings are consistent with the existence of alpha-adrenergic receptors in CTM.