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

Abstract

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. I.v. norepinephrine (NE); (3 .times. 10-9, 6 .times. 10-9, 1.2 .times. 10-8 and 2.4 .times. 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 1/3 of dogs. Propranolol prevented this initial relaxation. CTM responses induced by NE were not significantly altered by atropine, tripelennamine, bilateral vagotomy, curarization and complete tracheal transection below transducer sites; unrelated to passive constriction of cervical trachea associated with airway elongation; and independent of reflexes initiated by elevations of systemic arterial pressure. The moles per kilogram doses of acetylcholine exceeded those of NE when their i.v. administration caused equal CTM contractions in the same dog. Apparently, .alpha.-drenergic receptors exist in CTM.