Noradrenergic and non-noradrenergic relaxation at basal and high tone levels in the guinea-pig tracheal smooth muscle.

Responses to the beta-adrenoceptor agonist, 1-isoprenaline, and to transmural (electrical) stimulation of the guinea-pig tracheal muscle were evaluated at basal and high levels of carbachol or potassium-induced (K+) tone. The sensitivity of guinea-pig tracheal preparations to 1-isoprenaline was reduced by approximately 7-fold when high level tone was induced by carbachol or K+. Propranolol also shifted the dose-response curves to the right, but was apparently more potent in preparations with basal tone than in tissues where high tone was induced by carbachol or K+. Transmural (electrical) stimulation of tracheal strips produced an initial contraction followed by a longer lasting relaxation. At high level of tone produced by carbachol or K+, the contractile component was almost abolished while the relaxant response was unaltered. Relaxation induced by transmural stimulation was decreased to similar extents by propranolol at both basal and high tone levels. These results suggest that the potency of 1-isoprenaline and propranolol is affected by tonic conditions of tracheal muscle but the proportion of noradrenergic to non-noradrenergic components in the relaxation induced by neural stimulation may be unaltered at basal and elevated tone levels.