Relaxant β‐Receptors in the Trachea, but not Prejunctional α2‐Receptors in the Tracheal Cholinergic Neurones, are Subjected to Homologous Desensitization

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Abstract
Guinea‐pigs were pretreated with either isoprenaline, terbutaline or the α2 agonist B‐HT 920 in order to asses the hypothesis that β‐ and α2 receptors in trachea are subjected to homologous desensitization. In these experiments the β receptor activity was investigated on tracheal ring preparations contracted with carbacholine. Both in the isoprenaline and the terbutaline pretreated group the relaxant responses to β agonists were diminished. Pretreatment with B‐HT 920 did not affect the sensitivity of the trachea to β stimulation. In order to asses the responsiveness of α2 receptors the trachea was contracted by electrical field stimulation in the presence of propranolol. During these conditions contractions were mediated by activation of cholinergic neurones and inhibitory effects of alpha stimulation were due to inhibition of the cholinergic neurotransmission by stimulation of prejunctional α2 receptors. In these tests neither isoprenaline, terbutaline nor B‐HT 920 pretreatment affected the responsiveness of α stimulation to inhibit the electrically induced contractions.