Relaxant beta-receptors in the trachea, but not prejunctional alpha 2-receptors in the tracheal cholinergic neurones, are subjected to homologous desensitization

Guinea-pigs were pretreated with either isoprenaline, terbutaline or the alpha 2 agonist B-HT 920 in order to asses the hypothesis that beta-and alpha 2 receptors in trachea are subjected to homologous desensitization. In these experiments the beta receptor activity was investigated on tracheal ring preparations contracted with carbacholine. Both in the isoprenaline and the terbutaline pretreated group the relaxant responses to beta agonists were diminished. Pretreatment with B-HT 920 did not affect the sensitivity of the trachea to beta stimulation. In order to asses the responsiveness of alpha 2 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 alpha 2 receptors. In these tests neither isoprenaline, terbutaline nor B-HT 920 pretreatment affected the responsiveness of alpha stimulation to inhibit the electrically induced contractions.