Pharmacological isolation of a single beta-adrenoceptor subtype makes Schild plots which have slopes of 1 possible for selective beta-adrenoceptor antagonists in carbachol-contracted guinea-pig trachea

The effects of selective beta-adrenoceptor agonists and antagonists on isometric relaxation of carbachol-contracted, guinea-pig isolated trachea were studied. An attempt was made to pharmacologically isolate the beta 1- and beta 2-adrenoceptors by use of appropriate selective agonists and antagonists. The slopes of the Schild plots for butoxamine vs noradrenaline and terbutaline as well as for practolol vs terbutaline did not differ significantly from 1, the predicted value for a competitive antagonist acting at a single receptor type. These data differ from a previous report suggesting that slopes of less than 1 are characteristic for selective beta-adrenoceptor antagonists in carbachol-contracted guinea-pig trachea. They support the hypothesis that the slopes in this preparation are due to interaction of the agonist with both beta 1- and beta 2-adrenoceptors rather than the state of contraction of the trachea.