Inhibition of rabbit intestine mediated by α‐ and β‐adrenoceptors

Abstract

1 . The effects of some α- and β-adrenoceptor agonists and antagonists were studied on isolated segments of rabbit intestine in an attempt to characterize the two types of inhibitory response produced by sympathomimetic amines. 2 . Phenylephrine, an α-adrenoceptor agonist, produced an inhibition of rapid onset, from which recovery occurred despite the continued presence of the drug. On washout there was an overshoot in contraction height. Isoprenaline, a β-adrenoceptor agonist, produced an inhibition of slow onset which was maintained throughout the presence of the drug and there was no overshoot on washout. 3 . Adrenaline resembled phenylephrine more closely than it resembled isoprenaline, in that it showed more affinity for α-adrenoceptors, whereas noradrenaline, and the transmitter released on periarterial nerve stimulation, behaved more like isoprenaline, although both types of receptor were affected. 4 . Adenosine-5′-triphosphate produced an inhibition resembling that produced by an α-adrenoceptor agonist, whereas the dibutyryl analogue of cyclic adenosine 3′,5′-monophosphate (cyclic 3′,5′-AMP) produced an inhibition resembling that produced by a β-adrenoceptor agonist. 5 . In critical concentrations theophylline augmented and imidazole inhibited β-adrenoceptor mediated responses, as well as responses to dibutyryl cyclic AMP. However, additional actions of theophylline and imidazole were also demonstrated. 6 . Responses mediated by α-adrenoceptors, but not those mediated by β-adrenoceptors, were blocked by membrane stabilizers, quinidine being the most potent of those studied. 7 . The results are discussed in relation to the possible mechanisms of action of α- and β-adrenoceptor agonists.