Alpha- and beta-adrenoceptor-mediated responses of the guinea-pig ileum and the effects of neuronal uptake inhibition

Summary The effects of noradrenaline and isoprenaline were examined on preparations of guinea-pig ileum, in which contractions were induced by three different methods; by transmural electrical stimulation, by exogenous carbachol and by potassium depolarization. Alpha- or beta-adrenoceptor-mediated responses were examined by construction of cumulative concentration-response curves in the presence of propranolol (10^–6 M) and phentolamine (5×10^–6 M) respectively. Stimulation of alpha-adrenoceptors by noradrenaline virtually abolished the twitches from transmural stimulation, but only partially inhibited the carbachol- and potassium-induced contractions. The effects on the last two preparations were attributed to a postsynaptic inhibition at alpha-adrenoceptors on the longitudinal smooth muscle. In the transmurally-stimulated preparation there was an additional pre-synaptic alpha-adrenoceptor-mediated inhibition of cholinergic transmission. The maximum beta-adrenoceptor-mediated inhibition of all three preparations to noradrenaline and isoprenaline was of the same magnitude and attributed only to a post-synaptic action on longitudinal smooth muscle.The predominant post-synaptic beta-adrenoceptor-mediated (carbachol-contracted ileum) and pre-synaptic alpha-adrenoceptor-mediated (transmurally-stimulated ileum) relaxations were significantly (P<0.05) potentiated by the neuronal uptake inhibitor desmethylimipramine. These receptors may therefore be considered to be closely associated with the sympathetic innervation. The effect on the post-synaptic alpha-adrenoceptor-mediated relaxation was equivocal. Additional minor excitatory responses were identified as a direct alpha-adrenoceptor-mediated contractile response to noradrenaline and as a beta-adrenoceptor-mediated potentiation of transmural stimulation by isoprenaline, possibly due to facilitation of cholinergic transmitter release.