Isolated longitudinal muscle strips from the chicken rectum responded to isoprenaline, adrenaline and noradrenaline with a prolonged relaxation. The concentrations required to produce 50% of the maximum relaxation were 1.3 x 10(-8) M for isoprenaline, 1.7 x 10(-8) M for adrenaline and 10(-6) M for noradrenaline. The relaxing potency of isoprenaline is about equal to that of adrenaline, but more than 50 times that of noradrenaline. 2. Propranolol, 3.4 x 10(-6) M, blocked the isoprenaline-induced relaxation, and in the presence of this drug the responses to adrenaline and noradrenaline were converted into small, transient relaxations. The residual relaxation was blocked by phentolamine, 2.6 x 10(-6) M. 3. These catecholamines suppressed spontaneous spike discharge and produced membrane hyperpolarization. Propranolol, 3.4 x 10(-6) M, prevented the inhibitory effects of isoprenaline, and reduced but did not completely abolish those of adrenaline and noradrenaline. 4. Adrenaline and noradrenaline, but not isoprenaline, reduced membrane resistance in some preparations. 5. In the rectal muscle of the chicken, the beta-adrenoceptor mediates a prolonged relaxation and the alpha-adrenoceptor a fast and short-lasting relaxation which is usually obscured by the beta-response and unmasked only after blockade of the beta-adrenoceptors. The alpha- and beta-mediated relaxations are each associated with the suppression of spontaneous spike activity.
