THE EFFECTS OF α‐ AND β‐ADRENOCEPTOR ACTIVATION ON TENSION AND MEMBRANE PROPERTIES OF THE LONGITUDINAL SMOOTH MUSCLE OF THE CHICKEN RECTUM

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Abstract
1 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 times 10−8 m for isoprenaline, 1.7 times 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 times 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 times 10−6 m. 3 These catecholamines suppressed spontaneous spike discharge and produced membrane hyperpolarization. Propranolol, 3.4 times 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 β-adrenoceptor mediates a prolonged relaxation and the α-adrenoceptor a fast and short-lasting relaxation which is usually obscured by the β-response and unmasked only after blockade of the β-adrenoceptors. The α- and β-mediated relaxations are each associated with the suppression of spontaneous spike activity.