Studies on the Relaxing Actions Mediated by Stimulation of Adrenergic α‐and β‐Receptors in Taenia Coli of the Rabbit and Guinea Pig

Abstract

In experiments on taenia coli from rabbits, a study was made of the mechanisms whereby catecholamines induce a relaxing effect. In a low concentration (≤2.1 × 10–⁶ g/ml), l‐phenylephrine selectively stimulated adrenergic a‐receptors; this effect was blocked completely by the adrenergic α‐blocking agent dibenamine. In a low concentration (≤ 3.7 × 10–⁸ g/ml), 1‐isoprenaline selectively stimulated β‐receptors; this effect was blocked completely by the adrenergic β‐receptor blocking agent sotalol. In higher concentrations, phenylephrine and isoprenaline stimulated both adrenergic α‐and β‐receptors; their relaxing effect could only be blocked completely by a combination of dibenamine and sotalol. In taenia coli from rabbits, the threshold concentration for β‐receptor‐mediated relaxation was lower than that for relaxation induced via a‐receptors; in taenia coli from guinea pigs the reverse was found. The relaxant effect induced via adrenergic a‐receptors was blocked selectively by factors such as cold treatment, digitalis glycosides, desoxycorticosterone, which could be assumed to decrease the ionic gradients of K+ and Na+ across the cell membrane or reduce its ionic permeability. — On successive reduction of the carbohydrate reserves of the muscle, the relaxation induced via adrenergic β‐receptors was inhibited more rapidly than that induced via α‐receptors. Theophylline and puromycin, which inhibit the enzymatic hydrolysis of cyclic AMP, potentiated selectively the β‐receptors mediated relaxation.