THE EFFECTS OF ADRENERGIC AGONISTS AND ANTAGONISTS ON VESICOURETHRAL SMOOTH-MUSCLE OF RABBITS

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Abstract
The effects of .alpha. and .beta. adrenergic agonists and antagonists on isolated smooth muscle preparations from the rabbit bladder body, bladder base and proximal urethra were studied. The predominance of .alpha. receptors in the proximal urethra and the bladder base was observed via contraction of the areas by norepinephrine and blockade by phentolamine. .alpha.-Receptor-mediated contractile activity could be unmasked in the bladder body when .beta. receptors were blocked with propranolol. Isoproterenol, 1 .times. 10-10 to 3 .times. 10-7 M, had a strong, dose-related relaxant effect on the bladder body, but little effect on the bladder base or proximal urethra. Selective .beta.-2 agonists i.e., terbutaline, salbutamol and ritodrine elicited tissue responses similar to those of isoproterenol. The pD2 [negative log of the molar concentration of an agonist producing 50% of the maximal response] values for isoproterenol, terbutaline, salbutamol and ritodrine were 8.59, 7.87, 7.34 and 6.52, respectively. Dobutamine, a selective .beta.-1 agonist, did not relax the tissues. The nonselective .beta. receptor blocker, propranolol and the selective .beta.-2 receptor blocker, butoxamine, competitively antagonized the relaxant effects of the 4 active .beta. agonists; but atenolol, a selective .beta.-1 receptor blocker, was inactive. Due to the selective action of the agonists and antagonists, apparently .beta.-2 receptors mediate relaxation of the vesicourethral smooth muscles of the rabbit and the participation of .beta.-1 receptors in the areas is insignificant.