Abstract
β-Adrenoceptors in the guinea pig taenia caecum were investigated. In control preparations, catecholamines caused relaxation with the following rank order of potency: isoprenaline>adrenaline>noradrenaline. However, in the presence of 10-6M phentolamine, 3×10-4M atenolol and 10-4M butoxamine, the rank order of potency of the agonist was: isoprenaline>noradrenaline>adrenaline. CGP12177 caused graded relaxation of the guinea pig taenia caecum. This response was antagonized by bupranolol and SchiId plots of the data revealed a pA2 value of 5.61. The isoprenaline-induced increase in cyclic AMP levels was inhibited by propranolol. However, propranolol did riot significantly affect the CGP12177-induced increase in cyclic AMP levels. These results suggest that β-adrenoceptor-mediated relaxations of the guinea pig taenia caecum predominantly involve β2 and β3-adrenoceptors, whereas CGP12177-induced relaxation is mediated solely through β3-adrenoceptors, and suggest that the production of cyclic AMP contributes to the β3-adrenoceptor-mediated relaxation of the guinea pig taenia caecum. The concentration-response curves for noradrenaline and adrenaline were unaffected by propranolol or phentolamine. However, the responses to noradrenaline and adrenaline were antagonized by hupranolol, and Schild plots of the data revealed pA2 values of 5.53 and 6.10, respectively. These results suggest that the relaxant responses to noradrenaline and adrenaline in the guinea pig taenia caecum are mainly mediated by β3-adrenoceptors, and that in the guinea pig taenia caecum noradrenaline arid adrenaline behave as a β3-selective adrenoceptor agonist.
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