ABSTRACT-13-Adrenoceptors in the guinea pig taenia caecum were investigated by measuring relaxation responses to agonists and by a radioligand binding assay using [3H]CGP 12177. The rightward shift of the isoprenaline concentration-response curve was observed by butoxamine, a p2-selective antagonist, and the pA2 value for butoxamine was 6.46. In control preparations, catecholamines caused relaxation with the following rank order of potency: isoprenaline > adrenaline > noradrenaline. However, in the presence of 10-6 M phentolamine, 3 x 10-4 M atenolol and 10-4 M butoxamine, the rank order of potency of the agonists was: isoprenaline > noradrenaline > adrenaline. CGP 12177 caused graded relaxation of the guinea pig taenia caecum, and this response was not influenced by 10-6 M phentolamine, 3 X 10-4 M atenolol, 10-4 M butoxamine or 10-6 M propranolol. The Scatchard plot of the specific [3H]CGP 12177 binding to microsomal fractions from the guinea pig taenia caecum showed two affinity sites of the receptor: high affin ity (KD=0.64 nM) and low affinity (KD=142.21 nM) sites. The pKD value of the high affinity site of [3H] CGP 12177 was in agreement with its pA2 value, and that of the low affinity site was in agreement with its pD2 value. These results suggest that isoprenaline-, noradrenaline and adrenaline-induced relaxations of the guinea pig taenia caecum predominantly involve /2 and (33-adrenoceptors, whereas CGP 12177-induced relaxation is mediated solely through 33-adrenoceptors.

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