POSSIBLE MECHANISMS OF BETA-ADRENOCEPTOR-MEDIATED RELAXATION INDUCED BY NORADRENALINE IN GUINEA-PIG TAENIA CECUM

The mechanisms of the beta-adrenoceptor-mediated relaxation induced by noradrenaline in guinea pig taenia caecum were investigated. Noradren aline caused graded relaxation of this preparation. However, the conce ntration-response curves for noradrenaline were unaffected by proprano lol (similar to 10(-5) M) or phentolamine (similar to 10(-5) M). The r esponses to noradrenaline were antagonized in a concentration-dependen t manner by bupranolol, and Schild plots of the data revealed a pA(2) value of 5.53. Also, bupranolol antagonized responses to isoprenaline, and Schild plots of the data revealed the pA(2) value to be 8.53. Nor adrenaline significantly increased the cyclic AMP level in this prepar ation. Bupranolol (10(-4) M) significantly decreased the cyclic AMP re sponse elicited by noradrenaline, whereas propranolol (10(-5) M) produ ced no effect. These results suggest that the relaxant response to nor adrenaline in guinea pig taenia caecum is mainly mediated by beta(3)-a drenoceptors (or atypical beta-adrenoceptors) and that in guinea pig t aenia caecum noradrenaline behaves as a beta(3)-selective adrenoceptor agonist.