Fenoterol, a β
2-adrenoceptor selective agonist, belongs to the arylethanolamine class. To understand the receptor subtypes responsible for β-adrenoceptor-mediated relaxation of guinea pig taenia caecum, we investigated the effect of fenoterol. Fenoterol caused concentration-dependent relaxation of the guinea pig taenia caecum. Propranolol, bupranolol and butoxamine produced shifts of the concentration-response curve for fenoterol. Schild regression analyses carried out for propranolol, butoxamine and bupranolol against fenoterol gave pA
2 values of 8.41, 6.33 and 8.44, respectively. However, in the presence of 3 × 10
-4 M atenolol, 10
-4 M butoxamine and 10
-6 M phentolamine to block the β
1-, β
2- and α-adrenoceptor effects, respectively, Schild regression analysis carried out for bupranolol against fenoterol gave pA
2 values of 5.80. These results suggest that the relaxant response to fenoterol in the guinea pig taenia caecum is mediated by both the β
2- and the β
3-adrenoceptors.
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