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Characteristics of beta-adrenoceptor in the tracheal smooth muscle of Suncus murinus (suncus) were studied in comparison with those of rats and guinea pigs. Isoproterenol induced concentration-dependent relaxation of suncus trachea. However, the maximal relaxation was very small, and about 1000 times higher concentration was necessary compared to the case of the guinea pig. The order of the sensitivity to isoproterenol and the maximal relaxation was guinea pig much greater than rat greater than suncus. Tracheae from younger suncus were more sensitive to isoproterenol, but the maximal relaxation was not significantly different from the results using adult animals. Forskolin, a direct activator of adenylate cyclase, relaxed tracheae to similar extents in the three species. Though the affinity of specific [3H]dihydroalprenolol binding was not significantly different, the maximal number of binding sites was in the order of rat greater than guinea pig greater than suncus. However, the differences in density of the binding among the three species were not as great as differences in isoproterenol-induced relaxation. These results suggest that 1) beta-adrenoceptor-mediated relaxation of tracheal smooth muscle is not well-developed in the suncus and 2) the deficient relaxation is probably caused by insufficient coupling between adrenoceptors and adenylate cyclase.
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