Nature of the beta-adrenoreceptors in the tracheal smooth muscle of the adult pig

Temperature dependence of beta 1-adrenoceptor-mediated responses examined by use of partial agonists

The inotropic and chronotropic responses of guinea pig atria, and the relaxation responses of guinea pig intestine, trachea, lung, uterus and vas deferens to catecholamines have been examined at bath temperatures of 38 degrees C and 30 degrees C. Hypothermia resulted in a supersensitivity of cardiac tissues with a decrease in isoprenaline EC50 and an increase in the maximum response to the partial agonist, salbutamol. Ileum responses to isoprenaline were potentiated at 30 degrees C but no partial agonist could be found on this tissue. Responses of the lung and vas deferens to partial agonists were not affected by temperature, while uterine responses were inhibited by hypothermia. The trachea was supersensitive to isoprenaline at 30 degrees C, however this was not due to a change in beta-adrenoceptor sensitivity but an inhibition of COMT. Partial agonist responses of trachea were similar at both temperatures. beta-Adrenoceptor supersensitivity was therefore observed only where responses are mediated primarily by beta 1-adrenoceptors and supports the concept that beta 1- but not beta 2-adrenoceptors exhibit hypothermia-induced supersensitivity.