Temperature dependence if beta-receptor mediated relaxation in rabbit intestine

The relaxation induced by isoproterenol, A beta-stimulant, appears more markedly at 25 degrees C (pD2 7.98) than at 37 degree C (pD2 6.88) in rabbit intestine. The isoproterenol-induced relaxation was antagonized by treatment with propranolol but not phentolamine at both 25 degree and 37 degrees C, indicating that the relaxation was mediated via adrenergic beta-receptor. The relaxations induced by papaverine, theophylline and MgCl2 were not influenced by temperature change. Basal tissue cyclic AMP content was higher at 25 degrees C than at 37 degrees C, and isoproterenol increased the tissue cyclic AMP content to a similar extent at 25 degrees C and 37 degrees C in the presence of theophylline. The binding of 3H-dihydroalprenolol (3H-DHA) to beta-receptor did not solve the reason of higher sensitivity to isoproterenol at 25 degrees C than at 27 degrees C. At 37 degrees C isoproterenol was more effective in producing a relaxation in the presence than the absence of ascorbic acid (0.057 mM). It is thought that the temperature dependency of the isoproterenol-induced relaxation may be mainly due to the breakdown of isoproterenol in a high temperature.