Enhancement of contractile responses to partial alpha-adrenoceptor agonists during warming in rat aorta

We examined the effects of warming on the contractile responses to full and partial alpha-adrenoceptor agonists in rat aorta. The contractions elicited by norepinephrine and methoxamine were not affected during warming (40 degrees C, 42 degrees C), whereas those induced by clonidine and St 587 were significantly enhanced. KCl-induced contractions of rat aorta were not affected by warming. The dissociation constants of clonidine and St 587 at 40 degrees C were not different from those at 37 degrees C. At 40 degrees C, the receptor occupancy-contractile response curve of clonidine was a hyperbolic curve similar to that of methoxamine at 37 degrees C, although at 37 degrees C the curve was almost linear. The responses of St 587 at both 37 degrees C and 40 degrees C were related inversely hyperbolic to the receptor occupancy, but the receptor occupancy-contractile response curve was shifted to the left and upward during warming. Clonidine and St 587 elicited equal responses at lower fractional occupancies at 40 degrees C than at 37 degrees C. The relative efficacies of clonidine and St 587 to methoxamine were significantly augmented during warming. It is suggested that the contractile responses to partial alpha-adrenoceptor agonists in rat aorta are enhanced during warming, and that this effect is related to the intrinsic efficacy of the agonists rather than to any function of their relative selectivity for alpha 1- or alpha 2-adrenoceptors. Such enhancement is due to augmentation of the efficacy rather than to augmentation of the affinity of the agonists.