Comparison of H2 receptor agonists in rabbit atria and aorta

In an effort to compare the H2 receptors responsible for the chronotropic and vasorelaxant effects of histamine, rabbit right atria and aortae were studied using four H2 receptor agonists and the H2 receptor antagonist cimetidine. While the efficacies of the various agonists were not similar in the two tissues, the order of potency in both atria and aortae was impromidine greater than histamine greater than 4-methylhistamine greater than dimaprit. In addition, cimetidine, an H2 receptor antagonist, inhibited the chronotropic and relaxant effects of impromidine and histamine in a consistent fashion in both atria and aorta. The results obtained in the present study are consistent with the concept that the H2 receptors in rabbit atria and aortae represent one type of receptor and not two isoreceptors as is the case for beta-adrenergic receptors in these tissues.