Stimulatory (H1) and inhibitory (H2) histamine receptors in gallbladder muscle

The nature of histamine receptors in gallbladder muscle and examined using specific histamine-receptor agonists and antagonists. The H2-receptor antagonist, metiamide, augmented the contractile response to histamine indicating that gallbladder muscle possessed stimulatory H1 receptors and inhibitory H2 receptors. The independent inhibitory character of H2 receptors was confirmed by (1) induction of relaxation with histamine after H1-receptor blockade and the suppression of this relaxation with metiamide, and (2) induction of relaxation with a specific H2-receptor agonist, 4-methyl histamine and the suppression of this relaxation with metiamide. Further, blockade of H2 but not of H1 receptors augmented the response to the octapeptide of cholecystokinin. The nature of this effect was such that the apparent affinity of the octapeptide for its own receptor was increased. The finding raised the possibility that in their native unoccupied state, H2 receptors may modify the response to hormonal agents.