HISTAMINE-RECEPTORS IN THE MYENTERIC PLEXUS-LONGITUDINAL MUSCLE OF THE GUINEA-PIG ILEUM - H-1-RECEPTOR-MEDIATED AND H-2-RECEPTOR-MEDIATED POTENTIATION OF THE CONTRACTILE RESPONSE TO ELECTRICAL-STIMULATION

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  • 1 January 1982
    • journal article
    • research article
    • Vol. 223  (1) , 177-182
Abstract
The effect of histamine on the contractile response to low frequency-electrical field stimulation in the myenteric plexus-longitudinal muscle preparation of the guinea pig ileum was investigated. By blocking the direct increase in smooth muscle tone caused by histamine with low concentrations of pyrilamine (10-9-5 .times. 10-8 M), a dose-dependent, histamine-induced potentiation of the twitch response to electrical stimulation was observed. Blocking the direct actions of histamine with concentrations of pyrilamine > 10-7 M resulted in a biphasic histamine dose-response curve: lower histamine concentrations produced a dose-dependent decrease of the twitch response; higher concentrations produced a potentiation. The potentiating effect of histamine was inhibited by high concentrations of H2-receptor antagonists. Tiotidine (ICI 125, 211) had a pA2 of 5.25, 100 times greater than its pA2 in isolated guinea pig atria. Blockade of the actions of the H2-receptor agonists dimaprit and tetrahydrozoline also required greater antagonist concentrations. The selective H1-receptor agonist, 2-(2-thiazolyl)-ethylamine, also enhanced the response to electrical stimulation. The potentiating effect of histamine could be blocked by hexamethonium (10-7-10-5 M) but not by atropine. Atropine (10-9-10-8 M) did prevent the decrease in the contraction amplitude induced by histamine in the presence of 10-7 M pyrilamine. The purinergic antagonist theophylline, adrenergic antagonists or depletion of endogenous catecholamines were without effect. Tetrodotxin (10-6 M) inhibited the augmentation induced by histamine. Histamine potentiates the acetylcholine-mediated contractile response to electrical field stimulation of guinea-pig ileum via H1- and what may be an H2-receptor subtype.