A Question of the Specificity of Rabbit Atrial Chronotropic Histamine Receptors and Agents Which Affect Their Activity

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Abstract
The histamine chronotropic response of rabbit atria appears to be controlled by both H1 and H2 receptors and can be partially blocked by metiamide (an H2 antagonist) or diphenhydramine (an H1 antagonist), while both 2- and 4-methylhistamine (H1 and H2 agonists, respectively) stimulated the chronotropic response. At low agonist concentrations, the simultaneous presence of both H1 and H2 blockers causes considerably less calculations of individual inhibition data, apparently some sites behave as if they have both H1 and H2 properties. Additional compounds were tested for specific action on H1 and H2 receptors: 2-(2-pyridyl)ethylamine, a reported H1 agonist, appears to stimulate rabbit atria by releasing norepinephrine (NE) and guinea pig atria by releasing both NE and histamine. Dimaprit, a reported H2 agonist, may stimulate histamine receptors directly but has a nonspecific depressant action on rabbit atria which interferes with its use as an agonist in this species.