Histamine H2 receptor subtypes in the guinea pig right atrium.

Inotropic and chronotropic responses to histamine and to selective histamine H2 receptor agonists (4-methyl histamine, dimaprit, impromidine) and antagonists (metiamide and cimetidine) were studied in retrogradely superfused guinea pig right atrium preparations. All agonists tested were acting as chronotropic and, except impromidine, as inotropic agonists; impromidine had a dual inotropic action. Affinities of the antagonists varied, showing approximately 20-fold more activity on chronotropism. The findings suggest the existence of subtypes of histamine H2 receptors in the development of chronotropic and inotropic responses.