Histamine, a nonselective histamine receptor agonist, activates simultaneously both H1 and H2 receptors in the guinea pig trachea and lung strip. The resulting contraction is due to the prevalence of H1 receptors, because they are blocked by the selective H1antagonist diphenhydramine. The H2 receptor antagonists cimetidine and ranitidine increased the sensitivity of both tissues to histamine by affecting primarily the amplitude of contractions induced by high histamine concentrations. Since the lung strips were more sensitized by ranitidine and low concentrations of the other studied antagonists (diphenhydramine, dithiadene, stobadine) than the tracheal smooth muscle, it is inferred that the density of H2 receptors is higher in peripheral than central airways. From all the studied histamine receptor antagonists only dithiadene was able to unmask the relaxation induced by H2 receptor activation indicative of its highest H1 selectivity. In the light of the concentration-dependent antihistaminic effect of stobadine, i.e. potentiation in low and inhibition in high concentrations, stobadine is suggested to belong to antihistaminics with no histamine receptor subtype selectivity

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