Imidazole action on guinea pig heart

Imidazole (IM) exerts a positive inotropic effect on the guinea pig heart in a dose-dependent manner. This effect is not controlled by any of histamine receptors. Both, mepyramine - a H1-histamine receptor blocker, and cimetidine - a H2-histamine receptor blocker have no influence on IM responses. A positive inotropic action of IM on the heart has been shown in both, normal Ringer-Locke solution and in solutions with calcium deficit, however, the relative increase in contraction amplitude was higher at lower calcium concentrations than in solutions with normal calcium content. In calcium - free solution IM produces a marked delay of heart arrest, but fails to restore the heart action stopped by absence of calcium in the perfusion medium. The effects of IM and theophylline are additive. It is concluded that the action of IM on the guinea pig heart is not connected with histamine receptors, and a possible role of IM as a regulator of free calcium ions is suggested.