THE ROLE OF H1 AND H2‐RECEPTORS IN THE CORONARY VASCULAR RESPONSE TO HISTAMINE OF ISOLATED PERFUSED HEARTS OF GUINEA‐PIGS AND RABBITS

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Abstract
1 The effects of histamine on the isolated perfused hearts of guinea-pigs and rabbits were examined. Records of contractile force, heart rate and coronary perfusion pressure were obtained. 2 Histamine exerted positive inotropic and chronotropic effects which were antagonized by burimamide and attributed to stimulation of H2-receptors. 3 The coronary vascular response to histamine differed between guinea-pigs and rabbits. In guinea-pig hearts, three phases were apparent: (a) An initial vasodilatation preceding any effects on heart force and rate was antagonized by mepyramine and therefore mediated by histamine H1-receptors in the coronary circulation. (b) A secondary vasoconstriction was attributed to the increased myocardial compression during the positive inotropic and chronotropic responses. (c) The final, more predominant, component was a prolonged vasodilatation probably associated with the increased metabolic activity of the heart. 4 The latter two components were abolished together with the myocardial responses by burimamide. The remaining coronary vascular response was biphasic, consisting of a vasodilatation immediately followed by vasoconstriction. Both were antagonized by mepyramine and therefore mediated by H1-receptors. 5 The coronary vascular response of rabbit hearts was similar but no direct vasodilatation was observed and it was concluded that histamine receptors in the coronary vasculature involve only vasoconstriction.