ON THE HISTAMINE RECEPTOR OF THE CANINE MYOCARDIUM AND CORONARY VASCULATURE

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To indentify the histamine-receptors in the canine myocardium, experiments were performed in the canine heart-lung preparation supported by a donor. Smaller doses of histamine produced only an increase in the coronary blood flow. Positive inotropic and chronotropic effects appeared with larger doses. The increase in the coronary blood flow was associated with only a minimal increase in the myocardial oxygen consumption. Pretreatment of the preparation with a prototype H1-receptor antagonist, mepyramine, resulted in an abolishment of the positive chronotropic effect and a partial inhibition of the coronary vasodilatatory effect, indicating that the histamine-receptors in the dog atrium subserving the chronotropic effect are to be classified as H1-type. After a representative H2-receptor antagonist, metiamide, the positive chronotropic effect remained unaffected, while there was a partial inhibition of the coronary vasodilatatory effect. A combined use of both the H1- and H2-receptor antagonists brought about a complete suppression of the positive inotropic and the coronary vasodilatatory effects of histamine. These findings indicate that the histamine-receptors in the coronary vasculature belong to the same type of receptors as those in the peripheral blood vessels, and that those in the canine ventricular myocardium cannot be classified either as H1 or H2.
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