A COMPARISON OF THE EFFECTS OF TYRAMINE AND EPHEDRINE ON ATRIAL CONTRACTIONS IN RABBITS AND GUINEA-PIGS

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Bertler, Carlsson and Rosengren (1) found that the pressor effect of tyramine is decreased in animals that have previously been reserpinized, and Burn and Rand (2) have reported that in such animals the change in blood pressure, response of vessel wall, or nictitating membrane to tyramine recovers after the treatment of noradrenaline. It has also been reported that the catecholamine content of various organs decreases after the administration of reserpine (3-5). The generally accepted explanation of these findings has been that the decreased response to tyranime in reserpinized animals may be due to the decrease in endogenous catecholamine available to tyramine after reserpine administration. On the other hand, tyramine and ephedrine show similar pharmacological effects both <I>in vivo</I> and <I>in vitro</I>. The action mechanism of ephedrine also remains to be clarified, despite the many investigations that have been carried out since Chen and Schmidt (6) reported on the pharmacological effects of this drug. Ephedrine is also effective as a bronchodilator and, in some cases, as a pressor drug. Clinically, however, a decrease in response to ephedrine after repeated administration has often been encountered, especially in blood pressure response, but tyramine yields to tachyphylaxis scarcely. It is generally held that mechanism of this so-called “tachyphylaxis” phenomenon is due to receptor occupation by ephedrine (7). We have previously carried out a series of experiments on rabbit and guinea-pig atria with a number of drugs assumed to have the ability to mobilize and activate endogenous catecholamine (8, 9). We have used the similar experimental procedure in our attempt to clarify the action mechanisms of tyramine and ephedrine, and to explain the mechanism of tachyphylaxis phenomenon that displays following repeated administration of ephedrine. In the present experiments also, reserpine and nicotine were used as experimental means, reserpine for its reduction of endogenous catecholamine, and nicotine for its indirect activation.

A COMPARISON OF THE EFFECTS OF TYRAMINE AND EPHEDRINE ON ATRIAL CONTRACTIONS IN RABBITS AND GUINEA-PIGS

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