Effects of ethylephrine on the rat atrial pacemaker

Ethylephrine, assayed in isolated rat atria, as a dose-chronotropic response curve showed a typical bell-dome shape of the sympathomimetic amines. Yet, on the same basis, it was less powerful than epinephrine, norepinephrine or isoproterenol. Pretreatment with reserpine provoked supersensitivity and increase in the maximum. As well, previous administration of pargyline to the animals resulted in augmented accelerating effects, either in normal or reserpinized preparations. Cocaine or phentolamine shifted the dose-response curve to the left. On the contrary, propranolol, produced a marked action, decelerating the effects of ethylephrine and also decreased the maxima with higher doses. It is concluded that ethylephrine: a) is a direct-acting sympathomimetic amine; b) it brings beta-receptor stimulation; c) a certain degree of alpha-receptor decelerating effect is also involved; d) it is a good substrate of monoaminoxidase.