Abstract
The effect of epinephrine, norepinephrine, isoproterenol, and ephedrine on normal sinus rhythm has been studied by direct perfusion of the sinus node through its own artery, and compared to the results of stellate stimulation. The maximal sinus tachycardia produced by each of these substances was about the same, and resembled the response to stellate stimulation. To produce maximal tachycardia the doses of epinephrine and norepinephrine were about equal, but were about ten times the dose of isoproterenol required, and one-tenth the dose of ephedrine required. Duration of action of each of the catecholamines was similar, but that of ephedrine was about ten times longer. Dichloroisoproterenol perfused into the sinus node effectively blocked the response to moderate doses of all the test substances but incompletely blocked that to larger doses or the effect of direct stellate stimulation. Subsidence of the effect of the intranodal catecholamines followed a characteristic decay curve.
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