Effects of l-Epinephrine and l-Nor-Epinephrine on Cardiac Excitability

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Abstract
L-epinephrine and l-norepinephrine, in equimolar doses, affect the excitability of the dog heart in a manner that is quantitatively and qualitatively similar. Both agents increase heart rate in vagotomized dogs and both can produce ventricular tachycardia. Auricular, ventricular and A-V conduction velocity is increased more markedly by epinephrine than by norepinephrine. Both amines produce similar slight shortening of the relative and absolute refractory periods of auricle and ventricle. Resting excitability in both chambers is markedly altered by either agent and the lowering and subsequent elevation of diastolic thresholds that results from admn. of these agents varies with the volume and rate of injn. The primary increase in excitability is temporally related to the elevation of serum K that follows epinephrine injn. Hepatectomy abolishes this change in serum K but merely decreases the magnitude of the threshold changes produced by epinephrine. Following hepatectomy the effect of norepinephrine on diastolic thresholds is essentially unchanged.

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