THE EFFECTS OF EPINEPHRINE, NOREPINEPHRINE, AND ACETYLCHOLINE ON THE ELECTROGRAM OF THE ISOLATED FROG HEART

¹ Department of Pharmacoloqy, Jefferson Medical College, Philadelphia 7, Pa.

In surface electrograms of the isolated frog heart acetylcholine depresses the QRS-complex, and inverts or decreases the negativity of the T-wave. These changes are seen even at councentrations which do not alter the heart rate.

Epinephrine and norepinephrine increase the magnitude of the QRS-complex, and increase the amplitude and negativity of the T-wave. These effects can occur without a rate change.

The electrical responses produced by epinephrine, norepinephrine, and acetylcholone precede the chronotropic changes and seem to involve different mechanisms.

There is a mutual antagonism between the electrical and chronotropic changes produced by the sympathetic agents and those produced by acetylcholine. This antagonism depends on the ratio of the concentrations of the drugs present simultaneously.

Epinephrine is one and a half times more effective than norepinephrine in overriding the acetyicholine effect on the electrogram.

Acetylcholine, present in mixtures with epinephrine or norepinephrine in ratios of 1:10 to 1:20, was effective in antagonizing the electrical effects of the latter.

The findings described here suggest a possible role of acetylcholine in the frog heart for reducing or preventing large electrical responses to epinephrine and norepinephrine.