Abstract

The heart rate in the presence of S-A rhythm in the heart-lung preparation of the dog, as well as in the heart in situ, is reduced by veratramine. This negative chronotropic action of veratramine is not abolished or prevented by atropine.

The heart rate in the presence of A-V rhythm in the anesthetized dog is influenced by veratramine in qualitatively the same way as in the presence of S-A rhythm in the intact circulation and in the heart-lung preparation. After A-V rhythm has been established in the anesthetized dog by clamping off the S-A node, the heart rate is increased by the sympathomimetic amines l-epinephrine, l-norepinephrine, isopropylarterenol and tuaminoheptane. Veratramine antagonizes the positive chronotropic action of the sympathomimetic amines in the presence of A-V rhythm.

In the anesthetized dog in the presence of A-V rhythm, veratramine in a dose of 0.06 to 0.1 mgm./kgm. body weight is capable of producing asystole lasting for short periods, while, when the S-A node is the dominant pacemaker, asystole has not been observed under the same conditions and in this dosage range.

In the heart-lung preparation veratramine does not influence the increased heart rate resulting from spontaneous auricular fibrillation, or from the increased spontaneity of the ventricles in ventricular tachycardia caused by ouabain.

Veratramine, in doses which exert strong antiaccelerator action, or which cause a marked negative chronotropic effect in the presence of S-A rhythm, appears to possess little, if any, quinidine-like antiarrhythmic properties.