Effects of Sympathomimetic Amines on the Transmembrane Potential of Toad Heart

The effects of the sympathomimetic amines (adrenaline and noradrenaline) upon the transmembrane potential of both atrial and ventricular fibers of the toad were studied. Transmembrane potentials were recorded by glass-pipette microelectrode.1. In the atrial fibers, these amines increased the resting potential at an early stage. An increase was also observed on the overshoot and duration of the action potential. At the same time a slowing of the terminal phase of depolarization and a slowing of the initial phase of repolarization was observed in some cases.At a later stage, these changes were replaced by a marked shortening of the action potential duration especially after a longer exposure to higher concentrations of the amines. This shortening of the action potential duration at the later stage was occasionally followed by fibrillation or flutter. All these later effects were abolished by atropine. This suggests that a liberation of acetylcholine or some allied substance from the atrium in an amount sufficient to mask the effects of adrenaline occurs.2. Similar changes were observed in the ventricular fiber as a result of the administration of sympathomimetic amines.3. Premedication of the animal with reserpine sensitized both atrium and ventricle to the sympathominetic amines.4. Acetylcholine and sympathominetic amines are antagonistic and counteract each other in their effect on the duration of the action potential.5. The effect of adrenaline on the atrial membrane potential was not essentially altered by changing the cationic concentrations of the external fluid.6. The mode of action of adrenaline on the membrane potential is discussed on the basis of the results obtained.