Abstract
Some years ago, Bulbring and Burn (1) showed that the choline acetylase activity in aceton-dried powder from the atrium which had ceased to beat was low, but the activity was increased when acetylcholine was added before incubation and the increase was proportional to the amount of acetylcholine added. Thus, they considered that the endogenously liberated acetylcholine sustained the building up of excitement in the pacemaker and as a result automatic contractions were initiated and maintained. Kottegoda (2) reported that a high concentration of acetylcholine had a stimulant action on the rabbit atrium in the presence of atropine.
On the other hand, much information about the electrophysiology and pharmacology of the cardiac cells has been obtained by the use of the intracellular microelectrode. The effects of neurohumors such as acetylcholine and sympathomimetic amines on the transmembrane potentials of the atrial fibers were studied by many workers. Acetylcholine produces a marked acceleration of the repolarization in atrial muscle of the dog (3), cat (4), rat (5) and rabbit (6). Acetylcholine causes a slight increase in steepness of the depolarization in the dog atrium (3) ; carbamylcholine does the same in the cat atrium (4). In the atrium of the dog (3), cat (4) and rat (5) acetylcholine slightly increases the resting potential. Adrenaline lowers the resting potential of the rat atrium (5), while in the dog atrium the resting potential is elevated by adrenaline and noradrenaline (7). In the dog auricular tissue (8) and chick embryo heart (9), the amines increase the rate of repolarization and shorten the total duration of action potential without significantly affecting the resting potential. Recordings of the transmembrane potential from the specific pacemaker area gave light on the origin of impulse formation in the heart. In the sinoatrial node of the isolated rabbit atrium (6, 10), acetylcholine slows the atrial rate and decreases the slope of diastolic depolarization, and increases the resting potential in pacemaker fibers. On the contrary, adrenaline applied to the same region accelerates the heart rate, and increases the slope of diastolic depolarization of the pacemaker fibers. In Purkinje fibers of the sheep heart, rate acceleration produced by adrenaline is mainly due to an increase in slope of the prepotential (11).
The present experiment deals with the effects of catecholamines and their potent releaser, reserpine, on the transmembrane potentials of pacemaker and non-pacemaker fibers in the isolated spontaneously beating atria of rabbits. It was hoped to find out whether catecholamines, which are continuously synthesized and liberated in the cardiac tissue, are concerned with the initiation and maintenance of the atrial rhythmicity.
