In the course of studies in this laboratory on the mode of action of catecholamines on the transmembrane potential of the extirpated atrium of rabbit (1), some of the poten tials recorded from the pacemaker area (West) responded differently to reserpine and adrenaline from the typical pacemaker potentials. The potential under this study was recorded from the area which was somewhat wider than the triangular pacemaker area shown schematically by West (2). This potential was characteristic in a notching of the upstroke and a slow rate of depolarization, and was very sensitive to reserpine and adre naline. The notched potential in the sinoatrial node of rabbit's heart was at first reported by West (2), who studied the distribution of the potential. Although the similar notched potentials have been reported in the atrioventricular node of mammalian heart (3, 4), the smooth muscle of uterus (5) and the sinoauricular funnel of amphibian heart (6), the exact nature of the potentials remains to be settled. In the present report the responses of the notched potential recorded from the sinoatrial node of rabbit's heart were studied. It is expected that the responses of the potential to reserpine and adrenaline might have some contribution to elucidate the mechanism of the pacemaker activity.
METHODSAlbino rabbits, weighing 1.8 to 2.2 kg, were killed through haemorrhage by cutting of both common carotid arteries. The heart was extirpated and the right auricle was separated from the heart. Immediately thereafter, the auricle was fixed on the cork plate immersed in warm (30°C) and 02 saturated modified Tyrode solution. The volume of the solution in the bath was 50 ml. The composition of the modified Tyrode solution is as follows : NaCI 8.6 g, KCl 0.2 g, CaCl, 0.2 g, MgCl2 0.05 g, NaH,P04 0.2 g, Na2HPO4 1.6 g and glucose 1.0 g in 1000 ml. The preparation always showed a rhythmical auto matic contraction for 8 to 14 hours.A glass capillary tube filled with 3 molar KCl solution was used as a different elect rode. As an indifferent electrode a platinum wire immersed in the nutrient solution of the bath was used. The potential changes of the single muscle fiber were recorded follow ing the method of Matsumura and Takaori (7). The potential changes amplified through d.c. amplifier were photographed from a cathode-ray oscilloscope by use of a continuous recording camera. When it was required, the potentials were recorded by use of an ink writing oscillograph.