Frequency and vagal effects

¹ Donner Laboratory of Medical Physics, University of California, Berkeley, California

Effects of frequency of stimulation were studied in the isolated vagal heart preparation of Rana pipiens. Normal inhibitory effects were readily obtainable by stimulating the vagus nerve at pulse rates between 10 pulses/sec. and 100 pulses/sec. for a period of 10 seconds. A little above 100 pulses/sec. or below 10 pulses/ sec. the heart was usually less responsive to the same period of vagal stimulation. Below 5 pulses/sec. or above 200 pulses/ sec. the isolated preparation was practically nonresponsive to vagal stimulation for the 10-second period. The heart was responsive to the lower pulse rate but not to the higher rate when stimulated for a longer period. Arrhythmia usually was induced by frequent application of high-frequency stimulation. This suggests that the pacemaker is somehow temporarily impaired by high-frequency bombardment. The inhibitory effect is proportional to the duration of the impulse for any optimal frequency. The observed inhibitory vagal phenomenon results primarily from the liberation of an acetylcholine-like substance from the vagal nerve endings. The amount of this substance liberated can be assayed quantitatively. It was estimated that each optimal volley liberated approximately 2.5 x 10^–11 gm of acetylcholine-like substance.