Negative chronotropic effects of vagal stimulation with different modalities in the rabbit.

The negative chronotropic effects of efferent stimulations of the vagus nerves, after their sectioning at the neck, were studied in urethane anesthetized rabbits. Vagal stimulators were triggered by electrocardiographic P waves in order to deliver synchronized trains of stimuli. Three stimulation parameters were changed, one at a time: number of stimuli per cycle (St/c), delay between triggering waves and stimulating trains (P-St), and delay between stimuli within the trains (St-St). Stimulations were repeated for the right (R), the left (L) and both (R+L) vagus nerves: measurements were made before (control) and during each trial, 30 sec after beginning (steady state responses). The results of the experiments indicate that, in the presence of efferent vagal activity, heart rate is related with St/c, with parabolic functions, tending to saturation for 12-14 St/c. No such relation was seen with different P-St, nor with different stimulation frequencies (St-St). Results were similar for R and L, whilst the function curves for R+L were shifted upwards, but each point was lower than the sum of R plus L: this was interpreted as partial occlusions of the effects. We suggest that the acetylcholine released by the vagal volleys on the heart elicits accumulation of a 'restraining factor', which is utilized and partially reset in a fixed portion of each cardiac cycle.