Effect of frequency and impulse pattern on the non-cholinergic cardiac response to vagal stimulation in the toad, Bufo marinus.

In the toad, Bufo marinus, stimulation of the vagosympathetic trunk to the heart in the presence of cholinergic and adrenergic blockade results in cardiac slowing. This study investigates the importance of impulse pattern and frequency of neural stimulation in determining this cardiac response. When 360 stimuli were delivered to the heart either continuously at 3 Hz for 2 min, 4 Hz for 1.5 min or 6 Hz for 1 min, or in pulses at 6 Hz for 1 s every 2 s, over a 2 min period, there were no significant differences in the size of the chronotropic responses observed. However, when 360 stimuli were delivered in pulses at 6 Hz for 0.5 s every 1 s over 2 min, the resulting cardiac slowing was significantly greater than in response to the other stimulus regimens. In addition, the cardiac slowing in response to 8 Hz for 0.5 s every 1 s over 2 min was significantly greater than the response to 4 Hz continuously for 2 min. The results provide evidence to support the suggestion that the non-cholinergic, non-adrenergic cardiac response to stimulation of the vagosympathetic trunk is peptidergic in origin, and that the frequency of impulses is important in the gain of the response.