EFFECT OF FREQUENCY AND IMPULSE PATTERN ON THE NONCHOLINERGIC CARDIACRESPONSE 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 resu lts in cardiac slowing. This study investigates the importance of impu lse pattern and frequency of neural stimulation in determining this ca rdiac response. When 360 stimuli were delivered to the heart either co ntinuously at 3 Hz for 2 min, 4 Hz for 1.5 min or 6 Hz for 1 min, or i n 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 obs erved. 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 signific antly greater than in response to the other stimulus regimens. In addi tion, 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 continuousl y for 2 min. The results provide evidence to support the suggestion th at the non-cholinergic, non-adrenergic cardiac response to stimulation of the vagosympathetic trunk is peptidergic in origin, and that the f requency of impulses is important in the gain of the response.