Some parameters of descending inhibition during colonic propulsion

Isolated segments of rabbit distal colon were stimulated by distension of intraluminal balloons, with the aim of investigating the electrophysiological pattern of descending inhibition (DI) during the peristaltic reflex. Incidence, duration, and latency of DI were studied by varying the parameters of stimulation. A linear correlation was found between stimulus duration and DI duration for stimuli up to 10 s long. DI duration increased with the distance of the recording site from the stimulus up to 3 cm. A more complex relation was found between balloon volume and DI duration. Propagation velocity decreased with the distance from the stimulus. The relation between ascending excitation and DI was also studied: ascending excitation could always be suppressed by DI. DI was examined after addition of anticholinergics, ganglionic blocking agents, and tetrodotoxin to the organ bath. The results show that DI parameters are partially related to the stimulus characteristics and that propagation of DI is neurally mediated through nonadrenergic inhibitory nerves.