Propulsion in both small and large intestine is largely mediated by the peristaltic reflex; despite this, transit through the shorter colon is at least 10 times slower. Recently we demonstrated that elongating a segment of colon releases nitric oxide (NO) to inhibit peristalsis. The aims of this study were to determine if colonic elongation was physiologically significant, and whether elongation activated polarized intrinsic neural reflexes. Video imaging monitored fecal pellet evacuation from isolated guinea-pig colons full of pellets. Recordings were made from the circular muscle (CM) and longitudinal muscle (LM) in flat sheet preparations using either intracellular microelectrode or Ca(2+) imaging techniques. Full colons were 158.1 +/- 6.1% longer than empty colons. As each pellet was expelled, the colon shortened and pellet velocity increased exponentially (full 0.34, empty 1.01 mm s(-1)). In flat sheet preparations, maintained circumferential stretch generated ongoing peristaltic activity (oral excitatory and anal inhibitory junction potentials) and Ca(2+) waves in LM and CM. Colonic elongation (140% of its empty slack length) applied oral to the recording site abolished these activities, whereas anal elongation significantly increased the frequency and amplitude of ongoing peristaltic activity. Oral elongation inhibited the excitation produced by anal elongation; this inhibitory effect was reversed by blocking NO synthesis. Pelvic nerve stimulation elicited polarized responses that were also suppressed by NO released during colonic elongation. In conclusion, longitudinal stretch excites specific mechanosensitive ascending and descending interneurons, leading to activation of polarized reflexes. The dominance of the descending inhibitory reflex leads to slowed emptying of pellets in a naturally elongated colon.