An Enteric Occult Reflex Underlies Accommodation and Slow Transit in the Distal Large Bowel

Background & Aims: Transit of fecal material through the human colon takes ≥30 hours, whereas transit through the small intestine takes 24 hours. The mechanisms underlying colonic storage and slow transit have yet to be elucidated. Our aim was to determine whether an intrinsic neural mechanism underlies these phenomena. Methods: Recordings were made from circular muscle (CM) cells and myenteric neurons in the isolated guinea pig distal colon using intracellular recordings and Ca²⁺ imaging techniques. Video imaging was used to determine the effects of colonic filling and pellet transit. Results: Circumferential stretch generated ongoing oral excitatory and anal inhibitory junction potentials in the CM. The application of longitudinal stretch inhibited all junction potentials. N-ω-nitro-L-arginine (100 μmol/L) completely reversed the inhibitory effects of longitudinal stretch suggesting that nitric oxide (NO) inhibited interneurons controlling peristaltic circuits. Ca²⁺ imaging in preparations that were stretched in both axes revealed ongoing firing in nNOS +ve descending neurons, even when synaptic transmission was blocked. Inhibitory postsynaptic potentials were evoked in mechanosensitive interneurons that were blocked by N-ω-nitro-L-arginine (100 μmol/L). Pellet transit was inhibited by longitudinal stretch. Filling the colon with fluid led to colonic elongation and an inhibition of motility. Conclusions: Our data support the novel hypothesis that slow transit and accommodation are generated by release of NO from descending (nNOS +ve) interneurons triggered by colonic elongation. We refer to this powerful inhibitory reflex as the intrinsic occult reflex (hidden from observation) because it withdraws motor activity from the muscle.

Abbreviations used in this paper: CM, circular muscle, CMMP, circular muscle myenteric plexus, EJP, excitatory junction potential, FEPSP, excitatory postsynaptic potential, IJP, inhibitory junction potential, IPSP, inhibitory postsynaptic potential, LM, longitudinal muscle, L-NA, N-ω-nitro-L-arginine, PPADS, pyridoxal phosphate-6-azophenyl-2,′4′-disulfonic acid, TS, transmural stimulation