Regional differences in the nitrergic innervation between the proximal and the distal colon in rats

Background & Aims: Functional differences in the inhibitory neural pathway between the proximal and the distal colon are unknown. Methods: We investigated the nonadrenergic, noncholinergic (NANC) relaxation, nitric oxide synthase (NOS) synthesis, and NOS messenger RNA (mRNA) expression of the myenteric plexus in the proximal and the distal colon in rats. Results: Transmural nerve stimulation of the neuromuscular preparations from the proximal colon showed greater NANC relaxations than those from the distal colon. NANC relaxations were abolished by the NO biosynthesis inhibitor (N-G-nitro-L-arginine methyl ester) in the proximal and the distal colon, suggesting mediation by NO released from the myenteric plexus. The average number of NOS-immunoreactive cells was significantly higher in the tissue from the proximal colon than in the tissue from the distal colon. Western and Northern blot analyses showed a higher density of the immunoreactive NOS band and the NOS mRNA band in the tissue from the proximal colon than in that from the distal colon. Conclusions: These observations indicate that the number of NOS-containing neurons and the NOS activity are increased in the myenteric plexus of the proximal colon compared with the distal colon, resulting in greater NANC relaxation in the proximal colon. These findings may explain the physiological role of the proximal colon as an organ for fecal storage and absorption of excess fluid.