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

Background & Aims: Functional differences in the inhibitory neural pathway between the proximal and the distal colon are unknown. Methods: We investig ated the nonadrenergic, noncholinergic (NANC) relaxation, nitric oxide synt hase (NOS) synthesis, and NOS messenger RNA (mRNA) expression of the myente ric plexus in the proximal and the distal colon in rats. Results: Transmura l nerve stimulation of the neuromuscular preparations from the proximal col on showed greater NANC relaxations than those from the distal colon. NANC r elaxations were abolished by the NO biosynthesis inhibitor (N-G-nitro-L-arg inine methyl ester) in the proximal and the distal colon, suggesting mediat ion by NO released from the myenteric plexus. The average number of NOS-imm unoreactive cells was significantly higher in the tissue from the proximal colon than in the tissue from the distal colon. Western and Northern blot a nalyses 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 dista l colon. Conclusions: These observations indicate that the number of NOS-co ntaining 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 phys iological role of the proximal colon as an organ for fecal storage and abso rption of excess fluid.