The localization and distribution of regulatory peptides was studied in separated epithelium, lamina propria, submucosa, and external muscular layer from 16 specimens of human bowel. Immunoreactive enteroglucagon, gastric inhibitory polypeptide, and neurotensin were almost confined to the epithelial fraction (97.5 ± 2.2%, 97.5 ± 4.2%, and 99.3 ± 1.1% of their respective total content, mean ± SEM) and were only localized in endocrine cells. Vasoactive intestinal polypeptide-, substance P-, and bombesinlike peptides were virtually restricted to the nonepithelial layers (99.6 ± 0.2%, 99.6 ± 0.2%, and 100%) and were demonstrated exclusively in nerves. A particularly rich vasoactive intestinal polypeptide and substance P-immunoreactive nerve supply was seen in the nonepithelial mucosa, which contained the highest concentrations of these peptides, while bombesin was mainly recovered from the external muscle (87.7 ± 2.7%). Somatostatin, measured with an antiserum highly specific for somatostatin-14, was found throughout the wall, mainly in the epithelium (39.9 ± 5.2%) and lamina propria (29.5 ± 5.9%), but could be immunostained only in endocrine cells.
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