An electron microscopic study on the absorption of horseradish preoxidase (HPR) via rat rectal mucosa.

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Absorption of HRP via rat rectal mucosa was examined histochemically under electron microscope. At 1, 5, 15, and 60 min after infusion of HRP (1-5mg/cc) with a mixed emulsion of lipid (oleic acid 10 mM)-bile salt (sodium glycocholate 10 mM) as an adjuvant into rectal lumen through a catheter, rectum was fixed and chopped at 40-50 μm. These tissue slices were incubated in Karnovsky medium to detect exogenous HRP. They were then postfixed, dehydrated and embedded under ordinary conditions for the electron microscopic observafion. After only 1 min of administration, the uptake of HRP could be localized to the intercellular spaces of the epithelial cells. After 60 min, a lot of HRP was found in phagocytes of lamina propria, but not in the intercellular spaces. Without this adjuvant, no absorption of HRP could be found. During the course of these experiments, rectum retained its normal ultrastruuctural appearance. HRP uptake was accomplished directly through tight junctions of the epithelial cells rather than by endocytosis. It was assumed that this adjuvant made the tight junctions of rectal epithelium loose or leaky to some extent.
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