Mucus Gel Thickness and Turnover in the Gastrointestinal Tract of the Rat: Response to Cholinergic Stimulus and Implication for Mucoadhesion

The thickness of the mucus gel and its turnover rate were measured in the stomach, proximal jejunum, cecum and proximal colon of the rat, using microscopy and staining techniques. The specific mucus-secretory responses to carbachol-induced cholinergic stimulus in these locations were also studied. The mucus gel was found to be the thinnest (18 ±1 microns) in the cecum, and the thickest in the stomach (39 ±14 microns). The effect of carbachol on mucus secretion was profound and dose dependent in the stomach, and less profound, although still dose dependent, in the proximal jejunum. The least responsive organs were the cecum and the proximal colon, where no effect was observed after three doses of carbachol. Mucus secretion rate was significantly higher in the jejunum (1.1 ± 0.5 µg glucose equivalent min⁻¹ cm⁻²) than in the colon (0.5 ± 0.2 µg glucose equivalent min^−l cm⁻²). Also, the proximal jejunum was more responsive to the carbachol stimulus (mucus secretion rate of 5.4 ±2.2 µg glucose equivalent min⁻¹ cm⁻² after carbachol treatment) than the colon (mucus secretion rate of 1.0 ±0.4 µg glucose equivalent min^−l cm⁻² after carbachol treatment). In vitro mucoadhesion studies with Polycarbophil disks were performed in the mucosal tissues of the stomach, jejunum, cecum and proximal colon of the rat with and without cholinergic (carbachol) stimulus. The adhesion force in the cecum and the colon was significantly stronger than in the stomach and proximal jejunum when the studies were performed at pH 2. Carbachol treatment did not significantly change the mucoadhesion of Polycarbophil disks. It is concluded that in the gastrointestinal tract of the rat the colon and the cecum are more suitable locations for the mucoadhesion than the stomach and the jejunum because: (1) their mucus turnover is lower, (2) their sensitivity to mucus secretory stimulus is lower, and (3) their Polycarbophil adherence properties are stronger.