Role of the epithelium in the control of intestinal motility: implications for intestinal damage after anoxia and reoxygenation.

A vibration technique was used to dislocate the epithelium from the rat small intestine, in order to study the possible regulatory role of the epithelium on intestinal motility. Complete removal of the epithelium led to a slightly potentiated contraction of the longitudinal smooth muscle by the muscarinic agonist methacholine (pD2. 6.5 +/- 0.1 vs. 6.2 +/- 0.2). The maximal beta-adrenergic response expressed relative to the relaxation by 0.5 mM dibutyryl cyclic AMP increased from 55.9 +/- 9.0% to 72.6 +/- 9.1% by this treatment. Efforts were made to relate these observations to the endothelium-dependent relaxation in blood vessels, but no indication was found for a similar mechanism in the small intestine. Not only mechanical dislocation can be employed to affect the mucosal layer, but also intestinal ischemia has been reported to lead to mucosal damage. In this study we mimicked ischemia by applying in vitro anoxia and subsequent reoxygenation to isolated intestinal segments. When intestinal segments are isolated and kept in physiological buffer, xanthine dehydrogenase is converted slowly to xanthine oxidase, irrespective of whether the buffer is oxygenated or not. No evidence was found for oxygen radical damage after anoxia and reoxygenation. However, the intestinal mucosa was damaged both after normoxia, and after anoxia and reoxygenation. Anoxia and subsequent reoxygenation did not affect muscarinic contraction, but slightly increased the beta-adrenergic relaxation, which partly correlates with the effects of mechanical dislocation of the epithelium. The increased sensitivity of the smooth muscle after epithelial damage might be involved in motility changes during intestinal inflammatory diseases.

Intestinal motility disorder induced by peroxides: possible role of lipid peroxidation.

The effect of oxidative stress on the rat small intestine was investigated by pretreatment of isolated segments from the jejunum with hydrogen peroxide or cumene hydroperoxide. Both peroxides induced responses in the small intestine, viz. a contraction followed by a slow relaxation. The contraction could be blocked by the cyclooxygenase inhibitor indomethacin and the phospholipase A2 inhibitor quinacrine, suggesting a role for prostaglandins in the response. Pretreatment of intestinal segments with the peroxides diminished the muscarinic cholinergic response to methacholine. The lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA) and the antioxidant butylated hydroxytoluene (BHT) both protected against the damage induced by cumene hydroperoxide, but did not influence the effect of hydrogen peroxide on the muscarinic response. In contrast to hydrogen peroxide, cumene hydroperoxide induced lipid peroxidation in intestinal membranes, which could also be blocked by NDGA or BHT. We conclude that cumene hydroperoxide alters the muscarinic response in the rat jejunum by the induction of lipid peroxidation, whereas the damage by hydrogen peroxide is probably induced intracellularly.

Modulation of oxidative stress in the gastrointestinal tract and effect on rat intestinal motility.

The amounts of different factors, which are involved in oxygen free radical production or in protection against oxygen radicals, were determined in different parts of the gastrointestinal tract (GI-tract). Glutathione and superoxide dismutase were present in lower amounts within the small intestine compared with the stomach and the large intestine. In the small intestine glutathione peroxidase and catalase both prevailed in the intestinal muscle compared to the mucosa, whereas in the large intestine both enzymes are equally distributed among the mucosa and muscle. Xanthine oxidase was mainly present in the small intestinal mucosa. Taken together, these results suggest that the large intestine is better provided with protective enzymic and non-enzymic factors against oxidative stress than the small intestine. The protective capacity of different intestinal preparations against lipid peroxidation in liver microsomes was assessed, and particularly the mucosal fractions from the small intestine showed a marked protection against lipid peroxidation, which is not easily explained with the presence of the enzymes measured in this study. Pretreatment of intestinal segments with hydrogen peroxide or cumene hydroperoxide resulted in a damaged contractile response of the longitudinal smooth muscle to methacholine in all parts of the GI-tract, expressed in a lower pD2 value and a decreased maximal response. Pretreatment with these peroxides also decreased contractions after depolarization with K+. The large intestine is more sensitive to hydrogen peroxide and cumene hydroperoxide than the small intestine, which parallels with the sensitivity to lipid peroxidation. The results obtained with hydrogen peroxide also correlate well with the catalase activity in the various segments of the intestine. In conclusion, oxidative stress in the GI-tract alters intestinal motility, especially in the large intestine. Probably this does not occur at the level of muscarinic receptors.