Structure and function relationships in the colon. The role of the pericryptal sheath

High doses of ionising radiation result in a reduction in fluid absorption in rat descending colon after four days. This reduction in fluid absorption may be due to increased leakiness of the crypt epithelial cells and loss of the pericryptal sheath. There is loss of the cell adhesion molecules E-cadherin and OB cadherin from the crypt epithelial cells and the pericryptal sheath after 8Gy ionising radiation. Smooth muscle actin staining indicated that the pericryptal sheath was disrupted after radiation. These changes were preceded by dose dependent increases in the apoptotic signalling proteins cytochrome c and caspase-3. Loss of the pericryptal sheath and loss of crypt epithelial adhesion coincides with increased permeability as shown by increased leakage of FITC dextran in radiated crypts. Further investigation of cytochrome c release in Caco-2 cells after targeted radiation with a charged particle microbeam showed that it may be a 'bystander response' and that an important initiating event in release is the generation of free radicals. Low dietary Na+ causes an upregulation of plasma angiotensin II and aldosterone and increased fluid absorption in the colon. Previously work on aldosterone effects in the colon has concentrated on the epithelial Na+ channels and pumps. Angiotensin II has also been shown to upregulate various cytokines in the heart and other tissues. Investigation of the effects of low Na+ diet on the pericryptal sheath in rat descending colon showed increases in angiotensin II and transforming Growth factor β receptors and collagen type IV and smooth muscle actin. The increases in descending colonic sheath matrix and myofibroblast microfilaments were not seen in the caecum and suggest that an important factor in increased fluid absorption after low Na+ diet may be increased barrier function of the pericryptal sheath. Similar changes to the sheath components also occur at 4-11 days after ionising radiation and may be involved in the coincident recovery of fluid absorption. In addition to the structural changes that have been investigated a new technique using fluorescence recovery after photobleaching of FITC dextran has been developed in order to quantify crypt fluid absorption rates and may be potentially applied to look at colonic dysfunction in variety of conditions. Similarly, using a new fluorescent Na+ indicator, the Na+ concentration in the pericryptal space has been measured for the first time and confirms the presence of a hypertonic compartment involved in colonic fluid absorption. These findings indicate that the structural changes in the pericryptal sheath that occur during colonic dysfunction have important functional consequences and the use of quantitative techniques using fluorescent dyes may be useful in characterising these consequences. (author)