Radiation induced cytochrome c release causes loss of rat colonic fluid absorption by damage to crypts and pericryptal myofibroblasts

Background-Therapeutic or accidental exposure to radiation commonly causes gastrointestinal disturbances, including diarrhoea. Rats subjected to whole body ionising radiation at a dose of 8 Gy lose their capacity to absorb fl uid via the descending colon after four days. After seven days, fluid absor ption recovers to control levels. Aims-To investigate the effect of ionising radiation on colonic permeabilit y together with its effect on mitochondria dependent apoptotic signals and intercellular adhesion molecules. Methods-Rats were irradiated with doses of 0-12 Gy. Colonic permeability wa s measured by accumulation of fluorescein isothiocyanate (FITC) dextran in crypt lumens. Changes in levels of cytochrome c, caspase 3, E and OB cadher in, beta -catenin smooth muscle actin, and collagen IV were assessed using immunocytochemistry with confocal microscopy. Results-Cytosolic cytochrome c increased after 8 Gy (t(1/2) 1.4 (0.6) hours ) and peaked at approximately six hours. Caspase 3 increased more slowly, p articularly in crypt epithelial cells (t(1/2) 57 (14.5) hours). Pericryptal myofibroblasts disintegrated within 24 hours as was evident from loss of O B cadherin and smooth muscle actin. This coincided with increased crypt per meability to dextran. Intercellular adhesion between crypt luminal cells wa s not lost until day 4 when both beta -catenin and E-cadherin were minimal. The half maximal dose-response for these effects was in the range 2-4 Gy. Recovery of colonic transport was concurrent with recovery of pericryptal s mooth muscle actin and OB cadherin. The pan caspase inhibitor Z-Val-Ala-Asp .fluoromethylketone (1 mg/kg per day) had a small effect in conserving the pericryptal sheath myofibroblasts and sheath permeability but had no system ic therapeutic effects. Conclusions-These data suggest that radiation damage to the colon may be in itiated by mitochondrial events. Loss of crypt fluid absorption and increas ed permeability coincided with decreased intercellular adhesion between cry pt epithelial cells and loss of pericryptal sheath barrier function.