Exposure of the digestive tract to ionising radiation results in both morphological and functional alterations of the small intestine with the terminal ileum being a particular site of injury. The aim of this work was to characterize, in parallel, epithelial barrier properties of rat ileum as determined by the passage of different sized molecules and spatial localization junctional proteins following increasing radiation dose.

Animals were exposed to a single hemi-body irradiation (6Ð12 Gy) under gaseous anaesthesia (isoflurane 2.5 %, 0.4 l min^-1) and studied 3 days after exposure. After euthanasia (overdose of sodium pentobarbitone), samples of terminal ileum were placed in Ussing chambers for measurements of ¹⁴C-mannitol, FITC-Dextran-4 kDa and ³H-Dextran-70 kDa fluxes, short-circuit current (I_sc) and transepithelial resistance (R_T). Ileal samples were treated either for immunohistochemical analyses of junctional proteins (ZO-1, β-catenin) and pan-cytokeratin using confocal microscopy or for standard histological analyses with haematoxylin-eosin-saffron staining. All experiments were conducted according to the French regulations for animal experimentation (Ministry of the Agriculture Act No. 87848, October 19, 1987).

Following a 6 Gy irradiation I_sc was increased from 26 ± 5 mA cm^-2 (controls N = 22) to 64 ± 16 mA cm^-2 (irradiated N = 5) (means ± S.E.M., P < 0.01, one-way ANOVA). R_T was increased from 35 ± 3 Ω cm² (controls N = 22) to 50 ± 3 Ω cm² (irradiated N = 5, P < 0.05), but permeability to mannitol and dextran was unchanged. Analysis of pan-cytokeratin and β-catenin staining showed a slight increase of cell size with nevertheless a well-structured network in both villi and crypts. Standard histology confirmed these results.

At 3 days after 8 Gy R_T was decreased by 50 % concomitant with increased permeability to mannitol ( X 3), Dextran-4 kDa ( X 3) and Dextran-70 kDa ( X 2). A reduced and modified pattern of β-catenin staining was observed, suggesting a relocalization of this protein. The effect of irradiation on ZO-1 was more marked with very little staining being detected in either villi or crypts. An amplification of these effects occurred with increasing radiation dose where for ²ge³ 10 Gy R_T was quasi-null and mannitol and 4.4 kDa dextran permeability was markedly increased ( X 7). Major histological alterations with epithelial discontinuity were also observed.

In conclusion, these results demonstrate radiation dose-dependent differences in the response of rat small intestinal epithelium: at a lower dose (6 Gy) increased R_T was observed, whereas an opposite effect (decreased R_T, enhanced permeability) was obtained at higher doses (²ge³ 8 Gy). The latter were associated with a disorganization of the small intestinal epithelium.

All procedures accord with current National guidelines.