Effect of external abdominal irradiation on the dimensions and characteristics of the barriers to passive transport in the rat intestine.

Limited information is available on the effect of irradiation on the intestinal absorption of passively transported nutrients. In this study a previously validated in vitro technique was used to measure the uptake of fatty acids (FA), fatty alcohols and cholesterol into the jejunum, ileum and colon of control rats and animals exposed to cesium-137 source irradiation applied to the abdomen. The effective resistance of the intestinal unstirred water layer was measured with lauryl alcohol, and in control rats this resistance was lowest in the ileum, highest in the colon, and of intermediate value in the jejunum. Fourteen days after 600 rads, unstirred layer resistance was reduced by half in the colon when the bulk phase was stirred at 600 rpm, and in the jejunum, ileum and colon when the bulk phase was unstirred (0 rpm). The incremental change in the free energy of transfer (integral of delta Fw----l) was measured with a homologous series of saturated medium-chain length fatty acids; 14 days after 600 and 900 rads the value of integral of delta Fw----l in the jejunum rose significantly, and occurred when light and electron microscopic changes were minimal. Fourteen days after 300 rads, the uptake of FA 6:0-12:0 was reduced, but this decline in uptake appeared to be caused by a fall in the functional surface area of the membrane rather than a change in integral of delta Fw----l. The uptake of cholesterol into the jejunum, ileum and colon was unaffected by irradiation, suggesting that cholesterol and fatty acids may have different diffusion pathways through the membrane. Thus, external abdominal irradiation influences the dimensions and characteristics of the barriers to passive transport in the intestine of the rat, and thereby modifies the uptake of some but not all passively absorbed nutrients. These functional changes are not closely associated with morphological alterations.