Fractionated irradiation of the rat's abdomen with 1200 R, given in two equal exposures, produces changes in the enzyme activities of the epithelial brush border (lactase, invertase, maltase and leucineaminopeptidase (LAP)) of the small intestinal villi. In animals sacrificed at different times after the second irradiation three phases of enzyme activities could be discerned: (1) an initial phase where the activities increase above those in controls; (2) an intermediate phase where the activities are reduced; and (3) the final phase of return to normal of enzyme activity. The fall and subsequent recovery occurs earlier for maltase and invertase than for lactase and LAP. When the time interval between doses is short (6 to 24 hours), alterations of enzyme activities are analogous to, or more extensive than, those found after a single irradiation. When the interval is longer (48 or 72 hours), the initial phase of increased activity is absent. If the time interval is greatly increased (five days) the effect of fractionated doses is similar to that produced by a single 650 R dose.

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