An intestinal perfusion technique is reported for the study of the differential absorption of vitamin D3 and its active metabolite, 1,25dihydroxyvitamin D3, through intact jejunal segments of rats. Samples of introduced and collected perfusates, intestinal homogenates, and portal blood were assayed for [14C]vitamin D3 or [3H]1 ~~,25-dihydroxyvitamin D3 content at specified time intervals in control rats and in rats injected ip with cycloheximide (3 mg/kg body weight). Vitamin D3 uptake from the perfusates in cycloheximide-treated groups did not differ from controls.
However, an -2-fold increase of vitamin D3 retention in the perfused intestinal segments was observed after cycloheximide treatment. A 0.25-fold decrease was observed in the uptake of 1,25-dihydroxyvitamin D3 from the perfusates after cycloheximide treatment, and an -2.5-fold increase in its intestinal retention was noted. An increase in the active metabolite concentration was observed in the portal venous system 75 rnin after initiation of perfusion, with no detectable amounts being recorded prior to the first hour. The results suggest that intracellular binding proteins may be involved in the transport of labeled vitamin D3 and labeled 1,25-dihydroxyvitamin D3 through rat enterocytes. Further-