DIFFERENT ROUTES FOR THE TRANSPORT OF ALPHA-LACTALBUMIN IN THE RABBIT ILEUM

To identify the different pathways of proteins through the intestinal epithelium, the transport of [C-14]-radiolabeled alpha-lactalbumin was measured in the rabbit ileum mounted in Grass diffusion cells in both the presence and absence of glucose, galactose, or colchicine. The absorption of polyethylene glycol 4000 and the conductance of the tissue were not modified by the addition of 10 mmol/L galactose or 0.5 mmol/L colchicine, but were enhanced by the addition of 25 mmol/L glucose (by 37% and 75%, respectively). The total mucosal uptake of alpha-lactalbumin was not modified by the addition of 10 mmol/L galactose (3.09 +/- 1.39 versus 3.46 +/- 1.11 mug/h.cm2 in Ringer) and significantly enhanced by the addition of 25 mmol/L glucose, which is known to dilate tight junctions (5.41 +/- 1.34). Colchicine (0.5 mmol/L), a microtubule depolymeryzing agent, decreased the total uptake of alpha-lactalbumin by 35%. The fraction of intact protein (determined by high-pressure liquid chromatography and trichloroacetic acid precipitation) transported through the epithelium represented a small percentage of the total protein transported and this intact fraction was enhanced by the addition of 25 mmol/L glucose. The present results suggest that in control conditions alpha-lactalbumin is transported through a transcellular route, whereas in the presence of glucose an additional paracellular path way is stimulated.