Vitamin B12 absorption studied by vascular perfusion of rat intestine

The study of vitamin B12 release from the ileal enterocyte has been hampered by the fact that B12 does not cross the serosa of traditional everted ileal sacs. We studied this release by perfusing the superior mesenteric arteries of starved, heparinized, etherized rats and collecting perfusate from the superior mesenteric vein. The rats were fed 57CoB12 well before study. The standard perfusion medium was Krebs-Henseleit-NaHCO3 buffer containing glucose, dextran, albumin, propranolol, and dexamethasone. The preparation utilized glucose and O2, produced lactate, and was relatively impermeable to [14C]inulin, to D-xylose, and to 57CoB12 bound to an inert human IF. Glucose placed in the gut lumen was transported much more rapidly than D-xylose. Vitamin B12 emerged in the perfusate bound to a protein with a molecular size similar to that of TC II. IF could not be identified in the perfusate. Rat serum, independently of its unsaturated TC II content, increased the rate of transfer of B12 into the perfusate.