Gastric and pancreatic intrinsic factor-mediated absorption of cobalamin in the dog.

The effects of canine gastric and pancreatic intrinsic factors on uptake and subcellular localization of cobalamin have been investigated in vivo to determine whether these proteins could mediate the physiological absorption of cobalamin in the dog. Cyano [57Co]cobalamin was introduced into ileal loops in dogs under general anesthesia, either free (control) or bound to gastric or pancreatic intrinsic factor. At 2 h, total uptake of cobalamin by ileal mucosa was significantly enhanced after prior binding to either gastric or pancreatic intrinsic factor compared with controls. Displacement of receptor-bound cobalamin with EDTA showed that enhanced total uptake reflected increased internalization of cobalamin by both proteins. Findings after reorienting sucrose density gradient centrifugation of ileal mucosa from loops containing intrinsic factor-cobalamin complexes were consistent with a major lysosomal and perhaps endosomal localization of internalized cobalamin, in agreement with results after oral administration of cobalamin. In marked contrast, cobalamin was recovered predominantly in the soluble fractions and was not associated with particulate subcellular organelles in ileal mucosa from control loops. These findings suggest that both gastric and pancreatic intrinsic factors can promote the physiological absorption of cobalamin by receptor-mediated endocytosis in the dog.