The role of plasma transferrin in iron absorption in the rat.

The question of whether plasma transferrin acts as a carrier of iron from intestinal mucosal cells to the blood plasma was investigated in a series of experiments in the tat. By thoracic duct cannulation and collection of lymph during absorption it was confirmed that nearly all absorbed iron passes directly to portal blood. The total rate of transcapillary exchange of transferrin and the rate of passage of plasma transferrin into the extravascular compartment of the intestinal mucosa were then measured in separate groups of rats. The rate of exchange were insufficient for transferrin to act as the carrier of iron at the rate at which it passes from intestinal cells to portal plasma after ingesting a dose of inorganic iron. Some specific binding of transferrin and cellular uptake of iron by intestinal mucosa was observed after the intravenous injection of transferrin labelled with 59Fe and 125I. It is concluded from these experiments that absorbed iron passes from intestinal cells to portal plasma largely in a low molecular weight form. The passage of plasma transferrin into the interstitial fluid of the intestinal mucosa and binding to mucosal cells probably functions primarily to supply iron to the cells and not to act as a carrier of iron from the cells.