A animal model was developed whereby reticulocyte-rich blood was introduced into normal rats by exchange transfusion. Measurements of plasma iron turnover was made at similar plasma iron concentrations before and after exchange transfusions. High reticulocyte blood obtained from animals rendered iron deficient by diet or by treatment with phenylhydrazine resulted in a mean increase of 86% in internal iron exchange, while the plasma iron turnover was unaffected by exchange with normal red cells. Since iron input from reticuloendothelial cells could have increased due to breakdown of transfused cells, iron absorption was also measured. Within 1 hr and for a least 6 hr after exchange with high reticulocyte blood, mean absorption in six groups of animals was increased over control animals by 50%-130%. The increased plasma iron turnover and absorption was not mediated by a decrease in plasma iron or an increase in unsaturated iron-binding capacity. Indeed, a higher plasma iron and transferrin saturation augmented the movement of iron into the plasma from iron- donating tissues. It is proposed that the donation of iron by transferrin in some way immediately facilitates the procurement of more iron by transferrin.

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