Kinetic analysis of 59 Fe movement across the intestinal wall in duodenal rat segments ex vivo

Duodenal segments from iron-deficient and iron-adequate rats were luminally perfused ex vivo with solutions containing 1, 10, 50, 100, 200 and 500 micromol 59Fe/l. When duodenal tissue load and mucosal-to-serosal transport had reached a steady state, perfusion was continued without luminal 59Fe supply. Mobilization of 59Fe from the duodenal tissue into the serosally released absorbate followed first-order rate kinetics, which permitted calculation of the asymptotic maximum, the rate constant, and the initial mobilization rate for tissue-to-absorbate transfer. There was no evidence for adaptation of 59Fe tissue binding in iron-deficient segments. 59Fe tissue-to-absorbate transfer increased in proportion to the mobilizable fraction of recently absorbed iron in the tissue, which is indicative of simple diffusion or carrier-mediated transport below saturation. Regulation of the mucosal uptake step appears to determine the mobilizable 59Fe fraction and thus the adaptation of the overall iron absorption process to the demand. Iron retention in the duodenal tissue and iron transfer from here into the body appear not to be either regulated or rate limited.