Kinetic analysis of Fe-59 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 mu mol Fe-59/l. When duodenal tissue load and mucosal-to-serosal transport had reached a steady state, perfusion was continued without luminal Fe-59 Supp ly. Mobilization of Fe-59 from the duodenal tissue into the serosally relea sed absorbate followed first-order rate kinetics, which permitted calculati on of the asymptotic maximum, the rate constant, and the initial mobilizati on rate for tissue-to-absorbate transfer. There was no evidence for adaptat ion of Fe-59 tissue binding in iron-deficient segments. Fe-59 tissue-to-abs orbate transfer increased in proportion to the mobilizable fraction of rece ntly absorbed iron in the tissue, which is indicative of simple diffusion o r carrier-mediated transport below saturation. Regulation of the mucosal up take step appears to determine the mobilizable Fe-59 fraction and thus the adaptation of the overall iron absorption process to the demand. Iran reten tion in the duodenal tissue and iron transfer from here into the body appea r not to be either regulated or rate limited.