Passage of iron out of the intestinal mucosa of the rat.

To determine the effect of removal of the "lumen-to-mucosa" concentration gradient on the passage of iron (Fe) out of the intestine, duodenal loops of rats with loaded (FeL) or deficient (FeD) iron stores were perfused for 30 min with 5.0 mM solutions containing 59Fe. The test solutions were then removed and in vivo perfusion of the washed mucosa was continued for up to 60 min with saline or unlabelled FE-saline. Despite the greater initial uptake of Fe by FeD than FeL, continued perfusion with saline was associated with the appearance of similar quantities of 59Fe in the perfusate; in contrast, when perfusion was continued with Fe-saline, the loss of 59Fe into the perfusate was increased and was twice as great in FeD as in FeL. Iron transfer to the carcass was higher in FeD than in FeL, but continued perfusion with saline was associated with the appearance of increased amounts of 59Fe in the carcass of FeL but not FeD; the amount of 59Fe appearing in the carcass was greatly increased in both FeD and FeL by continued perfusion with Fe-saline. Thus, the amount of 59Fe available for entry into the intestinal lumen and carcass after removal of the lumen-to-mucosa concentration gradient is influenced by the size of the body iron stores and by the presence of iron in the fluid remaining in the lumen. Therefore, the amount of iron in the fluid remaining in the intestinal lumen after the administration of the Fe test dose must be carefully defined when estimating the kinetics of the intestinal absorption of iron.