Competitive nature of the intestinal transport mechanism for cobalt and iron in the rat.
Dose- and time-response studies were performed in iron-loaded and iron-deficient rats in order to define, (a) the kinetics of absorption of cobalt and iron, (b) the nature of the inhibitory effect of one metal on the absorption of the other, and (c) the effect of variations in body iron stores on these processes. The duodenum was perfused for 5-90 min with labeled solutions containing 5.0 mM iron or 5.0 mM cobalt. In iron-loaded rats, the rate of cobalt absorption was constant for 90 min whereas the rate of iron absorption fell after 30 min. In comparison to these results, the rate of absorption of both metals was increased in iron deficiency, and was more rapid in the first 30 min than in the 30-90 min period.To determine the response to varying doses of metal, we perfused duodenal loops for 30 min with 0.1-10.0 mM solutions of either iron or cobalt. In both iron-loaded and iron-deficient groups, a greater proportion of the metals was absorbed from smaller than from larger doses. When iron and cobalt were perfused together in iron-deficient animals, cobalt competitively inhibited iron absorption, and conversely, iron reduced cobalt absorption. The apparent maximum transport velocity was similar for both metals, but the affinity for cobalt was greater than iron. The results suggest that the absorption of cobalt and iron is mediated by a transport system in which two processes operate simultaneously; the first is limited largely by the concentration of available metal in the lumen of the intestine, whereas the second process depends upon the activity of a mechanism which displays saturation kinetics and competitive inhibition. The former process prevails when iron stores are replete, whereas the latter predominates when there is a need for iron, such as in iron deficiency.