Transport kinetics were examined for uptake of 65Zn from the lumen and for transport of mucosal 65Zn subsequent to uptake in the isolated, vascularly perfused intestines of rats fed either a zinc-deficient or zinc-adequate diet. Zinc depletion influenced the intestinal transport of zinc by stimulating a saturable uptake mechanism, reducing secretion of mucosal 65Zn into the lumen, and increasing the rate of 65Zn turnover in a rapidly absorbed mucosal zinc compartment. Uptake of 65Zn involved both saturable and nonsaturable processes. The saturable process was stimulated by zinc depletion with the apparent maximal transport rate for the saturable mechanism increasing from 60 to 180 nmol Zn X g-1 X 30 min-1. Most of the 65Zn taken up was not involved in the short-term secretion or absorption, and mucosal 65Zn retention was independent of dietary zinc status. Absorption of mucosal 65Zn was nonsaturable, involved a rapid exchanging zinc compartment, and was stimulated by zinc depletion. The half-life for 65Zn in this mucosal zinc compartment was approximately 24 min in the zinc-adequate group and 13 min in the zinc-depleted group.
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Intestinal 65Zn transport and metallothionein levels were examined in rats fed zinc-adequate and zinc-deficient diets and in rats subjected to an overnight fast. 65Zn uptake by intestines perfused with 1.5 microM 65Zn was greater in both zinc-deficient and fasted groups than in the control group. Mucosal retention of 65Zn was also greater in the zinc-deficient group but not in the fasted group. The greater 65Zn uptake in the fasted group was associated with a compartment that readily released 65Zn back into the lumen. Kinetic analysis of the rate of 65Zn transfer to the vascular space (absorption) showed that 65Zn absorption involved approximately 3% of mucosal 65Zn in a 40-min perfusion period. The half-life (t1/2) of this mucosal 65Zn rapid transport pool corresponded directly to changes in intestinal metallothionein levels. Both metallothionein and t1/2 were higher in the fasted group and lower in the zinc-deficient group than in controls. While the rate of 65Zn transport from this rapid transport pool decreased with increasing metallothionein level, the predicted pool size increased when the metallothionein level was elevated by fasting. These results indicate that the rate of zinc absorption is inversely related to intestinal metallothionein levels, but the portion of mucosal 65Zn available for absorption is directly related to intestinal metallothionein.
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