In Vitro Studies on the Metabolic Energy Requirements of Absorption of Zinc, Copper, Manganese and Iron in Rat
Trace mineral elements are either absorbed in the different segments of the intestine by the saturable non-linear transport at lower physiological concentrations or by simple linear diffusion at higher concentrations. The saturable transport that occurs through the action of specific transporters represents both facilitated diffusion and ATP-driven active transport. In this study, inverted duodenal sacs were prepared and filled with 0.15M Tris-Krebs buffer, pH 7.3 free from zinc and whose metabolic energy requirement of absorption was to be investigated until slightly distended. Sacs were incubated for 30 minutes at 37oC with continuous aeration in media containing the buffer and 2.0mM Zn as zinc acetate. The effects on zinc uptake produced by the addition of different metabolic inhibitors singly to the incubation media were investigated. In other experiments, duodenal sacs were filled with Cu, Mn or Fe-free Tris-Krebs buffer and incubated as for the first experiment except that the effect of only NaN3 was tested on Cu, Mn and iron absorption. NaCN, NaN3 and 2,4-DNP significantly enhanced the uptake of Zn into the duodenal sacs relative to the control sacs. NaF tended to produce a contrasting effect although it was not statistically significant. NaN3 had no significant effect on the uptake of Cu and Fe while the uptake of Mn was found to increase by the presence of the inhibitor in the incubation medium. The results suggested that the absorption of the metals was by passive process under the conditions used. Alternatively, energy may be required to keep out Zn and Mn either by inhibiting uptake or promoting removal into the lumen in in vivo situation.
KEYWORDS: Absorption, trace minerals, metabolic inhibitors, duodenal gut sac.