Transport characteristics of Ca and Mg were compared at three different levels of the absorptive process in male (120-170 g) Wistar rats. Balance studies in intact rats revealed that fractional Ca absorption decreased with increased Ca intake so that net Ca absorption remained constant. Fractional Mg absorption decreased modestly with increased Mg intake so that net Mg absorption increased proportionately with increased dietary Mg. Everted duodenal sacs demonstrated the presence of active Ca absorption with serosal-to-mucosal (S/M) ratio of 2.65 +/- 0.20 (n = 6), which was greater than unity (P less than 0.001). In contrast, the S/M for Mg did not exceed unity. Mucosal duodenal Ca uptake exhibited a large saturable (Michaelis constant of 4.80 +/- 0.34 mM, maximal velocity of 4.71 +/- 0.13 nmol.min-1.mg-1) and a small nonsaturable component (0.12 +/- 0.01 nmol.min-1.mg-1) in 5-wk-old rats (120 g). In 72-wk-old rats (600 g) the diffusional component of Ca uptake became predominant, and the slope increased significantly to 0.32 +/- 0.01 (P less than 0.05). Duodenal Mg uptake was completely concentration dependent and exhibited no age-related changes.
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