AJP - Gastrointestinal and Liver Physiology, Vol 259, Issue 5 720-G726, Copyright © 1990 by American Physiological Society
Comparison of calcium and magnesium absorption: in vivo and in vitro studies
L. L. Hardwick, M. R. Jones, R. K. Buddington, R. A. Clemens and D. B. Lee
Division of Nephrology, Veterans Administration Medical Center, Sepulveda, California 91343.
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.
