Uptake of (28)Mg by duodenal and jejunal brush border membrane vesicles in the rat.
Mg uptake was investigated with (28)Mg by a rapid filtration procedure in rat duodenal and jejunal brush border membrane (BBM) vesicles, prepared by CaCl(2)a or MgCl(2)b differential precipitation. At 1 mM Mg, 10 s uptakes were lower in jejunal vesicles (3.5(a) or 5.5(b) nmol/10 s per mg protein) than in duodenal vesicles (11.4(a) or 13.5(b) nmol/10 s per mg protein). The equilibrated 60 min uptakes were also lower in jejunum (11.0(a) or 26.6(b) nmol/60 min per mg protein) than in duodenum (l8.8(a) or 26.6(b) nmol/60 min per mg protein). The influence of medium osmolarity on 10s and 60 min uptakes of Mg indicated that Mg was 'transported' into osmotically active spaces. The effect of Mg concentration on the 10 s uptake suggested the existence of one single mechanism of transport in the duodenum, with an apparent K(T) of 1 mM, and of two mechanisms in the jejunum, with apparent K(T) values of 0.2 and 2-5 mM. Despite different amounts of calcium and magnesium in CaCl(2) and MgCl(2) precipitated vesicles, there were no large differences in magnesium uptakes depending on the mode of preparation of the vesicles. In contrast, calcium uptakes. measured with (45)Ca, were six to nine times higher in MgCl(2) prepared jejunal vesicles, and were always much higher than magnesium uptakes measured under the same conditions. At 0.1 mM calcium concentration, calcium uptake was depressed by 0.025 mM verapamil (50 percent) and by 0.1 mM ZnCl(2)(40-75 percent), while Mg uptakes were unaffected. L-leucine or L-phenylalanine (5 mM), two inhibitors of intestinal alkaline phosphatase, decreased Mg uptake by 30 to 40 percent at 1 mM Mg, but had no significant effect at 0.1 mM, and did not affect calcium uptakes at all. A possible involvement of alkaline phosphatase in magnesium uptake was ascertained in jejunal BBM vesicles treated with phosphatidylinositol-specific phospholipase C, which partially released alkaline phosphatase from the BBM. Calcium uptakes were unaffected by the treatment, while magnesium uptakes were significantly decreased at 1 mM Mg. These results confirm that magnesium and calcium are transported by distinct mechanisms in the jejunum.
Baillien
M
M
Laboratory of General and Comparative Biochemistry, University of Liège, Belgium.
Wang
H
H
Cogneau
M
M
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000493:pharmacokinetics; D004386:Duodenum / Q000378:metabolism* / Q000648:ultrastructure; D007583:Jejunum / Q000378:metabolism* / Q000648:ultrastructure; D008274:Magnesium / Q000493:pharmacokinetics*; D015636:Magnesium Chloride / Q000494:pharmacology; D008297:Male; D008871:Microvilli / Q000378:metabolism; D009994:Osmolar Concentration; D010042:Ouabain / Q000494:pharmacology; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D054801:Phosphoinositide Phospholipase C; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013806:Theophylline / Q000494:pharmacology
D015636:Magnesium Chloride; D010042:Ouabain; D013806:Theophylline; D000469:Alkaline Phosphatase; D010727:Phosphoric Diester Hydrolases; D054801:Phosphoinositide Phospholipase C; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D008274:Magnesium; D002118:Calcium