Different mechanism of magnesium and calcium transport across rat duodenum

Abstract

Concentration and voltage dependence of Mg transport across the rat duodenum was measured in an Ussing chamber. Mucosa (m) to serosa (s) Mg flux exhibits a cellular fraction comparable to that found for Ca. Mg sm flux is purely diffusive and probably restricted to the paracellular pathway. At all concentrations between 0.5 and 5 mmol/liter, Mg is secreted. Diffuse sm Mg flux is 3.5 times higher than the diffusive component of ms Mg transport. This prevalence of diffusive sm Mg flux over that from mucosa to serosa, which may be explained by an "anomalous solvent drag effect," is responsible for the Mg secretion observed. Mg 5 mmol/liter decreases ms Ca flux and abolishes Ca absorption. The voltage clamp experiments reveal that Mg has no effect on the cellular transport but only decreases diffusive ms Ca flux. 1 alpha,25-dihydroxyvitamin D3 has only a small effect on cellular Mg transport but remarkably stimulates ms Ca flux and increases Ca absorption by about 85%. Dexamethasone increases ms Mg flux but decreases ms Ca transport and hereby abolished Mg secretion or Ca absorption. In conclusion, Mg is secreted across the short-circuited duodenum whereas Ca is absorbed. There is evidence that both earth alkalines are transported by distinct cellular mechanisms. The data also demonstrate that diffusive movement across the paracellular pathway plays an important role on net Mg transport.

Molecular Cloning and Characterization of a Channel-like Transporter Mediating Intestinal Calcium Absorption

“…45 Ca 2ϩ uptake was assayed 3 days after injection of poly(A) ϩ or 1-3 days after injection of synthetic cRNA. For expression cloning, oocytes were incubated in modified Barth's solution (19) supplemented with 1 mM SrCl 2 (to avoid excessive loading of oocytes with Ca 2ϩ ) as well as penicillin, streptomycin, and gentamycin at 1 mg/ml.…”

“…For expression cloning, oocytes were incubated in modified Barth's solution (19) supplemented with 1 mM SrCl 2 (to avoid excessive loading of oocytes with Ca 2ϩ ) as well as penicillin, streptomycin, and gentamycin at 1 mg/ml. Standard uptake solution contained the following components: 100 mM NaCl, 2 mM KCl, 1 mM MgCl 2 , 1 mM CaCl 2 (including 45 Ca 2ϩ , NEN Life Science Products), and 10 mM Hepes, pH 7.5. Uptake was performed at room temperature for 30 min (for the expression cloning procedure, 2-h uptakes were employed), and oocytes were washed six times with ice-cold uptake solution plus 20 mM MgCl 2 .…”

“…Briefly, duodenal poly(A) ϩ RNA from rats fed a calcium-deficient diet (ICN Pharmaceuticals, Inc., Costa Mesa, CA) for 2 weeks was size-fractionated. A cDNA library was then constructed from the fractions of 2.5-3 kilobases (kb) 1 that stimulated 45 Ca 2ϩ uptake activity when expressed in oocytes. The RNAs synthesized in vitro from pools of ϳ500 clones were injected into oocytes, and the abilities of the pools to stimulate Ca 2ϩ uptake were assayed.…”

“…Steady-state currents were obtained as the average values in the interval from 135 to 145 ms after the initiation of the voltage pulses. For experiments involving voltage-clamped 45 …”

“…To achieve this goal, we took advantage of an expression cloning strategy by using Xenopus laevis oocytes as the expression system (19). Functional screening of a rat duodenal library by measuring 45 Ca 2ϩ uptake resulted in the isolation of a cDNA clone encoding a calcium transport protein (CaT1). Interestingly, CaT1 is structurally related to the recently reported renal apical calcium channel ECaC (20 CaT1 plays a key role in mediating Ca 2ϩ entry into the enterocytes as the first step of transcellular intestinal calcium absorption.…”