Different mechanism of magnesium and calcium transport across rat duodenum.

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.