Magnesium transport across colon ascendens of the rat.

Concentration and voltage dependence of unidirectional Mg fluxes across the rat colon ascendens were measured in a modified Ussing chamber. Mucosa (M) to serosa (S) Mg flux exhibits a cellular component, whereas SM flux is totally diffusive. At all the concentrations between 0.125 and 8 mmol/liter MS Mg transport is higher than the flux in the opposite direction, resulting in Mg absorption. In contrast to Mg, in Ca transport a cellular component is involved in both directions across the tissue, 1 alpha, 25-Dihydroxyvitamin D3 has no influence on the Mg transport. Mg (5 mmol/liter) remarkably decreases MS Ca flux and reduces Ca absorption by 70%. The parallel decrease in MS Ca flux with that of the simultaneously measured paracellular marker mannitol and the voltage clamp experiments reveal that Mg has no influence on cellular Ca transport but only reduces diffusive MS Ca flux, possibly by decreasing transepithelial fluid absorption. The experiments demonstrate that the colon ascendens of the rat is capable of absorbing Mg at rates comparable to that found for Ca. There is evidence that Mg and Ca are transported by separate cellular mechanisms. Diffusive movement across the paracellular route plays an important role on net transport of both earth alkali ions.