Characterization of two mechanisms of (28)Mg uptake in rat jejunal brush border membrane vesicles.

Uptakes of (28)Mg at 10 s were measured at 0.1 and 1mM MgCl(2), to mainly represent one or other of the two uptake mechanisms earlier shown to be present in rat jejunal brush border membrane vesicles, one with an apparent KT of 0.2 mM, the other in the millimolar range. Both mechanisms had an optimal temperature close to 28 degrees C, inactivation at 37 degrees C being more acute for the low affinity mechanism (55 percent, P < 0.01). Both mechanisms were equally stimulated by an electrical potential difference (negative inside the vesicles) imposed by a potassium gradient and not affected by the nature of the anion accompanying magnesium. At 0.1 mM MgCl(2), the uptake was increased by an outwardly directed proton gradient, pH 8.2 outside and 7.4 inside (38 percent, P < 0.05), but not depressed when the gradient was in the opposite direction, pH 6.6 outside and 7.4 inside. It was trans-stimulated by magnesium, strongly inhibited by amiloride and to a smaller extent by furosemide, but uninfluenced by 0.1 mM NaCl or by 100 mM NaCl, NaSCN or KCl. A slight but significant inhibition (20-30 per cent) was recorded in the presence of 0.1 mM CoCl(2), NlCl(2) or BaCl(2). At 1 mM MgCl(2), the uptake was not influenced by pH gradient, was not trans-stimulated by Mg and was not affected by furosemide. A 40 percent inhibition by amiloride was, however, recorded. Also 100 mM NaCl or KCl doubled the uptake, while 1 mM NaCl or 100 mM NaSCN did not affect it. In contrast, all the divalent cations tested produced an inhibition (from 60 to 12 percent) in the following order: Co > or = Mn > Ca > or = Ni> Ba > Sr, when used at the same concentration as magnesium. The results showed that cobalt and calcium were not true competitors. In conclusion, two distinct mechanisms would operate magnesium entry at the brush border: (1) an electrogenic high affinity Mg/Mg,H exchange, sensitive to amiloride and furosemide, and (2) an electrogenic low affinity mechanism, inhibited by the presence of several divalent cations and dependent on the presence or activity of alkaline phosphatase.