CHARACTERIZATION OF MG2-BORDER MEMBRANE-VESICLES OF RABBIT ILEUM STUDIED WITH MAG-FURA-2( TRANSPORT IN BRUSH)

Mg2+ transport in rabbit ileal brush border membrane vesicles (BBMV) w as characterized by means of a modified mag-fura-2 technique. In the p resence of an i > o Na+ gradient, BBMV showed a saturable Mg2+ uptake with a K-m of 1.64 mmoll(-1). There was no evidence of an overshoot. K +, Li+, and choline(+) were as effective as Na+ in stimulating Mg2+ tr ansport. In contrast, only a small amount of Mg2+ transport was observ ed in the presence either of an o > i Na+ gradient, or in an Na+ equil ibrium or in the absence of Na+. Moreover, the findings that Na+ efflu x was not stimulated but inhibited by outside Mg2+ and that the nonflu orescent amiloride-analogues DMA and EIPA did not affect Mg2+ transpor t do not favour the idea of an Mg2+/Na+ antiport system. At Cl- equili brium, independent of the Na+ gradient, the rate of Mg2+ transport was markedly suppressed compared with the transport rate noted in the pre sence of an i > o Cl- gradient, The stimulating effect of inside anion s could be enhanced by SCN- and decreased by SO42-. Furthermore, nonfl uorescent anion transport antagonist H-2-DIDS stimulated Mg2+ transpor t. These findings indicate that Mg2+ transport can be modulated by ins ide anions. Mg2+ transport appeared to be electroneutral because it wa s not dependent on membrane potential. Mg2+ transport was neither stim ulated by Bay K8644, a Ca2+ channel agonist, nor inhibited by verapami l, diltiazem, nifedipine and imipramine, the Ca2+ channel antagonists. It, therefore, seems unlikely that Mg2+ uses the Ca2+ transport syste m. (C) 1998 Elsevier Science B.V.