Characterization of Mg 2+ transport in brush border membrane vesicles of rabbit ileum studied with mag-fura-2

Abstract

Mg 2+ transport in rabbit ileal brush border membrane vesicles (BBMV) was characterized by means of a modified mag-fura-2 technique. In the presence of an i> o Na + gradient, BBMV showed a saturable Mg 2+ uptake with a K m of 1.64 mmol l −1. There was no evidence of an overshoot. K +, Li +, and choline + were as effective as Na + in stimulating Mg 2+ transport. In contrast, only a small amount of Mg 2+ transport was observed in the presence either of an o> i Na + gradient, or in an Na + equilibrium or in the absence of Na +. Moreover, the findings that Na + efflux was not stimulated but inhibited by outside Mg 2+ and that the nonfluorescent amiloride-analogues DMA and EIPA did not affect Mg 2+ transport do not favour the idea of an Mg 2+/Na + antiport system. At Cl − equilibrium, independent of the Na + gradient, the rate of Mg 2+ transport was markedly suppressed compared with the transport rate noted in the presence of an i> o Cl − gradient. The stimulating effect of inside anions could be enhanced by SCN − and decreased by SO 2− 4. Furthermore, nonfluorescent anion transport antagonist H 2-DIDS stimulated Mg 2+ transport. These findings indicate that Mg 2+ transport can be modulated by inside anions. Mg 2+ transport appeared to be electroneutral because it was not dependent on membrane potential. Mg 2+ transport was neither stimulated by Bay K8644, a Ca 2+ channel agonist, nor inhibited by verapamil, diltiazem, nifedipine and imipramine, the Ca 2+ channel antagonists. It, therefore, seems unlikely that Mg 2+ uses the Ca 2+ transport system.