Characterization of Mg2+ transport in brush border membrane vesicles of rabbit ileum studied with mag-fura-2.
Mg2+ 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 Mg2+ uptake with a Km of 1.64 mmol l-1. There was no evidence of an overshoot. K+, Li+, and choline+ were as effective as Na+ in stimulating Mg2+ transport. In contrast, only a small amount of Mg2+ 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 Mg2+ and that the nonfluorescent amiloride-analogues DMA and EIPA did not affect Mg2+ transport do not favour the idea of an Mg2+/Na+ antiport system. At Cl- equilibrium, independent of the Na+ gradient, the rate of Mg2+ 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 SO2-4. Furthermore, nonfluorescent anion transport antagonist H2-DIDS stimulated Mg2+ transport. These findings indicate that Mg2+ transport can be modulated by inside anions. Mg2+ transport appeared to be electroneutral because it was not dependent on membrane potential. Mg2+ transport was neither stimulated by Bay K8644, a Ca2+ channel agonist, nor inhibited by verapamil, diltiazem, nifedipine and imipramine, the Ca2+ channel antagonists. It, therefore, seems unlikely that Mg2+ uses the Ca2+ transport system.
Jüttner
R
R
Institut für Klinische Physiologie, Klinikum Benjamin Franklin, Freie Universität Berlin, Hindenburgdamm 30, 12200 Berlin, Germany.
Ebel
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H
D000818:Animals; D000838:Anions / Q000378:metabolism / Q000494:pharmacology; D017920:Antiporters / Q000378:metabolism; D002120:Calcium Channel Agonists / Q000494:pharmacology; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives* / Q000378:metabolism; D007082:Ileum / Q000378:metabolism* / Q000648:ultrastructure*; D007413:Intestinal Mucosa / Q000378:metabolism* / Q000648:ultrastructure*; D017136:Ion Transport / Q000187:drug effects; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D008871:Microvilli / Q000378:metabolism; D011817:Rabbits; D012965:Sodium Chloride / Q000378:metabolism
D000838:Anions; D017920:Antiporters; D002120:Calcium Channel Agonists; D005456:Fluorescent Dyes; C046250:sodium-magnesium antiporter; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D012965:Sodium Chloride; D008274:Magnesium; D016257:Fura-2