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J Biol Chem, Vol. 275, Issue 6, 3772-3780, February 11, 2000
From the Department of Physiology and Biophysics, School of
Medicine, Case Western Reserve University,
Cleveland, Ohio 44106-4970
Upon activation of specific cell signaling,
hepatocytes rapidly accumulate or release an amount of
Mg2+ equivalent to 10% of their total
Mg2+ content. Although it is widely accepted that
Mg2+ efflux is Na+-dependent,
little is known about transporter identity and the overall regulation.
Even less is known about the mechanism of cellular Mg2+
uptake. Using sealed and right-sided rat liver plasma membrane vesicles
representing either the basolateral (bLPM) or apical (aLPM) domain, it
was possible to dissect three different Mg2+ transport
mechanisms based upon specific inhibition, localization within the
plasma membrane, and directionality. The bLPM possesses only one
Mg2+ transporter, which is strictly
Na+-dependent, bi-directional, and not
inhibited by amiloride. The aLPM possesses two separate
Mg2+ transporters. One, similar to that in the bLPM because
it strictly depends on Na+ transport, and it can be
differentiated from that of the bLPM because it is unidirectional and
fully inhibited by amiloride. The second is a novel
Ca2+/Mg2+ exchanger that is unidirectional and
inhibited by amiloride and imipramine. Hence, the bLPM transporter may
be responsible for the exchange of Mg2+ between hepatocytes
and plasma, and vice versa, shown in livers upon specific metabolic
stimulation, whereas the aLPM transporters can only extrude
Mg2+ into the biliary tract. The dissection of these three
distinct pathways and, therefore, the opportunity to study each
individually will greatly facilitate further characterization of these
transporters and a better understanding of Mg2+ homeostasis.
Differential Localization and Operation of Distinct
Mg2+ Transporters in Apical and Basolateral Sides of
Rat Liver Plasma Membrane*
*
This work was supported by National Institutes of Health
Grant HL 18708.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence and reprint requests should be addressed:
Dept. Physiology and Biophysics, School of Medicine, Case Western
Reserve University, 10900 Euclid Ave., Cleveland, OH 44106-4970. Tel.:
(216) 368-3400; Fax: (216) 368-3952; E-mail: axs15@po.cwru.edu.
Copyright © 2000 by The American Society for Biochemistry and Molecular Biology, Inc.
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