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Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4970
The plasma
membrane of mammalian cells possesses rapid
Mg2+ transport mechanisms. The
identity of Mg2+ transporters is
unknown, and so are their properties. In this study,
Mg2+ transporters were
characterized using a biochemically and morphologically standardized
preparation of sealed rat liver plasma membranes (LPM) whose
intravesicular content could be set and controlled. The system has the
advantages that it is not regulated by intracellular signaling
machinery and that the intravesicular ion milieu can be designed. The
results indicate that 1) LPM retain
trapped intravesicular total Mg2+
with negligible leak; 2) the
addition of Na+ or
Ca2+ induces a concentration- and
temperature-dependent efflux corresponding to 30-50% of the
intravesicular Mg2+;
3) the rate of flux is very rapid
(137.6 and 86.8 nmol total Mg2+ · µm
2 · min1
after Na+ and
Ca2+ addition, respectively);
4) coaddition of maximal
concentrations of Na+ and
Ca2+ induces an additive
Mg2+ efflux;
5) both
Na+- and
Ca2+-stimulated
Mg2+ effluxes are inhibited by
amiloride, imipramine, or quinidine but not by vanadate or
Ca2+ channel blockers;
6) extracellular
Na+ or
Ca2+ can stimulate
Mg2+ efflux in the absence of
Mg2+ gradients; and
7)
Mg2+ uptake occurs in LPM loaded
with Na+ but not with
Ca2+, thus indicating that
Na+/Mg2+
but not
Ca2+/Mg2+
exchange is reversible. These data are consistent with the operation of
two distinct Mg2+ transport
mechanisms and provide new information on rates of Mg2+ transport, specificity of the
cotransported ions, and reversibility of the transport.
sodium/magnesium antiporter; calcium/magnesium antiporter; hepatocytes
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