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1 Department of Veterinary
Physiology,
The potential difference (PD)-dependent
component of transcellular Mg2+
uptake in sheep rumen epithelium was studied. Unidirectional 28Mg2+
fluxes were measured at various transepithelial PD values, and the
unidirectional mucosal-to-serosal
28Mg2+
flux (JMgms) was
correlated with the PD across the apical membrane
(PDa) determined by mucosal
impalement with microelectrodes.
PDa was found to be 
54 ± 5 mV, and JMgms was 65.9 ± 13.8 nmol · cm
2 · h1
under short-circuit conditions. Hyperpolarization of the ruminal epithelium (blood-side positive) depolarized
PDa and, most noticeably, decreased JMgms. Further experiments were
performed with cultured ruminal epithelial cells (REC). With the aid of the fluorescence probe mag-fura 2, we measured the intracellular free
Mg2+ concentration
([Mg2+]i)
of isolated REC under basal conditions at various extracellular Mg2+ concentrations
([Mg2+]e)
and after alterations of the transmembrane voltage. Basal [Mg2+]i
was 0.54 ± 0.08 mM. REC suspended in media with
[Mg2+]e
between 0.5 and 7.5 mM showed an increase in
[Mg2+]i
that was dependent on
[Mg2+]e
and that exhibited a saturable component (Michaelis-Menten constant = 1.2 mM; maximum
[Mg2+]i = 1.26 mM). Membrane depolarization with high extracellular K+ (40, 80, or 140 mM
K+) and the
K+ channel blocker quinidine (50 and 100 µM ) resulted in a decrease in
[Mg2+]i.
On the other hand, hyperpolarization created by
K+ diffusion (intracellular
K+ concentration > extracellular
K+ concentration) in the presence
of valinomycin induced a 15% increase in
[Mg2+]i.
None of the manipulations had any effect on intracellular Ca2+ concentration and
intracellular pH. The results support the assumption that the membrane
potential acts as a principal driving force for
Mg2+ entry in REC and suggest that
the pathway for this electrodiffusive Mg2+ uptake across the luminal
membrane is a channel or a carrier.
sheep rumen; epithelial cells; magnesium transport; intracellular magnesium; mag-fura 2
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