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3
cotransporter, rkNBC, expressed in oocytes
Departments of 1 Physiology and Biophysics and 2 Pharmacology, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106-4970
Recently, we
reported the cloning and expression of the rat renal electrogenic
Na+-HCO
3
cotransporter (rkNBC) in Xenopus
oocytes [M. F. Romero, P. Fong, U. V. Berger, M. A. Hediger, and
W. F. Boron. Am. J. Physiol. 274 (Renal Physiol. 43): F425-F432,
1998]. Thus far, all NBC cDNAs are at least 95% homologous.
Additionally, when expressed in oocytes the NBCs are
1) electrogenic,
2)
Na+ dependent,
3)
HCO3 dependent, and
4) inhibited by stilbenes such as
DIDS. The apparent
HCO3:Na+
coupling ratio ranges from 3:1 in kidney to 2:1 in pancreas and brain
to 1:1 in the heart. This study investigates the cation and voltage
dependence of rkNBC expressed in
Xenopus oocytes to better understand
NBC's apparent tissue-specific physiology. Using two-electrode voltage clamp, we studied the cation specificity, Na+ dependence, and the
current-voltage
(I-V)
profile of rkNBC. These experiments indicate that
K+ and choline do not stimulate
HCO3-sensitive currents via rkNBC, and
Li+ elicits only 3 ± 2% of
the total Na+ current. The
Na+ dose response studies show
that the apparent affinity of rkNBC for extracellular
Na+ (~30 mM
[Na+]o)
is voltage and HCO3 independent,
whereas the rkNBC
I-V
relationship is Na+ dependent. At
[Na+]o
vmax (96 mM), the
I-V
response is approximately linear; both inward and outward
Na+-HCO3
cotransport are observed. In contrast, only outward cotransport occurs
at low
[Na+]o
(<1 mM
[Na+]o).
All rkNBC currents are inhibited by extracellular application of DIDS,
independent of voltage and
[Na+]o.
Using ion-selective microelectrodes, we monitored intracellular pH and
Na+ activity. We then calculated
intracellular [HCO3] and,
with the observed reversal potentials, calculated the stoichiometry of
rkNBC over a range of
[Na+]o
values from 10 to 96 mM at 10 and 33 mM
[HCO3]o. rkNBC stoichiometry is 2 HCO3:1
Na+ over this entire
Na+ range at both
HCO3 concentrations. Our results indicate that rkNBC is highly selective for
Na+, with transport direction and
magnitude sensitive to
[Na+]o
as well as membrane potential. Since the rkNBC protein alone in oocytes
exhibits a stoichiometry of less than the 3 HCO3:1 Na+ thought necessary for
HCO3 reabsorption by the renal
proximal tubule, a control mechanism or signal that alters its in vivo
function is hypothesized.
sodium/bicarbonate cotransport; NBC; Xenopus oocyte expression; intracellular pH; sodium transport; bicarbonate transport; kinetics; voltage clamp
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