Pathophysiology 18 (2011) 207–213
Molecular origin of Na
+
/Li
+
exchanger: Evidence against the involvement
of major cloned erythrocyte transporters
Svetlana V. Koltsova
a,b
, Yulia A. Trushina
c
, Olga A. Akimova
a,c
,
Pavel Hamet
a
, Sergei N. Orlov
a,b,c,∗
a
Research Centre, Centre hospitalier de l’Université de Montréal (CHUM) – Technopôle Angus, Montreal, PQ, Canada
b
Institute of General Pathology and Pathophysiology, Russian Academy of Medical Sciences, Moscow, Russia
c
Faculty of Biology, M.V. LomonosovMoscow State University, Russia
Received 1 September 2010; received in revised form 1 December 2010; accepted 1 December 2010
Abstract
Numerous studies have demonstrated heightened Na
+
/Li
+
countertransport (NLCT) activity in erythrocytes of patients with essential
hypertension or diabetic nephropathy. The same carrier also contributes to the therapeutic action of lithium salt, widely used in the treatment
of psychiatric disorders. However, the molecular origin of NLCT remains unknown. This study examined the role of major ion transporters in
NLCT by comparative analysis of its activity and that of ion transporters providing inwardly directed
86
Rb,
22
Na and
32
P fluxes. NLCT was
below the detection limit in rat erythrocytes and ∼50-fold higher in rabbits compared to humans. Unlike NLCT, the activities of Na
+
,K
+
-ATPase,
Na
+
,K
+
,2Cl
-
cotransporter and anion exchanger were somewhat similar in the erythrocytes of these species, whereas Na
+
,P
i
cotransport was
in 1:2:6 proportion in rats, humans and rabbits, respectively. Loading of erythrocytes with Li
+
for NLCT measurement did not affect the
activity of Na
+
,P
i
cotransporter. Keeping in mind that NLCT is much higher in rabbits vs humans and rats, we compared the set of membrane
proteins in these species using 2-dimensional gel electrophoresis. This approach revealed 174 common spots, whereas 132 proteins were
detected only in human and rabbit erythrocyte membranes. Among these proteins, we found 17 spots whose expression was higher by more
than 5-fold in rabbit compared to human erythrocytes. Thus, our results argue against the involvement of major ion transporters in NLCT.
They also show that comparative proteomics is a potent tool to identify the molecular origin of this carrier.
© 2011 Elsevier Ireland Ltd. All rights reserved.
Keywords: Na
+
/Li
+
countertransport; Na
+
,P
i
cotransport; Na
+
/Na
+
exchange; Na
+
/H
+
exchange; 2-Dimensional gel electrophoresis
1. Introduction
Since initial observations of altered monovalent ion trans-
port in erythrocytes from rats with spontaneous hypertension
[1–3], these cells have been widely employed for functional
characterisation in primary (essential) hypertension. Both
augmented baseline Na
+
/Na
+
exchange, measured as the
rate of
23
Na
+
o
/
22
Na
+
i
turnover [4] or Na
+
o
/Li
+
i
exchange
(Na
+
/Li
+
countertransport – NLCT) [5], abnormal kinetic
properties of Na
+
,K
+
,2Cl
-
cotransport [6], and increased
Na
+
/H
+
exchange triggered by artificially created electro-
∗
Corresponding author at: Centre de recherche, CHUM – Technopôle
Angus, 2901 Rachel est, Montreal, Quebec H1W 4A4, Canada.
Tel.: +1 514 890 8000x23615; fax: +1 514 412 7655.
E-mail address: sergei.n.orlov@umontreal.ca (S.N. Orlov).
chemical proton gradient (μ
H+
) [7] have been reported
in Caucasians with essential hypertension (for review, see
[8–15]).
In contrast to Na
+
,K
+
,2Cl
-
cotransport and Na
+
/H
+
exchange, the molecular origin of NLCT, discovered almost
50 years ago in human erythrocytes as Na
+
o
-dependent Li
+
efflux [16], remains unknown. This issue is important for
several reasons. (i) Side-by-side with essential hypertension,
increased maximal activity of NLCT and its affinity for Na
+
o
have been documented in patients with insulin-dependent
diabetes mellitus or diabetic nephropathy [12]. In patients
with essential hypertension, the activity of this carrier is
positively correlated with insulin resistance [17]. (ii) Studies
performed in French-Canadian siblings affected with
essential hypertension and dyslipidaemia have disclosed
that NLCT is under gender-dependent control of inheritable
0928-4680/$ – see front matter © 2011 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.pathophys.2010.12.001