Editorial Commentary
The Amiloride-Sensitive Endothelial Sodium Channel
and Vascular Tone
David G. Warnock
See related article, pp 1053–1059
I
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In the context of the current work, cortical stiffness is
defined as the force needed to indent the individual cell cortex
(eg, apical surface) for a fixed distance, and hence directly
reflects the mechanical rigidity of the submembraneous region.
The actin cystoskeleton is assumed to organize this subapical
structure; dynamic changes in the organization and expression
of the actin cytoskeleton directly regulates the submembraneous rigidity. The role of ionized calcium, nitric oxide, and
other vasoactive agents like angiotensin II in regulating cortical stiffness,3 and the responsivity to various classes of antihypertensive medications are obvious areas for future inquiry.
An important role of aldosterone in regulating cortical
stiffness, and the use of knock-down and gain-of-function
cellular models strongly supports the critical role of EnNaC
in maintaining vascular tone at the individual endothelial
cellular level. The current findings take on added relevance
with the recent description of vascular endothelial mineralocorticoid receptors, and the appreciation of the importance
of local renin-angiotension-aldosterone system in regulating
vascular tone.8 The parallel evolutionary significance of the
development of α and β subunits of the sodium channel and
the sodium-potassium ATPase has recently been reviewed.7
With elucidation of the autosomal dominant monogenic
gain-of-function defect in Liddle syndrome,6 the phenotypic
extreme of low-renin hypertension was defined,9 an example
par excellence of the Guytonian view of maintaining salt balance and homeostasis.10 This view emphasizes the importance
of salt intake and balance in maintaining the milieu intérieur,
and also identifies systemic hypertension with resulting pressure natriuresis as the effector arm of this control loop. What
has not been adequately addressed is the mechanism(s) by
which dietary salt intake, aldosterone status, and vascular
tone participate in the effector arm of the Guytonian servoloop. The new findings reported herein by Jeggle and colleagues provide an approach to defining the effector arm at
the level of individual vascular endothelial cell, whereas the
overall regulation of salt balance by the classically described
ENaC/Na-K-ATPase mechanisms7,11 focus at the level of
whole-organ physiology.
I introduce novel, in this comment, nomenclature for the
EpNaC and the EnNaC in an attempt to stimulate further
discussion and experimental effort. ENaC, as a term, would
seem to have use in describing the entire family of related
amiloride-sensitive sodium channels (even though the E classically refers to epithelial). Although there seem to be many
similarities, and both complexes are undoubtly part of the
ENaC/degenerin superfamily, there are important functional
distinctions emphasized in the current work that clearly warrant additional investigations and inquiries. The time has come
n this issue of Hypertension, Jeggle et al1 provide important new insights into the characteristics and functional
significance of an amiloride-sensitive sodium channel that
is expressed in vascular endothelial cells. This work extends
the previous analyses by this group,2,3 and using atomic force
microscopy and state-of-the art animal and cell models, provides provocative yet conclusive evidence for the central role
of this channel in regulation vascular reactivity and stiffness.
The Figure depicts the heteromeric structure of the
amiloride-sensitive sodium channel. Although the overall
topology of the channel is well described,4 there is no general
agreement about the subunit composition even within epithelial cells, much less in other tissues like vascular endothelial
cells that are the subject of the current report. The bulk of the
subunits’ structures are expressed in the intertwined disulfiderich extracellular domains, the role of which has not been
fully explained. The subunits of the epithelial sodium channel
(ENaC) were originally defined by now-classical expression
cloning studies by Canessa and Rossier.5 The human genomic
clones and the critical insight into the gain of function effect
that characterizes Liddle syndrome6 was based on analogy
to defects in the mechanosensor structures of degenerins in
Caenorhabditis elegans.7
Although longitudinal flow is certainly a feature of many
epithelial tubule systems, a direct connection among the
putative mechanical transduction function, the extracellular
domains, and regulation of apical sodium entry via ENaC
is not obvious. In contrast, there are pulsatile pressures and
flows in the systemic vascular system. Local regulation of
vascular tone in response to changes in pressure and shear
stress, an important feature of the microvascular circulation,
lending credance to the idea that the extensive extracellular
of the vascular endothelial sodium channel (EnNaC) domain
could serve as a mechanoforce transducer. Using atomic
force spectroscopy and nanoindentation analysis, the current
report describes the regulation of cortical stiffness through the
agency of the EnNaC, either related to apical sodium entry or
interactions with the cortical actin cytoskeleton.
The opinions expressed in this editorial are not necessarily those of the
editors or of the American Heart Association.
From the Division of Nephrology, Department of Medicine, University
of Alabama at Birmingham, Birmingham, AL.
Correspondence to David G. Warnock, Room 636 ZRB, UAB,
1720 2rd Ave South, Birmingham, AL 34294-0007. E-mail dwarnock@
uab.edu.
(Hypertension. 2013;61:952-954.)
© 2013 American Heart Association, Inc.
Hypertension is available at http://hyper.ahajournals.org
DOI: 10.1161/HYPERTENSIONAHA.113.00768
952
Warnock Vascular Tone and EnNaC 953
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Figure. Heteromeric structure of the amiloride-sensitive
sodum channel. There is general agreement that the channel
is assembled intracellularly and includes several subunits
with similar topology, including intracellular N and C termini,
transmembrane spanning regions, and large extracellular
domains (adapted from open source figure: http://en.wikipedia.
org/wiki/Epithelial_sodium_channel).
to clearly distinguish between EpNaC and EnNac as members
of the amiloride-sensitive ENaC family.
Further work to precisely define the subunit structures and
assembly, the possibility of alternative transcription start sites
or splicing, and even other subunits12 need to be fully explored.
As an example, in the original clinical phenotype of the autosomal dominant gain-of-function condition by Liddle et al,13
there was a striking effect of increased dietary sodium intake
(and presumably increased urinary sodium concentration) to
reduce the effectiveness of sodium channel blockade observed
with triamterene. In contrast, EnNaC activity as described in
the current report by Jeggle and colleagues was fully inhibited
by amiloride in the presence of normal extracellular sodium
concentrations of 140 mmol/L. A recent report by Kleyman
and colleagues describes a novel point mutation in the γ ENaC
subunit (γL511Q) that increases amiloride-sensitive currents
and largely eliminates the Na+ self-inhibition response, which
reflects downregulation of ENaC open probability by higher
extracellular Na+ concentrations. This sort of downregulation
would be appropriate for the amiloride-sensitive EpNaC in the
distal nephron but would not be relevant for EnNaC in the systemic circulation that is continuously bathed in relative higher
extracellular Na+ concentrations. Hence, sequence variations
in the γ ENaC subunit at the single nulceotide level could
have important conseqences for understanding the regulation
of EnNaC activity.
It is well recognized that renin-angiotension-aldosterone
system blockade is an important avenue for treating chronic
kidney disease and reducing mortality in cardiovascular disease. Major efforts have demonstrated the clinical benefit
of using mineralocorticoid receptor antagonism in cardiovascular disease, with novel new approaches coming to the
fore.14 Unfortunately, the cardioprotective effects associated
with mineralocorticoid receptor blockade nearly 15 years
ago15 are not readily available to patients with moderate-tosevere chronic kidney disease who can develop dose-limiting
hyperkalemia in the setting of aggressive renin-angiotensionaldosterone system blockade, such as described in the recent
ALTITUDE Trial.16
Previous consideration has been given to trying to separate
the effects of Na-channel blockers like amiloride or triamterene from the parallel decreases in K+ secretion that invariably accompany EpNaC blockade. Furthermore, the currently
available EpNaC blockers do not have a substantial effect on
reduction of systemic blood pressure, at least in the currently
used dosing regimens.17 An exception to this overall analysis
is provided by the salulatory responses to amiloride in adolescents with Liddle syndrome, who presumably have not developed the long-term vascular changes associated with chronic
hypertension, although this effect may represent a primary
renal effect rather than an effect on the vascular endothelium
in Liddle syndrome and is clearly predicated on control of
dietary salt intake.9,13
EnNaC may present a specific target for therapeutic intervention if its inhibition can be functionally separated from
EpNaC, and thus avoiding or at least minimizing impairment
of renal and intestinal K+ secretion. Recent developments of
novel 2-acyl-amiloride derivatives18,19 may provide a potential avenue for further development. An interesting analogue
would have reasonable bioavailabilty (oral, sublingual, or
transdermal), with hepatobiliary rather than renal clearance, and effectiveness as a noncompetitive inhibitor against
EnNaC at subnanomolar concentrations. Amiloride, at usual
oral dosing, achieves plasma levels that are well below the
IC50 for ENaC, with nearly 100-fold greater concentrations
present in the final urine.20 Appropriate preclinical and clinical models and trials will need to be carried out to deterimine
whether amiloride analogues can be developed that are renal
sparing, minimize hyperkalemia, and also provide effective
antihypertensive effects as well as longer term cardioprotective effects such has been described with mineralocorticoid
receptor blockade.
Sources of Funding
Preparation of this comment was supported by the Hilda B. Anderson
Endowed Chair in Nephrology at the University of Alabama at
Birmingham.
Disclosures
D.G. Warnock serves as a consultant to Parion Sciences Inc and
Gilhead Sciences Inc on the clinical development of amiloride
analogues.
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The Amiloride-Sensitive Endothelial Sodium Channel and Vascular Tone
David G. Warnock
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Hypertension. 2013;61:952-954; originally published online March 4, 2013;
doi: 10.1161/HYPERTENSIONAHA.113.00768
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