Barium Reduces Resting Blood Flow and Inhibits
Potassium-Induced Vasodilation in the Human Forearm
Matthew Dawes, PhD, MRCP; Christine Sieniawska, BSc, MPhil;
Trevor Delves, PhD, CChem, EurClinChem, FRSC; Rahul Dwivedi, MRCP;
Philip J. Chowienczyk, FRCP; James M. Ritter, MA, DPhil, FRCP
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Background—Increasing extracellular K⫹ concentration within and just above the physiological range hyperpolarizes and
relaxes vascular smooth muscle in vitro. These actions involve inwardly rectifying potassium channels (KIR) and Na⫹/K⫹
ATPase, which are inhibited, respectively, by Ba2⫹ and ouabain. The role (if any) of KIR in controlling human resistance
vessel tone is unknown, and we investigated this in the forearm.
Methods and Results—Blood flow was measured by plethysmography in healthy men. Drugs and electrolytes were infused
through the brachial artery. BaCl2 (4 ␮mol/min, also used in subsequent experiments) increased Ba2⫹ plasma concentration
in the infused forearm to 50⫾0.8 ␮mol/L (mean⫾SEM) and reduced blood flow by 24⫾4% (n⫽8, P⬍0.001) without causing
systemic effects. Ouabain (2.7 nmol/min), alone and with BaCl2, reduced flow by 10⫾2% and 28⫾3%, respectively (n⫽10).
Incremental infusions of KCl (0.05, 0.1, and 0.2 mmol/min) increased flow from baseline by 1.0⫾0.2, 2.0⫾0.4, and 4.2⫾0.5
mL/min per deciliter forearm, respectively. Responses to KCl (0.2 mmol/min) were inhibited by BaCl2, alone and plus
ouabain, by 60⫾9% and 88⫾6%, respectively (both Pⱕ0.01). In control experiments, norepinephrine (240 pmol/min)
reduced blood flow by 24⫾2% but had no significant effect on K⫹-induced vasodilation. BaCl2, alone or with ouabain, did
not significantly influence responses to verapamil or nitroprusside.
Conclusions—Ba2⫹ increases forearm vascular resistance. K⫹-induced vasodilation is selectively inhibited by Ba2⫹ and
almost abolished by Ba2⫹ plus ouabain, suggesting a role for KIR and Na⫹/K⫹ ATPase in controlling basal tone and in
K⫹-induced vasorelaxation in human forearm resistance vessels. (Circulation. 2002;105:1323-1328.)
Key Words: potassium 䡲 vasodilation 䡲 endothelium-derived factors 䡲 muscle, smooth 䡲 vasculature
S
relatively specific for these channels.9 Higher concentrations of Ba2⫹ (100 to ⬎1000 ␮mol/L) are needed to block
non-KIR potassium channels, including HERG (an inwardly
rectifying potassium channel present in human hippocampus and distinct from KIR2.1),10 ATP-sensitive
K-channels,11 Ca2⫹-activated K-channels,12 and KV channels.13 Soluble barium salts are rapidly absorbed from the
intestinal tract, and Ba2⫹ is toxic in systemically active
doses. Acute toxic effects of Ba2⫹, which include hypertension, are consistent with block of KIR.14 Human studies
identified a no-observed adverse-effect dose of 0.21 mg
barium/kg body wt per day.15,16 These data were used by
the US Environmental Protection Agency to calculate a
chronic oral reference dose of 0.07 mg/kg per day.17,18 In
one study, human volunteers were given up to 10 mg/L in
drinking water daily for up to 10 weeks. There were no
clinically significant adverse events.16 We calculated that
brachial artery infusion of BaCl2 could achieve a local Ba2⫹
concentration of 30 to 50 ␮mol/L in plasma of the infused
forearm without causing systemic toxicity.
trong inwardly rectifying potassium channels (a family of
channels termed KIR) influence the membrane potential
and contractile state of several kinds of vascular smooth
muscle and contribute to vasodilator responses to modestly
increased extracellular potassium ion concentrations ([K⫹]o
⬍15 mmol/L) in vitro.1– 4 Such responses could be physiologically or pathologically important because elevated [K⫹]o
occurs in exercising muscle and during myocardial or cerebral ischemia. K⫹ has also been implicated as an endothelium-derived hyperpolarizing factor (EDHF) in some5 but not
all6 blood vessels. Electrophysiological, molecular biological,
and gene knockout experiments have shown that one member
of the KIR channel family, KIR2.1, is needed for KIR currents
and for K⫹-induced vasodilation in rat cerebral arteries in
vitro, effects that are blocked by Ba2⫹ concentrations ⬍100
␮mol/L.7,8 It is not known, however, whether KIR channels
control human resistance vasculature under physiological
conditions in vivo, and the present study addresses this.
Ba2⫹ has been proposed as a probe of the functional role of
KIR channels,2 and concentrations of Ba2⫹ ⬍100 ␮mol/L are
Received January 9, 2002; accepted January 11, 2002.
From the Department of Clinical Pharmacology and Centre for Cardiovascular Biology and Medicine, King’s College, London, UK (M.D., R.D., P.J.C.,
J.M.R.), and SAS Trace Element Unit, Chemical Pathology, Southampton University Hospitals NHS Trust, Southampton, UK (C.S., T.D.).
Correspondence to Prof J.M. Ritter, Department of Clinical Pharmacology, Block 5, South Wing, St Thomas’ Hospital, Lambeth Palace Road, London
SE1 7EH, UK. E-mail [email protected]
© 2002 American Heart Association, Inc.
Circulation is available at http://www.circulationaha.org
DOI: 10.1161/hc1102.105651
1323
1324
Circulation
March 19, 2002
We measured effects of Ba2⫹ on K⫹-induced vasodilation
as well as on basal blood flow. Increasing [K⫹]o displaces KIR
channel– bound polyamines19; in the relevant concentration
range, this increases outward K⫹ current, hyperpolarizing the
membrane.4 Increasing [K⫹]o also hyperpolarizes vascular
smooth muscle by activating the electrogenic Na⫹/K⫹ pump.3
Brachial artery infusion of ouabain was therefore used in
some experiments to produce unilateral regional block of
Na⫹/K⫹ ATPase.20 –22 Experiments with vasodilators (verapamil and nitroprusside) and vasoconstrictors (norepinephrine)
that act by mechanisms distinct from KIR and the Na⫹/K⫹
pump were used to control for possible nonspecific effects.
Methods
Human Studies
Downloaded from http://circ.ahajournals.org/ by guest on June 17, 2017
The St Thomas’ Hospital Research Ethics Committee approved all
the studies, and subjects gave written informed consent. Studies were
performed in healthy normotensive (arterial pressure, 116⫾4/
70⫾2 mm Hg; mean⫾SEM), normocholesterolemic (mean serum
cholesterol, 3.6⫾0.2 mmol/L) men (mean age, 33⫾2 years) who
were taking no medication. Studies were performed in the morning
in a temperature-controlled room (24⫾1°C). The left brachial artery
was cannulated with a 27-gauge stainless steel needle (Cooper’s
Needleworks) after local anesthesia (⬍1 mL 1% lidocaine). Isotonic
saline (140 mmol/L sodium chloride ) with or without dissolved
drugs was infused at 1 mL/min. BaCl2 was administered through a
syringe filter (Acrodisc, 0.2 ␮m, Pall Corporation). Subjects rested
supine for 30 minutes, and saline was infused for 12 minutes before
measuring baseline flow. Blood flow (milliliter per minute per
deciliter of forearm volume) was measured simultaneously in both
arms by venous occlusion plethysmography23 with the use of
electrically calibrated strain gauges.24 During measurements, the
hands were excluded from the circulation by inflation of wrist cuffs
to 180 mm Hg. Upper arm cuffs were intermittently inflated to
40 mm Hg, and the mean of the last 5 readings of each infusion
period was used for analysis.
Plasma Ba2ⴙ Analysis
In preliminary dose-ranging studies, 19-gauge plastic cannulas were
inserted into the medial antecubital veins draining each forearm.
Venous blood (10 mL) was sampled at baseline, during the final 30
seconds of the infusion of BaCl2, and 1 hour after infusion for
determination of plasma Ba2⫹ concentration. Samples were immediately centrifuged (1600g for 5 minutes), and plasma was stored at
⫺18°C in Ba2⫹-free tubes. Samples were analyzed in duplicate in the
Trace Element Unit, Southampton Hospital, with the use of inductively coupled plasma (ICP) mass spectrometry (Perkin Elmer–Sciex
Elan 5000 ICP mass spectrometer), detection limit of 10 nmol/L,
with a 0.5-mL plasma sample. Routine serum chemistries including
creatinine, electrolytes, and liver function tests, in addition to Ba2⫹
concentration, were measured at baseline and 1 week after BaCl2
infusion.
In preliminary dose-finding experiments, cumulative infusions of
intra-arterial BaCl2 (0.25 to 2.0 ␮mol/min, each for 4 minutes) were
administered. From these pilot studies, a dose of BaCl2 of 4
TABLE 1.
Figure 1. Effects of ouabain and Ba2⫹ on forearm blood flow.
Percentage change in resting forearm blood flow (mean⫾SEM)
during brachial artery infusion of ouabain (2.7 nmol/min for 15
min; n⫽10), barium chloride (4 ␮mol/min for 6 min; n⫽8), and
ouabain and barium chloride coinfusion (n⫽10).
␮mol/min for up to 6 minutes was chosen for the following
protocols.
Effects of BaCl2 and Ouabain on Basal Forearm
Blood Flow
After measuring basal forearm blood flow as above, BaCl2 (4
␮mol/min) was infused into the brachial artery for 6 minutes and
blood flow was measured in both arms (n⫽8). In 6 of these subjects,
plasma Ba2⫹ determinations were made as above. In 10 separate
experiments, ouabain (2.7 nmol/min) was infused for 15 minutes and
blood flows were measured as described elsewhere.21 Ouabain was
continued for a further 3 minutes, coinfused with BaCl2 (4
␮mol/min).
Effects of Ba2ⴙ (With or Without Ouabain) on
Forearm Responses to Kⴙ or Verapamil
After measuring basal forearm blood flow, cumulative doses (0.05,
0.1, 0.2 mmol/min, n⫽8) of KCl were infused into the brachial
artery, each dose for 3 minutes. Venous blood samples (10 mL) were
taken from both arms during the final 30 seconds for determination
of plasma K⫹ concentration.
In separate experiments (n⫽8), KCl (0.2 mmol/min) was infused
for 3 minutes followed by saline for 15 minutes, during which blood
flow returned to baseline. BaCl2 (4 ␮mol/min) was then infused for
6 minutes: alone for 3 minutes and during a second 3-minute infusion
of KCl (0.2 mmol/min). In control experiments, after blood flow was
measured at baseline, verapamil (80 nmol/min) was infused with
saline followed sequentially by saline alone, BaCl2 alone, and
verapamil with BaCl2 (n⫽7).
In further separate experiments (n⫽6), blood flow was measured
as before at baseline and during KCl (0.2 mmol/min), followed by a
15-minute saline recovery period. Ouabain and BaCl2 were infused
as described above and continued during a final 3-minute KCl
infusion (0.2 mmol/min).
Lack of Effect of Norepinephrine on Forearm
Blood Flow Response to KCl
Baseline blood flow was measured, followed by KCl infusion for 3
minutes (0.2 mmol/min, n⫽5). Saline was then infused alone for a
Plasma Barium Concentrations
Mean (SEM) Ba⫹ Concentration, ␮mol/L
Arm
Baseline
Infused Arm
Noninfused Arm
1 h Postinfusion
1 wk Postinfusion
Mean (SEM)
0.3 (0.07)
50.00 (8.00)
1.20 (0.08)
0.80 (0.08)
0.07 (0.03)
Mean (⫾SEM) plasma barium concentrations (␮mol/L; n⫽6) at baseline, immediately after brachial artery
administration of BaCl2 (4 ␮mol/min for 6 min, both arms), 1 hour after stopping the infusion, and 1 week later.
(*P⬍0.005 concentration in infused arm vs baseline, Student’s paired t test).
Actions of Ba2ⴙ on Human Forearm Blood Flow
Dawes et al
1325
changes and had no significant effect on serum electrolytes
and liver function tests measured 1 week after the infusion.
Effects of Ba2ⴙ and Ouabain on Basal Flow
Figure 2. Effect of potassium on forearm blood flow. Increases
in blood flow above baseline (⌬FBF, mL · min⫺1 · dL forearm⫺1;
mean⫾SEM) during cumulative rising-dose infusions of potassium chloride are shown. P⬍0.0001 ANOVA; n⫽8.
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15-minute recovery period, followed by norepinephrine (240 pmol/min),
initially alone for 3 minutes, and then continued throughout a second
3-minute KCl infusion.
Lack of Effect of BaC12 Plus Ouabain on Forearm
Blood Flow Response to Nitroprusside
Baseline blood flow was measured during saline, followed by
nitroprusside (3.3 nmol/min or 33 nmol/min for 3 minutes, n⫽4).
Saline was then infused for a 15-minute recovery period, followed by
sequential cumulative additions of ouabain (2.7 nmol/min), BaCl2 (4
␮mol/min), and nitroprusside (3.3 or 33 nmol/min), with the same
timings used as above.
Materials and Drugs
Drugs were obtained from Baxter Healthcare (saline); David Bull
Laboratories (sodium nitroprusside); Antigen Pharmaceuticals (KCl,
lidocaine); Abbott Laboratories (norepinephrine); BDH Laboratories
(BaCl2 10% wt/vol); Jenapharm (ouabain); and Baker Norton
(verapamil).
Statistical Analysis
Data are presented as mean⫾SEM. Vasodilator responses were
calculated as increases of blood flow (mL/min per deciliter of
forearm tissue) above the immediately preceding baseline.25,26 Vasoconstrictor responses were calculated as percentage decrease of
blood flow ratio of the infused to noninfused arm.27,28 Differences
between means were evaluated for statistical significance by means
of Student’s paired t test (2-sided) or repeated-measures ANOVA, as
appropriate. Differences were considered significant at a level of
P⬍0.05.
Results
Neither arterial blood pressure nor forearm blood flow in the
noninfused arm changed significantly during any study,
consistent with lack of systemic hemodynamic effects of the
infused agents at the doses used. BaCl2 caused no acute ECG
TABLE 2.
In pilot experiments, cumulative, rising doses of BaCl2 (0.25,
0.5, 1.0, and 2.0 ␮mol/min, n⫽6), each infused through the
brachial artery for 4 minutes, had no significant effect on
baseline flow (percent change in baseline flow, ⫺10⫾10%,
⫺9⫾7%, ⫺2⫾14%, and 1⫾15%, respectively, P⫽NS).
Plasma Ba2⫹ concentration at baseline (ie, before BaCl2
infusion) was 0.28⫾0.04 ␮mol/L and during the final 30
seconds of infusion (at 2.0 ␮mol/min) was 22⫾3.4 ␮mol/L
and 1.7⫾0.9 ␮mol/L, in the infused and noninfused arms
respectively.
BaCl2 (4 ␮mol/min for 6 minutes) reduced baseline blood
flow by 24⫾4% (n⫽8, P⬍0.001, Figure 1). Plasma Ba2⫹
concentration in venous blood draining the infused arm was
50⫾8 ␮mol/L during the final 30 seconds of the infusion
(Table 1).
Ouabain (2.7 nmol/min) reduced baseline blood flow by
10⫾2% (n⫽10, P⬍0.05) and coinfusion of BaCl2 with
ouabain reduced baseline flow by 28⫾3% (n⫽10, P⬍0.0005
compared with the effect of ouabain alone, Figure 1).
Effect of Kⴙ on Forearm Blood Flow
All subjects reported warmth and tingling in the infused arm
during KCl infusion, and in preliminary experiments discomfort limited the maximum dose that was consistently tolerated
to 0.2 mmol/min. KCl (0.05, 0.1, and 0.2 mmol/min) increased blood flow by 1.03⫾0.24, 1.99⫾0.4, and 4.21⫾0.46
mL/min per deciliter forearm over baseline (n⫽8, P⬍0.0001,
ANOVA; Figure 2). The concentration of K⫹ in plasma from
venous blood from the infused arm at the end of the infusion
of KCl was 6.3⫾0.2 mmol/L, whereas the concentration in
plasma from venous blood draining the noninfused arm was
4.0⫾0.2 mmol/L (n⫽6, P⬍0.0001).
Effects of Ba2ⴙ(With or Without Ouabain) on
Forearm Response to Kⴙ or Verapamil
KCl (0.2 mmol/min) alone increased forearm blood flow by
3.53⫾0.56 mL/min per deciliter forearm over baseline (n⫽8,
Table 2) and by 1.43⫾0.28 mL/min per deciliter forearm
when coinfused with BaCl2 (60⫾9% reduction; P⫽0.01,
Figure 3A). Verapamil (80 nmol/min) increased forearm
blood flow by 4.50⫾0.84 mL/min per deciliter forearm when
infused with saline and by 4.46⫾0.86 mL/min per deciliter
forearm when infused with BaCl2 (n⫽7, Table 3 P⫽NS). In
separate experiments, KCl (0.2 mmol/min) increased forearm
blood flow by 4.49⫾0.68 mL/min per deciliter forearm over
Effects of Ba2ⴙⴞKⴙ on Forearm Blood Flow
Mean (SEM) Forearm Blood Flow (mL 䡠 min⫺1 䡠 dL forearm⫺1)
Arm
Baseline 1
K⫹ (0.2 mmol/min)
Baseline 2
Ba2⫹ (4 ␮mol/min)
K⫹ and Ba2⫹
Infused
3.34 (0.45)
6.87 (0.66)*
3.69 (0.35)
2.86 (0.43)*
4.29 (0.44)†
Noninfused
3.26 (0.36)
3.22 (0.33)
3.49 (0.39)
3.51 (0.46)
3.62 (0.41)
⫺1
⫺1
Mean (⫾SEM) blood flow (mL 䡠 min 䡠 dL forearm ) is shown before and during brachial artery infusions of KCl
(0.2 mmol/min) and BaCl2 (4 ␮mol/min) separately and in combination (n⫽8).
*P⬍0.01 compared with preceding baseline; †P⬍0.001 compared with K⫹ alone.
1326
Circulation
TABLE 3.
March 19, 2002
Effects of Ba2ⴙⴞVerapamil on Forearm Blood Flow
Mean (SEM) Forearm Blood Flow (mL 䡠 min⫺1 䡠 dL forearm⫺1)
Arm
Baseline 1
Verapamil (80
nmol/min)
Baseline 2
Ba2⫹ (4 ␮mol/min)
Verapamil and Ba2⫹
Infused
2.10 (0.22)
6.60 (1.03)†
2.59 (0.31)
1.85 (0.23)*
6.31 (0.98)†
Noninfused
2.55 (0.30)
2.28 (0.28)
2.50 (0.54)
2.38 (0.46)
2.47 (0.43)
⫺1
⫺1
Mean (⫾SEM) blood flow (mL 䡠 min 䡠 dL forearm ) is shown before and during brachial artery infusions of
verapamil (80 mmol/min) and BaCl2 (4 ␮mol/min) separately and in combination (n⫽7).
*P⬍0.05, †P⬍0.01 compared with preceding baseline.
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baseline when infused alone and by 0.57⫾0.23 mL/min per
deciliter forearm when coinfused with BaCl2 and ouabain
(88⫾6% reduction, n⫽6, P⬍0.005; Figure 3B and Table 4).
Responses to KCl were measured before and during
vasoconstriction with norepinephrine. Norepinephrine (240
pmol/min) reduced baseline blood flow by 24⫾2% (n⫽5,
P⬍0.05). KCl (0.2 mmol/min) increased forearm blood flow
by 3.01⫾0.45 mL/min per deciliter forearm over baseline
when infused alone and by 3.79⫾0.64 mL/min per deciliter
of forearm when coinfused with norepinephrine (P⫽NS,
Figure 3C).
Vasodilator responses to nitroprusside were measured in
the presence and absence of Ba2⫹ and ouabain (Table 4).
Nitroprusside (3.3 and 33 nmol/min) increased forearm blood
flow, respectively, by 4.13⫾0.95 and 8.55⫾1.1.4 mL/min per
deciliter of forearm when infused with saline alone and by
4.19⫾0.39 and 9.30⫾2.06 mL/min per deciliter of forearm
when coinfused simultaneously with ouabain and BaCl2
(P⫽NS; Figure 3D).
Discussion
2ⴙ
Ba as a Probe of KIR Function in Human
Forearm Resistance Vasculature
Electrophysiological and molecular studies have established
the presence of KIR2.1 channels in isolated vascular smooth
muscle.7,8,23 However, KIR channels are blocked by Ca2⫹ and
Mg2⫹,9 each present in millimolar concentrations in extracellular fluid, and other signal transduction mechanisms could
overwhelm any contribution to basal tone from KIR in vivo.
Ba2⫹ can be used to probe the functional role of KIR channels.2
We investigated the possible role of KIR channels in human
resistance vasculature in vivo by using brachial artery administration of BaCl2 with bilateral measurement of forearm
blood flow. Doses were selected to produce concentrations
Figure 3. Specificity of inhibition of K⫹-induced vasodilation by Ba2⫹⫾ouabain. Each panel shows the increase above baseline in forearm blood flow (⌬FBF, mL · min-1 · dL forearm-1; mean⫾SEM) during brachial artery infusion of vasodilators (KCl or nitroprusside) alone
(solid bars) and with (open bars) vasoconstrictor (Ba2⫹⫾ouabain or norepinephrine). Doses used were: KCl (0.2 mmol/min); BaCl2 (4
␮mol/min); ouabain (2.7 nmol/min); norepinephrine (240 pmol/min); nitroprusside (3.6 nmol/min). a, Responses to KCl⫾BaCl2 (n⫽8;
*P⫽0.01). b, Responses to KCl⫾BaCl2 with ouabain (n⫽6; **P⬍0.005. c, Responses to KCl⫾norepinephrine (n⫽5). d, Effects of
nitroprusside⫾BaCl2 with ouabain (n⫽4).
Dawes et al
Actions of Ba2ⴙ on Human Forearm Blood Flow
1327
TABLE 4. Comparison of Effects of BariumⴙOuabain on Vasodilator Responses to Potassium
Versus Nitroprusside
Mean (SEM) Forearm Blood Flow (mL 䡠 min 䡠 dL forearm⫺1)
Baseline 1
Vasodilator
Baseline 2
Ouabain
Ouabain and Ba2⫹
Ouabain and Ba2⫹
and Vasodilator
KCl protocol
3.72 (0.27)
8.21 (0.75)
4.35 (0.48)
4.02 (0.40)
3.32 (0.34)
3.88 (0.21)
NP protocol
4.0 (0.53)
8.12 (1.47)
4.23 (0.40)
3.72 (0.29)
3.15 (0.24)
7.34 (0.59)
Absolute forearm blood flow (mean⫾SEM) in response to brachial artery infusions of vasodilators comprising KCl
(0.2 mmol/min; n⫽6) or nitroprusside (NP) (3.6 nmol/min; n⫽4). Each vasodilator was studied on a separate
occasion. Vasodilators were infused alone, and with a coinfusion of ouabain (2.7 nmol/min) and BaCl2 (4 ␮mol/min).
Downloaded from http://circ.ahajournals.org/ by guest on June 17, 2017
that are pharmacologically active locally in the infused
forearm but not systemically, and no significant effects on
blood pressure or on blood flow in the contralateral (noninfused) forearm were observed. The dose of Ba2⫹ administered
during each study was less than the daily reference dose, and
no adverse effects were observed. The mean concentration of
Ba2⫹ measured in plasma from the infused arm was 50⫾0.8
␮mol/L, sufficient to block KIR channels.2– 4,7–9 Ouabain and
other drugs were similarly administered through the brachial
artery in doses that are not active systemically20 –22 and
produced no change in blood flow in the noninfused arm.
A major concern with this approach is the possibility that
Ba2⫹ and ouabain, in the doses studied, could have effects in
addition to those on KIR and Na⫹/K⫹ ATPase. Bilateral
measurement of forearm blood flow coupled with brachial
artery administration of subsystemically active doses circumvents confounding from drug actions on the central nervous
system or heart, which cause bilateral changes in forearm
blood flow.26 Thus, central effects of ouabain resulting in
increased vascular resistance cannot explain its unilateral
effects on the infused forearm. Similarly, actions of Ba2⫹ on
the heart or central nervous system cannot explain its unilateral effects in the infused arm. Concentrations of Ba2⫹ in the
range of 100 to ⬎1000 ␮mol/L are needed to block other
potassium channels,10 –13 which have not been implicated in
K⫹-induced vasodilation and are unlikely to underlie the
effect of Ba2⫹ described here. However, it is not possible to
exclude completely some other property of Ba2⫹ in the
physiological milieu of the human forearm. In particular,
physicochemical properties shared with Ca2⫹ raise the theoretical possibility that Ba2⫹ could exert some of the effects we
observed by influencing [Ca2⫹]i signaling. We sought effects
of Ba2⫹ on responses to verapamil, an antagonist of L-type
voltage-dependent Ca2⫹-channels, which are active under
physiological conditions, to address this possibility. The lack
of effect of Ba2⫹, in a dose that inhibited K⫹-responses, on
responses to verapamil argues against an action of Ba2⫹ on
[Ca2⫹]i signaling under the experimental conditions in our
studies. Furthermore, doses of Ba2⫹ and ouabain that almost
abolished K⫹-induced vasodilation had no significant effect
on vasodilator responses to nitroprusside, ruling out effects of
these antagonists on cGMP-mediated vasodilation. Finally,
we considered the possibility that the effects of these antagonists on K⫹-induced vasodilatation were simply the result of
vasoconstriction. However, a dose of norepinephrine that
caused a similar reduction in baseline blood flow as did Ba2⫹
plus ouabain had no significant effect on K⫹-induced vaso-
dilation. The main new positive findings of the present study,
namely that Ba2⫹ constricts forearm resistance vasculature
under basal physiological conditions, selectively inhibits
K⫹-induced vasodilation, and virtually abolishes K⫹ responses when combined with ouabain, can thus best be
explained by selective actions of Ba2⫹ and ouabain, in the
doses used, on, respectively, KIR and Na⫹/K⫹ ATPase in the
infused forearm.
Implications of Tonic KIR and Naⴙ/Kⴙ ATPase
Activity in Resistance Vasculature In Vivo
Physiological conditions that influence KIR-channel activity
include membrane potential and K⫹, Ca2⫹, and Mg2⫹ concentrations.2,4 Consequently, unequivocal evidence of the presence and function of KIR2.1 channels in arterial smooth
muscle in vitro4,7,8 does not, of itself, prove the functional
importance of these channels under in vivo conditions. The
modest vasoconstriction caused in the forearm by ouabain
(10⫾2% reduction in basal blood flow, consistent with
previous work20 –22) and greater effect of Ba2⫹, demonstrated
here for the first time, are consistent with tonic activity of K⫹
on Na⫹/K⫹ ATPase and, more effectively, on KIR-channels in
vascular smooth muscle in forearm resistance vessels in vivo.
Tonic K⫹-related vasodilation could have been underestimated by the inhibition observed (24⫾4% by Ba2⫹ alone,
28⫾3% by Ba2⫹ plus ouabain) because, due to concerns
related to toxicity, we did not perform prolonged infusions.
Vasoconstriction caused by Ba2⫹ may therefore not have
reached a plateau. Even so, Ba2⫹-induced vasoconstriction
was substantial and compares with a maximum vasoconstrictor effect of L-NG-monomethyl-L-arginine (which blocks basal
endothelium-derived nitric oxide synthesis in this vascular
bed) of ⬇50% reduction of basal blood flow.29 Another
limitation of the study is that it was not practicable to
investigate the effect of Ba2⫹ on K⫹-induced responses
throughout a dose range of KCl infusions because responses
to lower doses of KCl were small and variable and larger
doses could not be used as these caused forearm discomfort.
Tonic vasodilator activity of K⫹ within the physiological
range could influence arterial blood pressure. Dietary supplementation with potassium salts has been reported to lower
blood pressure in patients with essential hypertension.30,31
Conversely, elevated blood pressure has been described in
association with low dietary K⫹.32 The ability of a small
increase in [K⫹]o to hyperpolarize vascular smooth muscle
could contribute to the hypotensive effect of potassium salts.
Local plasma K⫹ concentrations (mean, 6.3⫾0.2 mmol/L)
1328
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March 19, 2002
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achieved in venous blood draining the infused arm are in the
range observed in voluntary muscle during exercise,4 supporting a role for KIR and for Na⫹/K⫹ ATPase activation in
regional blood flow responses during exercise.4 It is likely
that mechanisms similar to those we have observed in
forearm vasculature also operate in vivo in cerebral and
coronary vessels (which are perfused by plasma of the same
composition as are forearm vessels) and in which KIRchannels have been detected in vitro.2,3 If so, K⫹-induced
vasodilation could be important in pathological conditions
such as myocardial or cerebral infarction, in which local
elevations of [K⫹]o could reduce resistance vessel tone in and
around the infarct.4 Furthermore, hyperpolarization-induced
vasorelaxation by activation of Na⫹/K⫹ ATPase is reported to
be upregulated when nitric oxide bioavailability is impaired,33,34 so K⫹-induced vasodilation could provide a compensatory mechanism in diseases in which the L-arginine/
nitric oxide mechanism is impaired, especially if the KIR
mechanism also proves to be upregulated in such disorders.
In conclusion, Ba2⫹ constricts forearm resistance vessels,
and K⫹-induced vasodilation is selectively inhibited by Ba2⫹
and almost abolished by Ba2⫹ plus ouabain. These findings
support a role for KIR and Na⫹/K⫹ ATPase in controlling basal
tone and in K⫹-induced vasorelaxation in human forearm
resistance vessels in vivo. Such a role has important implications for the (patho-) physiological regulation of resistance
vessel tone in humans.
Acknowledgments
This work was supported by the British Heart Foundation. We thank
Dr Alison Jones (Guy’s Poisons Unit) for advice on the toxicity of
barium salts.
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Barium Reduces Resting Blood Flow and Inhibits Potassium-Induced Vasodilation in the
Human Forearm
Matthew Dawes, Christine Sieniawska, Trevor Delves, Rahul Dwivedi, Philip J. Chowienczyk
and James M. Ritter
Downloaded from http://circ.ahajournals.org/ by guest on June 17, 2017
Circulation. 2002;105:1323-1328; originally published online February 25, 2002;
doi: 10.1161/hc1102.105651
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