Nitric Oxide and Cardiac Autonomic Control in Humans
Saqib Chowdhary, Julian C. Vaile, Janine Fletcher, Hamish F. Ross,
John H. Coote, Jonathan N. Townend
Downloaded from http://hyper.ahajournals.org/ by guest on June 15, 2017
Abstract—Cardiac autonomic control is of prognostic significance in cardiac disease, yet the control mechanisms of this
system remain poorly defined. Animal data suggest that nitric oxide (NO) modulates cardiac autonomic control. We
investigated the influence of NO on the baroreflex control of heart rate in healthy human subjects. In 26 healthy male
volunteers (mean age, 23⫾5 years), we measured heart rate variability and baroreflex sensitivity during inhibition of
endogenous NO production with NG-monomethyl-L-arginine (L-NMMA) (3 mg/kg per hour) and during exogenous NO
donation with sodium nitroprusside (1 to 3 mg/h). Increases from baseline (⌬) in high-frequency (HF) indexes of heart
rate variability were smaller with L-NMMA in comparison to an equipressor dose of the control vasoconstrictor
phenylephrine (12 to 42 ␮g/kg per hour): ⌬root mean square of successive RR interval differences (⌬RMSSD)⫽23⫾32
versus 51⫾48 ms (P⬍0.002); ⌬percentage of successive RR interval differences ⬎50 ms (⌬pNN50)⫽5⫾15% versus
14⫾12% (P⬍0.05); and ⌬HF normalized power⫽⫺2⫾7 versus 9⫾8 normalized units (P⬍0.01), respectively. Relative
preservation of these indexes was observed during unloading of the baroreflex with sodium nitroprusside compared with
a matched fall in blood pressure produced by a control vasodilator, hydralazine (9 to 18 mg/h): ⌬RMSSD⫽⫺8⫾8
versus ⫺24⫾15 ms (P⬍0.001); ⌬pNN50⫽⫺6⫾11% versus ⫺15⫾19% (P⬍0.01); ⌬HF normalized power⫽⫺7⫾13
versus ⫺13⫾11 normalized units (P⬍0.05), respectively. The change in cross-spectral ␣-index calculated as the square
root of the ratio of RR interval power to systolic spectral power in the HF band (although not ␣-index calculated in the
same way for the low-frequency bands or baroreflex sensitivity assessed by the phenylephrine bolus method) was
attenuated with L-NMMA compared with phenylephrine (⌬⫽4⫾8 versus 14⫾15 ms/mm Hg, respectively; P⬍0.02) and
with sodium nitroprusside compared with hydralazine (⌬⫽⫺7⫾6 and ⫺9⫾7 ms/mm Hg, respectively; P⬍0.05). In
conclusion, these data demonstrate that NO augments cardiac vagal control in humans. (Hypertension.
2000;36:264-269.)
Key Words: nitric oxide 䡲 heart rate 䡲 baroreceptors 䡲 autonomic nervous system 䡲 blood pressure
T
he powerful influence of autonomic control on the
natural history of cardiac disease is evidenced by large
trials showing that reduced heart rate variability (HRV) and
baroreflex sensitivity (BRS) are independent indicators of
adverse prognosis.1,2 However, the mechanisms controlling
cardiovascular autonomic function remain poorly defined.
The initial suggestion that nitric oxide (NO) may be an
important mediator in cardiac autonomic control came from
the demonstration of discrete neuronal populations that possess NO synthase at numerous sites within known cardiac
autonomic pathways.3 Animal evidence suggests that the NO
synthesized at these sites is active in modulating activity
within both limbs of the autonomic nervous system. NO
appears to act as a sympatholytic agent, decreasing activity
within sympathoexcitatory brain stem nuclei and reducing
central sympathetic outflow,4,5 as well as attenuating cardiac
responses to sympathetic stimulation.6 Conversely, NO increases activity in central vagal motoneurons7 and enhances
the cardiac response to vagal stimulation.6,8 In addition, NO
donors may increase sinoatrial nodal discharge rate directly.9
However, the results of animal experiments have not been
consistent, and the influence of NO on human cardiac
autonomic control remains to be determined.
Cardiac autonomic control can be assessed noninvasively from the measurement of HRV and of BRS. This
study was designed to determine the effects on these
indexes of the systemic administration of (1) the inhibitor
of endogenous NO synthesis NG-monomethyl-L-arginine
(L-NMMA) and (2) the exogenous NO donor sodium
nitroprusside in healthy human subjects. Investigation of
the autonomic effects of these agents is hampered by their
influence on blood pressure and consequently on baroreceptor loading. We have therefore made comparisons with
the effects of equal changes in blood pressure produced by
vasoactive drugs that do not act via the NO pathway,
namely, phenylephrine and hydralazine.
Received November 22, 1999; first decision December 27, 1999; revision accepted February 28, 2000.
From the Departments of Cardiovascular Medicine (S.C., J.C.V., J.N.T.) and Physiology (J.F., H.F.R., J.H.C.), University of Birmingham, UK.
Correspondence to Dr S. Chowdhary, Department of Cardiovascular Medicine, Queen Elizabeth Hospital, Birmingham B15 2TH, UK. E-mail
[email protected]
© 2000 American Heart Association, Inc.
Hypertension is available at http://www.hypertensionaha.org
264
Chowdhary et al
Nitric Oxide and Cardiac Autonomic Control
265
TABLE 1. Blood Pressure, RR Interval, RR Interval Variability, and BRS (␣-Index)
at Baseline and During Steady-State Drug Infusion
Baseline
Index
Mean arterial
pressure, mm Hg
RR interval, ms
Infusion
Phenylephrine
L-NMMA
Phenylephrine
L-NMMA
80⫾7
81⫾8
91⫾8¶
91⫾9¶
1080⫾175¶
957⫾170
979⫾170
1112⫾214¶
SDNN, ms
66⫾31
70⫾25
97⫾52㛳
86⫾37§
RMSSD, ms
66⫾41
71⫾37
117⫾81¶
94⫾52*§
39⫾29
46⫾24
53⫾32¶
pNN50, %
2
LF power, ms
902⫾643
861⫾713
LF power, % nU
28⫾12
24⫾12
19⫾11¶
23⫾13
LF CCV, %
3.0⫾1.3
2.8⫾1.0
2.7⫾1.2
2.9⫾1.2
2922⫾2873
3228⫾2607
8060⫾8733㛳
4917⫾4680†
HF power, ms2
1142⫾966
51⫾27
1106⫾788
Downloaded from http://hyper.ahajournals.org/ by guest on June 15, 2017
HF power, % nU
65⫾13
69⫾13
73⫾14㛳
68⫾15*
HF CCV, %
5.1⫾2.8
6.0⫾2.5
8.3⫾5.6¶
6.0⫾3.0*
5214⫾4341
5524⫾3552
12035⫾11024㛳
8746⫾6720*§
49⫾30
41⫾32
29⫾21㛳
42⫾34*
␣-HF, ms/mm Hg
26.8⫾9.8
25.1⫾10.2
39.3⫾20.1㛳
29.5⫾11.8†
␣-LF, ms/mm Hg
10.9⫾3.0
10.9⫾3.3
15.0⫾8.9
15.2⫾7.2
Total power, ms2
LF/HF ratio, %
CCV indicates coefficient of component variance.
*P⬍0.05, †P⬍0.01.
§P⬍0.05, 㛳P⬍0.01, ¶P⬍0.001, infusion vs baseline value.
Methods
Subjects
Twenty-six healthy male volunteers aged 18 to 36 years (mean age,
23⫾5 years) were studied. All subjects were normotensive (supine
cuff blood pressure measurement ⬍140/90 mm Hg at initial screening visit), with no symptoms or signs of cardiovascular disease and
on no medication. Subjects were asked to abstain from food or drink
for at least 2 hours before the study and from caffeine and alcohol for
24 hours. Experimental protocols were approved by the South
Birmingham local research ethics committee, and individual written
consent was obtained.
Procedures
All protocols were of a single-blind, random-order, crossover design.
Subjects attended an initial habituation and training visit to our
dedicated clinical autonomic research laboratory. At this visit they
were trained to breathe to an audio signal set close to the individual’s
resting respiratory frequency. All studies were performed at the same
time of day and with an ambient temperature of 24⫾1°C.
A standard 3-lead ECG signal was amplified, processed (highfrequency [HF] signal noise filter ⬎500 Hz), and digitized at 500 Hz
with the use of a National Instruments NB/MI0/16XH/18 analog-todigital converter board (National Instruments Corporation). A continuous arterial pressure signal was obtained with the Portapres
device (TNO Biomedical Instrumentation) and was similarly digitized. Respiratory excursion was recorded from the amplified output
of a standard strain gauge attached to an elastic strap around the
subject’s chest. All signals were displayed on the screen of a
personal computer running Laboratory View 5.0 software (National
Instruments Corporation).
At the start of each study, a venous cannula was inserted into an
antecubital vein for drug administration. Subjects were rested for 30
minutes, after which selected periods from all 3 signals were stored
to disk during breathing at the predetermined frequency. Two
5-minute recordings were taken during a 30-minute normal saline
infusion and an additional 2 at steady-state (⬍10% deviation in the
mean heart rate and arterial pressure over two 60-second periods, 5
minutes apart) during the vasoactive infusion. Results were calculated as the mean of the 2 recordings. Target increments and
decrements in mean arterial pressure were assessed from a continuously updated 60-second integrated mean of the Portapres signal and
by intermittent arm cuff sphygmomanometry with an automated
oscillometric system. All drugs were diluted in normal saline or 5%
glucose immediately before the experiment.
Protocol 1: Effects of Inhibition of NO Synthesis on HRV
Fourteen subjects were randomly assigned to receive intravenous
infusions of either L-NMMA (3 mg/kg per hour) or a control
infusion of the pressor agent phenylephrine (12 to 42 ␮g/kg per hour)
during the first of 2 studies. The second agent was given during a
separate study visit 7 to 14 days later. At each study visit the subject
rested semisupine, and data were acquired at baseline and during the
drug infusion after the target rise in mean arterial pressure of
⬇10 mm Hg had been achieved.
Protocol 2: Effects of Exogenous NO on HRV
In a similar experimental design, 12 subjects received random-order
infusions of the NO donor sodium nitroprusside (1 to 3 mg/h) and, as
a control, hydralazine (9 to 18 mg/h) on separate days. Each infusion
was titrated to achieve a drop in mean arterial pressure of 5 to
10 mm Hg. Initial experiments showed that in the supine position the
dose of hydralazine required to achieve this target was occasionally
associated with headache and nausea. Using a mild degree of
orthostatic stress considerably reduced the cumulative dose required
and side effects. Both experiments were therefore performed with
subjects at 30° of head-up tilt during all phases of the experiment.
Recordings were made, as before, at baseline and during a 60- to
90-minute infusion of each of the hypotensive agents.
To test the effect of time on this protocol, in 6 volunteers we
assessed changes in mean arterial pressure and HRV during normal
saline infusion (30 mL/h) at 30° head-up tilt. Data were acquired at
baseline (30 minutes) and at 2 hours. No significant changes over
time were observed in mean arterial pressure, RR interval, or any
HRV index.
266
Hypertension
August 2000
Measurement of BRS
BRS was assessed by 2 methods. The ␣-index, describing the
transfer function of variability in the systolic pressure signal to
variability in the RR interval, was assessed by cross-spectral analysis
in protocols 1 and 2. In addition, we used the Oxford method of
measuring the RR interval response to a transient rise in blood
pressure generated by an intravenous bolus injection of phenylephrine.10 The value of the regression line of RR interval against the
preceding systolic blood pressure was accepted as baroreflex gain
only if the correlation coefficient was ⬎0.8. Quoted values represent
the mean of at least 3 results.
Data Analysis
Initial Processing
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The ECG series for analysis were coded so that the investigator
performing the analysis was blinded to the vasoactive agent under
study. All ECG series were reviewed and if necessary edited to
exclude ectopic and artifact signals. The RR intervals before and
after any ectopic beats were replaced by interpolation from the
previous and following sinus intervals. No signal containing ⬎1% of
ectopic beats was used for analysis. R waves were detected by an
individually adjusted threshold, and the fiducial point was determined by fitting a quadratic polynomial to sequential groups of 7
data samples. HRV was analyzed off-line on data lengths of 256 RR
intervals.
Time Domain Analysis
We used the standard time domain measures of standard deviation of
RR interval values (SDNN), root mean square of successive RR
interval differences (RMSSD), and percentage of successive RR
interval differences ⬎50 ms (pNN50). Overall variability is expressed by SDNN, whereas those indexes based on successive
differences in RR intervals, ie, pNN50 and RMSSD, assess HF
(“beat-to-beat”) variation associated with respiratory sinus arrhythmia mediated principally by the vagus nerve.11
Frequency Domain Analysis
Stationarity of the time series was tested by calculation of the mean
and variance of the first and last 128 beats of each recording period
to verify a difference of ⬍10% in the values for each time series.
Power spectral analysis was performed with the use of the Burg
algorithm (autoregressive modeling), with a model order between 8
and 20 selected to minimize the Akaike information criterion. The
power at each underlying frequency was quantified by decomposing
the total variability signal according to the method of Zetterberg.
Powers at low frequency (LF) (centered at ⬇0.1 Hz) and at HF
(corresponding to the observed respiratory frequency) were thus
determined. Power was expressed in absolute and normalized units
(nU) (power/total power ⬎0.04 Hz)12 and as the coefficient of
component variance (square root of power/mean RR interval).13
Baroreflex Sensitivity
In the assessment of BRS by cross-spectral analysis, the recordings
of RR interval and systolic blood pressure were interpolated (with a
cubic spline) and then resampled at 1 Hz to produce a uniform time
base. A cross spectrum of 256 samples was then analyzed by fast
Fourier transforms with the use of a Hanning window on successive
overlapping records of 64 samples each. The ␣-index was calculated
as the square root of the ratio of RR interval power to systolic
spectral power in both the HF (␣-HF) and LF bands (␣-LF).14 In each
case the ␣-index was computed only when the squared coherence
between these 2 signals was ⬎0.5.
Statistical Analysis
Data for mean arterial pressure and RR interval were compared by a
2-tailed paired Student’s t test. Differences between groups for time
and frequency domain indexes of HRV and BRS were determined
with the Wilcoxon signed rank test for paired data. Statistical
significance was defined by a P value ⬍0.05. Numerical values are
expressed as mean⫾SD.
Figure 1. Changes from baseline for indexes of HRV and BRS
(␣-index). *P⬍0.05, **P⬍0.01, ***P⬍0.001 for change with
L-NMMA vs that with phenylephrine. Abs indicates spectral
power in absolute units; norm, power in normalized units
(nU⫽power/total power ⬎0.04 Hz).
Results
Effects of Inhibition of Endogenous NO on HRV
The frequency of metronomic breathing was within the range
of 0.17 to 0.22 Hz. Baseline values for mean arterial pressure,
RR interval, and indexes of HRV were not significantly
different between the groups before administration of
L-NMMA or phenylephrine (Table 1). Infusion of L-NMMA
and phenylephrine resulted in similar rises in mean arterial
pressure (11⫾5 and 11⫾3 mm Hg, respectively). Both agents
increased the mean RR interval, but this increase tended to be
lower with L-NMMA (⫹101⫾55 ms) than with phenylephrine (⫹156⫾110 ms) (P⫽0.08).
However, measures of HF, vagally mediated HRV in both
time and frequency domains were significantly lower during
L-NMMA infusion than during phenylephrine (absolute values are shown in Table 1). A comparison of the magnitude of
change in HRV from baseline values emphasizes the markedly discrepant responses between the 2 equipressor infusions
(Figure 1). The increases in HF HRV in both domains (ie,
RMSSD, pNN50, and HF power) were significantly smaller
in response to L-NMMA than to an equipressor phenylephrine infusion. The fall in LF power in normalized (although
Chowdhary et al
Nitric Oxide and Cardiac Autonomic Control
267
TABLE 2. Blood Pressure, RR Interval, RR Interval Variability, and BRS (␣-Index) at Baseline
and During Steady-State Drug Infusion
Baseline
Index
Mean arterial pressure, mm Hg
Hydralazine
Infusion
SNP
Hydralazine
SNP
82⫾6
82⫾5
75⫾6¶
73⫾6¶
839⫾93
814⫾71
628⫾74¶
717⫾70‡¶
SDNN, ms
51⫾14
47⫾12
24⫾13¶
42⫾11†
RMSSD, ms
35⫾17
29⫾14*
11⫾9¶
22⫾8†㛳
pNN50, %
16⫾20
10⫾16
2⫾4¶
4⫾5*㛳
694⫾317
601⫾323
193⫾164¶
617⫾443†
52⫾20
53⫾16
65⫾19㛳
61⫾17
RR interval, ms
LF power, ms2
LF power, % nU
LF CCV, %
3.1⫾0.8
2.9⫾0.8
1.9⫾0.9㛳
3.3⫾1.2*
HF power, ms2
796⫾873
567⫾633*
154⫾281¶
336⫾318*§
HF power, % nU
44⫾20
42⫾16
31⫾17㛳
36⫾16
HF CCV, %
3.1⫾1.2
2.8⫾1.1
1.5⫾1.2㛳
2.8⫾1.1†
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Total power, ms2
2820⫾1569
2325⫾1244
700⫾681¶
1887⫾1030†
LF/HF ratio, %
163⫾111
169⫾132
331⫾308㛳
282⫾309
␣-HF, ms/mm Hg
13.9⫾8.3
13.2⫾9.5
4.7⫾2.5¶
6.3⫾3.8¶
␣-LF, ms/mm Hg
9.4⫾3.4
9.5⫾3.1
5.3⫾3.7§
7.2⫾2.5§
SNP indicates sodium nitroprusside; CCV, coefficient of component variance.
*P⬍0.05, †P⬍0.01, ‡P⬍0.001 vs hydralazine.
§P⬍0.05, 㛳P⬍0.01, ¶P⬍0.001, infusion vs baseline value.
not absolute) units was significantly less with L-NMMA than
with phenylephrine infusion (Figure 1). The LF/HF ratio was
higher during L-NMMA than during phenylephrine administration (Table 1).
Effects of Exogenous NO on HRV
Baseline levels of mean arterial pressure and RR interval
were not significantly different before administration of
sodium nitroprusside or hydralazine. Indexes of HRV were
comparable at baseline except for RMSSD and HF absolute
power, which were lower in the sodium nitroprusside group
(Table 2).
The hypotensive responses to infusions of sodium nitroprusside and hydralazine were well matched (change in mean
arterial pressure⫽⫺8⫾2 and ⫺7⫾3 mm Hg, respectively)
but produced discrepant effects on heart rate, with less
acceleration during sodium nitroprusside than during hydralazine infusion (change in mean RR interval⫽⫺98⫾58 versus
⫺212⫾81 ms, respectively; P⬍0.0001). The HRV responses
were also significantly different. HF, vagally mediated indexes of HRV in the time domain (ie, RMSSD and pNN50)
were significantly greater during sodium nitroprusside infusion than during hydralazine infusion (Table 2). It can be seen
that despite the fall in blood pressure, there was relative
preservation of HF HRV with sodium nitroprusside. In the
frequency domain, the pattern is less clear because of a large
reduction in total power with hydralazine. In absolute units,
both HF and LF powers appeared to be higher with sodium
nitroprusside than hydralazine, although there were no significant differences when the change in total power was
corrected for by the use of normalized units. However, when
changes from baseline are examined, it can be seen that the
effects of the 2 agents on HF variability in both domains were
discrepant with smaller changes during sodium nitroprusside
than during hydralazine administration (Figure 2).
Effects of NO on BRS
Cross-spectral analysis of RR interval and systolic blood
pressure variability revealed that the ␣-LF index was not
calculable because of noncoherence for 25% of data in
protocol 1 and 14% of data in protocol 2, whereas ␣-HF was
calculable for all data. Baseline values of ␣-HF and ␣-LF
were not significantly different for either protocol. ␣-HF
(although not ␣-LF) was both significantly lower and increased less during infusion of L-NMMA than during infusion of phenylephrine (Table 1 and Figure 1). Sodium
nitroprusside resulted in significantly less attenuation of
␣-HF (although not of ␣-LF) than did hydralazine (Figure 2).
Assessment of BRS by the phenylephrine bolus method
revealed similar baseline values before L-NMMA and phenylephrine infusion (17.4⫾6.3 and 15.6⫾6.1 ms/mm Hg, respectively; P⫽0.2). When L-NMMA and phenylephrine infusion were compared, values were not significantly different
(18.6⫾4.7 and 18.4⫾6.5 ms/mm Hg, respectively; P⫽0.9).
Compared with baseline, BRS did not change significantly in
response to L-NMMA, whereas there was a significant (18%)
rise in BRS with phenylephrine (P⫽0.02).
Discussion
Direct recording of cardiac vagal and sympathetic activity is
not possible in humans, but there is good evidence that HF
indexes of HRV in both time and frequency domains reflect
vagal modulation of heart rate in the conscious human. This
includes the demonstration that HF variability is proportional
to efferent vagal activity in anesthetized dogs15 and in
humans is strongly correlated with heart rate acceleration in
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August 2000
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Figure 2. Changes from baseline for indexes of HRV and BRS
(␣-index). *P⬍0.05, **P⬍0.01, ***P⬍0.001 for change with sodium
nitroprusside vs that with hydralazine. Abbreviations are as defined
in Figure 1 legend (nU⫽power/total power ⬎0.04 Hz).
response to atropine.13,16 Furthermore, HF variability is virtually abolished by atropine.16,17 We have shown that despite
a rise in blood pressure equal to that obtained with phenylephrine, systemic inhibition of NO synthase by L-NMMA
was associated with less bradycardia and lower measures of
HF HRV. We suggest that this is a result of removal of a tonic
excitatory effect of NO on baroreflex-mediated cardiac vagal
activity. In support of this, we found that infusion of the
exogenous NO donor sodium nitroprusside was associated
with less tachycardia and relative preservation of cardiac
vagal control (as measured by HF HRV) compared with a
control infusion of hydralazine. These data constitute the first
controlled evidence of an important role for NO in cardiac
vagal control in human subjects.
Although we have shown that NO appears to modulate
cardiac vagal control of heart rate, we cannot exclude the
possibility of a sympathetic nervous influence on our results.
LF power has been used as an index of sympathetic activity
(especially when expressed in normalized units),18 but there is
undoubtedly a large vagal component to this oscillation.17
Similarly, use of the LF/HF ratio as a measure of “sympathovagal balance”12 has been the subject of recent debate.19
Thus, we are reluctant to draw any firm conclusions on the
relative activity of the sympathetic nervous system during
these drug infusions.
The observed effect of NO on vagal activity during a sustained
rise or fall in blood pressure could have been mediated by effects on
either the sensitivity or “gain” of the baroreflex or a change in the
reflex set point. When BRS was measured by cross-spectral
analysis, ␣-HF was higher during phenylephrine than L-NMMA
infusion and fell less during sodium nitroprusside than during
hydralazine administration, suggesting an enhancement of baroreflex gain by NO. Changes in ␣-LF were directionally similar but
were not statistically significant (possibly because of the loss of
analyzable data through noncoherence). There may also be differences in the information held within each ␣-index. Since ␣-HF is
determined at a frequency at which sympathetic influence is not
effective, this index represents gain in the vagal limb of the
baroreflex,20 while the ␣-LF index is open to influence by both
limbs of the autonomic nervous system.20 The Oxford method of
measuring BRS, which is historically the “gold standard,” failed to
show a clear influence of NO. Thus, our results do not allow firm
conclusions to be drawn on the possible effects of NO on human
baroreflex gain.
Previous information on the effects of NO on autonomic
activity in humans is limited. Studies comparing the response
of muscle sympathetic nerve activity to an infusion of
L-NMMA with appropriate baroreflex controls indicated an
inhibitory action of endogenous NO on sympathetic nerve
activity in humans.21 Castellano et al22 studied the effects of
L-NMMA on HRV in 7 healthy volunteers. No controls for
alterations in blood pressure were studied, making the results
difficult to interpret, but in agreement with our results, there
were no significant changes in absolute values of HF or LF
RR interval powers or in cross-spectral BRS despite a clear
rise in blood pressure.
Our data appear to indicate that NO exerts a tonic enhancing effect on human cardiac vagal control. The site of action
cannot be determined from our results, but animal data
suggest that this may be due to actions of NO synthesized
within neurons at a number of sites. NO has been shown to
increase neuronal activity within the nucleus tractus solitarius5 (the primary relay site for baroreceptor afferent fibers),
as well as enhancing the activity of medullary vagal motoneurons.7 In the efferent limb of the reflex, NO has been shown
to enhance the bradycardic effects of efferent vagal8 and
muscarinic stimulation.6 “Indirect” vagal activity (the ability
of the efferent vagus to antagonize sympathetic cardiac
responses) is also enhanced by NO.23
In vitro studies have shown that NO may also have direct
nonneurally mediated effects on the sinus node, increasing
heart rate by stimulating the If current.9 In our study the NO
donor (sodium nitroprusside) caused less tachycardia than the
non–NO-dependent agent hydralazine, while inhibition of
NO synthesis caused less bradycardia than the ␣1-adrenergic
agonist phenylephrine. Thus, any direct effects of NO on the
sinus node in humans would appear to have been overwhelmed by opposing effects of NO on cardiac autonomic
control.
There are a number of limitations to our study. It is not
possible for us to be certain that the stimulus to the baroreceptor was identical between our study drugs and their
Chowdhary et al
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controls. We chose to use mean arterial pressure to control for
baroreceptor stimulus because there is good evidence that this
value (rather than pulse pressure or rate of pressure change)
is the main determinant of baroreceptor discharge.24 Furthermore, we did not measure central aortic pressure directly but
assessed blood pressure in the upper limb. However, the
avoidance of invasive procedures permitted a more accurate
estimation of resting cardiac autonomic tone.
It is also possible that the vasoactive drugs we used may
exert different mechanical effects on the arterial baroreceptors. In dogs phenylephrine was found to constrict carotid
arteries, resulting in augmentation of the baroreceptor response.25 However, in humans phenylephrine appears to
cause dilatation rather than constriction of the carotid artery.26
Furthermore, in dogs there was no significant difference in
baroreceptor firing whether intraluminal pressure was raised
by phenylephrine, angiotensin II, or an aortic balloon,27 and
Hirooka et al28 showed that the baroreceptor response was
independent of the effects of drugs on aortic diameter. We
therefore believe that it is unlikely that any mechanical
effects on arterial baroreceptors could explain our results.
Clinical Implications
Our study provides the first demonstration that both endogenous and exogenous NO modulate baroreflex-mediated cardiac vagal control in humans, as indicated by measures of
HRV and BRS. The clinical relevance of this finding is
emphasized by the clear demonstration that both of these
measures of cardiac autonomic control are of prognostic
importance in patients with cardiac disease.1,2 Further work is
required to assess whether these prognostic markers can be
favorably influenced by manipulation of the NO pathway in
patients with cardiac disease. Finally, until recently it was
common to assess BRS in humans with the use of nitrovasodilators to unload the reflex. Our data raise doubts about the
results obtained by this method because of the possibility of
a direct autonomic effect of the NO donor itself.
Acknowledgments
This research was supported by a grant from the British Heart
Foundation. Dr Chowdhary is a British Heart Foundation Junior
Research Fellow, and Dr Townend is a British Heart Foundation
Senior Lecturer.
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Nitric Oxide and Cardiac Autonomic Control in Humans
Saqib Chowdhary, Julian C. Vaile, Janine Fletcher, Hamish F. Ross, John H. Coote and
Jonathan N. Townend
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Hypertension. 2000;36:264-269
doi: 10.1161/01.HYP.36.2.264
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