A Test of the “Epinephrine Hypothesis” in Humans
David S. Goldstein, Anna Golczynska, John Stuhlmuller, Courtney Holmes, Robert F. Rea,
Ehud Grossman, Jacques Lenders
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Abstract—According to the “epinephrine hypothesis,” circulating epinephrine taken up by sympathetic nerves is coreleased
with norepinephrine during sympathetic stimulation and binding of coreleased epinephrine to presynaptic
b-adrenoceptors augments exocytotic release of norepinephrine, contributing to high blood pressure. This study
examined whether infusion of a physiologically active amount of epinephrine affects subsequent vascular responses and
the estimated rate of entry of norepinephrine into regional venous plasma (norepinephrine spillover). Each of 3
experiments included intravenous infusion of 3H-norepinephrine, measurements of forearm vascular resistance, and
intra-arterial infusion of epinephrine (3 ng/min per deciliter forearm volume). In experiment 1, subjects underwent lower
body negative pressure (LBNP225 mm Hg) before and after intra-arterial epinephrine; in experiment 2, LBNP and
intra-arterial yohimbine before and after intra-arterial epinephrine; and in experiment 3, intravenous nitroprusside before
and after intra-arterial epinephrine. In all subjects, intra-arterial epinephrine produced ipsilateral pallor and decreased
forearm vascular resistance. Ipsilateral venous epinephrine increased by 10-fold. Epinephrine did not affect forearm
vasoconstrictor responses to LBNP or vasodilator responses to intra-arterial yohimbine or intravenous nitroprusside; did
not affect venous norepinephrine levels or norepinephrine spillover during LBNP, yohimbine, LBNP during yohimbine,
or nitroprusside; and did not increase venous epinephrine levels during any of these manipulations. Loading of forearm
sympathetic terminals with epinephrine therefore does not augment subsequent neurogenic vasoconstriction or
norepinephrine release in the human forearm in response to sympathetic stimulation. The findings are inconsistent with
the epinephrine hypothesis. (Hypertension. 1999;33:36-43.)
Key Words: epinephrine n norepinephrine n nervous system, sympathetic n yohimbine n nitroprusside
S
everal studies have reported that epinephrine can augment neurogenic vasoconstriction.1,2 Majewski and coworkers3,4 proposed a model to explain this. According to the
“epinephrine hypothesis,” sympathetic nerve terminals take
up circulating epinephrine by neuronal uptake (uptake-1);
sympathetic stimulation coreleases epinephrine with norepinephrine, the sympathetic neurotransmitter; the coreleased
epinephrine binds to b-adrenoceptors on sympathetic terminals; and binding of coreleased epinephrine to presynaptic
b-adrenoceptors augments norepinephrine release during
sympathetic stimulation.
The epinephrine hypothesis helps to explain how excessive
adrenomedullary hormonal system activity can contribute to
the development of essential hypertension by augmenting
sympathoneural norepinephrine release.3–5 More generally,
the hypothesis provides a mechanism whereby endogenous
compounds taken up into nerve terminals can undergo corelease with the transmitter and prolong or exaggerate release
of the transmitter by binding to facilitatory presynaptic
receptors.
Previous studies about the epinephrine hypothesis in humans have led to different conclusions. Floras and coworkers1,6 reported that 30 minutes after brachial intra-arterial
epinephrine infusion (50 ng/min), lower body negative pressure (LBNP) elicited a larger forearm vasoconstrictor response than before the epinephrine infusion, consistent with
the epinephrine hypothesis. In contrast, Stein et al7 reported
that intra-arterial epinephrine infusion at the same dose did
not augment forearm norepinephrine spillover.
Previous studies did not assess whether epinephrine augments norepinephrine spillover responses to LBNP or to other
stimuli that increase sympathoneural exocytosis. In the present study we hoped to obtain this information and test the
epinephrine hypothesis more directly in 3 experiments. In
each, epinephrine was infused intra-arterially into the brachial
artery for a prolonged period (40 minutes) at a physiologically active dose to load sympathetic terminals. Regional
vascular and neurochemical responses to manipulations expected to increase norepinephrine release were assessed
before and after cessation of intra-arterial epinephrine. The
Received May 28, 1998; first decision June 24, 1998; revision accepted September 11, 1998.
From the Clinical Neuroscience Branch, National Institute of Neurological Disorders and Stroke, Bethesda, Md (D.S.G., A.G., C.H.); Food and Drug
Administration, Department of Health and Human Services, Rockville, Md (J.S.); Mayo Clinic, Rochester, Minn (R.F.R.); Department of Internal
Medicine D, Chaim Sheba Medical Center, Tel Ha-Shomer, Israel (E.G.); and Department of Internal Medicine, St Radboud University Hospital
Nijmegen, Nethlerlands (J.L.).
Correspondence to Dr David S. Goldstein, Building 10, Room 6N252, NINDS, NIH, 10 Center Dr MSC-1620, Bethesda, MD 20892-1620. E-mail
[email protected]
© 1999 American Heart Association, Inc.
Hypertension is available at http://www.hypertensionaha.org
36
Goldstein et al
January 1999
37
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Figure 1. Flow diagrams for the 3 experiments in this study. Each included intravenous infusion of 3H-norepinephrine (3H-NE), measurements of FVR, and intra-arterial infusion of epinephrine (EPI) (3 ng/min per milliliter forearm volume). In experiment 1, the subjects
underwent LBNP (225 mm Hg) to stimulate sympathetic outflows before and after intra-arterial infusion of EPI. In experiment 2, LBNP
was applied in the setting of intra-arterial yohimbine (YOH) to block local a2-adrenoceptors, and with LBNP and YOH before and after
intra-arterial infusion of EPI. In experiment 3, the subjects underwent intravenous infusion of nitroprusside (NIP) to stimulate sympathetic outflows before and after intra-arterial infusion of EPI.
study used 2 systemic, 1 local, and a combination of systemic
and local stimuli of sympathetically mediated exocytosis:
LBNP,8 intravenous nitroprusside,9 intra-arterial yohimbine,10 and LBNP in the setting of intra-arterial yohimbine.
By blocking presynaptic a2-adrenoceptors, yohimbine augments norepinephrine release for a given amount of sympathetic traffic.10 3H-Norepinephrine was infused to distinguish
norepinephrine spillover from clearance as determinants of
plasma norepinephrine levels.
Methods
Subjects
Forty healthy, nonobese adult volunteers participated in this study
after giving written informed consent. The protocol of the study was
approved by the Intramural Research Board of the National Institute
of Neurological Disorders and Stroke.
In all subjects, the medical history, physical examination, and
routine laboratory tests (including complete blood count; serum
glucose; renal, liver, and thyroid function tests; plasma cortisol;
urinalysis; and ECG) showed no evidence of cardiovascular or any
other diseases. None of the subjects took any medication for at least
2 weeks before the study. Subjects were instructed to refrain from
smoking cigarettes or drinking alcohol or caffeine-containing beverages for at least 12 hours before the experiment.
Experimental Procedures
Epinephrine was given intra-arterially at a dose (3 ng/dL forearm
volume per minute) that pilot studies demonstrated produced clear
local vasomotor effects (hand pallor and decreased forearm vascular
resistance [FVR]) without systemic hemodynamic changes.3H-Norepinephrine was given intravenously (1 mCi/mL, 0.75 mL/min) to
quantify norepinephrine spillover. Stimuli in each subject were
repeated beginning 30 minutes after cessation of the 40-minute
intra-arterial infusion of epinephrine.
Each experiment was performed in the morning in a quiet room at
a temperature of 22°C to 23°C, with the subject supine. A brachial
artery was cannulated for intra-arterial blood pressure monitoring
and for local infusions and blood sampling. In the same arm, the
antecubital vein was cannulated for blood sampling. The antecubital
vein of the other arm was cannulated for systemic infusions. Forearm
blood flow (FBF) was measured by strain-gauge, venous occlusion
plethysmography (D.E. Hokanson) and expressed as milliliters of
blood flow per deciliter forearm tissue per minute. Hand blood flow
was excluded by a wrist cuff inflated to '100 mm Hg above systolic
pressure. For each FBF determination at least 5 measurements were
performed, and the results were averaged.
Experiment 1
This experiment was designed to determine effects of epinephrine
loading of sympathetic terminals in the forearm on regional vasoconstrictor and norepinephrine spillover responses to sympathetic
stimulation induced by LBNP. Each of the 8 subjects received
3
H-norepinephrine intravenously for 20 minutes. FBF was measured
and blood sampled from the artery and antecubital vein of the same
arm (BL-1, Figure 1). With the 3H-norepinephrine infusion continuing, the subject underwent LBNP at 225 mm Hg for 20 minutes.
FBF was measured and blood sampled from the artery and antecubital vein of the same arm (LBNP-1). The intravenous infusion of
3
H-norepinephrine was then stopped. Epinephrine was infused intraarterially for 40 minutes. At the end of the epinephrine infusion, FBF
was measured. Ten minutes later, an antecubital venous blood
38
Epinephrine Hypothesis
sample was obtained, and the intravenous infusion of 3H-norepinephrine was restarted. After 20 minutes, FBF was measured and blood
sampled from the artery and antecubital vein of the same arm (BL-2).
Finally, with the 3H-norepinephrine infusion continuing, the subject
underwent LBNP again for 20 minutes. FBF was measured and
blood sampled from the artery and antecubital vein of the same arm
(LBNP-2).
The catechols in 1-mL aliquots of plasma were assayed by batch
alumina extraction followed by high-pressure liquid chromatography
with electrochemical detection.13 The limit of detection was '5
pg/mL for each catechol.3H-Norepinephrine in plasma and the
infusate was measured in fractions of column effluent by liquid
scintillation spectrometry.14
Experiment 2
Data Analysis
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This experiment was designed to determine effects of blockade by
yohimbine of regional a2-adrenoceptors and effects of epinephrine
loading of sympathetic terminals on regional vasoconstrictor and
norepinephrine spillover responses to sympathetic stimulation induced by LBNP. Each of the 8 subjects received 3H-norepinephrine
intravenously for 20 minutes. FBF was then measured and blood
sampled from the artery and antecubital vein of the same arm (BL-1,
Figure 1). With the 3H-norepinephrine infusion continuing, the
subject received yohimbine intra-arterially (1.3 mg/kg per minute
bolus for 5 minutes, then 0.3 mg/kg per minute for 10 minutes). FBF
was measured and blood sampled from the artery and antecubital
vein of the same arm (yohimbine-1). The subject then underwent
LBNP at 225 mm Hg for 20 minutes, with the yohimbine infusion
continuing. FBF was measured and blood sampled from the artery
and antecubital vein of the same arm (LBNP-1). The intravenous
infusion of 3H-norepinephrine was stopped. Epinephrine was then
infused intra-arterially for 40 minutes. At the end of the epinephrine
infusion, FBF was measured. Ten minutes later, an antecubital
venous blood sample was obtained, and the intravenous infusion of
3
H-norepinephrine was restarted. After 20 minutes, FBF was measured and blood sampled from the artery and antecubital vein of the
same arm (BL-2). Finally, with the 3H-norepinephrine infusion
continuing, the subject underwent intra-arterial infusion of yohimbine and LBNP again (LBNP-2). FBF was measured and blood
sampled from the artery and antecubital vein of the same arm.
Experiment 3
This experiment was designed to determine effects of epinephrine
loading of sympathetic terminals in the forearm on regional vasomotor and norepinephrine spillover responses to sympathetic stimulation induced by intravenous nitroprusside. Each of 7 subjects
received 3H-norepinephrine intravenously for 20 minutes. FBF was
then measured and blood sampled from the artery and antecubital
vein of the same arm (BL-1, Figure 2). With the 3H-norepinephrine
infusion continuing, the subject underwent intravenous infusion of
nitroprusside (increasing doses beginning at 0.5 mg/kg per minute) to
achieve a clear increase in pulse rate and small (target, 5 mm Hg)
decrease in mean arterial pressure for 20 minutes. FBF was measured
and blood sampled from the artery and antecubital vein of the same
arm (nitroprusside-1). The intravenous infusion of 3H-norepinephrine was stopped. Epinephrine was then infused intra-arterially for 40
minutes. At the end of the epinephrine infusion, FBF was measured,
and in 4 subjects antecubital blood was obtained from both arms. Ten
minutes later, in the other 3 subjects, an antecubital venous blood
sample was obtained. The intravenous infusion of 3H-norepinephrine
was restarted. After 20 minutes, FBF was measured and blood
sampled from the artery and antecubital vein of the same arm (BL-2).
Finally, with the 3H-norepinephrine infusion continuing, the subject
underwent nitroprusside intravenous infusion again at the previously
established dose for 20 minutes. FBF was measured and blood
sampled from the artery and antecubital vein of the same arm
(nitroprusside-2).
Supplementary Experiment
Each of 7 subjects underwent intravenous nitroprusside infusion at
increasing doses (0, 0.3, 0.65, and 1.3 mg/kg per minute), with
measurements of total body and forearm norepinephrine spillover at
each dose and, in 6 subjects, with peroneal skeletal sympathetic
nerve traffic (muscle sympathetic nerve activity [MSNA]) measured
simultaneously by microneurography.11,12
Assays
Forearm spillover of norepinephrine was calculated with the following equation15:
Norepinephrine Spillover5Q[V2A~12E)]
where Q is the regional plasma flow, V is the concentration of
norepinephrine in regional venous plasma, A is the concentration of
norepinephrine in arterial plasma, and E is the regional extraction
fraction of 3H-norepinephrine.
Results are presented as mean6SEM. We used t tests for paired
samples to examine effects of intra-arterial epinephrine on hemodynamics and levels of catechols. In the supplementary experiment, an
ANOVA for repeated measures was done to assess the dose
relatedness of hemodynamic and neurochemical changes. A P value
,0.05 defined statistical significance.
Results
Forearm Blood Flow and Vascular Resistance
LBNP significantly decreased FBF and increased FVR, with
or without concurrent intra-arterial yohimbine infusion,
whereas intra-arterial epinephrine, intra-arterial yohimbine,
and intravenous nitroprusside all decreased FVR (Figure 2).
Infusion of epinephrine intra-arterially did not affect responses of FBF or of FVR to LBNP, LBNP in the setting of
intra-arterial yohimbine, or intravenous nitroprusside.
Plasma Epinephrine
In all subjects, the arterial epinephrine concentration exceeded the venous epinephrine concentration (Figure 3). At
the end of the intra-arterial infusion of epinephrine, the mean
ipsilateral venous plasma epinephrine concentration averaged
'10 times the contralateral concentration (1.6260.27 versus
0.1660.08 nmol/L; n54). Across the 3 experiments, the
mean values for arterial and venous epinephrine at BL-2
significantly exceeded those at BL-1 (t53.02, 2-tailed
P50.007; t52.87, 2-tailed P50.005, respectively).
LBNP elicited relatively small, statistically nonsignificant
increases in arterial and venous epinephrine concentrations,
with similar epinephrine responses before and after intra-arterial epinephrine infusion, both with and without concurrent
yohimbine administration. Nitroprusside increased epinephrine concentrations inconsistently. Venous plasma epinephrine concentrations during nitroprusside infusion were similar
before and after intra-arterial epinephrine infusion.
Plasma Norepinephrine
Exposure to LBNP increased arterial and venous norepinephrine levels (t56.47, P,0.001; t53.67, P50.008, respectively; Figure 4). Nitroprusside increased arterial and venous
norepinephrine levels in all 7 of the 7 subjects who received
it (P50.008 by the nonparametric sign test). Yohimbine
given intra-arterially increased venous plasma norepinephrine
levels in all 6 subjects who received it (t56.05, P50.002) but
did not affect arterial norepinephrine levels.
Goldstein et al
January 1999
39
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Figure 2. Mean values (6SEM) for FBF (left) and FVR (right). Top, In experiment 1, measurements were made at baseline (BL-1, BL-2)
and during LBNP (LBNP-1, LBNP-2) before and at the end of intra-arterial infusion of epinephrine (EPI) for 40 minutes. Middle, In
experiment 2, measurements were made at baseline (BL-1, BL-2), during intra-arterial infusion of yohimbine (YOH-1, YOH-2), and during LBNP (LBNP-1, LBNP-2) before and after intra-arterial EPI. Bottom, In experiment 3, samples were obtained at baseline (BL-1,
BL-2) and during administration of nitroprusside (NIP-1, NIP-2) before and at the end of intra-arterial EPI.
After intra-arterial infusion of epinephrine, arterial and
venous norepinephrine levels were similar to the corresponding levels before epinephrine infusion, at baseline, during
LBNP, during yohimbine, during LBNP in the setting of
yohimbine, and during nitroprusside.
Forearm Norepinephrine Spillover
Across the 3 experiments, forearm norepinephrine spillover at BL-2 averaged 23% higher than at BL-1 (t53.59,
2-tailed P50.002). Whereas LBNP increased arterial (“total body”) norepinephrine spillover significantly (t54.41,
P50.003), LBNP failed to increase forearm norepinephrine spillover (Figure 5), regardless of epinephrine
infusion.
Intra-arterial yohimbine increased forearm norepinephrine spillover similarly before and after intra-arterial epinephrine infusion. LBNP in the setting of yohimbine failed
to increase forearm norepinephrine spillover before or
after intra-arterial epinephrine infusion. Systemic infusion
of nitroprusside increased forearm norepinephrine spillover substantially (t53.88, P50.008); however, intra-arterial epinephrine infusion did not enhance the
nitroprusside-induced increment in forearm norepinephrine spillover.
Supplementary Experiment
Nitroprusside given intravenously produced dosedependent increases in pulse rate (F530.2, P50.0001),
40
Epinephrine Hypothesis
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Figure 3. Mean values (6SEM) for concentrations of epinephrine in arterial (light cross-hatching) and venous (dark crosshatching) plasma. The same experiments as in Figure 2 were
performed. Ipsilateral antecubital venous blood was obtained 10
minutes after cessation of intra-arterial epinephrine infusion (EPI
inf. i.a.). Other abbreviations are as in Figure 2 legend.
arterial plasma norepinephrine (F514.3, P50.0001), total
body norepinephrine spillover (F516.8, P50.0001), and
MSNA (F54.2, P50.03). Dose-related changes in mean
arterial pressure, arterial plasma epinephrine, and forearm
norepinephrine spillover were not statistically significant
(Table). Intravenous nitroprusside at the highest dose
elicited an '3-fold increase in MSNA but only an '60%
increase in forearm norepinephrine spillover.
Figure 4. Mean values (6SEM) for concentrations of norepinephrine in arterial (light cross-hatching) and venous (dark
cross-hatching) plasma. The same experiments as in Figure 2
were performed. Ipsilateral antecubital venous blood was
obtained 10 minutes after cessation of intra-arterial epinephrine
infusion (EPI inf. i.a.). Other abbreviations are as in Figure 2
legend.
Discussion
Several studies have reported that administration of physiologically active amounts of epinephrine enhances neurogenic
vasoconstrictor responses or enhances release of norepinephrine during sympathetic stimulation. The epinephrine hypothesis explains this as follows: Sympathetic nerve terminals
take up circulating epinephrine; sympathetic stimulation
coreleases the removed epinephrine with norepinephrine; the
Goldstein et al
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Figure 5. Mean values (6SEM) for the estimated rate of entry
(spillover [SO]) of norepinephrine (NE) in the forearm (FA). The
same experiments as in Figure 2 were performed. Other abbreviations are as in Figure 2 legend.
coreleased epinephrine binds to b-adrenoceptors on sympathetic terminals; and the binding of coreleased epinephrine to
the b-adrenoceptors augments further norepinephrine release,
enhancing neurogenic vasoconstriction during sympathetic
stimulation.
In the present experiments to test the epinephrine hypothesis, we used a physiologically active dose of intra-arterial
epinephrine (3 ng/min per deciliter forearm volume), as
indicated by pallor of the hand and by increased FBF in all
subjects. The 40-minute infusion increased ipsilateral venous
January 1999
41
epinephrine levels by 10-fold without affecting contralateral
venous epinephrine levels, indicating effective local concentrations without detectable increases in epinephrine levels in
the systemic circulation.
The epinephrine hypothesis predicts that loading of local
sympathetic terminals with epinephrine should enhance vasoconstrictor responses to subsequent sympathetic stimulation; however, intra-arterial epinephrine failed to augment
subsequent vasoconstrictor responses to LBNP. Even in the
setting of intra-arterial infusion of yohimbine, to block
possible autoinhibition of norepinephrine release by a2adrenoceptors, epinephrine loading failed to augment LBNPinduced increases in FVR. Analogously, the epinephrine
hypothesis predicts that during intravenous nitroprusside
infusion, the extent of forearm vasodilation before epinephrine infusion should exceed that after epinephrine infusion
because of enhanced reflexive release of norepinephrine from
sympathetic terminals; however, the extent of nitroprussideinduced forearm vasodilation remained the same after as
before intra-arterial epinephrine infusion.
Because of extraction of circulating epinephrine in passage
through forearm tissues, arterial plasma epinephrine levels
normally substantially exceed antecubital venous levels. According to the epinephrine hypothesis, after cessation of
intra-arterial epinephrine infusion, epinephrine corelease with
norepinephrine during sympathetic stimulation should decrease the magnitude of the arteriovenous decrement in
plasma epinephrine levels. In all 3 experiments, however, the
magnitude of the arteriovenous decrement in plasma epinephrine levels remained about the same after as before intra-arterial epinephrine infusion. The possibility remains that
locally released epinephrine mainly undergoes extraneuronal
uptake and metabolism before it can enter the venous drainage. A study in which intra-arterial infusion of 3H-epinephrine is used could test this.
Finally, and most importantly, the epinephrine hypothesis
predicts that epinephrine loading of sympathetic terminals
should enhance norepinephrine release during manipulations
that increase exocytosis as a result of binding of coreleased
epinephrine to stimulatory b-adrenoceptors on the terminals.
In the present study, however, epinephrine loading did not
augment arteriovenous increments in plasma norepinephrine
levels or forearm norepinephrine spillover responses to
LBNP, intra-arterial yohimbine, LBNP during intra-arterial
yohimbine, or intravenous nitroprusside.
Inadequate numbers of subjects in the 3 experiments
probably do not explain the negative results. For instance, in
experiment 3, when we considered forearm norepinephrine
spillover as the dependent measure, at a significance level (a)
of 0.05, 7 subjects, a population standard deviation (s) of 1.0
pmol/min per deciliter, and a sought response to nitroprusside
after intra-arterial epinephrine administration 50% larger than
the response to nitroprusside before epinephrine administration (d, 4.2 pmol/min per deciliter), f5d/s54.2, the power
would exceed 0.95 by far.16
LBNP reflexivity decreases FBF, and forearm norepinephrine spillover depends to some extent on regional blood
flow.15 Thus, the failure to increase forearm norepinephrine
spillover during LBNP could have resulted from combined
42
Epinephrine Hypothesis
Effects of Intravenous Nitroprusside on Hemodynamic Variables, Plasma Levels of Catechols, and Peroneal Muscle
Sympathetic Nerve Activity
HR,*
bpm
MAP,
mm Hg
NEa,*
nmol/L
EPIa,
nmol/L
Total Body NE
Spillover,*
nmol/min
Forearm NE
Spillover,*
pmol/(min z dL)
MSNA,†
bursts/min
0.00
6663
8363
1.660.4
0.560.3
4.461.3
2.060.6
1964
0.30
7365
8263
2.260.4
0.460.1
5.661.3
3.160.9
2363‡
0.65
7465
8365
2.560.5
0.560.2
6.561.7
2.961.1
3067
1.30
8464
8165
3.060.4
0.560.1
7.961.6
3.060.8
3465
NIP Dose,
mg/(kg z min)
Values are mean6SEM. NIP indicates nitroprusside; HR, heart rate; MAP, mean arterial pressure; NEa, arterial norepinephrine; EPIa, arterial
epinephrine; and NE, norepinephrine.
*Significant dose effect by ANOVA, P,0.05.
†Significant dose effect by ANOVA, P50.0001.
‡Data for 1 outlyer deleted.
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influences of vasoconstriction, which would have decreased
norepinephrine spillover, and increased exocytotic release of
norepinephrine, which would have increased norepinephrine
spillover. The flow dependence of norepinephrine spillover
has led investigators to devise a flow-independent index of
norepinephrine release into the extravascular space (extravascular appearance rate [EAR]),17 on the basis of the formula:
EAR5Norepinephrine Spillover/(12E), where E is the regional extraction fraction of 3H-norepinephrine. When we
apply the formula to the data in the present study, intravenous
nitroprusside increased forearm EAR from 4.962.0 to
19.6610 pmol/min per deciliter, or '5-fold, before epinephrine loading and from 4.260.8 to 10.663.2 pmol/min per
deciliter after epinephrine loading, indicating no epinephrinerelated enhancement of norepinephrine release into the extravascular space. When we apply the same formula to the
data in experiment 1, LBNP at 225 mm Hg decreased EAR
nonsignificantly by 30% before epinephrine loading and
increased EAR nonsignificantly by 53% after epinephrine
loading. In experiment 2, LBNP in the setting of intra-arterial
yohimbine increased EAR nonsignificantly by 11% before
epinephrine loading and nonsignificantly by 24% after epinephrine loading. Thus, on correction for flow dependence of
norepinephrine spillover, LBNP did not increase the responses of EAR to stimuli that increase exocytosis. The
potentially confounding effects of decreased blood flow also
would not explain the present finding of a larger proportional
increase in peroneal MSNA than in the forearm norepinephrine spillover during intravenous nitroprusside infusion, since
nitroprusside increases FBF.
Consistent with the epinephrine hypothesis, Floras and
coworkers1,6 reported that after intra-arterial infusion of
epinephrine, LBNP elicited a larger forearm vasoconstrictor
response than before the infusion. In contrast, Stein et al7
used intra-arterial infusion of isoproterenol to stimulate forearm norepinephrine spillover and observed no delayed facilitatory effects of intra-arterial isoproterenol in either normotensive or borderline hypertensive subjects. Thompson et al18
reported that intravenous epinephrine did not augment cardiac norepinephrine spillover in humans. Adrenalectomized
humans have normal vascular and plasma norepinephrine
responses to various stimuli,8 which also argues against a
modulatory role of circulating epinephrine in neurogenic
vasoconstriction. Preclinical studies designed to test the
epinephrine hypothesis comprehensively have also failed to
confirm it.19 –21
The basis for the absence, in the present study, of LBNPinduced increases in values for indices of local norepinephrine release, despite consistent LBNP-induced sympathoneural stimulation9 and forearm vasoconstriction, remains
obscure. Others22 have noted relatively small increases in
forearm norepinephrine spillover during LBNP at
215 mm Hg. Analogously, in the present study,
nitroprusside-induced proportional increases in directly recorded MSNA exceeded those in forearm norepinephrine
spillover. Most of 3H-norepinephrine entering the forearm
undergoes removal by nonneuronal cells,23 whereas most of
endogenously released norepinephrine undergoes removal by
neuronal uptake. Perhaps in the forearm, assumptions about
mixing of endogenous and 3H-norepinephrine may not apply.
Total body norepinephrine spillover and arterial and venous plasma epinephrine levels all were higher 30 minutes
after cessation of intra-arterial epinephrine infusion than
before the infusion. A small amount of epinephrine could
have remained in the stopcock apparatus after the intra-arterial infusion; however, this seems unlikely, because fluid
lacking epinephrine was infused continuously via the same
apparatus during the 30 minutes after cessation of intra-arterial epinephrine infusion. Retention of epinephrine in the
stopcock would also not explain the higher value for total
body norepinephrine spillover after than before intra-arterial
epinephrine. Stein et al7 also reported higher total body
norepinephrine spillover after than before intra-arterial epinephrine. They speculated that the increase in total body
norepinephrine spillover after intra-arterial epinephrine could
have resulted from effects outside the forearm of epinephrine
that had entered the systemic circulation. The present results
cast doubt on this explanation, because whereas intra-arterial
epinephrine infusion increased venous epinephrine ipsilaterally by 10-fold, the infusion did not increase venous epinephrine contralaterally at all; in other words, little if any of the
locally infused epinephrine entered the systemic circulation.
Persson et al24 reported that after cessation of systemic
epinephrine infusion, elevated MSNA and plasma norepinephrine persisted, by mechanisms that were obscure. Thus, 3
studies by different groups have obtained evidence for sus-
Goldstein et al
tained stimulatory effects of infused epinephrine on sympathetic outflows. A possibility is that intra-arterial epinephrine
somehow alters afferent information to the central nervous
system, increasing sympathoneural and adrenomedullary outflows. One could test this by measuring MSNA after brachial
intra-arterial infusion of epinephrine. In the present study,
this was done successfully in 5 subjects, and in 4 of the 5,
MSNA was higher at BL-2 than at BL-1; however, the mean
increase (15%) was not statistically significant, and when the
low number of subjects is considered, the data are
inconclusive.
In summary, because epinephrine loading failed to augment responses of FVR, regional venous epinephrine levels,
or indices of norepinephrine release during exposure to
stimuli that increase sympathetically mediated exocytosis, the
present findings are inconsistent with the epinephrine hypothesis in humans.
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A Test of the ''Epinephrine Hypothesis'' in Humans
David S. Goldstein, Anna Golczynska, John Stuhlmuller, Courtney Holmes, Robert F. Rea, Ehud
Grossman and Jacques Lenders
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Hypertension. 1999;33:36-43
doi: 10.1161/01.HYP.33.1.36
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