Download Cardiovascular effects and cardiopulmonary

Clinical Science (2002) 103, 535–542 (Printed in Great Britain)
Cardiovascular effects and cardiopulmonary
plasma gradients following intravenous
infusion of neuropeptide Y in humans:
negative dromotropic effect on
atrioventricular node conduction
Bengt ULLMAN*, John PERNOW†, Jan M. LUNDBERG‡, Hans AH STRO= M§
and Lennart BERGFELDT†
*Department of Cardiology, So$ der Hospital, SE-118 83 Stockholm, Sweden, †Department of Cardiology, Thoracic Clinics,
Karolinska Hospital, Stockholm, Sweden, ‡Division of Pharmacology, Department of Physiology and Pharmacology, Karolinska
Institutet, Stockholm, Sweden, and §Department of Clinical Physiology, Thoracic Clinics, Karolinska Hospital, Stockholm, Sweden
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Neuropeptide Y (NPY) is co-released with noradrenaline from sympathetic nerves, has a strong
vasoconstrictive action, and causes an attenuation of parasympathetic action in animal
experiments. The plasma level of NPY is greatly elevated in patients with congestive heart
failure, but the clinical relevance of this finding is unclear. Central haemodynamic effects, cardiac
conduction system electrophysiology and coronary sinus blood flow were therefore studied in
two sets of experiments, each carried out on seven healthy men. In the first series, NPY was
given intravenously at doses of 3, 10 and 30 pmol : min−1 : kg−1, and in the second it was given as
a bolus injection of 90, 200 or 900 pmol/kg, which resulted in plasma concentrations similar to
those seen in heart failure patients. During continuous infusion of NPY, systemic blood pressure
increased slightly, but myocardial perfusion, cardiac output, pulmonary arterial pressure,
cardiac conduction intervals and atrioventricular (AV) node functional measures remained
unchanged. In contrast, the bolus injection of NPY evoked prolongation and block (in four out
of seven subjects) of AV node conduction, but did not affect haemodynamic variables, apart from
a minor increase in systemic blood pressure. Impaired AV node conduction is a novel observation,
which might reflect a baroreceptor-mediated vagal reflex, or – more likely – an NPY-induced
direct negative dromotropic effect, caused by a reduction of the L-type calcium current as
observed in vitro, or a combination of the two.
INTRODUCTION
Neuropeptide Y (NPY) is co-released with the classical
transmittor substance noradrenaline from sympathetic
nerve terminals. Immunohistochemical studies have
shown that NPY-immunoreactive fibres are present in
high density in the mammalian heart and around blood
vessels (especially arteries) [1]. The highest density is
found around the coronary arteries, followed by the
conduction system and the myocardium [1–3]. Systemic
NPY infusions in the pig have revealed that certain
regions, such as skeletal muscle, kidney and spleen, are
very sensitive to NPY [4]. Furthermore, threshold
vasoconstrictor effects in the human renal circulation are
Key words : atrioventricular node, electrophysiology, haemodynamics, humans, neuropeptide Y.
Abbreviations : AV, atrioventricular ; CHF, congestive heart failure ; CSBF, coronary sinus blood flow ; NPY, neuropeptide Y ;
NPY-LI, NPY-like immunoreactivity ; PCV, pulmonary capillary venous.
Correspondence : Dr Bengt Ullman (e-mail bengt.ullman!sos.sll.se).
# 2002 The Biochemical Society and the Medical Research Society
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B. Ullman and others
obtained upon NPY infusion at plasma concentrations of
approx. 260 pmol\l [5]. The vasoconstrictor effect
of NPY potentiates α-adrenoceptor-mediated vasoconstriction in vitro [6], but is unaltered after α- and βadrenoceptor blockade [7,8]. NPY infusion into coronary
arteries in the rabbit, guinea pig, dog and pig causes a
sustained decrease in coronary perfusion [2,9–12]. Intracoronary NPY infusion in patients with angina pectoris
without coronary stenosis produces transient myocardial
ischaemia with typical ECG changes, decreased arterial
blood pressure and bradycardia [13]. In addition to
coronary vasoconstriction, NPY also inhibits vagal parasympathetic neurotransmission in the dog heart, as
reflected by a decreased vagal-induced bradycardia [14].
More recent experimental studies show that relatively
low concentrations of NPY have electrophysiological
effects on the myocardium both in vitro [15] and in vivo
[16], but the potential synergistic effects of noradrenaline
and its co-transmittor NPY on electrophysiological
function under pathological conditions have been little
studied [17].
Plasma NPY-like immunoreactivity (NPY-LI) is elevated in humans during sympathetic activation, such as
heavy physical exercise [18,19], and in patients with acute
myocardial infarction [20]. Severe congestive heart failure
(CHF) is associated with markedly elevated (up to
2000 pmol\l) plasma levels of NPY-LI [21,22]. It is not
clear, however, whether this is of clinical relevance, or
just an epiphenomen.
The aim of the present study was therefore to evaluate
whether doses of exogenous NPY that result in plasma
levels found in patients with CHF have any pharmacological effects on central haemodynamics, myocardial
blood flow or cardiac electrophysiology (the conduction
system) in healthy men. In addition, we wanted to
determine the possible clearance of NPY over the
pulmonary and coronary vascular beds.
METHODS
Subjects
This study was performed as two separate sets of
experiments on a total of 14 healthy male volunteers.
Their mean age was 28 years (range 23–35 years), height
180 cm (range 170–187 cm), and weight 79 kg (range
65–92 kg). They were all non-smokers and were not
on any medication. They were fully informed about
the procedures. The study protocol was approved by the
Ethics Committee of the Karolinska Hospital.
Catheterization and cardiovascular
measurements
All catheterizations were performed in the morning with
the subject non-sedated and fasting. No tea or coffee was
ingested during a period of 12 h prior to the study.
# 2002 The Biochemical Society and the Medical Research Society
A thermodilution catheter was inserted percutaneously
via a medial cubital vein and advanced into the coronary
sinus. The position of the tip of the catheter was verified
by injection of contrast medium.
Teflon catheters were inserted percutaneously into the
brachial artery and the femoral vein. A 7F Swan-Ganz
catheter was inserted through the femoral vein and
advanced to the pulmonary artery. The pulmonary
and brachial arterial catheters were connected to electromechanical transducers (Siemens-Elema, EMT 34). The
pressures were recorded on a Mingograph (Sicor) and
processed in a computer. The mid-thoracic level was used
as the zero reference for pressures. Cardiac output was
measured according to the Fick principle. Stroke volume
was calculated from cardiac output and heart rate.
Pulmonary capillary venous (PCV) pressure could not be
obtained in two patients, for whom PCV pressure was
substituted by diastolic pulmonary artery pressure.
Blood samples for measurements of oxygen saturation
were drawn from the pulmonary artery, the brachial
artery and the coronary sinus. Coronary sinus blood
flow (CSBF) was measured by the continuous thermodilution technique [23] using isotonic saline at room
temperature. The flow was calculated using computerized
on-line integration of the temperature curve. The mean
value of three measurements was used. Myocardial
oxygen consumption was calculated according to the
Fick principle from CSBF and the oxygen difference
between arterial and coronary sinus blood.
Two 6F quadripolar electrode catheters (USCI)
were introduced through the right femoral vein using
10-cm, 7F introductory sets. One catheter was positioned
high on the lateral wall in the right atrium for pacing and
recording, and one was positioned over the tricuspid
valve in the His bundle position for recording of the low
right atrial activity and His bundle potential. Right atrial
pacing was performed with a current that was 2–4
times the diastolic threshold. The following electrophysiological variables were assessed, with estimated
detection levels of interventions within parentheses [24] :
intra-atrial conduction time (PA interval ; 15 %) ;
atrioventricular (AV) node conduction time (AH interval ; 10 %) ; His-Purkinje conduction time (HV
interval ; 15 %) ; Wenckebach cycle length ( 10 %) ;
effective refractory period of the AV node ( 20 %),
intraventricular depolarization and conduction time
(QRS interval; 10 %) ; and intraventricular depolarization, conduction and repolarization time [QT interval,
which was heart rate corrected according to Bazett (QTc
l QT\NRR), in s (5 %)]. Surface ECG leads I, II, V1,
V2 and V6 were recorded together with intracardiac
signals on a multi-channel ink jet recorder (SiemensElema, Solna, Sweden) at a paper speed of 100 mm\s.
All conduction intervals were measured by an experienced technician on a digitizer table working in point
mode with a resolution of 10 lines\mm and an accuracy
Cardiovascular effects of neuropeptide Y in humans
During the infusion of the low dose of NPY
(3 pmol : min−" : kg−"), only blood samples for determination of clearance were collected, whereas haemodynamic and electrophysiological variables were determined during infusion of the two higher doses. Pacing
was stopped for 10 min during each NPY infusion for the
assessment of the conduction intervals at a spontaneous
rate, and the evaluation of AV node function. At the end
of the last infusion, atropine was given intravenously
(0.04 mg\kg) in order to evaluate the influence of NPY
on vagal tone. The study protocol is shown in Figure 1
(upper panel).
In the second part of the study, NPY was given as
bolus injections into the femoral vein at doses of 90, 200
and 900 pmol\kg. Blood samples for analysis of NPY-LI
in the femoral vein were drawn at 0, 1, 2, 5 and 10 min
after each injection. The study protocol is shown in
Figure 1 (lower panel). In this part of the study, neither
cardiac output nor pulmonary arterial pressure were
determined.
Figure 1 Study protocols for the continuous infusion (upper
panel) and bolus injection (lower panel) of NPY
‘ Time ’ is the time (min) in relation to the start of the first infusion/injection ; NPY
inf 3, 10 and 30 represent NPY infusion at 3, 10, and 30 pmol : min−1 : kg−1 ;
NPY bolus 1, 2 and 3 represent bolus injections of 90, 200, and 900 pmol/kg.
A, intravenous injection of atropine (0.04 mg/kg) ; RAP, right atrial pacing ;
H, haemodynamic measurement ; E (EPS), electrophysiological study. The time
points at which blood samples were taken are indicated by arrows.
of p0.635 mm (Calcomp 2000 ; Digitizer Products Division). The average of at least three intervals was
calculated. The Wenckebach cycle length was determined
by incremental atrial pacing from just above the basic
rate. The effective refractory period of the AV node was
determined by the extrastimulus (S1S2) technique after 8beat trains of regular atrial pacing at 100 beats\min with
an accuracy of 10 ms.
Synthetic human NPY was dissolved in saline containing 0.5 % human albumin and sterile filtered through
a Millipore filter prior to infusion. NPY was administered
as a continuous infusion at three different concentrations
in seven men, and as bolus injections at three different
doses in a separate group of seven men.
Experimental protocols
Following catheterization, the subjects were allowed
to rest in the supine position for 30 min. Pressures
in the brachial artery, pulmonary artery and PCV
were measured, and cardiac output and CSBF were
determined. Blood samples from the brachial artery,
pulmonary artery and coronary sinus were drawn
simultaneously. Right atrial pacing was started at
100 beats\min to assure a constant heart rate.
In the first part of the study, NPY was infused into the
femoral vein at a constant rate of 1 ml\min for 20 min at
three different doses (3, 10 and 30 pmol : min−" : kg−").
Analysis of plasma NPY-LI
Blood samples were collected in parallel from the brachial
artery, pulmonary artery and coronary sinus during
protocol 1, and from the femoral vein during protocol 2.
The blood samples were put into pre-chilled heparinized
tubes. The samples were placed in an ice bath and
centrifuged at 3000 rev.\min for 10 min at 4 mC. Plasma
was frozen at k70 mC until assayed for NPY-LI (within
2 months). Plasma NPY-LI was determined by RIA after
extraction with acid ethanol. "#&I-labelled human NPY
was used as a tracer and synthetic human NPY as the
standard [25]. HPLC characterization of human plasma
in connection with stress (physical exercise) [19] or NPY
infusion [26] has revealed that one main peak corresponding to NPY-(1–36) is recognized by the antiserum.
This has also been substantiated by recent characterization of the N-1 antiserum, showing that it does not
cross-react with NPY fragments comprising residues
1–21, 16–36 or 26–36 to any significant extent ( 0.1 %).
An even higher cross-reactivity (136 %) is observed for
NPY-(2–36) than for NPY-(1–36), however. The stability
of NPY in samples handled according to this protocol is
well documented [25]. The limit of detection for plasma
NPY-LI was 10 pmol\l using 0.5 ml of plasma. The coefficient of variation between duplicate samples in the
same assay is 7 % for concentrations below 200 pmol\l.
Statistical methods
Variables are presented as meanspS.E.M. Statistical
analyses were performed by ANOVA for repeated
measurements. A probability level of P 0.05 was
regarded as statistically significant.
# 2002 The Biochemical Society and the Medical Research Society
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B. Ullman and others
RESULTS
Continuous infusion – protocol 1
Baseline cardiac index and CSBF increased, while stroke
volume decreased slightly, during right atrial pacing
(Table 1). Arterial and pulmonary artery pressures
remained unchanged.
During NPY infusion, mean arterial blood pressure
increased slightly, but this was not statistically significant.
Oxygen consumption, cardiac index, pulmonary artery
pressure, PCV pressure and total systemic resistance did
not change. Mean CSBF and myocardial oxygen consumption were higher during NPY infusion, but these
changes were not significant (Table 1). The spontaneous
heart rate remained unchanged. The conduction interval,
the Wenckebach cycle length and the effective refractory
period of the AV node did not change during NPY
infusion (Table 2). After the atropine injection, mean
heart rate increased from 67 to 126 beats\min (Figure 2).
The plasma levels of NPY-LI in the brachial artery,
pulmonary artery and coronary sinus are shown in Figure
3. At baseline, the plasma NPY-LI level was
35p7 pmol\l in the pulmonary artery, 48p10 pmol\l in
the brachial artery and 38p6 pmol\l in the coronary
sinus. During the first, second and third infusions (3, 10
and 30 pmol : min−" : kg−"), plasma NPY was elevated 6fold, 20-fold and 60-fold respectively. At baseline before
atrial pacing the mean plasma NPY-LI level was somewhat higher in the coronary sinus (55p15 pmol\l) than
in arterial blood (32p4 pmol\l), but the difference was
not significant (P l 0.06). During the first and second
NPY infusions, mean plasma NPY-LI was higher (27 %)
in arterial blood than in blood from the coronary sinus,
while at the highest NPY dose no significant difference
was found. Mean plasma NPY-LI was higher in systemic
Table 1
Table 2 Results from electrophysiological studies in seven
healthy men before and during intravenous infusion of NPY at
10 pmol : min−1 : kg−1 (NPY10) and 30 pmol : min−1 : kg−1
(NPY30)
The conduction intervals were obtained during sinus rhythm. RR, ventricular cycle
length ; PA, intra-atrial conduction time ; AH, AV node conduction time ; HV, HisPurkinje conduction time ; QRS, ventricular depolarization time ; QTc, ventricular
depolarization and repolarization time corrected for heart rate (QTc l
QT/NRR) ; Wb, stimulation interval at Wenckebach block in the AV node ; ERPAVN, effective refractory period of the AV node. No statistically significant
differences compared with baseline were seen.
Interval (ms)
Variable
Baseline
NPY10
NPY30
RR
PA
AH
HV
QRS
QTc
Wb
ERP-AVN
919p70
39p4
76p6
46p2
99p4
392p7
366p19
387p36
964p51
40p3
74p5
47p1
97p4
386p8
365p24
410p37
903p49
40p4
75p7
47p1
96p4
389p8
380p24
397p34
than in pulmonary arterial blood during infusion of all
three doses, but a significant difference (25 %) was only
found between the two lowest NPY doses (Figure 3).
Bolus injection – protocol 2
After each injection of NPY (90, 200 and 900 pmol\kg),
the greatest difference compared with baseline was noted
1 min after injection. The bolus injections of NPY
resulted in a dose-dependent, statistically significant,
increase in mean arterial blood pressure compared with
Haemodynamics and CSBF in seven healthy men during continuous infusion of NPY
HR, heart rate ; MAP, mean arterial blood pressure ; CI, cardiac index ; SV, stroke volume ; PPA, pulmonary artery pressure ; PCW,
pulmonary capillary wedge pressure ; Rs,tot, total systemic resistance ; Rcor, coronary resistance ; RAP, right atrial pacing. NPY
was infused over 10 min at 10 (NPY10) and 30 (NPY30) pmol : min−1 : kg−1. None of the differences between RAP and
RAPjNPY10 or RAPjNPY30 are statistically significant.
Parameter
Baseline
RAP
RAPjNPY10
RAPjNPY30
HR (beats/min)
MAP (mmHg)
O2 consumption (ml/min)
CI (litres : min−1 : kg−1)
SV (ml)
PPA (mmHg)
PCW (mmHg)
Rs,tot (mmHg : l−1 : min−1)
CSBF (ml/min)
Myocardial O2 consumption (ml/min)
Rcor (mmHg : l−1 : min−1)
69p4
100p5
313p18
3.59p0.16
106p5
14p1
8p1
14p1
91.4p4
12.3p0.6
1.09p0.06
100p0
103p4
307p23
4.22p0.37
88p7
13p2
7p1
13p1
111p5
14.9p0.9
0.94p0.06
100p0
107p4
313p19
4.27p0.23
89p5
12p2
7p2
13p1
124p8
16.1p1.3
0.92p0.07
100p0
111p3
314p16
4.16p0.29
87p7
13p2
6p1
14p1
137p14
18.4p2.1
0.87p0.1
# 2002 The Biochemical Society and the Medical Research Society
Cardiovascular effects of neuropeptide Y in humans
Figure 2 Heart rate during intravenous infusion of NPY at 3
(NPY 3), 10 (NPY 10) and 30 (NPY 30) pmol : min−1 : kg−1
At the end of the last infusion, atropine (0.04 mg/kg) was given intravenously.
Values are meanspS.E.M.
Figure 3 Plasma NPY-LI levels in pulmonary arterial (PA),
arterial (A) and coronary sinus (SC) blood during intravenous
NPY infusion at 3, 10 and 30 pmol : min−1 : kg−1
Blood samples were drawn simultaneously during spontaneous sinus rhythm
without pacing. Values are meanspS.E.M. Differences were calculated as PAkA
and AkSC ; *P 0.05.
Figure 4 ECG and arterial blood pressure recordings after
intravenous bolus injection of NPY in one subject who
developed a second-degree AV block
other three only at the highest dose of NPY. Along the
same lines, the mean AV node conduction time (the AH
interval) was increased compared with baseline (Table 3).
The His-ventricular conduction time (the HV interval),
QRS and QT intervals were not altered after the NPY
injections.
The plasma NPY-LI level measured in femoral venous
blood was dose-dependently elevated by the NPY
injections (Table 3).
There was a weak but significant correlation between
the change in mean arterial blood pressure compared
with baseline and the change in the mean AV node
conduction time (Spearman non-parametric correlation
test ; P 0.05 and R l 0.22).
DISCUSSION
baseline (Table 3). Four out of seven subjects developed
second-degree AV block after the injections. All blocks
occurred in the AV node and resulted in single blocked
atrial impulses (Figure 4). One subject developed a
second-degree AV block after each injection, and the
During continuous infusion of NPY, mean arterial
pressure increased, while other haemodynamic and
electrophysiological variables remained unchanged. In
contrast, bolus injections of NPY increased the mean AV
Table 3 Arterial pressure, coronary resistance and electrophysiological variables after
intravenous injection of NPY at three doses : 90 pmol/kg (1), 200 pmol/kg (2) and 900 pmol/kg
(3)
All values were obtained at right atrial pacing at 100 beats/min. MAP, mean arterial blood pressure ; Rcor, coronary resistance ;
AH, AV node conduction time ; HV, His-Purkinje conduction time ; QT, intraventricular depolarization, conduction and
repolarization time ; QS, intra-ventricular depolarization and conduction time. Time intervals (AH, HV, QT and QS) are given
as a percentage of the baseline value. Significance of differences : *P 0.05, **P 0.01 compared with baseline.
Parameter
Baseline
Dose 1
Dose 2
Dose 3
MAP (mmHg)
Rcor (mmHg : l−1 : min−1)
AH ( %)
HV ( %)
QT ( %)
QS ( %)
NPY-LI (pmol/l)
94p2
0.68p0.08
100p0
100p0
100p0
100p0
18p4
97p3
0.60p0.08
116p9*
100p2
101p1
102p1
291p72**
102p4
0.63p0.06
106p6
100p2
101p1
102p2
837p212**
104p6*
0.58p0.05
126p12*
102p3
101p1
101p2
3920p1950**
# 2002 The Biochemical Society and the Medical Research Society
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B. Ullman and others
node conduction time and resulted in AV node block in
four out of seven subjects, in addition to mildly increased
mean arterial pressure. At plasma levels similar to those
seen in patients with CHF, NPY thus had very modest
haemodynamic effects, but significant electrophysiological effects, on the healthy human heart.
Cardiovascular effects of NPY
We have shown previously that patients with severe
CHF have plasma NPY-LI values of up to 2000 pmol\l
[21]. Similar levels have also been found in patients with
hypertension [27]. This level (2000 pmol\l) was therefore
chosen as a maximal target for the infusion studies, since
the overall hypothesis was to test the haemodynamic and
electrophysiological effects of NPY at observed clinical
plasma levels. The only significant cardiovascular effect
of the continuous NPY infusion was elevation of
systemic blood pressure. This effect is probably due to
peripheral vasoconstriction, since cardiac output was
unchanged. In the second protocol, the same total
amounts of NPY were administered as bolus injections in
order to achieve even higher plasma concentrations of
the peptide. The bolus injections evoked increases in
arterial blood pressure, but did not significantly affect
coronary blood flow or vascular resistance. The present
human data are in accordance with findings in the pig, in
which intravenous NPY infusion caused peripheral but
no coronary vasoconstriction, at similar plasma levels
of NPY-LI [12]. This observation suggests that circulating NPY at this plasma level does not regulate coronary
blood flow in healthy men. However, the concentration
of endogenous NPY that is close to its receptors after
release from nerve terminals may be even higher. It may
thus be difficult to relate circulating levels to interstitial
concentrations of endogenous substances.
However, the effect on AV node conduction, which
was impaired, is a novel and unexpected observation, and
has not been described previously after intravenous or
intracoronary bolus injections in humans. The positive
correlation between mean arterial blood pressure and AV
node conduction time suggests that this might be a
baroreceptor-mediated vagal reaction. However, the
increase in blood pressure was minor, and apparently not
sufficient to elicit a vagal reflex, which also would seem to
contrast with the results of animal experiments, in which
NPY had an inhibitory effect on cardiac vagal action.
Another explanation, based on results from in vitro
experiments in guinea pig cardiac myocytes and phaeochromocytoma cells, is offered [16,28,29]. According to
these experiments, NPY inhibits L-type calcium channel
activity through a mechanism that involves a G-proteinmediated pathway in guinea pig ventricular cardiomyocytes [16,28], and an intracellular Ca#+- and protein
kinase C-dependent pathway in phaeochromocytoma
cells [29]. As stated in the latter reference, both
catecholamine synthesis and release is inhibited by NPY,
# 2002 The Biochemical Society and the Medical Research Society
probably via the activation of different NPY receptor
subtypes and different calcium-ion channels. Thus,
whereas the direct vascular effects of NPY generally are
mediated via the Y receptor subtype, other receptors
"
such as the Y receptor may be involved in prejunctional
#
regulation of transmitter release. Taken together, in vitro
data support the hypothesis that NPY actually regulates
sympathetic neuronal function, according to these
authors [29]. Since it is well known – and has been
exploited therapeutically in clinical cardiology for many
years – that inhibition of L-type calcium channel activity
impairs AV node conduction, the experimental data
above thus offer an alternative explanation for our results.
The physiologically beneficial effect of such NPY modulation on catecholamine synthesis and release at the AV
node level would apparently be to counteract any
exaggerated sympathetic activity that would threaten the
ventricles by resulting in too rapid conduction of atrial
impulses and hence too frequent activation of the
ventricles.
Our observations are in contrast to those of some
previous in vivo experiments, in which exogenous NPY
in doses of 5–7 nmol\kg had no effect on cardiac
conduction intervals in dogs, but the vagal effects on the
sinus and AV nodes were attenuated [14,30]. Intravenous
bolus injection of NPY at even higher doses (approximately 15 nmol\kg) into the femoral vein increased mean
arterial pressure, reduced coronary blood flow and also
inhibited the response to vagal stimulation in dogs [31].
When atropine was given after 20 min infusion of NPY at
30 pmol : min−" : kg−" in our study the average heart rate
increased from 67 to 126 beats\min, which is not less
than the expected response of approximately j46 to
60 % [32,33]. This finding together with the unaltered
resting heart rate and AV node function, Table 2,
Figure 3, suggests that NPY in doses according to our
protocol 1 does not have any major effect on the cardiac
cholinergic tone. This may not be surprising, however,
since considerably higher doses of NPY were given in the
above mentioned animal experiments, which demonstrated an inhibitory effect on vagal action.
The absence of any detectable effect on cardiac
electrophysiology in the healthy heart during continuous
infusion does not exclude the possibility that the high
circulating levels of NPY observed in patients with heart
disease are of biological importance. First, the circulating
plasma levels of endogenous NPY may not properly
reflect the local concentration close to the receptor site at
the neuro-effector junction. Secondly, the sensitivity to
an agonist may differ between healthy and diseased
tissue. Thus, when comparing the effects of the sodiumchannel blocking substances carbamazepine and disopyramide in healthy subjects and patients with cardiac
disease, we found no or only mild effects in the healthy
heart, but moderate to severe effects in the sick heart
[34–36].
Cardiovascular effects of neuropeptide Y in humans
Gradients of NPY between different
compartments
The concentration of NPY near the site of release is
probably much higher than in venous plasma because of
local degradation by peptidases [37], and the local
concentration of NPY in the myocardium during severe
heart failure is unknown. During intravenous administration of NPY, there is probably a lower interstitial
concentration in the myocardium than in arterial plasma,
as NPY is a relatively large peptide (Mr approx. 4200),
resulting in a possible diffusion gradient into the myocardium during intravenous infusion. Earlier studies have
shown net cardiac release of NPY in humans, especially
upon exercise and hypoxia [38]. In accordance with
previous observations, we found slightly higher plasma
NPY-LI levels in coronary sinus than in arterial
plasma before right atrial pacing and NPY infusion, but
the difference was not significant (P l 0.06). The
NPY-LI overflow from the heart is enhanced during
sympathetic nerve stimulation in pigs [12] and dogs [39],
suggesting release of NPY from the myocardium during
sympathetic activation. Furthermore, one study has
demonstrated that a portion of coronary constriction
evoked by sympathetic nerve stimulation was resistant to
adrenergic receptor blockade and correlated with the
release of NPY [40].
Approx. 50 % of NPY is cleared over the splanchnic
vascular bed during NPY infusion in humans [5], but
possible clearance over the coronary circulation is unknown. During NPY infusion in the present study, the
mean plasma NPY-LI level was lower in coronary sinus
blood than in arterial blood at an infusion dose of
10 pmol : min−" : kg−", which indicates clearance of the
peptide over the coronary vascular bed. At the highest
dose no difference was found, possibly due to saturation
of NPY degradation. Surprisingly, higher plasma levels
were found in the brachial artery than in the pulmonary
artery. This suggests production of NPY-LI in the lung.
One possible explanation for this may be production of
long NPY-like fragments [e.g. NPY-(3–36)], which can
occur, at least in vitro [37]. Although this particular
fragment was not available for testing, NPY-(2–36) has a
higher cross-reactivity with the N-1 antiserum than
NPY-(1–36). Therefore the higher levels of NPY-LI in
systemic than in pulmonary arterial plasma may indicate
production of NPY fragments in the pulmonary circulation, with high cross-reactivity to the antiserum
used. Further analysis, e.g. by reverse-phase HPLC or
gel-permeation chromatography, is necessary to test this
hypothesis.
Conclusions
Continuous intravenous infusion of NPY into healthy
men at doses resulting in plasma concentrations observed
in patients with severe CHF increased systemic blood
pressure, but did not significantly influence coronary
blood flow or other haemodynamic variables. Neither
was there evidence of any significant electrophysiological
effect, nor of inhibition of cholinergic effects on the
heart. In contrast, bolus injections impaired AV node
conduction, possibly due to a NPY-induced reduction of
the L-type calcium current. This novel finding suggests
that NPY can have a modulating (counteractive) role in
the sympathetic nervous system, at least in some tissues.
Evidence for NPY-LI production in the lung was
obtained during NPY infusion.
The electrophysiological effect of NPY observed in the
present study is intriguing, but it remains to be established, preferably by the use of a specific NPY
antagonist, whether the high levels of NPY in heart
failure patients have any pathophysiological effects.
ACKNOWLEDGMENTS
This study was supported by the Swedish Medical
Research Council (10857, 14X-6554), the Swedish Heart
Lung Foundation, the Swedish Society of Medicine, and
the Karolinska Institutet.
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Received 12 July 2002; accepted 13 August 2002
# 2002 The Biochemical Society and the Medical Research Society