Adrenomedullin-epinephrine cotreatment enhances cardiac output and left

Am J Physiol Heart Circ Physiol 302: H1584–H1590, 2012.
First published February 3, 2012; doi:10.1152/ajpheart.00887.2011.
Adrenomedullin-epinephrine cotreatment enhances cardiac output and left
ventricular function by energetically neutral mechanisms
Thor Allan Stenberg,1,2 Anders Benjamin Kildal,1 Ole-Jakob How,1 and Truls Myrmel1,2
1
2
Surgical Research Laboratory, Department of Clinical Medicine, Faculty of Health Sciences, University of Tromsø, and
Department of Cardiothoracic and Vascular Surgery, University Hospital of North Norway, Tromsø, Norway
Submitted 6 September 2011; accepted in final form 2 February 2012
Stenberg TA, Kildal AB, How OJ, Myrmel T. Adrenomedullinepinephrine cotreatment enhances cardiac output and left ventricular
function by energetically neutral mechanisms. Am J Physiol Heart
Circ Physiol 302: H1584 –H1590, 2012. First published February 3,
2012; doi:10.1152/ajpheart.00887.2011.—Adrenomedullin (AM) used therapeutically reduces mortality in the acute phase of experimental
myocardial infarction. However, AM is potentially deleterious in
acute heart failure as it is vasodilative and inotropically neutral. AM
and epinephrine (EPI) are cosecreted from chromaffin cells, indicating
a physiological interaction. We assessed the hemodynamic and energetic profile of AM-EPI cotreatment, exploring whether drug interaction improves cardiac function. Left ventricular (LV) mechanoenergetics were evaluated in 14 open-chest pigs using pressure-volume
analysis and the pressure-volume area-myocardial O2 consumption
(PVA-MV̇O2) framework. AM (15 ng·kg⫺1·min⫺1, n ⫽ 8) or saline
(controls, n ⫽ 6) was infused for 120 min. Subsequently, a concurrent
infusion of EPI (50 ng·kg⫺1·min⫺1) was added in both groups
(AM-EPI vs. EPI). AM increased cardiac output (CO) and coronary
blood flow by 20 ⫾ 10% and 39 ⫾ 14% (means ⫾ SD, P ⬍ 0.05 vs.
baseline), whereas controls were unaffected. AM-EPI increased CO
and coronary blood flow by 55 ⫾ 17% and 75 ⫾ 16% (P ⬍ 0.05,
AM-EPI interaction) compared with 13 ⫾ 12% (P ⬍ 0.05 vs.
baseline) and 18 ⫾ 31% (P ⫽ not significant) with EPI. LV systolic
capacitance decreased by ⫺37 ⫾ 22% and peak positive derivative of
LV pressure (dP/dtmax) increased by 32 ⫾ 7% with AM-EPI (P ⬍
0.05, AM-EPI interaction), whereas no significant effects were observed with EPI. Mean arterial pressure was maintained by AM-EPI
and tended to decrease with EPI (⫹2 ⫾ 13% vs. ⫺11 ⫾ 10%, P ⫽ not
significant). PVA-MV̇O2 relationships were unaffected by all treatments. In conclusion, AM-EPI cotreatment has an inodilator profile with
CO and LV function augmented beyond individual drug effects and is not
associated with relative increases in energetic cost. This can possibly take
the inodilator treatment strategy beyond hemodynamic goals and exploit
the cardioprotective effects of AM in acute heart failure.
left ventricular energetics; conductance catheter
ADRENOMEDULLIN (AM) is a 52-amino acid peptide originally
isolated by its ability to elevate cAMP in rat platelet assays (20).
This peptide participates in cardiovascular homeostasis as an
autocrine or paracrine factor with vasodilative and chronotropic
properties (15). AM administration consistently augments cardiac
output (CO) and facilitates forward flow in vivo (5, 29, 34), but
experimental findings with respect to the direct cardiac effects of
AM are diverging (14, 41). Evidence has suggested, however, that
AM is inotropically neutral within what is potentially the therapeutic dose range (24). AM reduces ischemia-induced arrhythmias, limits infarct size, attenuates ischemia-reperfusion injury,
and lowers mortality in rodent models (25, 30, 31, 33). The
Address for reprint requests and other correspondence: T. A. Stenberg, Dept.
of Clinical Medicine, Univ. of Tromsø, NO-9037 Tromsø, Norway (e-mail:
thor.allan.stenberg@uit.no).
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cardioprotective effects are therapeutically attractive, and the
hemodynamic profile of AM is advantageous in experimental
heart failure models (35). The hypotensive effect, however, is
possibly detrimental in ischemic heart failure, and profound hypotension after AM administration to patients with acute myocardial infarction has been reported (16).
Interestingly, AM and catecholamines are both secreted
from chromaffin cells through Ca2⫹-dependent regulated exocytosis after cholinergic stimulation (17). Adrenal medullary
basal catecholamine release and catecholamine levels in
plasma are both increased by AM (1, 43), whereas AM release
from chromaffin cells is augmented by dibutyryl cAMP, a
membrane-permeable analog of cAMP (21). AM release from
cultured rat vascular smooth muscle cells is also augmented by
epinephrine (EPI) (40). Thus, there is evidence indicating a
physiological interaction between AM and catecholamines
such as EPI. Furthermore, cAMP is a well-recognized second
messenger in AM signal transduction, and both negative and
positive modulation of ␤-adrenergic signaling by AM has been
reported in vitro (10, 13). It is presently unknown whether AM
can modulate hemodynamic and inotropic responses to ␤-adrenergic stimulation in vivo.
In addition to hemodynamic and potentially cytoprotective
effects, AM appears to be involved in insulin regulation, fatty
acid mobilization, and glucose metabolism (13, 26). These
metabolic effects of AM, either alone or in combination with
EPI, can potentially influence left ventricular (LV) energetics
due to alterations in the concentration of free fatty acids in
plasma (23). There is evidence indicating that AM has favorable energetic properties (29), but the relation between total
mechanical work and myocardial O2 consumption (MV̇O2) has
not been quantified.
In the present study, we assessed the hemodynamic and
contractile effects of potentially therapeutic levels of AM in a
porcine model. In addition, we analyzed whether low-dose AM
infusion modulates the hemodynamic and contractile effects of
␤-adrenergic stimulation by a low-dose EPI infusion in physiologically intact animals. The effects of both individual and
combined treatments on LV energetics were quantified through
the assessment of total mechanical work and MV̇O2 within the
pressure-volume area (PVA)-MV̇O2 framework.
METHODS
The experimental protocol was approved by the local steering
committee of the Norwegian Animal Research Authority and was
conducted in compliance with the National Institutes of Health Guide
for the Care and Use of Laboratory Animals (1996). Fourteen castrated male pigs (Norwegian Landrace, Sus scrofa domesticus) weighing 27 ⫾ 2 kg were habituated to the animal facilities for 4 –7 days
and fasted overnight before the experiment with free access to water.
The premedication, anesthetic protocol, surgical preparation of the
0363-6135/12 Copyright © 2012 the American Physiological Society
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HEMODYNAMIC INTERACTIONS WITH AM-EPI COTREATMENT
open-chest model, conductance volumetric technique, and LV energetic assessment method have been previously detailed (28).
Instrumentation. Animals were anesthetized, tracheostomized, and
ventilated via a volume-controlled ventilator (Servo 900 D, ElemaSchönander). A 7-Fr balloon catheter for preload reductions and
transient caval occlusions was placed in the inferior caval vein. The
hemiazygos vein was ligated to avoid mixture of systemic blood into
the coronary sinus. Transit-time flow probes (Medi-Stim PB series,
Medi-Stim) were placed on the pulmonary trunk, coronary, left
femoral, and left carotid arteries for measurements of CO, coronary
blood flow (CBF), muscular blood flow, and cerebral blood flow. The
great cardiac vein was catheterized through the superior caval vein
and coronary sinus for blood sampling. A 7-Fr dual-field combined
pressure-conductance catheter (CD Leycom) was inserted into the LV
via the right carotid artery to measure LV volume and pressure.
Intravascular volume was maintained with a glucose-enriched (1.25
g/l) saline infusion (20 ml·kg⫺1·h⫺1). Animals received 150 mg
amiodaron to prevent arrhythmias, and 2,500 IE heparin was administered to prevent catheter clotting.
Experimental protocol. In preliminary experiments, the porcine
dose-response relationship to intravenous infusions of human AM1–52
(10 –150 ng·kg⫺1·min⫺1, Bachem) was established (Fig. 1). Low-dose
AM (15 ng·kg⫺1·min⫺1) was chosen to achieve a moderate increase in
CO while minimizing heart rate (HR) and mean arterial pressure
(MAP) alterations, thus aiming at a potentially tolerable level in
settings with acute ischemia and hemodynamic instability.
The isolated effect of AM and the interaction between AM and a
concurrent infusion of EPI (AM-EPI) were compared with the individual effect of EPI in a time-matched control group receiving saline
as vehicle. After baseline measurements, the AM group (n ⫽ 8)
received a continuous infusion of AM, whereas the control group (n ⫽
6) received a continuous infusion of saline. A second set of measurements was obtained 120 min after baseline recordings. Subsequently,
both groups received a concurrent infusion of EPI (50 ng·kg⫺1·min⫺1)
with the third set of measurements obtained at steady state (⬃15 min).
LV energetics. LV MV̇O2 was plotted as a function of PVA as
described by Suga (39). The PVA-MV̇O2 relation at each measurement set was assessed by preload alteration and differing levels of
steady-state LV mechanical work. MAP and CBF were allowed to
reach uninfluenced levels between the successive preload reductions
to ensure unimpaired LV function. At each preload, steady-state
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pressure-volume data were recorded for 10 s with simultaneous blood
sampling from the great cardiac vein to calculate cardiac O2 extraction. PVA and MV̇O2 were calculated as previously described (28).
LV function and ventriculoarterial matching. Indexes of global LV
function [preload recruitable stroke work (PRSW)], inotropy [slope
(Ees) and volume axis intercept (V0) of the end-systolic pressurevolume relationship (ESPVR)], and lusitropy [curve fitting constant
(␣) and diastolic stiffness constant (␤) of the end-diastolic pressurevolume relationship (EDPVR)] derived from the pressure-volume
loops were evaluated by transient caval occlusions, whereas indexes
derived at steady state [peak positive and negative derivatives of LV
pressure (dP/dtmax and dP/dtmin, respectively), and isovolumic time
constant (␶)] were assessed using the initial steady-state recording at
each measurement set. PRSW, which incorporates both systolic and
diastolic properties, is a relatively load-insensitive index of global LV
function (9). The ESPVR was defined as follows: ESP ⫽ Ees(ESV ⫺
V0), where ESP and ESV are end-systolic pressure and volume,
respectively. The EDPVR was defined by the following exponential
relation: EDP ⫽ ␣e(␤EDV) where EDP and EDV are end-diastolic
pressure and volume, respectively. Due to covariance between Ees and
V0, as well as nonlinearities in the ESPVR, positive inotropic effects
may manifest as increased slope (Ees) or a leftward ESPVR shift (V0).
Importantly, with the EDPVR being nonlinear in nature, there is no
agreed upon method to compare EDPVRs without logarithmic transformation (3). Calculating capacitance yields variables that reflect
altered LV function by integrating altered slope (Ees and ␤) and/or
shifts in the intercept (V0 and ␣) (37). LV systolic capacitance
(ESV120) and LV diastolic capacitance (EDV20) were defined as
follows: ESV120 ⫽ (120/Ees) ⫹ V0 and EDV20 ⫽ [ln(20/␣)]/␤.
Arterial elastance (Ea) was calculated as follows: ESP/stroke volume.
Systemic vascular resistance (SVR) was calculated as mean systemic
perfusion pressure/CO and expressed as dyn·s·cm⫺5. Ventriculoarterial (VA) matching was assessed by the ratios of Ea/Ees and PRSW/
SVR (12).
Statistics. Groups and drug interactions were assessed by two-way
repeated-measurements ANOVA (least-squares linear regression with
dummy variables, subject identifier as random effect). Pooled PVAMV̇O2 data were analyzed by two-way repeated-measurements analysis of covariance (ANCOVA; dummy variables and random effect as
above). Tukey’s highly significant difference test was used to adjust
for multiple comparisons, and P values of ⬍0.05 were considered
statistically significant. Statistical analysis was performed using JMP
7 (SAS Institute) with absolute or relative data (i.e., percent change
from baseline) consistent with the data presented. Values are reported
as means ⫾ SD.
RESULTS
Fig. 1. Data from preliminary experiments showing the dose-response relationship to incremental adrenomedulin (AM) infusions from 10 ng·kg⫺1·min⫺1
(n ⫽ 4) to 150 ng·kg⫺1·min⫺1 (n ⫽ 2) as percent changes from baseline (BL)
values. MAP, mean arterial pressure; CO, cardiac output; HR, heart rate; CBF,
coronary blood flow.
The hemodynamic variables were matched between groups
at baseline with the exception of a higher HR in the control
group (62 ⫾ 7 vs. 88 ⫾ 6 beats/min, P ⬍ 0.05). LV systolic
and diastolic function, LV energetics, and VA matching were
similar in both groups at baseline. Hemodynamic and LV
functional data are shown as percent changes from baseline
values in Fig. 2. Corresponding absolute data and PVA-MV̇O2
parameters are shown in Tables 1–3.
LV function and VA matching. The effects of AM, EPI, and
AM-EPI on LV function are shown in Figs. 2 and 3. LV
function, as assessed by dP/dtmax, showed that AM alone had
no observable effect on contractility. AM-EPI increased dP/
dtmax by 32 ⫾ 14% compared with baseline (P ⬍ 0.05),
whereas the control group had an initial ⫺14 ⫾ 12% decrease
with saline (P ⬍ 0.05) and with EPI dP/dtmax was still ⫺2 ⫾
14% [P ⫽ not significant (NS)] compared with baseline.
Interaction analysis showed a significant interaction with AMEPI (P ⬍ 0.05). Global LV function, as measured by PRSW,
AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00887.2011 • www.ajpheart.org
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HEMODYNAMIC INTERACTIONS WITH AM-EPI COTREATMENT
Fig. 2. Hemodynamics and left ventricular
(LV) function shown as percent changes from
BL values. A: CO and HR were augmented to
a greater extent with AM-epinephrine (EPI)
cotreatment (AM-EPI) than with the individual
drugs. MAP was also preserved with AM-EPI.
B: CBF was greatly enhanced by AM-EPI
cotreatment, whereas carotid (Car) and femoral (Fem) blood flows were less affected.
C: LV systolic capacitance (ESV120), peak
positive derivative of LV pressure (dP/dtmax),
and preload recruitable stroke work (PRSW)
results showing that AM-EPI cotreatment improved LV function to a greater degree than
the individual drugs. Open circles, control
group; filled circles, AM group. Within-group
differences: *P ⬍ 0.05 vs. BL values and †P ⬍
0.05 vs. AM or control; between-group differences: ‡P ⬍ 0.05; interaction: §P ⬍ 0.05 (by
two-way repeated-measures ANOVA).
AM-EPI / EPI
showed that AM had no effect on LV function when administered alone. With AM-EPI, PRSW increased by 40 ⫾ 14%
compared with baseline values (P ⬍ 0.05), whereas EPI alone
produced a nonsignificant 23 ⫾ 27% increase. Through interaction analysis using absolute data, an interaction between
AM-EPI was observed (P ⬍ 0.05; Table 2). ESV120 decreased
by ⫺37 ⫾ 22% with AM-EPI compared with baseline (P ⬍
0.05), whereas it was unaltered with the individual drugs (19 ⫾
18% vs. 16 ⫾ 42%, AM vs. EPI, P ⫽ NS). The effect of
AM-EPI cotreatment was a greater reduction in ESV120 compared with EPI infusion in the control group, and a significant
interaction was detected (P ⬍ 0.05; Fig. 2 and Table 2).
AM-EPI / EPI
AM-EPI / EPI
VA matching was assessed using the ratios of Ea/Ees and
PRSW/SVR. Afterload, as assessed by Ea, predominantly reflecting arterial compliance, showed no systematic changes. The ratio
of Ea/Ees was unaltered across all conditions. SVR primarily
reflects peripheral vasoconstriction, and, in contrast to Ea, SVR
was reduced with AM, EPI, and AM-EPI (⫺14 ⫾ 13%, ⫺23 ⫾
10%, and ⫺34 ⫾ 14%, P ⬍ 0.05 vs. baseline by one-way
ANOVA). The ratio of PRSW/SVR increased with AM, EPI, and
AM-EPI (22 ⫾ 18%, 58 ⫾ 44%, and 120 ⫾ 41%, P ⬍ 0.05). A
significant interaction was detected with PRSW/SVR, and thus
forward flow facilitation was augmented to a greater extent by
AM-EPI than by the individual drugs (P ⬍ 0.05).
Table 1. Hemodynamics
AM Group
Mean arterial pressure, mmHg
Mean pulmonary artery pressure, mmHg
Central venous pressure, mmHg
Cardiac output, ml/min
Heart rate, beats/min
End-systolic volume, ml
End-diastolic volume, ml
End-systolic pressure, mmHg
End-diastolic pressure, mmHg
Coronary blood flow, ml/min
Carotid blood flow, ml/min
Femoral blood flow, ml/min
Ea, mmHg/ml
SVR, dyn 䡠 s 䡠 cm⫺5
Control Group
Baseline
AM
AM-EPI
Baseline
Control
EPI
93 ⫾ 8
17 ⫾ 2
6⫾1
2,028 ⫾ 443
62 ⫾ 7‡
25 ⫾ 6
57 ⫾ 7
106 ⫾ 8
15 ⫾ 2
87 ⫾ 13
214 ⫾ 62
94 ⫾ 27
3.3 ⫾ 0.4
3,516 ⫾ 484
95 ⫾ 13
19 ⫾ 2
6⫾1
2,441 ⫾ 625*
80 ⫾ 15*
29 ⫾ 9
59 ⫾ 10
107 ⫾ 12
13 ⫾ 2
122 ⫾ 24*
227 ⫾ 55
111 ⫾ 39
3.6 ⫾ 0.6
3,047 ⫾ 705
94 ⫾ 14
20 ⫾ 4
8⫾3
3,106 ⫾ 648*†§
101 ⫾ 18*†
20 ⫾ 10
49 ⫾ 10
100 ⫾ 15
11 ⫾ 4
151 ⫾ 19*†
277 ⫾ 77
129 ⫾ 45*
3.3 ⫾ 0.6
2,315 ⫾ 615
89 ⫾ 10
19 ⫾ 2
6⫾1
2,318 ⫾ 331
88 ⫾ 6‡
20 ⫾ 5
45 ⫾ 7
96 ⫾ 10
12 ⫾ 2
108 ⫾ 23
176 ⫾ 18
95 ⫾ 43
3.7 ⫾ 0.3
2,888 ⫾ 307
83 ⫾ 8
22 ⫾ 1
6⫾1
2,222 ⫾ 371
93 ⫾ 5
22 ⫾ 3
45 ⫾ 6
90 ⫾ 9
11 ⫾ 2
120 ⫾ 32
188 ⫾ 22
85 ⫾ 21
3.9 ⫾ 0.8
2,818 ⫾ 566
78 ⫾ 6
21 ⫾ 4
6⫾1
2,612 ⫾ 366†
104 ⫾ 10*
18 ⫾ 3
42 ⫾ 6
86 ⫾ 8
11 ⫾ 2
127 ⫾ 40
218 ⫾ 23
101 ⫾ 22
3.5 ⫾ 0.7
2,231 ⫾ 357
Values are means ⫾ SD; n ⫽ 8 animals in the adrenomedulin (AM) group and 6 animals in the control group. EPI, epinephrine; AM-EPI, AM and EPI
cotreatment; Ea, arterial elastance; SVR, systemic vascular resistance. Within-group differences: *P ⬍ 0.05 vs. baseline and †P ⬍ 0.05 vs. AM or control;
between-group differences: ‡P ⬍ 0.05; interaction: §P ⬍ 0.05 (by two-way repeated-measures ANOVA).
AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00887.2011 • www.ajpheart.org
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HEMODYNAMIC INTERACTIONS WITH AM-EPI COTREATMENT
Table 2. LV function and ventriculoarterial matching
AM Group
Ees, mmHg/ml
V0, ml
ESV120, ml
PRSW, mmHg
dP/dtmax, mmHg/s
dP/dtmin, mmHg/s
␤, mmHg/ml
␣
EVD20, ml
␶, ms
Ea/Ees
PRSW/SVR, l/min
Control Group
Baseline
AM
AM-EPI
Baseline
Control
EPI
2.5 ⫾ 1.3
⫺26 ⫾ 21
29 ⫾ 9
57 ⫾ 7
1,410 ⫾ 208
⫺1,762 ⫾ 217
0.068 ⫾ 0.026
0.66 ⫾ 0.79
62 ⫾ 9
49 ⫾ 6
1.5 ⫾ 0.4
1.32 ⫾ 0.17
2.5 ⫾ 1.4
⫺24 ⫾ 27
35 ⫾ 10
60 ⫾ 12
1,459 ⫾ 265
⫺1,891 ⫾ 249
0.072 ⫾ 0.036
0.86 ⫾ 1.29
67 ⫾ 10
44 ⫾ 4
1.7 ⫾ 0.6
1.61 ⫾ 0.38
3.3 ⫾ 1.2
⫺19 ⫾ 22
23 ⫾ 12*†
80 ⫾ 13*†‡§
1,868 ⫾ 317*†§
⫺2,042 ⫾ 322*
0.092 ⫾ 0.048
0.45 ⫾ 0.51
55 ⫾ 10†
38 ⫾ 3
1.1 ⫾ 0.4
2.93 ⫾ 0.81*†§
2.0 ⫾ 0.6
⫺33 ⫾ 8
29 ⫾ 10
54 ⫾ 15
1,546 ⫾ 224
⫺1,908 ⫾ 254
0.083 ⫾ 0.028
0.47 ⫾ 0.34
50 ⫾ 8
40 ⫾ 5
1.9 ⫾ 0.4
1.50 ⫾ 0.37
1.9 ⫾ 0.5
⫺33 ⫾ 10
33 ⫾ 8
54 ⫾ 8
1,328 ⫾ 279*
⫺1,778 ⫾ 179
0.071 ⫾ 0.038
0.75 ⫾ 0.47
57 ⫾ 14
39 ⫾ 3
2.1 ⫾ 0.4
1.56 ⫾ 0.31
2.1 ⫾ 0.7
⫺34 ⫾ 14
30 ⫾ 8
62 ⫾ 5‡
1,511 ⫾ 312
⫺1,825 ⫾ 182
0.067 ⫾ 0.039
1.05 ⫾ 0.67
54 ⫾ 11
35 ⫾ 3
1.8 ⫾ 0.3
2.27 ⫾ 0.39*†
Values are means ⫾ SD; n ⫽ 8 animals in the AM group and 6 animals in the control group. LV, left ventricular; Ees, end-systolic elastance [slope of the
end-systolic pressure-volume relationship (ESPVR)]; V0, initial volume (volume axis intercept of the ESPVR); PRSW, preload recruitable stroke work; Ea/Ees
and PRSW/SVR, measures of ventriculoarterial matching (see text for details). Within-group differences: *P ⬍ 0.05 vs. baseline and †P ⬍ 0.05 vs. AM or
control; between-group differences: ‡P ⬍ 0.05; interaction: §P ⬍ 0.05 (by two-way repeated-measures ANOVA).
LV energetics. PVA-MV̇O2 relationships within each group
are shown as pooled scatterplots in Fig. 4. Mean PVA-MV̇O2
parameters are shown in Table 3. There was no observable
effect on the slope (i.e., cardiac energetic efficiency) or y-intercept (i.e., unloaded MV̇O2) of the PVA-MV̇O2 relationship
with AM, EPI, or AM-EPI. Individual slope and y-intercept
values showed no significant between- or within-group differences, and repeated-measurements ANCOVA of pooled PVAMV̇O2 data showed no significant main or interaction effects.
Thus, LV energetics were unaffected by AM, EPI, or AM-EPI.
DISCUSSION
Our study supports evidence suggesting physiologically relevant interactions between AM, ␤-adrenergic signaling, and
the sympathetic nervous system. As stated, an interrelationship
between the signaling and release of AM and catecholamines
has been observed in several models (1, 40, 43), and modulation of ␤-adrenergic signaling has been reported (10, 13). In
addition, increased sympathetic activity has been seen with
intravenous administration of AM (36). However, whether
cotreatment could produce hemodynamically relevant interaction effects has not been previously described. Our data show
that the increase in CO is paralleled by a chronotropic response, and altered HR could potentially explain the increased
contractility observed with AM-EPI. However, a higher absolute HR was seen in the control group at baseline, and with
AM-EPI and EPI, the HR was similar between groups, thereby
indicating that other mechanisms are responsible for the increase in LV systolic function during AM-EPI cotreatment.
AM can enhance the baroreceptor reflex response through
cAMP- and PKA-dependent signaling in the nucleus tractus
solitarius, the terminal site for primary baroreceptor afferents
(11). Increased HR and cardiac sympathetic nerve activity have
also been reported, with the effect being more pronounced with
AM than with pressure-matched nitroprusside administration
(4). Interestingly, the inotropic effect of CGRP has been attributed to indirect myocardial sympathetic activation through an increased interstitial concentration of norepinephrine (18). AM
belongs to the CGRP peptide superfamily and partly shares
receptor complexes with calcitonin receptor-like receptor (CL)
and receptor activity-modifying protein (RAMP)2 and RAMP3,
which constitute AM receptors, whereas CL and RAMP1 form
the CGRP receptor (15). To which extent AM may stimulate
myocardial sympathetic signaling remains to be clarified. Adrenal chromaffin cells used as models of catecholaminereleasing neurons have been shown to release AM through
cAMP-mediated mechanisms (22), and catecholamine release
after AM administration has also been reported (1). Thus, a
hypothetical feedback system regulating myocardial sympathetic activity may be envisioned.
AM reduces Ea and SVR and affects potent vasodilation in
resistance vessels (6, 24, 34), whereas preload is maintained
(24). Thus, in addition to the chronotropic response, reduced
afterload with concomitantly less effect on capacitance vessels
and sustained venous return may also explain why AM potently augments CO. It is uncertain whether AM has inotropic
properties in vivo as there are reports of both positive and
unaltered inotropy (24, 29). Positive inotropic effects in vivo
Table 3. LV energetics
AM Group
Pressure-volume area, J 䡠 beat
MV̇O2, J 䡠 beat⫺1 䡠 100 g⫺1
y-Intercept
Slope
R2
⫺1
䡠 100 g
⫺1
Control Group
Baseline
AM
AM-EPI
Baseline
Control
EPI
0.64 ⫾ 0.13
1.76 ⫾ 0.26
0.51 ⫾ 0.12
1.93 ⫾ 0.14
0.96 ⫾ 0.02
0.68 ⫾ 0.21
1.87 ⫾ 0.29
0.53 ⫾ 0.10
2.01 ⫾ 0.29
0.98 ⫾ 0.01
0.61 ⫾ 0.17
1.75 ⫾ 0.31
0.44 ⫾ 0.12
2.19 ⫾ 0.42
0.98 ⫾ 0.02
0.59 ⫾ 0.09
1.58 ⫾ 0.28
0.35 ⫾ 0.12
2.17 ⫾ 0.68
0.97 ⫾ 0.04
0.55 ⫾ 0.09
1.55 ⫾ 0.37
0.39 ⫾ 0.18
2.00 ⫾ 0.72
0.95 ⫾ 0.05
0.51 ⫾ 0.09
1.44 ⫾ 0.39
0.38 ⫾ 0.10
2.07 ⫾ 0.48
0.96 ⫾ 0.03
Values are means ⫾ SD; n ⫽ 8 animals in the AM group and 6 animals in the control group. MV̇O2, myocardial O2 consumption; y-intercept, unloaded MV̇O2;
slope, contractile efficiency; R2, coefficient of determination.
AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00887.2011 • www.ajpheart.org
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HEMODYNAMIC INTERACTIONS WITH AM-EPI COTREATMENT
Fig. 3. Pressure-volume loops from a representative experiment in each group. A: AM
group; B: control group. AM-EPI improved
LV function, as shown by the leftward shift
of the end-systolic pressure-volume relationship together with an increased slope (see
text for details).
AM-EPI
MVO2 (J•beat-1•100 g-1)
Fig. 4. Pooled scatterplots of the pressurevolume area myocardial O2 consumption
(PVA-MV̇O2) relationships within each
group from all experiments. A: AM group;
B: control group. No significant differences
were detected by repeated-measures analysis of covariance (see text for details).
bining lipolytic drugs that can increase free fatty acids in
plasma could potentially induce a mechanoenergetic inefficiency that would be unwanted therapeutically in the ischemically challenged heart (23). This is the first study to characterize LV energetics using total mechanical work related to MV̇O2
with systemic administration of AM. Favorable energetic properties of AM have been reported with an increase in contractility paralleling a reduction in MV̇O2, thus implying an increased mechanoenergetic efficiency (29). However, afterload
reduction and relatively less energetic cost with volume versus
pressure work may explain the finding, and the relationship
between total mechanical work and MV̇O2 must be assessed to
describe any influence on LV energetics (39). We found no
evidence of altered mechanoenergetic efficiency or unloaded
MV̇O2 with AM, EPI, or AM-EPI.
The vascular response after AM administration is heterogeneous with increased blood flow in the heart, lungs, kidneys,
adrenal glands, and spleen, whereas the splanchnic bed is
relatively unaffected (19). In agreement with earlier studies (7,
44) showing that AM infusion augments CBF, we found that
low-dose AM produced a relatively large increase in CBF. This
effect was further enhanced by AM-EPI cotreatment, and the
extent of CBF elevation suggests that the drugs interact.
Importantly, PVA-MV̇O2 data indicate that the observed increase is not metabolically induced, as unloaded MV̇O2, contractile efficiency, and total mechanical work were similar
between groups. In vitro studies (2, 42) have shown that AM
MVO2 (J•beat-1•100 g-1)
have predominantly been observed with relatively load-sensitive measures of LV function. AM in doses ranging from 20 to
200 ng·kg⫺1·min⫺1 confer no inotropic effect, as assessed by
the ESPVR (24); thus, AM is probably inotropically neutral
within the potentially therapeutic dose range. We found no
evidence of any inotropic effect with low-dose AM, and
preliminary dose-response experiments revealed no inotropic
effect with doses ranging from 10 to 150 ng·kg⫺1·min⫺1 (data
not shown). AM-EPI cotreatment caused moderate elevations
of dP/dtmax and PRSW, with both being larger than with EPI as
the only active drug, and a significant AM-EPI interaction was
thus observed. The ESPVR was unaltered, as assessed by Ees
and V0, and there were no differences in ␣ or ␤ describing the
EDPVR. The calculation of systolic capacitance accounts for
the potential covariance between Ees and V0 (3, 37), and the
reduction in ESV120 with AM-EPI exceeded the sum of the
individual drug effects, thus implying that the end-systolic
stiffness (i.e., contractility) was increased through AM-EPI
interaction. As shown in Fig. 3, AM-EPI produced a leftward
shift of the ESPVR together with increased Ees, thereby indicating an improved contractile performance within the measured pressure range. The functional parameters dP/dtmax,
PRSW, and ESV120 were unaltered by AM, thereby indicating
that the interaction effect is synergistic in nature.
AM is metabolically active and confers lipolytic properties
in vitro (13), and catecholamines and sympathomimetic drugs
are known to increase free fatty acids in plasma (27). Com-
AM-EPI
PVA (J•beat-1•100 g-1)
AJP-Heart Circ Physiol • doi:10.1152/ajpheart.00887.2011 • www.ajpheart.org
PVA (J•beat-1•100 g-1)
HEMODYNAMIC INTERACTIONS WITH AM-EPI COTREATMENT
mediates coronary vasodilation through nitric oxide (NO) synthase (NOS) and EDHF pathways. In the isolated rat aorta, AM
causes endothelium-dependent vasodilation by Ca2⫹-mediated
activation of the phosphatidylinositol 3-kinase (PI3K)/Akt
pathway, which, in turn, activates endothelial NOS (eNOS)
and NO production (32). In endothelial cells, cAMP accumulation and mobilization of intracellular Ca2⫹ stores via phospholipase C and inositol 1,4,5-trisphosphate formation have
been observed after AM exposure (38). In addition to PI3K
activation, increased levels of intracellular Ca2⫹ in endothelial
cells can also produce vasodilation through the EDHF pathway
with opening of endothelial small- and intermediate-conductance Ca2⫹-activated K⫹ channels inducing hyperpolarization
of vascular smooth muscle (8). Interestingly, NO production in
canine myocardial microvessels after exposure to AM or isoproterenol is substantially enhanced in subjects with heart
failure despite eNOS downregulation, an effect abolished by
NOS, PKA, and PI3K inhibition (45). Thus, cAMP and PI3K
appear to serve as compensatory pathways that can sustain or
augment NO production in situations with eNOS downregulation and endothelial dysfunction. Furthermore, as both pathways have been implicated in AM and ␤-adrenergically induced vasorelaxation (45), they could potentially serve to
explain the substantial augmentation of CBF observed with
AM-EPI cotreatment.
Conclusions. The main observations presented in this study
is that low-dose AM is inotropically neutral and increases CO
primarily through vasodilation and chronotropy, that low-dose
AM-EPI cotreatment has the hemodynamic profile of an inodilator with CO and LV function augmented beyond individual
drug effects, and that this functional enhancement is not
associated with disproportionate increases in energetic expenditure.
Thus, low-dose AM-EPI cotreatment has a hemodynamic
profile that is attractive in ischemic heart failure and circulatory
compromise with cardiac unloading, increased CO and inotropy, maintained systemic perfusion pressure, and mechanoenergetic neutrality. In addition, AM is potentially attractive in
settings of ischemia and heart failure due to its effect on infarct
size, arrhythmias, and mortality. In combination with an inotrope, this can take the inodilator treatment strategy beyond
purely hemodynamic goals by exploiting the pleiotropic effects
while supporting the circulation with an inotrope. The chronotropic effect of AM-EPI, however, is a concern in acute heart
failure and should be evaluated in future studies.
ACKNOWLEDGMENTS
The authors are indebted to the staff at the Surgical Research Laboratory for
providing excellent technical assistance.
GRANTS
This work was financially supported by the Northern Norway Regional
Health Authority.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
AUTHOR CONTRIBUTIONS
T.A.S., A.B.K., O.-J.H. and T.M. conception and design of research;
T.A.S., and A.B.K. performed experiments; T.A.S. and T.M. analyzed data;
T.A.S., A.B.K., O.-J.H., and T.M. interpreted results of experiments; T.A.S.
prepared figures; T.A.S. drafted manuscript; T.A.S. and T.M. edited and
H1589
revised manuscript; T.A.S., A.B.K., O.-J.H., and T.M. approved final version
of manuscript.
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