48 | JANUARY 2010 | volUme 7
Baker IDI Heart and
St Kilda Road Central,
Melbourne, Vic 3008,
Australia (X.-J. Du).
(R. A. D. Bathgate,
C. S. Samuel). Alfred
Heart Center, the Alfred
Hospital and Central
Australia (A. M. Dart).
Drug Discovery Biology,
Monash Institute of
Sciences & Department
(R. J. Summers).
Cardiovascular effects of relaxin: from basic
science to clinical therapy
Xiao-Jun Du, Ross A. D. Bathgate, Chrishan S. Samuel, Anthony M. Dart and Roger J. Summers
Abstract | Although substantial advances have been achieved in recent decades in the clinical management
of heart diseases, new therapies that provide better or additional efficacy with minimal adverse effects are
urgently required. Evidence that has accumulated since the 1990s indicates that the peptide hormone relaxin
has multiple beneficial actions in the cardiovascular system under pathological conditions and, therefore,
holds promise as a novel therapeutic intervention. Clinical trials for heart failure therapy using relaxin
revealed several beneficial actions. Here we review findings from mechanistic and applied research in this
field, comment on the outcomes of recent phase I/II clinical trails on patients with heart failure, and highlight
settings of cardiovascular diseases where relaxin might be effective.
Du, X.-J. et al. Nat. Rev. Cardiol. 7, 48–58 (2010); published online 24 November 2009; doi:10.1038/nrcardio.2009.198
The peptide hormone relaxin was named for its action
of relaxing the female reproductive tract,1 and was
first sequenced in the 1970s.2 Early research on relaxin
focused on extracellular matrix (ECM) remodeling in the
reproductive system, which promoted collagen degrada-
tion and smooth muscle hypertrophy or proliferation.1
RLN1 and RLN2, the human relaxin genes encoding
H1 and H2 relaxin, respectively, were discovered in the
1980s.3,4 RLN2 is the equivalent of the single relaxin gene
expressed in the corpus luteum or placenta during preg-
nancy of most other species and its product is generally
referred to as relaxin.5
Over the past two decades, there has been rapid progress
in our understanding of the pleiotropic actions of relaxin
in the cardiovascular system. In addition to continuous
research on the regulation of the vascular system by relaxin
during the course of pregnancy, studies have examined
cardiovascular pharmacology, signaling mechanisms and
its protective effects against disease.1,6 Consequently, clini-
cal studies were undertaken to examine changes in endog-
enous relaxin in patients with cardiovascular diseases and
whether exogenously administered relaxin has beneficial
therapeutic effects in patients with heart failure (HF).
Thus, it is timely to summarize progress in the field that
indicates beneficial actions of relaxin in cardio vascular
pathology. In this Review, we discuss basic knowledge of
relaxin signaling mechanisms, findings from experimen-
tal animal models of cardiovascular diseases, and findings
from clinical trials on relaxin. To facilitate translation of
basic research into therapeutic use, we highlight areas
where further research is warranted to address clinically
Relaxin signaling: basic science
Relaxin consists of A and B chains connected by disulfide
bonds. Relaxin is not only a hormone of pregnancy, but is
a paracrine or autocrine factor in many tissues in males
and females and acts in the endometrium, mammary
gland, decidua, placenta, prostate and heart.1,5–7 Relaxin
regulates collagen metabolism in many tissues,8 as well
as gene expression of matrix metalloproteinases (MMP),
tissue inhibitors of metalloproteinases, insulin-like
growth factor (IGF) binding protein 1, vascular endo-
thelial growth factor (VEGF), basic fibroblast growth
factor, c-fos and cyclooxygenase 2.1,7,9–11
Relaxin family peptide receptors
In 2002, the endogenous receptor for relaxin, relaxin
family peptide receptor (RXFP) 1, was identified and
deorphanized by searching genomic databases for related
glycoprotein receptors.12 The four currently identified
receptors for relaxin family peptides are G protein-
coupled receptors (GPCRs),13 and are strikingly different
from the tyrosine kinase receptors for the closely related
insulin and IGF peptides (Box 1).14–16 Unlike these recep-
tors, RXFP1 has a large extracellular domain with mul-
ti ple leucine-rich repeats (LRR) in addition to the seven
transmembrane domain core of a typical GPCR.17
RXFP1 binds relaxin with high affinity,18,19 and sig-
naling involves three distinct mechanisms.19–22 Primary
ligand binding involves the B-chain residues of the
peptide and the LRRs of the ectodomain.23 Lower affin-
ity binding occurs in the transmembrane exoloops.20,21,24
Finally, activation of signaling depends on the low density
lipoprotein class A (LDLa) module, as receptors lacking
this region bind peptides, but do not generate cyclic
High-affinity binding comprises an angle of 45 ° to
five of the parallel pleated sheets of the LRRs and occurs
The authors declare no competing interests
NATURE REVIEws | CARDiology
VOLUME 7 | JANUARY 2010 | 49
through a hydrogen-bonding network between the
receptor and two residues of the relaxin peptide-binding
motif, stabilized by hydrophobic interactions between the
relaxin peptide binding motif (Ile-B20) and a cluster of
residues from neighboring LRRs. studies with chi meric
RXFP1/RXFP2, however, suggest that the secondary
binding sites reside in exoloop 2.20,21 Both primary and
secondary binding are necessary for receptor activation
and it is likely that compounds that bind to the secondary
binding site alone will be receptor antagonists.
How the LDLa module directs cAMP signaling is
unknown, but probably involves specific side-chain
interactions. A mutant of RXFP1, which has the LDLa
module replaced by the second ligand binding domain
of the LDL receptor, LB2, binds relaxin normally but
does not activate cAMP signaling.25 Additionally, muta-
tions in the RXFP1 LDLa module alter cAMP signaling
but not binding.25 Importantly, isolated LDLa modules
do not rescue the function of LDLa-less RXFP1, but act as
RXFP1 antagonists.22 Thus, RXFP1 activation is complex
and creation of small molecule agonists may require
novel approaches that target the receptor–G- protein
interface directly or produce conformational changes
in receptor structure to cause or enhance signaling
by allosteric mechanisms.
Signal transduction pathways
Multiple signal transduction pathways are induced in
response to relaxin (Figure 1). Many, but not all, pathways
involve interaction between relaxin and RXFP1.
cAMP is important for RXFP1 signaling in native cells
and cell lines that respond to relaxin, and involves distinct
patterns of G-protein coupling (Box 2).26 RXFP1 initially
couples to Gαs to increase cAMP production,26 and to
GαoB to negatively modulate this effect. subsequently, the
receptor recruits Gαi3 to activate a Gαi3–phosphoinositide
3-kinase (PI3K)–protein kinase C (PKC) ζ pathway,
producing a further surge in cAMP.26–28 Gαi3 recruit-
ment involves residues in the carboxy terminus of the
receptor.29 Coupling occurs rapidly (within minutes) and
involves membrane lipid rafts; the delay in cAMP signal-
ing (about 20–30 min) might be related to the activation
of PI3K and the trans location of PKCζ to the cell surface
to activate adenylyl cyclase.28
These pathways are activated in cells recombinantly
expressing RXFP1 and also in those that endogenously
express the receptors, including THP-1, MCF-7, preg-
nant human myometrial and mouse mesangial cells.28
In rat atria, the Gi/Go inhibitor pertussis toxin inhibited
relaxin-induced cAMP accumulation, as well as chrono-
tropic and inotropic responses.30 The cAMP response is
affected by the cell background on which the receptors
are expressed and often reflects a subset of these three
pathways. For example, relaxin activates only Gαs in
T-47D cells but both Gαs and GαoB in Colo 16 and rat
renal fibroblasts.31 These differences may help to explain
the varied physiological responses exerted by relaxin in
different target tissues and among species.
Mitogen activated protein kinases (MAPKs) are activated
by relaxin in human endothelial stromal cells, THP-1 cells
and primary human smooth muscle cells from coronary
artery and pulmonary artery that show increased extra-
cellular signal-regulated kinase (ERK) 1/2 phosphoryla-
tion and MAPK kinase phosphory lation with relaxin.32,33
In cardiac myocytes treated with hydrogen peroxide, there
is a protective decrease in apoptosis linked to increases in
ERK1/2 phosphorylation in response to relaxin.34
In human lower uterine segment fibroblasts, relaxin
causes tyrosine phosphorylation of a 220 kDa protein
without associated increase in cAMP.35 Tyrosine kinase
and MAPK inhibitors prevent relaxin-mediated increases
in cAMP accumulation (second wave) in EsC and
THP-1 cells,35 indicating positive feedback from MAPK
into the cAMP response. This feedback may involve
trans activation of a tyrosine kinase receptor by RXFP1
and activation of adenylyl cyclase, but further work is
required to determine the exact mechanism.
Altered gene transcription
VEGF expression in human endometrial cells is increased
by H2 relaxin and involves cAMP and, possibly, MAPKs.
VEGF is involved in regulating menstrual-cycle-specific
blood vessel growth in the uterine endometrium and
Relaxin is a naturally occurring peptide hormone with a broad array
of cardiovascular actions demonstrated in experimental models
Cardiovascular tissues are equipped with relaxin receptors that are activated
by circulating relaxin or regionally generated relaxin and mediate a range of
bioactivities via diverse signaling pathways
Experimental research has documented multiple cardioprotective actions
of relaxin against key disease components, including myocardial injury,
vasoconstriction, oxidative stress, fibrosis and inflammation
Clinical trials have also documented vasodilatory action of relaxin in patients
with acute heart failure
Clinical trials have demonstrated that relaxin has a good safety profile when
administered either intravenously or subcutaneously over a short or long period
Box 1 | Receptors for relaxin family peptides
The relaxin family peptide receptors (RXFPs) are family A, G-protein-coupled
RXFP1 and RXFP2 are leucine rich repeat (LRR)-containing GPCRs whereas
RXFP3 and RXFP4 are small peptide receptor-like GPCRs
RXFP1 is the relaxin receptor and has 10 LRRs in the ectodomain and an amino
terminal low density lipoprotein class A (LDLa) module
Relaxin binds to RXFP1 via the Arg-X-X-X-Arg-X-X-Ile/Val motif in the peptide B
chain, but signal activation involves three distinct mechanisms
Primary ligand binding occurs between the peptide B-chain residues of relaxin
and the LRRs of the receptor
Secondary lower-affinity binding occurs between the transmembrane exoloops
of the receptor and residues in the A chain of the peptide
Finally, signal activation involves the LDLa module of the receptor
50 | JANUARY 2010 | volUme 7
may also be important for maintaining the differen tiated
pheno type of these cells.11
Relaxin increases levels of nitric oxide (NO) both acutely
and chronically,36 and several studies link this increase
to therapeutically relevant outcomes.37–47 In a rat stroke
model, relaxin reduces infarct size in a NO-dependent
manner,48 and decreases leukocyte recruitment to areas
of inflammation. These effects may involve increased
expression and activity of nitric oxide synthase (NOs).36
Relaxin can activate endo thelial NOs (eNOs) directly
by Gβγ subunits and the PI3K pathway, and inducible
NOs (iNOs) by cAMP-mediated activation of protein
kinase A, which in activates IκB, thereby increasing iNOs
expression via NFκB.36 Relaxin also increases MMP acti-
vity, converting ‘big’ endothelin (ET) to ET1–32, which acts
on ETB receptors to increase NO production.36 Relaxin
may up regulate the endothelial ETB receptor;49 however,
this finding is controversial.50 The specific pathways
involved in NOs activation still remain to be elucidated
and are likely highly dependent on cell type.
Interaction with GRs
Relaxin upregulates glucocorticoid receptor (GR) mRNA
expression and the number of GR binding sites in human
cervical carcinoma cells (HeLa), spleen fibroblasts and
HEK293 cells.51 Porcine relaxin co- immunoprecipitates
with GR in both cytosolic and nuclear fractions of HeLa
cells. The effect is independent of RXFP1, as inactivated
relaxin—which cannot interact with RXFP1—still causes
this effect in intact cells.51 Comparison of porcine relaxin
and a modified relaxin peptide that does not interact with
RXFP1 shows that both molecules inhibited endotoxin-
stimulated tumor necrosis factor and interleukin-6 secre-
tion, effects mediated by GR.52 The full implications of
this interaction remain to be elucidated.
Cardiovascular effects of relaxin
Relaxin helps maintain tissue homeostasis and protects the
injured myocardium during various patho physiological
processes, underpinning the increasing interest in relaxin
as a therapeutic agent. Experimental findings from animal
models suggest that provision of relaxin aids in the protec-
tion and/or treatment of the failing heart through mul-
tiple actions, and clinical trials have revealed the efficacy
of relaxin in patients with acute HF.
Physiology and pathophysiology: animal studies
Experimental studies in various animal models of cardio-
vascular disease have demonstrated cardioprotective effects
of relaxin (Table 1, Figure 2). Most experimental animal
studies to date involve pro vision of exogenous relaxin.
Future studies of relaxin or relaxin receptor gene-targeted
mice subjected to different disease models may provide
further insight on the role of endogenous relaxin.
Relaxin promotes flow and vasodilation in rodent coro-
nary arteries,42,45,46 and in the vascular beds of other
organs.53,54 Relaxin also vasodilates human systemic
resistance arteries in vitro,55 and reduces vascular resis-
tance in rat models of hypertension.56,57 An earlier study
showed that relaxin lowered blood pressure in spon-
taneously hypertensive rats, but not in control rats.58 This
vasodilation involves increased NO production from the
endothelium via the ETB receptor, through ERK1/2 and
NF-κB activation,49 and is enhanced by antagonism of
ET-1, catecholamine or angiotensin II-mediated vaso-
constriction.39,49 Relaxin additionally vasodilates small
renal arteries of pregnant and nonpregnant female rats
and of male rats,54,59,60 where low infusion rates produce
vasodilation and hyperfiltration and reduce myogenic
acti vity,61 causing increases in effective renal plasma
flow and glomerular filtration rate (GFR). Activation
of gelatinase activity in renal arteries during pregnancy
cleaves big ET to form ET1-32, which potently activates
the endothelial ETB receptor–NO pathway.54,59,60 These
findings led to the clinical evaluation of relaxin as a
Figure 1 | Signaling pathways activated by the relaxin receptor RXFP1. Interaction of
relaxin with RXFP1 causes coupling to Gαs that activates adenylyl cyclase to increase
cAMP. RXFP1 also couples to GαoB to modulate this effect and to Gαi3 to release Gβγ
to activate PI3K and cause translocation of PKCζ to the cell membrane, to activate
adenylyl cyclase. The mechanism involving Gαi3 involves lipid rich signaling platforms
and an interaction with the carboxy terminus of the relaxin receptor. The activation of
PI3K also results in ERK1/2 activation and an increase in eNOS expression. The
rise in intracellular cAMP levels also activates PKA and CREB to cause changes in
gene expression for various proteins, including iNOS. In addition there is evidence
that RXFP1 transactivates tyrosine kinases and that relaxin directly interacts with
the GR to alter gene transcription. Abbreviations: cAMP , cyclic AMP; CRE, cAMP
response element; CREB, cAMP-responsive element-binding protein 1; eNOS,
endothelial nitric oxide synthase; ERK, extracellular signal-regulated kinase; GR,
glucocorticoid receptor; GRE, glucocorticoid response element; iNOS, inducible nitric
oxide synthase; NO, nitric oxide; PDE, phosphodiesterase; PKA, protein kinase A;
PKB, protein kinase B; PKC, protein kinase C; PI3K, phosphoinositide 3-kinase;
RTK, receptor tyrosine kinase; RXFP1, relaxin family peptide receptor 1.
Altered gene transcription
NATURE REVIEws | CARDiology
VOLUME 7 | JANUARY 2010 | 51
vaso dilator in patients with acute HF,62,63 and in pre-
eclampsia, characterized by hypertension, fluid retention
and albumin uria during pregnancy.60
Relaxin also increases expression of the angiogenic
cytokine VEGF in a cAMP-dependent fashion.11 In
in vivo experimental models, including the infarcted
heart, relaxin stimulates angiogenesis at ischemic
wound sites and induces expression of VEGF and basic
fibroblast growth factor isoforms in cells collected from
wound sites.64,65 These studies indicate that relaxin speeds
the healing process in ischemic wounds. Interestingly,
studies in a mouse model of myocardial infarction show
elevated vessel densities in the border regions after
Chronotropic and inotropic effects
In vitro and in vivo studies in rodents show that relaxin
causes concentration-dependent chronotropic and ino-
tropic effects in the atria,46,66–70 which contain a high
density of relaxin receptors.71 However, no positive
inotropic effects were observed in atria from sheep72 or
humans,6 consistent with a lack of relaxin binding in the
atria of these species. Clinical studies do not report any
relaxin-induced changes in heart rate of patients with
Effects on tissue injury
Relaxin is cardioprotective in guineapig, rat and pig
models of ischemia–reperfusion.41,42,45 Treatment with
relaxin before ischemia–reperfusion reduced markers
of inflammation, cell death, oxidative stress and calcium
overload, and markedly reduced infarct size, ventricular
arrhythmias and overall mortality.42,45 In a rat model of
isoproterenol-induced cardiac injury, relaxin ameliorated
myocardial injury, inflammatory cell infiltration and
subsequent fibrosis, thereby alleviating severe ventri-
cular dysfunction.39 In 2007, a study with cultured rat
cardiomyocytes showed that apoptosis, in response to
H2O2-mediated oxidative stress, was significantly inhi-
bited by relaxin at nanomolar concentrations, owing to an
increased Bcl2/Bax ratio and increased Akt expression.34
Another potentially beneficial cardioprotective effect
of relaxin is its antifibrotic activity. Collagens are ECM
proteins that tether beating myocytes, transfer con-
tractile force and provide the tensile properties of the
myo cardium. Following injury, fibroblast activation and
proliferation causes increased collagen production and,
ultimately, interstitial fibrosis. Fibrosis is increased by
hormonal factors (such as growth factors and cytokines),
tissue injury or increased biomechanical overload, and
influences ventricular dysfunction, remodeling, and
arrhythmogenesis. Continuous infusion of relaxin for
2–4 weeks,39,73–75 or relaxin gene delivery by viral con-
structs,65,76,77 consistently inhibits or reverses cardiac
fibrosis in models of transgenic cardiomyopathy, hyper-
tension, isoprenaline-induced cardiac toxicity, diabetic
cardiomyopathy and myocardial infarction. Additionally,
male relaxin gene-deficient mice develop age-related
cardiac fibrosis, ventricular stiffening and diastolic dys-
function,78 suggesting that relaxin is an important endo-
genous regulator of collagen turnover during ageing.
Inhibition of fibrogenesis or reversal of established
fibrosis can reduce ventricular stiffening and improve
diastolic function.79 Notably, although relaxin reduces
aberrant collagen accumulation, it does not affect basal
collagen content in healthy tissues, highlighting its safety
for therapeutic use.
The antifibrotic effects of relaxin provoke the sugges-
tion that it may facilitate and improve stem-cell-based
therapies for myocardial regeneration, particularly
in ischemic heart disease. stem cell survival in the
infarcted heart is low and scar formation in the post-
ischemic, infarcted myocardium limits stem-cell homing,
proliferation and overall viability, and forms a barrier
against the proper integration of implanted and native
stem cells.80 studies in pigs and rats with chronic myo-
cardial infarction demonstrated that the addition of skel-
etal myoblasts virally transfected with relaxin lowered
scar density, reduced apoptosis of engrafted cells and
increased microvascular density in the infarcted region,
effects associated with an improved ventricular con-
tractile function.65,77 In vitro studies have demonstrated
that relaxin, added to the culture system or secreted by
virally transfected skeletal myoblasts,81,82 facilitates cell–
cell coupling, differentiation and functional maturation
of neonatal cardiomyocytes.
Changes in endogenous relaxin: clinical studies
Clinical studies have examined changes in circulating
relaxin and its possible role as a diagnostic marker for
patients with cardiovascular diseases (Table 2). Findings
have been inconsistent, as described below.
In a study of 27 patients with moderate or severe HF
(NYHA Class II or IV) owing to dilated or ischemic cardio-
myopathy, circulating relaxin levels were higher than that
seen in the 13 control patients with suspected coronary
artery disease but no structural cardio vascular disease,
and correlated well with the severity of HF.83 An inverse
correlation between relaxin plasma levels and levels of the
vasoconstrictor ET-1 was also identi fied in the 14 patients
with severe HF,83 implying an anti vasoconstrictive action
of relaxin. The relationship between the two proteins was
confirmed in cell studies.49,83 serial monitoring of relaxin
levels showed good reproduci bility, but also a significant
decline of plasma relaxin levels after vasodilatory therapy
and hemodynamic improvement in the 14 patients with
Box 2 | Relaxin family peptide receptor signaling via cyclic AMP
Cyclic AMP is a major signaling pathway activated in cells expressing relaxin
family peptide receptor 1
Cyclic AMP signaling is complex and may involve up to three G-proteins, G
GαoB and Gαi3
The C-terminal 10 amino acids and Arg752, in particular, are required for G
signaling, which also involves interaction of the receptor with membrane lipid
The contribution of the various G-proteins to signaling varies with the cell type
in which the receptor is expressed
52 | JANUARY 2010 | volUme 7
severe HF.83 Furthermore, increased myocardial relaxin
mRNA and protein levels were also identified in tissue
from failing human hearts.83 This finding agrees with
reports showing elevated relaxin expression or content
in rodents with pressure overload hypertrophy,84
and in rodents with catecholamine over-stimulation.39
Interestingly, pro hormone convertase-1, that converts
pro-relaxin to relaxin, was present in heart tissue but was
found at lower levels in failing myocardium.83
High circulating relaxin levels were also reported
in patients with severe HF (n = 87; 75% with NYHA
Class III–IV) owing to left ventricular systolic dysfunc-
tion resulting from a variety of etiologies, such as coro-
nary heart disease and pulmonary obstructive disease.85
However, circulating relaxin measurements were highly
variable and were not compared with those in control
patients. Furthermore, no relationship was observed
between circulating relaxin and prognosis indicators,
such as rehospitalization rate and mortality, or circu-
lating levels of B-type natriuretic peptide (BNP), a
validated prognostic biomarker in patients with HF.85
A study of 245 patients with end-stage renal disease
who had high incidences of coronary artery disease (64%),
hypertension (90%) and mortality (43.6%, mainly cardio-
vascular causes) during the follow-up period (1,140 days)
indicated that elevated blood relaxin levels might be a
prognostic indicator.86 In male but not female patients
(odds ratio 0.64) with serum relaxin levels higher than
the median (28.8 pg/ml), a threefold increase in cardiac
death was observed. However, as with the study described
above, large variations in relaxin levels were reported and
no comparison was made with control individuals.
In 41 patients with aortic valve stenosis and stable or
moderate HF, circulating relaxin levels were not different
to those in 88 patients with aortic valve stenosis and no
HF, or those in 11 individuals with no heart disease.87
Additionally, 42 hypertensive individuals without HF
had lower levels of circulating relaxin, compared with 40
normotensive controls; the relaxin level was weakly but
significantly correlated with systolic and diastolic blood
pressures, but not to other measures of cardiac function.88
In both studies,87,88 plasma relaxin levels were much
higher and more variable than reported previously.83
Additionally, relaxin assays in arterial and coronary
sinus blood samples indicated relaxin release from dis-
eased hearts in two studies.83,87 Thus, changes in circulat-
ing (and cardiac) relaxin may be associated with disease
etio logy, severity of HF, prognosis and patient sex.
The inconsistent results on circulating levels of
relaxin from these studies might also be in part due to
metho dological reasons. Although a few assay methods
(enzyme-linked immunosorbent assay [ELIsA], enzyme
immunoassay or radioimmunoassay) are commercially
available, ELIsA kits from Immundiagnostik GmbH—
which have a detection limit of 0.4 pg/ml and good
specificity—were used in all these studies discussed.
Nevertheless, the reports differed significantly in the
detection limit (0.4 versus 12 pg/ml) and the normal
value derived from healthy subjects. Additionally, dif-
ferences between men and women, as reported in a study
using the same assay,89 were either not observed or not
indicated. studies using a different relaxin assay showed
variation in relaxin levels during the estrous cycle,90 and
in some of the aforementioned studies, some patients
Table 1 | Cardiovascular protection afforded by relaxin in various disease models
Modelsinflammation Tissue injury/
Ischemia and reperfusion
DecreasedDecreased DecreasedNA DecreasedNA
Ischemia and reperfusion
Ischemia and reperfusion
Decreased DecreasedDecreasedNADecreased NA
DecreasedDecreasedNANA NA NA
NANA UnchangedDecreasedNA Unchanged
NA NA NA NANA Decreased
NA DecreasedNA NANADecreased
NA NA NADecreased NA NA
NANA NA DecreasedNA NA
cardiac toxicity (rat)39
Decreased Decreased DecreasedDecreased DecreasedDecreased
NA NA Decreased Decreased NAUnchanged
Myocardial infarction (pig)64
NA DecreasedDecreased DecreasedNANA
Myocardial infarction (rat)76
NA Decreased Decreased DecreasedNA NA
Abbreviations: NA, information not available; SHR, spontaneously hypertensive rat.
NATURE REVIEws | CARDiology
VOLUME 7 | JANUARY 2010 | 53
were premenopausal and this factor was apparently not
considered. Finally, the immunoreactive materials mea-
sured by the ELIsA in all these studies were not verified
and possibly include prorelaxin and relaxin.
Changes in circulating relaxin levels in patients with
HF, therefore, remain unclear and further inves tigation
on the role of endogenous relaxin in heart disease is
warranted. Clinical studies are necessary to confirm fin-
dings in patients with cardiomyopathy and severe HF,
and to explore potential relationships in other settings
of heart disease.83 Given that circulating relaxin is elimi-
nated mainly by the kidneys and, to a lesser extent, by the
liver,91 elevated circulating relaxin observed in patients
with renal failure86 might in part be a result of reduced
capacity for relaxin clearance. Changes in relaxin clea-
rance in the setting of heart disease, and the potential
contribution of these changes to levels of circulating
relaxin remain to be addressed.
Therapeutic use of relaxin: clinical studies
In 11 healthy individuals enrolled in an open-label
study in the UK, relaxin therapy (0.5–2 μg/kg/h intra-
venously for 5 h) caused no change in systolic blood
pressure or GFR.92 In 92 patients with systemic sclerosis,
however, relaxin therapy (25 μg/kg/day subcutaneously,
6–12 months) caused a fall in systolic blood pressure;
this effect was particularly strong in the 10 patients with
systolic blood pressure greater than 140 mmHg.93 The
vasodilator properties of relaxin, its antagonism of angio-
tensin II, and the possible beneficial effect of relaxin
on renal function,36 underpin its potential therapeutic
benefit in patients with cardiovascular disease, particu-
larly HF. The first reports of clinical trials of relaxin
therapy in patients with HF were published in 2009.
Phase I clinical studies using relaxin investigated its
vasodilator effects in patients with compensated or acute
HF.62,63 A study enrolled 16 patients with stable NYHA
Class II/III HF and with left ventricular ejection frac-
tion (LVEF) less than 35%.62 All patients received an
angiotensin-converting-enzyme (ACE) inhibitor or an
angiotensin receptor blocker (ARB), as well as a β-blocker
and diuretics, and received 24 h relaxin (intravenously) at
one of three dosage regimens while undergoing hemo-
dynamic monitoring. No adverse events occurred and
hemodynamic measurements indicated a vasodilator
action, with falls in pulmonary capillary wedge pres-
sure, systemic vascular resistance with maintained
systolic blood pressure, and a trend toward increased
cardiac index attributable to higher stroke volume, since
heart rate was unchanged. Creatinine levels also fell, but
with a rebound elevation at day 9 in patients receiving
the highest dose (960 μg/kg/day). The greatest effects
on pulmonary capillary wedge pressure and levels of
N-terminal proBNP occurred at doses of 10–100 μg/kg/
day whereas effects on cardiac index was more evident
at higher doses.62 The trial investigators suggested that
different dose–response curves for venous and arterial
dilator properties of relaxin may explain the bell-shaped
In the recent Pre-RELAX-AHF study, 234 patients
with acute HF and mild-to-moderately impaired renal
function (eGFR 30–75 ml/min/1.73m2) but maintained
systolic blood pressure (>125 mmHg) were allocated to
receive placebo or one of four relaxin intravenous dosage
regimens.63 Patients receiving inotropes or intravenous
vasodilators were excluded. There was a high incidence
of atrial fibrillation or flutter (39–60%) and about half the
cohort had a baseline LVEF lower than 40% (not mea-
sured in approximately 30% of patients). ACE inhibitor
or ARB use at enrollment was 60–70%, and β-blocker
use was 50–60%, in keeping with the current standard
management of patients without systolic dysfunction.63
The trial assessed a composite of a number of clinical
end points, including renal impairment and changes in
blood pressure. Relaxin was associated with a border line
and significant improvement in self-reported breathless-
ness when administered at a dose of 30 μg/kg/day (intra-
venously). Cardiovascular death or re admission by day
60 or death by day 180 was also improved, although all-
cause death or readmission rate was not. This dose was
Figure 2 | A summary of the cardiac protective actions of relaxin based on experimental studies in a variety of animal
models. In the text boxes, and indicate decreased and increased, respectively. Abbreviations: Ang II, angiotensin II;
ANP, atrial natriuretic peptide; CF, cardiac fibroblast; ET-1, endothelin-1; NO, nitric oxide.
via ET-1/Ang II
CF activation and
Stem cell survival
Tissue injury, cell death
54 | JANUARY 2010 | volUme 7
also associated with a trend to clinical improvement of
HF and reduced requirement for diuretics by day 5. No
significant differences between serum creatinine at day
14 were apparent among the groups; however, the propor-
tion with an increase in creatinine ≥26 μmol/l tended to
be higher in groups treated with relaxin at higher doses,
compared with those receiving the placebo control. All
groups displayed a similar fall in systolic blood pres-
sure by 10–20 mmHg. For all relaxin groups, survival or
being out of hospital from baseline to day 60 was higher
than for the placebo group, although this study was not
powered to address this end point.
Taken together, these studies confirm the safety of
relaxin infusion (up to 960 μg/kg/day) and are consistent
with a vasodilator effect in patients with HF, but relaxin
therapy was not consistently accompanied by improve-
ment in renal function. The patient population enrolled
in the Pre-RELAX-AHF study is a good representation of
the general population of patients hospitalized with acute
HF to date. Based on findings from the Pre-RELAX-AHF
study, a further trial testing the dose of 30 μg/kg/day in
acute HF is planned (RELAX AHF-1). It is important
that patients in this trial receive current best practice
management at baseline, including for hypertension
(for example, ACE inhibitors if it is indicated and toler-
ated), and that the study is powered sufficiently to detect
worthwhile changes in ‘hard’ end points.
The HF studies to date have sensibly addressed potential
benefits from short-term treatment, but have not exa-
mined whether other properties of relaxin may be thera-
peutic. Indeed, relaxin treatment may also be beneficial
for situations other than vasodilation, given that relaxin
exerts other biological actions. The treatment of condi-
tions where cardiac fibrosis is the major pathophy siology
is intriguing, especially with the increasing availability
of imaging techniques, such as MRI, allowing both
patient selection and quantification of fibrosis in patchy
or diffuse form as a surrogate end point.94 Myocardial
stiffness in hypertensive patients with left ventricular
hypertrophy and fibrosis and presenting as HF with
normal LVEF is a situation with limited therapeutic
options that might benefit from the use of relaxin. such
patients could be included in further relaxin trials since
a substantial percentage of HF patients have a history of
hypertension and may not have a markedly suppressed
Research has established the pivotal role of cardiac
fibrosis in initiation and maintenance of arrhythmias,
owing to ectopic automaticity of myofibroblasts and
aberrant conductance of fibrotic myocardium.95,96 The
possibility that inhibition or regression of fibrosis would
be antiarrhythmic has never been examined. whereas
mineralo corticoid receptor blockade with one of the
known antifibrotic drugs eplerenone or spironolactone
is associated with 21–29% reduction in sudden cardiac
death,97,98 a direct antiarrhythmic effect cannot be
excluded, since conditional overexpression of mineralo-
corticoid receptors in mouse hearts leads to channel
remodeling and ventricular arrhythmias.99
Relaxin is also likely to be effective in patients with
acute myocardial infarction receiving primary coronary
intervention. This condition is associated with events in
which relaxin has been shown to exert beneficial effects
at therapeutic doses, including ischemia–reperfusion
injury, oxidative stress, cardiomyocyte death, arrhyth-
mias, vasoconstriction, angiogenesis and reactive or
Pharmacology and clinical efficacy
Pleiotropic effects of relaxin
Relaxin clearly activates a wide range of signaling path-
ways, including those involving cAMP, ERK1/2, tyrosine
kinases, NO, GR, VEGF and IGF binding protein-1. what
is less clear, however, is which pathways are important for
the improvements seen in relaxin-treated patients with
acute HF. Possible properties of relaxin that may under-
pin its therapeutic effects include vasodilation, antago-
nism of angiotensin II and possible improvements in
Table 2 | Studies that determined serum levels of (H2) relaxin in patients with cardiovascular diseases
etiologygroup size Sample
Changes in relaxin level
Dilated and ischemic
n = 40
Plasma ~2Relaxin levels elevated by 8–10 fold in patients with severe
HF and correlated with hemodynamic parameters
Chronic HF owing
to a variety of
n = 87
Plasma<2 Relaxin levels increased in patients with HF (median
89 pg/ml, range 11–644 pg/ml) but had no prognostic
value nor correlation with BNP
n = 245
SerumNo control Relaxin levels over the median (28.8 pg/ml, range
0–240 pg/ml) were associated with a 3-fold increase in
cardiovascular mortality in males but not in females
Aortic valve stenosis
and HF 87
n = 129
Plasma 42 (<12–100)Patients with HF showed similar relaxin levels as healthy
controls, but had transcardiac relaxin gradient of +6 pg/ml,
compared with -5 pg/ml for patients without HF, indicating
cardiac release of relaxin
n = 82
Serum49 ± 40Relaxin levels were lower in hypertensive patients and
negatively correlated with blood pressures
*All studies used the Immundiagostik AG (Bensheim, Germany) enzyme-linked immunosorbent assay kits for the relaxin assay. Abbreviations: BNP, B-type
natriuretic peptide; HF, heart failure.
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VOLUME 7 | JANUARY 2010 | 55
renal function. Both cAMP and NO are vasodilators and
would be expected to cause physiological antagonism
of angiotensin II. The cAMP response is complex and,
from the earliest studies of relaxin signaling, some cells
and tissues were found not to show a cAMP response
to relaxin whereas, in others, the response varies from
poor to easily measureable.26,28,30,100 The changes in cAMP
involve coupling of RXFP1 to at least three different
G-proteins (as described above),28,29 but the contribu-
tion of each of these combinations varies with cell type,
which explains some of the wide variations in responses
to relaxin observed in different tissues.101 Determination
of the role of cAMP in arteries and veins of different
diameter should shed further light.
NO signaling is undoubtedly involved in the vaso-
dilator effects of relaxin and its importance likely varies
in different vessels. As described above, NOs isoforms
are upregulated secondarily to activation of other sig-
naling pathways. Two paradigms provide additional
experimental support for the vasodilatory effects of
relaxin in vascular beds and tissues. The first is relaxin-
induced upregulation of vascular gelatinases in the
kidney, increased conversion of big ET to ET1-32, stimu-
lation of endothelial ETB receptors, increased synthesis
and release of NO, and relaxation of vascular smooth
muscle.36,54 Relaxin may also upregulate ETB receptors
to enhance this response.49 The second is upregulation
of iNOs by relaxin in vascular tissues.7,44,102,103 In preg-
nant and nonpregnant rats, relaxin causes renal vaso-
dilation and increases in GFR and renal plasma flow.104 In
humans, the clinical trials on systemic sclerosis showed
that relaxin increases GFR,105 but more-recent studies in
healthy humans showed markedly increased renal plasma
flow without changes in GFR.106 Improvements in kidney
function seem more likely in patients with HF, who
charac teristically display strong activation of both the
renin–angiotensin and adrenergic systems; however, this
effect was not seen in a clinical trial with patients receiv-
ing relaxin in addition to current standard medications.62
studies are clearly necessary to determine the mechanism
responsible for vasodilator effects in humans.
Pharmacodynamic effects of relaxin
One of the striking features of the clinical trial of relaxin
in HF was the bell-shaped dose–response curve for the
effects on dyspnoea and renal function.62,63 such curves
have been observed in a wide variety of pre clinical
studies, including cAMP responses in cells expres-
sing RXFP1,26,29,107 renal and cardiac hemo dynamic
responses,56,61,108 changes in plasma osmolality,108 and
MMP expression.109 These pheno mena have been
explained in various ways.
The bell-shaped dose–response relationships observed
in the studies with relaxin on renal hemodynamics in rats
and renal performance in humans can be explained by
relaxin exerting different effects in different vascular
beds.62 In humans, the effect was suggested to result from
relaxin having a greater vasodilatory effect in veins than
in arteries, where lower doses would reduce pulmonary
capillary wedge pressure without influencing cardiac
index, which would be increased by higher doses.62 This
phenomenon could exist if RXFP1 effector coupling is
better in veins than arteries, perhaps owing to a higher
RXFP1 density or a different pattern of G-protein coup-
ling in veins versus arteries, as observed in a variety of
other cell types.101
One of the problems in ascribing the bell-shaped
dose–response curve exclusively to hemodynamic con-
siderations is that similar phenomena occur in cells
and tissues where altered blood flow is not a potential
explana tion. For example, as mentioned above, bell-
shaped concentration–response curves are a common
feature of cAMP responses in cell systems express-
ing RXFP1,26,29,107 and of MMP expression in isolated
fibroblasts.109 Although the reason for the bell-shaped
dose–response curve is not clear at present, the lower
cAMP responses observed with higher concentrations
of relaxin acting at RXFP1 are not a result of receptor
desensi tization, since RXFP1 does not undergo signifi-
cant phosphorylation, interact with β-arrestin 1 or
β-arrestin 2, or become internalized.110
Evidence that RXFP1 receptors form oligomers is
emerging and this oligomerization may be respon-
sible for the negative cooperativity observed in recep-
tor binding studies.111–113 The accelerated dissociation
of ligand that occurs at higher concentrations shortens
the time the ligand remains bound to the receptor and
may influence the pattern and degree of activation of
Comparison of effects in humans and animals
The earliest experiments on reproduction clearly demon-
strated that the role of relaxin in humans differs from
that in many other species. In humans, plasma relaxin
levels peak in the first trimester of pregnancy and pro-
bably have a role in implantation, whereas, plasma
levels increase throughout pregnancy in many other
species, to maintain the quiescence of the uterus and
easy partu rition.5 In the heart, relaxin produces power-
ful chronotropic effects in rats and mice where there is a
correspondingly high concentration of RXFP1.68,71,114 In
human hearts, however, there is no equivalent response,
despite the presence of mRNA for RXFP1.12
Nevertheless, in the cardiovascular system, some of
the differences may be due, in part, to some confounding
factors such as medication. For example, the beneficial
effects of relaxin on the renal circulation seen in rats are
not always apparent in humans; however, many of the
human studies are conducted in patients treated with
a variety of drugs including ACE inhibitors (or ARBs),
vasodilators, β-blockers, diuretics and inotropes, all of
which can influence renal function.62,63 Furthermore,
treatment with ACE inhibitors influences the relaxin-
induced vasodilator response to relaxin in human small
systemic resistance arteries. In arteries from patients on
ACE inhibitors, indomethacin blocked, and phospho-
diesterase inhibition enhanced, responses to relaxin,
indicating that prostanoids and cAMP were important for
vasodilation. However, in patients not on ACE inhibitors,
the NO pathway was much more important.55
56 | JANUARY 2010 | volUme 7
Development of novel ligands acting at RXFP1
As outlined previously, the interaction between relaxin
and RXFP1 is complex and involves three stages; binding
of the relaxin B-chain to the LRRs, interaction with the
second extracellular loop of the seven transmembrane
region and, finally, an unknown action of the LDLa
module that is essential for signaling.20,22 The complex
binding process represents a major challenge for the
development of nonpeptide orthosteric agonists at the
RXFP1 as the three-stage binding process would be diffi-
cult to mimic with molecules other than peptides. Even
simple alterations of the receptor cause major disrup-
tions to RXFP1 signaling. Receptors lacking the LDLa
module bind relaxin normally, but are unable to signal.22
However, coexpression of the receptor lacking the LDLa
module and the LDLa module or addition of LDLa to
LDLa-less receptors does not restore signaling, which
suggests that the LDLa-receptor linkage is essential for
conformations required for signaling.22 Perhaps the best
approach to the development of nonpeptide agonists for
RXFP1 lies with allosteric interactions with the recep-
tor that may induce the correct conformations for sig-
naling without interacting with the orthosteric binding
sites. For development of antagonists, interaction with
any of the three sites identified as important for relaxin
binding and receptor signaling should block the response
to relaxin; however, to date no orthosteric antagonists for
RXFP1 have been successfully developed. The use of the
soluble ectodomain of RXFP1 as an antagonist depends
on high affinity binding to the LRR region, which acts
as a scaven ger for relaxin, and lowering its effective
concentration at intact RXFP1.12
Over the 80 years since the discovery of relaxin as an
endogenously produced reproductive peptide hormone,
most of the research has involved its reproductive action
and regulation of the ECM. However, the beneficial
actions of relaxin on the cardiovascular system have
been increasingly appreciated. Clinical trials for acute
HF have confirmed the safety record of relaxin therapy
and shown therapeutic efficacy. Clinical studies indicate
relaxin as a vasodilator in HF, but we need to also con-
sider other clinical situations suitable for relaxin therapy,
taking into consideration the pleiotropic actions of the
peptide. Acute myocardial infarction and cardiac fibro-
sis are clinical situations likely to benefit from relaxin
therapy and worthy of further investigation. Research
with relevant experimental models and selected patients
is warranted to generate proof-of-concept evidence and
define suitable conditions to fulfill the cardiovascular
potential of relaxin.
A search was conducted via PubMed for articles published
from 1990 that focussed on relaxin and heart disease.
Search terms were “relaxin” and “cardiovascular”. we
also searched for original articles of molecular and
pharmacological aspects of relaxin, relaxin receptors and
signaling. All articles are English language full-text papers.
1. Sherwood, O. D. Relaxin’s physiological roles and
other diverse actions. Endocr. Rev. 25, 205–234
Sherwood, O. D. & O’Byrne, E. M. Purification
and characterization of porcine relaxin. Arch.
Biochem. Biophys. 160, 185–196 (1974).
Hudson, P . et al. Structure of a genomic clone
encoding biologically active human relaxin.
Nature 301, 628–631 (1983).
Hudson, P . et al. Relaxin gene expression in
human ovaries and the predicted structure of a
human preprorelaxin by analysis of cDNA clones.
EMBO J. 3, 2333–2339 (1984).
Bathgate, R. A. D., Hsueh, A. J. &
Sherwood, O. D. Physiology and molecular
biology of the relaxin peptide family in Physiology
of Reproduction (eds Knobil, E. & Neill, J. D.)
679–770 (Elsevier, San Diego, 2006).
Samuel, C. S., Du, X. J., Bathgate, R. A. D.
& Summers, R. J. ‘Relaxin’ the stiffened heart
and arteries: The therapeutic potential for relaxin
in the treatment of cardiovascular disease.
Pharmacol. Therap. 112, 529–552 (2006).
Bani, D. Relaxin: a pleiotropic hormone.
Gen. Pharmacol. 28, 13–22 (1997).
Samuel, C. S. Relaxin: antifibrotic properties
and effects in models of disease. Clin. Med. Res.
3, 241–249 (2005).
Lenhart, J. A., Ryan, P . L., Ohleth, K. M.,
Palmer, S. S. & Bagnell, C. A. Relaxin increases
secretion of matrix metalloproteinase-2 and
matrix metalloproteinase-9 during uterine and
cervical growth and remodeling in the pig.
Endocrinology 142, 3941–3949 (2001).
10. Lenhart, J. A., Ryan, P . L., Ohleth, K. M.,
Palmer, S. S. & Bagnell, C. A. Relaxin increases
secretion of tissue inhibitor of matrix
metalloproteinase-1 and -2 during uterine and
cervical growth and remodeling in the pig.
Endocrinology 143, 91–98 (2002).
11. Unemori, E. N. et al. Relaxin stimulates
expression of vascular endothelial growth factor
in normal human endometrial cells in vitro and is
associated with menometrorrhagia in women.
Hum. Reprod. 14, 800–806 (1999).
12. Hsu, S. Y. et al. Activation of orphan receptors
by the hormone relaxin. Science 295, 671–674
13. Bathgate, R. A., Ivell, R., Sanborn, B. M.,
Sherwood, O. D. & Summers, R. J. International
Union of Pharmacology: Recommendations for
the nomenclature of receptors for relaxin family
peptides. Pharmacol. Rev. 58, 7–31 (2006).
14. Hawkes, C. & Kar, S. The insulin-like growth
structure, distribution and function in the central
nervous system. Brain Res. Rev. 44, 117–140
15. Pirola, L., Johnston, A. M. & Van Obberghen, E.
Modulation of insulin action. Diabetologia 47,
16. De Meyts, P ., Palsgaard, J., Sajid, w.,
Theede, A. M. & Aladdin, H. Structural biology
of insulin and IGF-1 receptors. Novartis Found.
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17. Hsu, S. Y. New insights into the evolution of the
relaxin-LGR signaling system. Trends Endocrinol.
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18. Bullesbach, E. E. & Schwabe, C. The relaxin
receptor-binding site geometry suggests a novel
gripping mode of interaction. J. Biol. Chem. 275,
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relaxin-binding site of the leucine-rich
G-protein-coupled receptor 7. J. Biol. Chem.
280, 14051–14056 (2005).
20. Halls, M. L. et al. Multiple binding sites revealed
by interaction of relaxin family peptides with
native and chimeric relaxin family peptide
receptors 1 and 2 (LGR7 and LGR8).
J. Pharmacol. Exp. Ther. 313, 677–687 (2005).
21. Sudo, S. et al. H3 relaxin is a specific ligand for
LGR7 and activates the receptor by interacting
with both the ectodomain and the exoloop 2.
J. Biol. Chem. 278, 7855–7862 (2003).
22. Scott, D. J. et al. Characterization of novel splice
variants of LGR7 and LGR8 reveals that receptor
signaling is mediated by their unique low density
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23. Scott, D. J. et al. Defining the LGR8 residues
involved in binding insulin-like peptide 3.
Mol. Endocrinol. 21, 1699–1712 (2007).
24. Hossain, M. A. et al. The A-chain of human
relaxin family peptides has distinct roles in the
binding and activation of the different relaxin
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25. Hopkins, E. J., Layfield, S., Ferraro, T.,
Bathgate, R. A. & Gooley, P . R. The NMR solution
structure of the relaxin (RXFP1) receptor
lipoprotein receptor class A module and
identification of key residues in the N-terminal
region of the module that mediate receptor
activation. J. Biol. Chem. 282, 4172–4184
26. Halls, M. L., Bathgate, R. A. & Summers, R. J.
Relaxin family peptide receptors RXFP1 and
RXFP2 modulate cAMP signaling by distinct
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NATURE REVIEws | CARDiology
VOLUME 7 | JANUARY 2010 | 57
27. Nguyen, B. T., Yang, L., Sanborn, B. M.
& Dessauer, C. w. Phosphoinositide 3-kinase
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28. Nguyen, B. T. & Dessauer, C. w. Relaxin
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29. Halls, M. L. et al. Relaxin family peptide receptor
(RXFP1) coupling to Gαi3 involves the C-terminal
Arg752 and localization within membrane raft
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30. Kompa, A. R., Samuel, C. S. & Summers, R. J.
Inotropic responses to human gene 2 (B29)
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31. Halls, M. L. et al. RXFP1 couples to the
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33. Zhang, Q., Liu, S. H., Erikson, M., Lewis, M.
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34. Moore, X. L. et al. Relaxin antagonizes
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35. Palejwala, S., Stein, D., wojtczuk, A., weiss, G.
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36. Conrad, K. P . & Novak, J. Emerging role of relaxin
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38. Masini, E. et al. Protective effects of relaxin in
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due to splanchnic artery occlusion.
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40. Boehnert, M. U., Hilbig, H. & Armbruster, F. P .
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preserving and reperfusion solution for liver
transplantation, shown in a model of isolated
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42. Bani, D., Masini, E., Bello, M. G., Bigazzi, M.
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This work was supported by National Health and
Medical Research Council (NHMRC) of Australia
Research Fellowships to X.-J. Du, A. M. Dart and
R. A. D. Bathgate; a National Heart Foundation of
Australia (NHFA)/NHMRC RD wright Fellowship to
C. S. Samuel; a NHMRC project grant 436,713
to R. J. Summers and R. A. D. Bathgate; a NHMRC
program grant 519,461 to P . M. Sexton,
A. Christopoulos and R. J. Summers; and a NHMRC
program grant A72600 to A. M. Dart.