Adrenomedullin and myocardial contractility in the rat

Adrenomedullin: Continuing to explore cardioprotection

Adrenomedullin (AM), a peptide isolated from an extract of human pheochromocytoma, comprises 52 amino acids with an intramolecular disulfide bond and amidation at the carboxy-terminus. AM is present in various tissues and organs in rodents and humans, including the heart. The peptide concentration increases with cardiac hypertrophy, acute myocardial infarction, and overt heart failure in the plasma and the myocardium. The principal function of AM in the cardiovascular system is the regulation of the vascular tone by vasodilation and natriuresis via cyclic adenosine monophosphate-dependent or -independent mechanism. In addition, AM may possess unique properties that inhibit aldosterone secretion, oxidative stress, apoptosis, and stimulation of angiogenesis, resulting in the protection of the structure and function of the heart. The AM receptor comprises a complex between calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein (RAMP) 2 or 3, and the AM-CLR/RAMP2 system is essential for heart development during embryogenesis. Small-scale clinical trials have proven the efficacy and safety of recombinant AM peptide therapy for heart failure. Gene delivery and a modified AM peptide that prolongs the half-life of the native peptide could be an innovative method to improve the efficacy and benefit of AM in clinical settings. In this review, we focus on the pathophysiological roles of AM and its receptor system in the heart and describe the advances in AM and proAM-derived peptides as diagnostic biomarkers as well as the therapeutic application of AM and modified AM for cardioprotection.

The Role of Adrenomedullin in Cardiovascular Response to Exercise - A Review

Adrenomedullin (ADM), the product of the vascular endothelial and smooth muscle cells, and cardiomyocytes, is considered to be a local factor controlling vascular tone, cardiac contractility and renal sodium excretion. The aim of this article was to review the existing data on the effect of different types of exercise on plasma ADM concentration in healthy men. The results of studies on the effect of dynamic exercise on the plasma ADM are contradictory. Some authors reported an increase in plasma ADM, while others showed a slight decrease or did not observe any changes. The inverse relationship between plasma ADM and mean blood pressure observed during maximal exercise support the concept that ADM might blunt the exercise-induced systemic blood pressure increase. Positive relationships between increases in plasma ADM and those in noradrenaline, atrial natriuretic peptide (ANP) or interleukin-6 observed during prolonged exercise suggest that the sympathetic nervous system and cytokine induction may be involved in ADM release. Increased secretion of ADM and ANP during this type of exercise may be a compensatory mechanism attenuating elevation of blood pressure and preventing deterioration of cardiac function. Studies performed during static exercise have showed an increase in plasma ADM only in older healthy men. Positive correlations between increases in plasma ADM and those in noradrenaline and endothelin-1 may indicate the interaction of these hormones in shaping the cardiovascular response to static exercise. Inverse relationships between exercise-induced changes in plasma ADM and those in cardiovascular indices may be at least partly associated with inotropic action of ADM on the heart. Interactions of ADM with vasoactive peptides, catecholamines and hemodynamic factors demonstrate the potential involvement of this peptide in the regulation of blood pressure and myocardial contractility during exercise.

Cohort of estrogen-induced microRNAs regulate adrenomedullin expression

Estrogen regulates the expression of many genes and has been correlated with differences in cardiac contraction; however, the underlying mechanisms remain poorly defined. Adrenomedullin (Adm = gene; AM = protein) is a multifunctional peptide with inotropic actions. Previous studies have demonstrated that estrogen enhances the expression of Adm, suggesting a relationship between AM and estrogen in cardiac contraction during physiological and pathological states. In this study, female mice in a mouse model of genetic Adm overexpression, abbreviated as Adm(hi/hi), were found to express 60 times more Adm in the heart than wild-type littermates, compared with the three-fold elevation of Adm previously reported in Adm(hi/hi) male hearts. Thus, this study sought to further investigate any functional consequences of increased cardiac Adm expression and begin exploring the mechanisms that regulate Adm expression in an estrogen-dependent fashion. This study revealed that heart function is enhanced in Adm(hi/hi) females, which along with Adm expression levels, was reversed following ovariectomization. Since the Adm(hi/hi) line was generated by the displacement of the 3' untranslated region (UTR), the native 3'UTR was examined for estrogen-induced microRNAs target sites to potentially explain the aberrant overexpression observed in Adm(hi/hi) female hearts. Using a bioinformatic approach, it was determined that the mouse Adm 3'UTR contains many target sites for previously characterized estrogen-induced microRNAs. This study also determined that the novel microRNA, miR-879, is another estrogen-induced microRNA that interacts with the 3'UTR of Adm to destabilize the mRNA. Together, these studies revealed that estrogen-induced microRNAs are important for balancing cardiac Adm expression in females.

The relationships between plasma adrenomedullin and endothelin-1 concentrations and Doppler echocardiographic indices of left ventricular function during static exercise in healthy men

Our previous study showed a significant relationships between static exercise-induced changes in plasma adrenomedullin (ADM) and those in endothelin-1 (ET-1), noradrenaline (NA) and pre-ejection period/left ventricular ejection time ratio (PEP/LVET) in older healthy men. It is hypothesized that ADM, ET-1, NA and adrenaline (A) may function as endogenous regulators of cardiac function by modulating myocardial contractility during static exercise. The present study was undertaken to assess the relationships between exercise-induced changes in plasma ADM, ET-1, NA, A concentrations and those in ascending aortic blood flow peak velocity (PV) and mean acceleration (MA) measured by Doppler echocardiography in 24 healthy older men during two 3-min bouts of handgrip at 30% of maximal voluntary contraction, performed alternately with each hand without any break between the bouts. Plasma ADM, ET-1, NA and A as well as heart rate (HR), blood pressure (BP), PV and MA were determined. During handgrip, plasma ADM, ET-1, NA and A as well as HR, BP increased, whereas PV and MA decreased. The increases in plasma ADM correlated positively with those in ET-1, NA and diastolic BP, and correlated negatively with changes in PV (r= −0.68) and MA (r= −0.62). The increases in plasma ET-1 correlated positively with those in NA and BPs and correlated negatively with changes in PV (r= −0.67) and MA (r= −0.60). The results of this study suggest that in healthy older men the exercise-induced changes in plasma ADM, ET 1 and catecholamines are related to alterations in left ventricular contractile state and may co-operatively counteract age-related deterioration of cardiac performance in men.

Intermedin elicits a negative inotropic effect in rat papillary muscles mediated by endothelial-derived nitric oxide

Intermedin (IMD) is a novel vasoactive peptide from the calcitonin gene-related peptide (CGRP) implicated in cardiac regulation, yet the contractile effects of IMD remain controversial, since previous studies in vivo and isolated cardiomyocytes documented contradictory results. We hypothesized cardiac endothelial cells involvement in IMD modulation of cardiac function as an explanation for these opposing observations. With this in mind, we investigated the direct action of increasing concentrations of IMD (10(-8) to 10(-6)M) on myocardial performance parameters in rat left ventricular (LV) papillary muscles with and without endocardial endothelium (EE) and in presence of receptor antagonists and intracellular pathways inhibitors. In LV papillary muscles with intact EE, IMD induced a concentration-dependent negative inotropic action (%decrease relative to baseline, at IMD concentration of 10(-6)M, active tension of 14 ± 4%, and maximum velocity of tension rise of 10 ± 4%). These effects were blunted by EE removal, AM receptor antagonist (AM(22-52)), and CGRP receptor antagonist (CGRP(8-37)). Additionally, nitric oxide (NO) synthase inhibition with N(G)-nitro-l-arginine (l-NAME) in muscles with and without EE and guanylyl cyclase inhibition with {1H-[1,2,4]oxadiazole-[4,4-a]-quinoxalin-1-one} not only blunted the negative inotropic action of IMD but also unmasked IMD-positive inotropic effect dependent on CGRP receptor PKA activation. Western blot quantification of phosphorylated cardiac troponin I (P-cTnI) in IMD-treated papillary muscles revealed a significant increase in P-cTnI when compared with untreated muscles, while in l-NAME-pretreated papillary muscles IMD failed to increase P-cTnI. Finally, we found that stimulation of both EE and microvascular endothelial cells with IMD significantly increased NO production by 40 ± 3 and 38 ± 3%, respectively, suggesting the role of cardiac endothelial cells in NO production upon IMD stimulation. Our findings establish IMD negative inotropic effect in isolated myocardium due to NO/cGMP pathway activation with concomitant thin myofilament desensitization by increase in cTnI phosphorylation and provide a coherent explanation for the previously reported contradictory results.

Effects of adrenomedullin on systolic and diastolic myocardial function

Adrenomedullin (AM) effects were studied in rabbit papillary muscles by adding increasing concentrations (10(-10) to 10(-6)M) either alone or after pre-treatment with l-NNA, indomethacin, AM22-52 (AM receptor antagonist), CGRP(8-37) (CGRP receptors antagonist), KT5720 (PKA inhibitor), as well as after endocardial endothelium (EE) removal. Passive length-tension relations were constructed before and after a single concentration of AM (10(-6)M). AM concentration-dependently induced negative inotropic and lusitropic effects, and increased resting muscle length (RL). At 10(-6)M, AT, dT/dt(max) and dT/dt(min) decreased 20.9+/-4.9%, 18.3+/-7.3% and 16.7+/-7.8%, respectively, and RL increased to 1.010+/-0.004L/L(max). Correcting RL to its initial value resulted in a 26.6+/-6.4% decrease of resting tension, indicating decreased muscle stiffness, also patent in the down and rightward shift of the passive length-tension relation. The negative inotropic effect of AM was dependent on its receptor, CGRP receptor, PKA, the EE and NO, while the effects of AM on myocardial stiffness were abolished by EE damage and NO inhibition. This latter effect represents a novel mechanism of acute neurohumoral modulation of diastolic function, suggesting that AM is an important regulator of cardiac filling.

Positive inotropic effects of epigallocatechin-3-gallate (EGCG) involve activation of Na+/H+ and Na+/Ca2+ exchangers

BACKGROUND

There is evidence that the tea catechin epigallocatechin-3-gallate (EGCG) modulates myocardial contractility. However, the underlying mechanisms remain to be determined.

AIMS

To study potential signalling pathways involved in EGCG-induced contractile parameters.

METHODS AND RESULTS

EGCG increased fractional shortening in rat cardiac myocytes and enhanced intracellular systolic Ca2+ concentrations. In isolated rat hearts, perfusion with EGCG resulted in significant, dose-dependent increase in peak systolic left ventricular pressure, as well as in contraction and relaxation velocities. Heart rate did not change. Inhibition of the beta1-receptor with metoprolol had no influence on the contractile effects of EGCG. Furthermore, levels of cAMP and phosphorylation of phospholamban did not change with EGCG, indicating that the beta-receptor pathway is not involved. The L-type Ca2+ channel inhibitors, nifedipine and gallopamil, failed to modulate EGCG-induced increase in contractility. However, the myocardial effects and intracellular calcium transients stimulated by EGCG were significantly reduced by the antagonist of the Na+/H+ exchanger (NHE) methyl-N-isobutyl amiloride (MIA), and by blocking of the reverse mode of the Na+/Ca2+ exchanger (NCX) by KB-R7943.

CONCLUSION

These results indicate that Ca2+-dependent positive inotropic and lusitropic effects of EGCG are mediated in part via activation of the Na+/H+ exchanger and the reverse mode of the Na+/Ca2+ exchanger in the rat myocardium.

Adrenomedullin reduces vascular hyperpermeability and improves survival in rat septic shock

OBJECTIVE

Current therapies of sepsis and septic shock require administration of a large volume of fluid to maintain hemodynamic stability. The vasoregulatory peptide adrenomedullin has been shown to prevent the transition to the fatal hypocirculatory septic state by poorly understood mechanisms. We tested the hypothesis that therapeutic administration of adrenomedullin would reduce vascular hyperpermeability, thereby contributing to improved hemodynamics and survival.

DESIGN

Prospective randomized controlled animal study.

SUBJECTS

Male Sprague-Dawley rats (270 g).

INTERVENTIONS

We used 4.8 x 10(3) U/kg of Staphylococcus aureus alpha-toxin, a pore-forming exotoxin, to induce vascular leakage and circulatory shock in rats. The infusion rate was 24 microg/kg per hour. Adrenomedullin was started 1 h after alpha-toxin administration.

MEASUREMENT AND RESULTS

Infusion of alpha-toxin in rats induced cardiocirculatory failure resulting in a 6-h mortality of 53%. alpha-Toxin provoked massive vascular hyperpermeability, which was indicated by an enrichment of Evans blue dye albumin in the tissues of lung, liver, ileum and kidney. Plasma fluid loss led to a significant hemoconcentration. Hemodynamic impairment observed after alpha-toxin infusion was closely correlated to vascular hyperpermeability. Therapeutic administration of 24 microg/kg per hour adrenomedullin reduced 6-h mortality from 53% to 7%. Stabilization of the endothelial barrier by adrenomedullin was indicated by reduced extravasation of albumin and plasma fluid and may have contributed to hemodynamic improvement.

CONCLUSIONS

These data suggest that adrenomedullin-related reduction of vascular hyperpermeability might represent a novel and important mechanism contributing to the beneficial effects of this endogenous vasoregulatory peptide in sepsis and septic shock.

Receptor activity-modifying proteins: RAMPing up adrenomedullin signaling

Adrenomedullin (AM) is a 52-amino-acid multifunctional peptide that circulates in the plasma in the low picomolar range and can exert a multitude of biological effects through an autocrine/paracrine mode of action. The mechanism by which AM transduces its signal represents a novel and pharmacologically tractable paradigm in G protein-coupled receptor signaling. Since its discovery in 1993, the study of AM has emerged into a new field of research with nearly 1800 publications that rivals the renown of other common factors like angiopoetin (1015 publications) and ghrelin (1550 publications). Despite the tremendous strides made in recent years toward unveiling the biochemical and cellular functions of AM, we are still lagging in our understanding of the essential roles of AM in normal and disease physiology. As discussed in this current review, a concerted effort to combine information from clinical, genomic, biochemical, and genetic mouse model sources can provide a focused view to help define the physiological functions of AM. Specifically, we find that certain conditions, such as pregnancy, cardiovascular disease, and sepsis, are associated with robust and dynamic changes in the expression of AM and AM receptor proteins, which together represent an elegant mechanism for altering the physiological responsiveness or function of AM. Thus, the modulation of AM signaling may be further exploited for therapeutic strategies in the management and treatment of human disease.

Adrenomedullin: molecular mechanisms and its role in cardiac disease

Adrenomedullin (AM) is a potent, long-lasting vasoactive peptide originally isolated from human pheochromocytoma. Since its discovery, serum and tissue AM expression have been shown to be increased in experimental models and in patients with cardiac hypertrophy, myocardial infarction and end-stage heart failure with several beneficial effects. Considerable evidence exists for a wide range of autocrine, paracrine and endocrine mechanisms for AM which include vasodilatory, anti-apoptotic, angiogenic, anti-fibrotic, natriuretic, diuretic and positive inotropic. Thus, through regulation of body fluid or direct cardiac mechanisms, AM has additive and beneficial effects in the context of heart disease. Notable molecular mechanisms of AM include cyclic adenosine monophosphate, guanosine-3',5'-monophosphate, PI3K/Akt and MAPK-ERK-mediated cascades. Given the endogenous and multifunctional nature of AM, we consider this molecule to have great potential in the treatment of cardiovascular diseases. In agreement, early experimental and preliminary clinical studies suggest that AM is a new and promising therapy for cardiovascular diseases.

The role of adrenomedullin in angiogenesis

Adrenomedullin (AM) is a 52 amino acid peptide originally isolated from human pheochromocytoma. It was initially demonstrated to have profound effects in vascular cell biology, since AM protects endothelial cells from apoptosis, promotes angiogenesis and affects vascular tone and permeability. This review article summarizes the literature data concerning the relationship between AM and angiogenesis and describes the relationship between vascular endothelial growth factor, hypoxia and AM and tumor angiogenesis. Finally, the role of AM as a potential target of antiangiogenic therapy is discussed.

The clinical relevance of adrenomedullin: a promising profile?

Adrenomedullin (AM) is a peptide that possesses potentially beneficial properties. Since the initial discovery of the peptide by Kitamura et al. in 1993, the literature has been awash with reports describing its novel mechanisms of action and huge potential as a therapeutic target. Strong evidence now exists that AM is able to act as an autocrine, paracrine, or endocrine mediator in a number of biologically significant functions, including the endothelial regulation of blood pressure, protection against organ damage in sepsis or hypoxia, and the control of blood volume through the regulation of thirst. Its early promise as a potential mediator/modulator of disease was not, however, entirely as a result of the discovery of physiological functions but due more to the observation of increasing levels measured in plasma in direct correlation with disease progression. In health, AM circulates at low picomolar concentrations in plasma in 2 forms, a mature 52-amino acid peptide and an immature 53-amino acid peptide. Plasma levels of AM have now been shown to be increased in a number of pathological states, including congestive heart failure, sepsis, essential hypertension, acute myocardial infarction, and renal impairment. These earliest associations have been further supplemented with evidence of a role for AM in other pathologies including, most intriguingly, cancer. In this review, we offer a timely review of our current knowledge on AM and give a detailed account of the putative role of AM in those clinical areas in which the best therapeutic opportunities might exist.

Direct coronary vasodilator action of adrenomedullin is mediated by nitric oxide

Increased circulating levels of adrenomedullin (ADM) cause peripheral vasodilatation and hypotension, accompanied by cardiac actions including tachycardia and increases in cardiac contractility, cardiac output, coronary conductance (CC) and coronary blood flow (CBF). It is unclear to what extent these cardiac effects are direct actions of ADM or secondary to the hypotension and altered cardiac loading. The direct cardiac actions of ADM were examined in conscious sheep previously implanted with aortic and coronary flow probes, and an indwelling left coronary artery cannula. Responses to infusion of ADM (0.5 microg kg(-1) h(-1) for 1 h) into the left coronary artery or jugular vein were compared (n=6). The effect of blockade of nitric oxide (NO) synthase with intracoronary (i.c.) N(omega)-nitro-l-arginine (l-NNA; 1.5 mg kg(-1) h(-1), infused for 2 h before and during ADM infusion, was assessed to determine whether the responses to ADM were mediated by NO (n=5). I.c. ADM caused large and sustained increases in CC (0.35+/-0.07-0.55+/-0.13 ml min(-1) mmHg-1, P<0.05) and CBF (28+/-6-42+/-9 ml min(-1), P<0.05), but had no effect on arterial pressure or indices of cardiac contractility (first differential of the upstroke of systole and peak aortic flow rate). Intravenous infusion of ADM had no effects. I.c. l-NNA, at a dose that abolished the coronary vasodilator action of acetylcholine, blocked ADM-induced coronary vasodilatation. In conclusion, ADM had a direct coronary vasodilator action that was mediated by release of endogenous NO and resulted in increased CBF. There was no evidence for a direct inotropic action of ADM.

Cell and molecular biology of the multifunctional peptide, adrenomedullin

Adrenomedullin (AM) is a recently discovered regulatory peptide involved in many functions including vasodilatation, electrolyte balance, neurotransmission, growth, and hormone secretion regulation, among others. This 52-amino acid peptide is expressed by specific cell types in many organs throughout the body. A complex receptor system has been described for AM; it requires at least the presence of a seven-transmembrane-domain G-protein-coupled receptor, a single-transmembrane-domain receptor activity modifying protein, and a receptor component protein needed to establish the connection with the downstream signal transduction pathway, which usually involves cyclicAMP. In addition, a serum-binding protein regulates the biological actions of AM, frequently by increasing AM functional attributes. Changes in levels of circulating AM correlate with several critical diseases, including cardiovascular and renal disorders, sepsis, cancer, and diabetes. Whether AM is a causal agent, a protective reaction, or just a marker for these diseases is currently under investigation. New technologies seeking to elevate and/or reduce AM levels are being investigated as potential therapeutic avenues.

Effects of adrenomedullin on human myocyte contractile function and beta-adrenergic response

BACKGROUND

Adrenomedullin has been demonstrated to cause systemic vasodilation, and increased plasma adrenomedullin levels have been observed in cardiovascular disease states such as heart failure. While adrenomedullin receptors have been localized to the myocardium, the effects of adrenomedullin on human myocyte contractility remained unknown.

METHODS AND RESULTS

Left ventricular myocytes were isolated from myocardial biopsies of patients (n = 16) undergoing elective coronary artery bypass surgery with normal left ventricular ejection fractions (51 +/- 1%). A total of 233 left ventricular myocytes were studied by videomicroscopy. Myocyte shortening velocity (microm/s) was measured at baseline and following the addition of either 3 nM, 30 nM, or 60 nM of adrenomedullin. The change in myocyte shortening velocity with increasing concentrations of adrenomedullin was computed. At all concentrations, adrenomedullin reduced myocyte shortening velocity from baseline values (P < 0.05). Next, the potential interaction of adrenomedullin with the beta-adrenergic receptor system was examined using 25 nM isoproterenol. The beta-adrenergic receptor-mediated increase in the myocyte shortening velocity was blunted with adrenomedullin (29 +/- 7 vs 63 +/- 13 microm/s, P < 0.05).

CONCLUSIONS

These unique findings demonstrate that adrenomedullin reduced contractility in isolated human left ventricular myocytes and exhibited a negative interaction with the beta-adrenergic receptor system. Past studies have shown that adrenomedullin induces nitric oxide synthesis and that nitric oxide can uncouple myocyte metabolism. Thus, while adrenomedullin causes systemic vasodilation, this peptide can also exert a negative contractile effect in human left ventricular myocytes.

Inhibition of the ubiquitin-proteasome pathway induces differential heat-shock protein response in cardiomyocytes and renders early cardiac protection

The effects of proteasome inhibition (PI) on heat-shock protein (HSP) expression in cardiomyocytes were investigated. Neonatal rat cardiomyocytes were incubated with MG132 (0.1-10 microM) for 1 h. Induction of various HSPs was determined by real-time PCR and Western blotting. PI induced a 2- to 3-fold increase in HSP27, HSP60, and HSP90, and a 18-fold increase in HSP70 mRNA expression, whereas HSP40 levels were unaffected. Western blotting revealed increased protein expression for HSP70 after PI. Similar results were obtained with MG262. HSP induction correlated with enhanced survival of neonatal cardiomyocytes after sublethal heat stress in XTT testing. In papillary muscles, pretreatment with MG132 (10 microM, 90 min) was associated with enhanced recovery of the contractile parameters after a 40-min hypoxia. In these proof-of-principle experiments, we show that PI induces differential heat-shock response in cardiomyocytes, accompanied by enhanced cell survival and functional recovery after various forms of stress.

A review of the biological properties and clinical implications of adrenomedullin and proadrenomedullin N-terminal 20 peptide (PAMP), hypotensive and vasodilating peptides

Adrenomedullin (AM), identified from pheochromocytoma and having 52 amino acids, elicits a long-lasting vasodilatation and diuresis. AM is mainly mediated by the intracellular adenylate cyclase coupled with cyclic adenosine monophosphate (cAMP) and nitric oxide (NO) -cyclic guanosine monophosphate (cGMP) pathway through its specific receptor. The calcitonin receptor-like receptor (CLCR) and receptor-activity modifying protein (RAMP) 2 or RAMP3 models have been proposed as the candidate receptor. AM is produced mainly in cardiovascular tissues in response to stimuli such as shear stress and stretch, hormonal factors and cytokines. Recently established AM knockout mice lines revealed that AM is essential for development of vitelline vessels of embryo. Plasma AM levels elevate in cardiovascular diseases such as heart failure, hypertension and septic shock, where AM may play protective roles through its characteristic biological activities. Human AM gene delivery improves hypertension, renal function, cardiac hypertrophy and nephrosclerosis in the hypertensive rats. AM decreases cardiac preload and afterload and improves cardiac contractility and diuresis in patients with heart failure and hypertension. Advances in gene engineering and receptor studies may contribute to further understandings of biological implication and therapeutic availability of AM.