Short-term modulation of the renin-angiotensin system does not alter plasma adrenomedullin concentration in humans

Increased concentrations of bioactive adrenomedullin subsequently to angiotensin-receptor/neprilysin-inhibitor treatment in chronic systolic heart failure

AIMS

The clinically investigated rationale for neprilysin (NEP)-inhibition by angiotensinreceptor-NEPinhibitor (ARNi) therapy is to induce elevations in endogenous natriuretic peptides. NEP, however, cleaves a broad spectrum of substrates, which partially hold significant implications in heart failure with reduced ejection fraction (HFrEF). The effect of NEP inhibition on these peptides has not been investigated thoroughly. This study explored the response of adrenomedullin (ADM) regulation to the initiation of ARNi.

METHODS

Seventy-four patients with stable HFrEF and initiation of ARNi were prospectively enrolled, 67 patients on continuous angiotensin-converting-enzyme inhibitor(ACEi)/angiotensin-receptor blocker (ARB) therapy served as control. Plasma bioactive-ADM (bio-ADM), mid-regional-pro-ADM (MR-proADM), B-typenatriuretic peptide (BNP) and N-terminal-pro-BNP (NT-proBNP) were determined at baseline, short-term, 1-year and 2-year follow up.

RESULTS

Following ARNi initiation both bio-ADM and MR-proADM concentrations were significantly increased at early and long-term follow up (bio-ADM [pg/mL]: 26.0 [interquartile range {IQR}: 17.7-37.5] vs. 50.8 [IQR: 36.5-78.1] vs. 54.6 [IQR: 42.0-97.1] vs. 57.4 [IQR: 48.5-161.6]; MR-proADM [nmol/L]: 0.87 [IQR: 0.64-1.12] vs. 1.25 [IQR: 0.93-1.79] vs. 1.42 [IQR: 0.95-1.90] vs. 1.60 [IQR: 1.12-2.46], P < .0001 for all). The ratios bio-ADM/MR-proADM and BNP/NT-proBNP increased during ARNi-therapy proving improved availability of bioactive peptides. The proportional increase of bio-ADM markedly exceeded BNP increase. Patients converted to ARNi showed similar biomarker patterns irrespective of baseline renin-angiotensin system blocker therapy, i.e. ACEi or ARB (P > .05 for all), indicating that activation of the ADM-axis arises particularly from NEPinhibition.

CONCLUSION

The significant increase of MR-proADM and bio-ADM together with an elevated bioADM/MR-proADM ratio suggest both enhanced formation and reduced breakdown of bioactive ADM following the initiation of ARNi. Activation of the ADM-axis represents a so far unrecognized effect of ARNi.

Adrenomedullin and cardiovascular diseases

The cardiovascular system is regulated by the autonomic nervous system, the renin-angiotensin-aldosterone system, nitric oxide (NO) and other factors including neuropeptides. Research in neurohumoral factors has led to the development of many cardiovascular drugs. Adrenomedullin (ADM), initially isolated from the adrenal gland, has diverse physiological and pathophysiological functions in the cardiovascular system. It is produced in many organs and tissues including the vasculature. ADM has numerous actions, including vasodilation, natriuresis, antiapoptosis and stimulation of NO production. It might play a protective role in various cardiovascular pathologies, and its plasma level is elevated in patients with hypertension and heart failure. Administration of ADM is a possible therapeutic approach for treating cardiovascular diseases. A number of studies have investigated the infusion of ADM in humans, which seems to be benficial in heart failure and myocardial infarction. Instead of ADM infusion, augmentation of its endogenous level is another possible strategy. Gene therapy is feasible in animal models, but its application in humans is limited. At present, the most promising clinical application of ADM is the use of the plasma level of mid-regional proadrenomedullin as a biomarker in cardiovascular diseases. It is a good marker of prognosis and survival in patients with coronary aretery disease or heart failure.

Adrenomedulline improves ischemic left colonic anastomotic healing in an experimental rodent model

BACKGROUND

Leakage from colonic anastomosis is a major complication causing increased mortality and morbidity. Ischemia is a well-known cause of this event. This study was designed to investigate the effects of adrenomedullin on the healing of ischemic colon anastomosis in a rat model.

METHODS

Standardized left colon resection 3 cm above the peritoneal reflection and colonic anastomosis were performed in 40 Wistar rats that were divided into four groups. To mimic ischemia, the mesocolon was ligated 2 cm from either side of the anastomosis in all of the groups. The control groups (1 and 2) received no further treatment. The experimental groups (3 and 4) received adrenomedullin treatment. Adrenomedullin therapy was started in the perioperative period in group 3 and 4 rats (the therapeutic groups). Group 1 and group 3 rats were sacrificed on postoperative day 3. Group 2 and group 4 rats were sacrificed on postoperative day 7. After careful relaparotomy, bursting pressure, hydroxyproline, malondialdehyde, interleukin 6, nitric oxide, vascular endothelial growth factor, and tumor necrosis factor alpha levels were measured. Histopathological characteristics of the anastomosis were analyzed.

RESULTS

The group 3 animals had a significantly higher bursting pressure than group 1 (p<0.05). Hydroxyproline levels in group 1 were significantly lower than in group 3 (p<0.05). The mean bursting pressure was significantly different between group 2 and group 4 (p<0.05). Hydroxyproline levels in groups 3 and 4 were significantly increased by adrenomedullin therapy relative to the control groups (p<0.05). When all groups were compared, malondialdehyde and nitric oxide were significantly lower in the control groups (p<0.05). When vascular endothelial growth factor levels were compared, no statistically significant difference between groups was observed. Interleukin 6 and tumor necrosis factor alpha were significantly decreased by adrenomedullin therapy (p<0.05). The healing parameters and inflammatory changes (e.g., granulocytic cell infiltration, necrosis, and exudate) were significantly different among all groups (p<0.05).

CONCLUSION

Adrenomedullin had positive effects on histopathologic anastomotic healing in this experimental model of ischemic colon anastomosis.

Expression of adrenomedullin and proadrenomedullin N-terminal 20 peptide in PC12 cells after exposure to nerve growth factor

Adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP) are multi-functional peptides derived from the same precursor, proadrenomedullin. We have studied the regulatory mechanism of expression of these peptides during neuronal differentiation of rat pheochromocytoma PC12 cells by nerve growth factor (NGF). The cellular levels of the peptides increased slightly, and then progressively decreased below the control by NGF. Immunoreactive (ir)-AM in the medium was transiently increased by NGF. Cytochemical staining showed that ir-AM and ir-PAMP were abundantly present in cytoplasm in the undifferentiated cells, and were decreased during culture with NGF. There was no preferential localization of ir-AM or ir-PAMP in neurites in comparison with in cytoplasm in the differentiated cells. Northern blot analysis showed that mRNA encoding these peptides, as detected as a band of 1.6 kb, increased more than three-fold at 1 h after the addition of NGF and then progressively decreased to one fifth of the control during 72 h. Degradation rate of the mRNA was slowed by NGF even when mRNA level is decreased after 72 h of NGF treatment. The transcription rate of their gene increased transiently and then decreased by the long-term treatment with NGF. These results demonstrate that expression of AM and PAMP is regulated by NGF along with time-dependent differentiation: AM gene transcription is transiently activated by NGF, whereas it was suppressed during neuronal differentiation of the cells.

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.

Regulation of production and secretion of adrenomedullin in the cardiovascular system

Adrenomedullin (AM) has multi-functional properties, of which the vasodilatory hypotensive effect is the most characteristic. AM and its gene are ubiquitous in a variety of tissues and organs, in the cardiovascular system, as well as the adrenal medulla. AM secretion, especially in cardiovascular tissues, is regulated mainly by mechanical stressors such as shear stress, inflammatory cytokines such as interleukin (IL)-1, tumor necrosis factor (TNF), and lipopolysaccharide (LPS), hormones such as angiotensin (Ang) II and endothelin (ET)-1, and metabolic factors such as hypoxia, ischemia, or hyperglycemia. Elevation of plasma AM due to overproduction in response to one or more of these stimuli in pathological conditions may explain the raised plasma AM levels present in cardiovascular and renal diseases such as congestive heart failure, myocardial infarction, hypertension, chronic renal failure, stroke, diabetes mellitus, and septic shock. In addition to shear stress, stretching of cardiomyocytes may be another mechanical stimulus for AM synthesis and secretion. Our recent studies have shown the importance of aldosterone and additional hormonal factor on AM secretion in vascular wall.

Plasma Adrenomedullin Is Closely Correlated with Pulse Wave Velocity in Middle-Aged and Elderly Patients

Arterial stiffness as measured by pulse wave velocity (PWV) is a major predictor of cardiovascular disease. Adrenomedullin (AM), a hypotensive peptide, works as a compensatory factor for arterial sclerosis. The aim of this study was to investigate the relationship between PWV and the plasma concentration of AM in risk-loading patients. One hundred and twenty-six inpatients aged 30 to 75 years with or without varying degrees of atherosclerosis were investigated. Patients with heart and/or renal failure were excluded. The PWV was measured using an automatic waveform analyzer, and the plasma AM level was measured using a newly developed, hypersensitive immunoenzymometric assay system. The PWV increased with the increasing number of cardiovascular risk factors and organ damage in the patients. A positive correlation between the PWV and AM level was observed (r=0.375, p<0.0001, n=126). Seventy-four patients were receiving antihypertensive medications; medication did not affect the correlation. Multivariate regression analysis revealed that the PWV was significantly and independently associated with age, systolic blood pressure, and AM level. These results indicate that the plasma AM concentration could serve as a marker of advanced arterial sclerosis as estimated by increased PWV.

High plasma adrenomedullin concentrations in patients with high-renin essential hypertension

Adrenomedullin (AM) is a novel peptide, first isolated from human phaeochromocytoma, which elicits a long-lasting vasorelaxant activity. Recently, it has been reported that endothelial cells produce AM and that immunoreactive AM plasma levels may be elevated in human arterial hypertension, although the exact pathophysiological role of AM remains to be established. The aim of our study was to determine the relationship between the components of the enin-angiotensin-aldosterone system (RAAS) and plasma AM levels in patients with low-, medium- or high- renin essential hypertension. The study groups included 10 patients with low-renin essential hypertension (average age 42+15 years), nine patients with medium-renin essential hypertension (46+13 years), 11 patients with high-renin essential hypertension (42+14 years) and 12 healthy subjects (43+11 years). Our results demonstrated that the mean AM values of all patients with essential hypertension were 10.85+3.14 pg/ml; there was a statistical correlation (r=0.705; p<0.001) between plasma renin activity (PRA) and AM levels in hypertensives. In patients with high-renin essential hypertension, plasma AM levels (14.2+2.2 pg/ml) were significantly higher (p<0.001) than those of healthy subjects (8.7+2.1 pg/ml), patients with medium-renin essential hypertension (8.5+1.4 pg/ml), and patients with low-renin essential hypertension (9.1+1.5 pg/ml). There was no statistical difference in AM concentrations between medium- and low-renin hypertensive patients. In conclusion, we have found that, in hypertensive patients, plasma AM levels were increased only in high-renin individuals, suggesting a role of AM in this particular form of human essential hypertension.

Adrenomedullin in the adrenal

Adrenomedullin (AM) was originally characterized in extracts of an adrenal medullary tumor. Since this original finding the peptide and its mRNA have also been found in the adrenal cortex, specifically, in the cells of the aldosterone-secreting zona glomerulosa. It is clear that the synthesis of AM is actively regulated in both cortex and medulla. Much research effort has been focused on identifying a role for AM in the adrenal gland. To date, no consistent effect on medullary catecholamine biosynthesis has been demonstrated. In the cortex the actions of AM are controversial and appear to depend on both the tissue preparation used and on the specific receptor population expressed in the individual gland. The results of further studies on the long-term actions of AM on adrenal growth and differentiation are awaited with interest.

Adrenomedullin and PAMP: discovery, structures, and cardiovascular functions

We discovered adrenomedullin (AM) from human pheochromocytoma tissue by monitoring the elevating activity of intracellular cyclic AMP (cAMP) in rat platelets in 1993. Since the discovery of AM, it has attracted intense interest from cardiovascular researchers because AM elicits multiple biological activities, including a potent and powerful hypotensive activity caused by dilatation of resistance vessels. AM is biosynthesized and secreted from tissues, including cardiovascular organs. In addition to AM, "proadrenomedullin N-terminal 20 peptide (PAMP)," another biologically active peptide, was found to be processed from the AM precursor. Plasma AM levels are increased in various cardiovascular and renal diseases. AM, therefore, seems to function as a novel system that controls circulation and body fluid, and may be involved in pathophysiological changes in cardiovascular diseases. Therefore, in this review we will focus on the structure of AM and its gene, distribution, receptor, and the physiological and pathological roles of AM in cardiovascular disease.

Proadrenomedullin-derived peptides in the paracrine control of the hypothalamo-pituitary-adrenal axis

Adrenomedullin (ADM) and proadrenomedullin N-terminal 20 peptide (PAMP) are widely distributed in various body tissues and organs, including the hypothalamo-pituitary-adrenal (HPA) axis. ADM and PAMP inhibit in vitro release of ACTH from pituitary corticotropes, and findings suggest that this effect may become relevant when an exceedingly high ACTH secretion must be counteracted. ADM directly supresses angiotensin-II- and K+-stimulated aldosterone secretion from ZG cells, acting through calcitonin gene-related peptide (CGRP) type 1 ADM(22-52)-sensitive receptors, the activation of which is likely to impair Ca2+ influx. In contrast, ADM stimulates medullary chromaffin cells to release catecholamines, which in turn enhance aldosterone secretion acting in a paracrine manner. Also this effect of ADM occurs via CGRP1 receptors, which are coupled with the adenylate cyclase-dependent cascade. There is indication that in vivo these two opposite effects of ADM on ZG may interact with each other when normal aldosterone secretion has to be restored. ADM exerts a mitogenic effect on rat ZG, acting via CGRP1 receptors that activate the tyrosine kinase-dependent mitogen-activated protein kinase cascade. These findings, along with the demonstration of a high level of ADM gene expression in adrenocortical adenomas and carcinomas, may suggest a role for ADM as adrenocortical growth stimulator and tumor promoter. PAMP, like ADM, suppresses aldosterone response of ZG cells to Ca2+-dependent agonists, but, in contrast with ADM, it inhibits catecholamine release by adrenal medulla. Both effects of PAMP are mediated by PAMP(12-20)-sensitive receptors, whose signaling mechanism is likely to involve the blockade of voltage-gated Ca2+ channels. The concentrations attained by ADM and PAMP in the blood rule out the possibility that they act as true circulating hormones. Conversely, their content in the hypothalamo-pituitary complex and adrenal gland is consistent with a paracrine mechanism of action, which may play an important role in pathophysiological conditions where the function of the HPA axis has to be reset.

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.

Adrenomedullin, a multifunctional regulatory peptide

Since the discovery of adrenomedullin in 1993 several hundred papers have been published regarding the regulation of its secretion and the multiplicity of its actions. It has been shown to be an almost ubiquitous peptide, with the number of tissues and cell types synthesizing adrenomedullin far exceeding those that do not. In Section II of this paper we give a comprehensive review both of tissues and cell lines secreting adrenomedullin and of the mechanisms regulating gene expression. The data on circulating adrenomedullin, obtained with the various assays available, are also reviewed, and the disease states in which plasma adrenomedullin is elevated are listed. In Section III the pharmacology and biochemistry of adrenomedullin binding sites, both specific sites and calcitonin gene-related peptide (CGRP) receptors, are discussed. In particular, the putative adrenomedullin receptor clones and signal transduction pathways are described. In Section IV the various actions of adrenomedullin are discussed: its actions on cellular growth, the cardiovascular system, the central nervous system, and the endocrine system are all considered. Finally, in Section V, we consider some unresolved issues and propose future areas for research.

Biological and clinical roles of adrenomedullin in circulation control and cardiovascular diseases

1. Adrenomedullin (AM) is found ubiquitously in tissues and organs, especially in cardiovascular tissues and in the kidney, lung and endocrine glands. It has multifunctional biological properties, of which, its effects on the control of circulation and body fluid volume regulation seem to be the most outstanding and characteristic. 2. Acute administration of a high dose of AM induces a vasodilator depressor response, cardiac inotropic effects, diuresis and suppression of aldosterone secretion in experimental animals. 3. Long-term continuous administration of a very low dose of AM causes vasodilation in sheep (0.5 microgram/kg per h) and hypotension in rats (0.8 microgram/kg per h). 4. The plasma concentration of AM increases under pathological conditions such as congestive heart failure, myocardial infarction and hypertensive and renal diseases. Under these disease conditions, AM may be produced in vascular endothelial and smooth muscle cells and in cardiac myocytes in response to volume expansion, hypertension and activated humoral factors, such as catecholamine and the renin-angiotensin system. 5. Increased AM in the circulating blood and cardiovascular tissues may counteract pathological deviation in the system that controls circulation and body fluid volume, acting against cardiovascular damage and disease. 6. Because of these beneficial properties in the cardiovascular system, AM and its pharmaceutical ligands should prove useful in the treatment of cardiovascular diseases.