Effect of adrenomedullin on renal hemodynamics and functions in dogs

Effects of dietary salt on adrenomedullin and its receptor mRNAs in rat kidney

There is accumulating evidence that adrenomedullin (ADM) is involved in the control of salt and water homeostasis. ADM is considered to act primarily in a paracrine fashion, and since the kidneys are target organs for ADM, we investigated the localization and regulation of ADM and ADM receptor (ADM-R) mRNAs in the kidney. mRNAs for ADM and ADM-R were colocalized in renal vessels, glomeruli, and inner medullary collecting ducts. ADM mRNA was also detected in proximal tubules, whereas ADM-R mRNA was found in distal convoluted tubules. By ribonuclease protection assay, the abundance of ADM mRNA was fourfold higher in cortex than in outer medulla and papilla. In isolated glomeruli, ADM mRNA was threefold higher compared with cortex. Conversely, ADM-R mRNA was fourfold higher in papilla than in renal cortex. This distribution of mRNAs for ADM and ADM-R suggests a cortical source of ADM and a preferential action of ADM in the papilla. Ten days of feeding a low-salt (0.02%) or a high-salt diet (4%) did not change ADM mRNA or ADM-R mRNA in any kidney zone.

Significance of increased plasma adrenomedullin concentration in patients with cirrhosis

BACKGROUND/AIMS

Adrenomedullin recently discovered in human pheochromocytoma is a potent vasodilatory peptide mainly derived from vascular endothelial and smooth muscle cells. Hyperdynamic circulation, ultimately leading to ascites formation, has been attributed to peripheral vasodilatation in liver cirrhosis. However, little is known about the role of adrenomedullin in this condition.

METHODS

Plasma adrenomedullin concentrations were measured by radioimmunoassay after extraction and purification in 28 cirrhotic patients without ascites, 12 cirrhotic patients with ascites and 10 healthy subjects.

RESULTS

Plasma adrenomedullin concentrations in cirrhotic patients with ascites (12.7+/-4.5 fmol/ml) were significantly higher than those in cirrhotic patients without ascites (8.2+/-2.3 fmol/ml, p<0.005) and healthy subjects (5.8+/-0.8 fmol/ml, p<0.005). Interestingly, plasma adrenomedullin concentrations were highest in patients with refractory ascites (n=5, 15.8+/-3.0 fmol/ml) and were positively correlated with the Child-Pugh score (r=0.44, p<0.01). Moreover, plasma adrenomedullin concentrations were positively correlated with plasma renin activity (r=0.63, p<0.0001), plasma aldosterone (r=0.60, p<0.0001) and plasma norepinephrine concentrations (r=0.60, p<0.0001), and negatively correlated with creatinine clearance (r=-0.61, p<0.0005) and urinary sodium excretion (r=-0.44, p<0.02). Stepwise multiple regression analysis using certain independent variables, including Pugh's score, vasoactive substances, renal function and hemodynamic parameters, showed that the adjusted R square was highest when plasma renin activity and creatinine clearance (standard coefficient=0.53, -0.49, respectively) were considered (adjusted R square=0.61, p<0.0001).

CONCLUSIONS

Plasma adrenomedullin concentrations increased with the progression of liver cirrhosis and were highest in cirrhotic patients with refractory ascites. In addition, elevated adrenomedullin was associated with activation of the renin-angiotensin-aldosterone and sympathetic nervous systems, and with functional renal impairment in cirrhosis. Considering the potent vasodilatory action of adrenomedullin, increased adrenomedullin may participate in the hyperdynamic circulation, ultimately leading to ascites formation, in patients with liver cirrhosis.

Central actions of adrenomedullin on cardiovascular parameters and sympathetic outflow in conscious rats

Adrenomedullin (ADM) is reported to be a peripherally acting hypotensive peptide, but its central actions are unclear. We investigated the effects of centrally administered ADM on blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity (RSNA) in conscious rats and sinoaortic-denervated (SAD) rats. We also investigated the receptors interacting with ADM using two putative antagonists. Intracerebroventricular administration of ADM in doses of 0.1 and 0.5 nmol/kg caused tachycardia and early inhibition of RSNA. Central ADM (1.0 nmol/kg) induced hypertension, tachycardia, and a decrease followed by an increase in RSNA. In SAD rats, increases in BP, HR, and RSNA at the late phase were enhanced by central ADM (1.0 nmol/kg), whereas the early decrease in RSNA remained. Thus the inhibition of RSNA via central ADM may be unrelated to the arterial baroreceptor reflex. Pretreatment with antagonists human calcitonin gene-related peptide-(8-37) and human ADM-(22-52) significantly suppressed the central actions of ADM. The findings suggest that ADM is involved as a neuropeptide in the receptor-mediated central regulation of the cardiovascular system and RSNA.

Proadrenomedullin-derived peptides

Posttranslational processing of the adrenomedullin gene product results in the formation of at least two biologically active peptides, adrenomedullin (AM) and proadrenomedullin N-20 terminal peptide (PAMP). Produced predominantly in the vasculature, both peptides are potent hypotensive agents, albeit via unique mechanisms of action. The gene is transcribed in a variety of other tissues including brain, pituitary, and kidney. Numerous actions have been reported most related to the physiologic control of fluid and electrolyte homeostasis. In the kidney, AM is diuretic and natriuretic, and both AM and PAMP inhibit aldosterone secretion by direct adrenal actions. In pituitary gland, both peptides at physiologically relevant doses inhibit basal ACTH secretion, again by apparently differing mechanisms. Additionally, AM antagonizes CRH-stimulated ACTH release. The peptides are produced in numerous brain sites, including hypothalamus and brainstem. Inhibition of AVP release has been reported and the physiologic significance of AM's ability to inhibit water drinking and salt appetite has been established. Thus the peptides appear to act in brain and pituitary gland to facilitate the loss of plasma volume, actions which complement their hypotensive effects in the blood vessel. Interestingly, direct cardiac effects (positive inotropism and chronotropism) and CNS actions (sympathostimulation) have been reported, leading to the hypothesis that these peptides also can exert important cardioprotective effects, helping to prevent vascular collapse during states of high AM secretion such as sepsis.

The intermediate form of glycine-extended adrenomedullin is the major circulating molecular form in human plasma

Adrenomedullin (AM), a potent vasodilator peptide, is processed from its AM precursor as glycine-extended AM (AM-gly), an intermediate form of AM. Subsequently, mature AM is converted from AM-gly by enzymatic amidation. Using two kinds of radioimmunoassay which recognize the entire AM molecule (E-AM-RIA) and C-terminal amide structure (C-AM-RIA), human plasma AM immunoreactivity was chromatographically characterized. In analyses of gel filtration and reverse phase high-performance liquid chromatography, most of the AM immunoreactivity measured by E-AM-RIA was eluted at a position identical to where mature AM and AM-gly emerged and was not recognized by C-AM-RIA. These data show that immunoreactive AM measured by E-AM-RIA is not amidated. When amidated by peptidylglycine alpha-amidating enzyme, the immunoreactive AM was converted to a form that can be detected by C-AM-RIA. These results indicate that most of the total AM immunoreactivity measured by E-AM-RIA represents immunoreactivity of AM-gly and that the concentration of immunoreactive mature AM in plasma is much lower than that of AM-gly. In practice, plasma concentration of AM-gly and mature AM in healthy volunteers was 2.7 +/- 0.18 fmol/ml and 0.48 +/- 0.05 fmol/ml, respectively. Furthermore, plasma concentration of AM-gly and total AM was significantly elevated in patients with hypertension compared to normotensive control. The present data indicate that most of circulating plasma AM immunoreactivity is occupied by AM-gly, an intermediate form of AM, which may reflect the process of production of AM in tissues.

Adrenomedullin(1–52) measured in human plasma by radioimmunoassay: plasma concentration, adsorption, and storage

We describe a specific and sensitive RIA for human adrenomedullin (AM)(1–52). The detection limit and the concentration required for 50% inhibition of binding were 0.1 and 1.2 fmol/tube, respectively. Cross-reactivities with AM(1–12), AM(13–52), calcitonin gene-related peptide, amylin, and other vasoactive hormones were negligible. AM immunoreactivity in normal subjects ranged from 2.7 to 10.1 pmol/L (n = 44). We investigated factors influencing the recovery and measurement of AM in the assay. Recovery of labeled AM (&gt;80%) was markedly higher than that of unlabeled AM (56%). Immunoreactivity of exogenous AM added to plasma decreased up to 70% over four freeze–thaw cycles, whereas endogenous AM was stable. Alkali-treated casein (1 g/L) reduced adsorption of AM to surfaces and significantly increased assay precision compared with bovine serum albumin (P &lt;0.0001). HPLC separation of extracted plasma verified the presence of AM(1–52). We suggest that considerable care is needed to ensure that accurate and reproducible results are obtained from studies quantifying this peptide.

Adrenomedullin: a possible autocrine or paracrine inhibitor of hypertrophy of cardiomyocytes

Adrenomedullin (AM), a potent vasodilator peptide, exists in the cardiac ventricle; however, the role of AM in the ventricular tissue remains unknown. In the present study, we investigated the production and secretion of AM in cultured neonatal rat cardiomyocytes, and we examined the effect of AM on de novo protein synthesis in these cells by measuring [14C]phenylalanine incorporation. The cardiomyocytes cultured with serum-free media secreted AM into the media in a time-dependent manner at the rate of 12.2+/-0.5 fmol/10(5) cells/48 hours (mean+/-SEM). Angiotensin II (1 micromol/L) or 10% fetal bovine serum significantly (P<.01) increased the AM secretion by 115% and 305%, respectively. In addition, Northern blot analysis of total RNA extracted from the myocytes disclosed the expression of prepro-AM mRNA of 1.6 kb. Synthetic AM at 1 micromol/L significantly reduced the 10(-6) mol/L angiotensin II- and 10% fetal bovine serum-stimulated [14C]phenylalanine incorporation into the cells, by 16% (P<.05) and 20% (P<.01), respectively. The inhibitory effect of AM on the angiotensin II-stimulated [14C]phenylalanine incorporation was abolished dose-dependently by a calcitonin gene-related peptide receptor antagonist, CGRP(8-37). Furthermore, blockade of the action of endogenous AM by either 10(-6) mol/L CGRP(8-37) or anti-AM monoclonal antibody significantly enhanced the basal and 10(-6) mol/L angiotensin II-stimulated [14C]phenylalanine incorporation. In summary, cultured neonatal rat cardiomyocytes produce and secrete AM, and the secreted AM inhibits the protein synthesis of these cells. Thus, AM may act on cardiomyocytes as an autocrine or a paracrine factor modulating the cardiac growth.

Cardiac production and secretion of adrenomedullin are increased in heart failure

Plasma adrenomedullin (AM) levels are reportedly increased in heart failure, but whether the cardiac production and secretion of AM is increased in heart failure remains unknown. To investigate the sites of production and secretion of AM in heart failure, we measured plasma AM levels and peptide and mRNA levels of AM in various tissues in rats with heart failure. We also examined whether the heart actually secretes AM into the circulation in patients with heart failure. We measured plasma and tissue AM levels by specific radioimmunoassay and AM mRNA by Northern blot analysis in rats with heart failure produced by aortocaval fistula. We also measured plasma AM levels in the coronary sinus and aorta in patients with left ventricular dysfunction before and after rapid right ventricular pacing. The increase in plasma AM levels in heart failure rats correlated with ventricular weight. Tissue AM levels were increased in the heart and lungs but not in the kidneys or adrenals of rats with heart failure. Similarly, tissue AM mRNA levels were also increased in the heart and lungs of heart failure rats. Plasma AM levels were higher in the coronary sinus than in the aorta in patients with left ventricular dysfunction. Rapid right ventricular pacing increased plasma atrial natriuretic peptide but not AM. These results suggest that plasma AM levels are increased in heart failure in proportion to the severity of heart failure and that cardiac production and secretion of AM is increased in heart failure rats. The lung may be another site for increased production of AM in heart failure rats. Human failing heart actually secretes AM into the circulation, and the regulation of AM secretion appears to differ from that of atrial natriuretic peptide.

Plasma concentrations of adrenomedullin correlate with the extent of pulmonary hypertension in patients with mitral stenosis

OBJECTIVE

To examine the pathophysiological significance of adrenomedullin in the pulmonary circulation by investigating the relation between plasma concentrations of adrenomedullin and central haemodynamics in patients with mitral stenosis.

METHODS

Plasma concentrations of adrenomedullin in blood samples obtained from the femoral vein, pulmonary artery, left atrium, and aorta were measured by a newly developed specific radio-immunoassay in 23 consecutive patients with mitral stenosis (16 females and seven males, aged 53 (10) years (mean (SD)) who were undergoing percutaneous mitral commissurotomy.

RESULTS

Patients with mitral stenosis had higher concentrations of adrenomedullin than age matched normal controls (3.9 (0.3) v 2.5 (0.3) pmol/l, p < 0.001). There was a reduction in adrenomedullin concentrations between the pulmonary artery and the left atrium (3.8 (0.2) v 3.2 (0.4) pmol/l, p < 0.001). The venous concentrations of adrenomedullin correlated with mean pulmonary artery pressure (r = 0.65, p < 0.001), total pulmonary vascular resistance (r = 0.83, p < 0.0001), and pulmonary vascular resistance (r = 0.65, p < 0.001). Plasma concentrations of adrenomedullin did not change immediately after percutaneous mitral commissurotomy; however, they decreased significantly one week later.

CONCLUSIONS

Plasma concentrations of adrenomedullin are increased in patients with mitral stenosis. This may help to attenuate the increased pulmonary arterial resistance in secondary pulmonary hypertension due to mitral stenosis.

Hormonal and humeral considerations in hypertensive disease

This article has discussed the classic hormonal causes of hypertension. Pheochromocytoma and hyperaldosteronism have been discussed, and a clinical approach to evaluation of patients for these problems has been presented. Other humeral factors that influence volume pressure homeostasis have been discussed. It is likely that pharmacologic agents affecting these other factors will become available to patients with hypertension in the future.

Increased plasma concentrations of adrenomedullin correlate with relaxation of vascular tone in patients with septic shock

OBJECTIVE

To investigate plasma concentrations of adrenomedullin in patients with septic shock and the potential association of these concentrations with relaxation of vascular tone.

DESIGN

Prospective, case series.

SETTING

Department of Emergency and Critical Care Medicine, Nara Medical University.

PATIENTS

Twelve patients who fulfilled the clinical criteria for severe sepsis or septic shock (as defined by the Members of the American College of Chest Physicians/Society of Critical Care Medicine Consensus Conference Committee) and 13 healthy volunteers.

INTERVENTIONS

Arterial blood samples were obtained via a 20-gauge cannula inserted into each patient's radial artery.

MEASUREMENTS AND MAIN RESULTS

After extraction and purification, plasma adrenomedullin was measured by radioimmunoassay. Systemic vascular resistance index, pulmonary vascular resistance, cardiac index, and stroke volume index were determined with a thermodilution catheter. The mean plasma concentration of adrenomedullin was markedly higher in patients than in controls (226.1 +/- 66.4 [SEM] vs. 5.05 +/- 0.21 fmol/mL, p < .01). Moreover, these concentrations correlated significantly with cardiac index, stroke volume index, and heart rate values, and correlated significantly with decreases in diastolic blood pressure, systemic vascular resistance index, and pulmonary vascular resistance index values.

CONCLUSIONS

Enhanced production of adrenomedullin in patients with septic shock may contribute to reduced vascular tone, hypotension, or both. More data are needed to clarify the role of adrenomedullin in the regulation of vascular tone in this patient population.

The effect of hypothermic cardiopulmonary bypass on plasma adrenomedullin in adult cardiac surgical patients

Cardiopulmonary bypass (CPB) can evoke a systemic inflammatory response, which is accompanied by an increase in plasma cytokines that may stimulate the production of adrenomedullin (AM), a potent vasodilator peptide. This study was undertaken to investigate whether CPB influenced plasma AM concentration in 10 patients undergoing cardiac surgical procedures. We found that the plasma AM concentration increased significantly after the commencement of CPB, with the greatest increase observed at weaning from bypass (P < 0.01). After CPB, plasma AM concentration declined but still exceeded baseline significantly 24 h postoperatively. The increase in the plasma AM concentration at weaning from CPB correlated significantly with aortic cross-clamp time (r = 0.74, P < 0.05). The authors conclude that the secretion of AM into circulation is augmented by CPB in patients undergoing cardiac surgery, which suggests a possible role of AM in cardiovascular regulation during and after surgery with CPB.

Adrenomedullin stimulates renin release and renin mRNA in mouse juxtaglomerular granular cells

The recently discovered peptide adrenomedullin (AM) alters blood pressure through effects on the resistance vessels. Moreover, AM modifies the secretion of corticotropin and aldosterone and could thereby indirectly influence blood pressure through the renin-angiotensin-aldosterone system. Although plasma AM and renin concentration have been found to directly correlate, a causal linkage between AM and renin has not been shown. The present study tested the influence of AM on renin secretion and renin gene expression by renal juxtaglomerular granular cells. Prominent expression and release of AM by vascular structures has been reported; therefore, we investigated the local expression of AM in juxtaglomerular structures. Renin release from isolated perfused rat kidneys was dose-dependently increased by AM (1 to 30 nmol/L), whereas renal perfusate flow rate increased up to 17% at a constant perfusion pressure of 100 mm Hg. In primary cultures of mouse granular cells, AM augmented renin release, renin mRNA accumulation, and cAMP production in a dose- and time-dependent manner (threshold values in the range 10 pmol/L to 1 nmol/L). By reverse transcription-polymerase chain reaction, significant expression of the AM gene was detected in microdissected rat glomeruli with afferent arterioles and in primary cultures of mesangial and granular cells. We conclude that AM is expressed in juxtaglomerular structures and that it has a direct stimulatory effect on renin secretion and renin mRNA abundance by receptors on juxtaglomerular cells, possibly through increases in cAMP. AM could act as an autocrine/paracrine stimulatory factor in the control of renin secretion and renin gene expression.

Different secretion patterns of adrenomedullin, brain natriuretic peptide, and atrial natriuretic peptide during exercise in hypertensive and normotensive subjects

The purpose of this study was to investigate the effect of exercise on plasma concentrations of adrenomedullin, brain natriuretic peptide (BNP), and atrial natriuretic peptide (ANP) in patients with essential hypertension (n = 15) and in normotensive controls (n = 10). Exercise consisted of two fixed workloads, 40 and 80 watts of work load using a supine bicycle ergometer. Plasma levels of all three peptides at rest were significantly higher in hypertensives than in controls. Plasma concentrations of ANP increased with exercise in both groups and had greater increments in hypertensive patients than in normotensives. Plasma concentrations of BNP increased only in patients with hypertension and the levels of increase correlated with basal plasma BNP levels (r = 0.94, p < 0.001) and with left ventricular mass (r = 0.62, p < 0.01) determined by echocardiography. In contrast, plasma adrenomedullin did not change with exercise in either group. These results suggest that secretion patterns of these peptides are regulated by different mechanisms and that the amount and kind of peptides mobilized by exercise may depend on the underlying diseases or pathophysiologic condition.

Potent and long-lasting vasodilatory effects of adrenomedullin in humans. Comparisons between normal subjects and patients with chronic heart failure

BACKGROUND

Adrenomedullin (ADM) is a recently discovered hypotensive peptide that has been isolated from human pheochromocytoma cells. Observations that ADM is produced from cardiovascular tissue and is found in plasma suggest that it may be important in the regulation of regional vascular resistance.

METHODS AND RESULTS

Limb vascular responses to ADM were examined in 10 healthy subjects and compared with those in 18 patients with chronic heart failure (CHF). The peptide increased forearm blood flow (FBF) from 2.7 +/- 0.3 to 11.8 +/- 0.9 mL.min-1.100 mL-1 in the control group and from 2.4 +/- 0.3 to 6.5 +/- 0.7 mL.min-1.100 mL-1 in the CHF group. The ADM-induced FBF increase was significantly impaired in the CHF group (P < .01). After cessation of the infusion, an increased FBF level was sustained for > 60 minutes in the control group, whereas in the CHF group the response returned to the baseline in < 30 minutes. The ADM infusion increased forearm skin blood flow in both groups (P < .05), whereas the skin blood flow response was impaired in the CHF group (P < .01). The role of nitric oxide in ADM-induced vasorelaxation was also studied in 11 healthy subjects and 6 patients with CHF. FBF and skin blood flow responses during ADM administration were significantly attenuated by NG-monomethyl-L-arginine administration in healthy control subjects (P < .05), whereas both flow responses remained the same in the CHF group.

CONCLUSIONS

These observations demonstrate that ADM exerts a potent and long-lasting vasodilatory effect on skeletal muscle arteries with involvement of nitric oxide-dependent mechanisms in normal human peripheral vasculature and that these vascular effects are significantly attenuated in patients with CHF, in part because of impaired production of nitric oxide in the forearm resistance vessels.

Hypotensive effect of chronically infused adrenomedullin in conscious Wistar-Kyoto and spontaneously hypertensive rats

1. The hypotensive effect of chronically infused human adrenomedullin (hAM), a potent vasodilator peptide that has been reported to have a natriuretic action, was examined in normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). 2. Conscious WKY rats and SHR were infused with 200 ng/h synthetic hAM for 14 days by means of osmotic minipumps. Control groups were infused at the same schedule with 0.9% saline. Systolic blood pressure (SBP) and daily urinary excretion of Na+ and K+ were measured before and during the infusion period. In addition, plasma renin activity (PRA), aldosterone and hAM concentrations were measured on day 14 of infusion. 3. A significant reduction in SBP was observed in hAM-treated SHR at day 2 and SBP remained significantly lower throughout the experiment compared with control SHR. Similarly, SBP in the hAM-treated WKY rats was found to be significantly lower than in control WKY rats during infusion. However, the hypotensive effect was not accompanied by any significant increase in urinary volume or Na+ excretion in hAM-treated rats of either strain. Chronic infusion with hAM significantly suppressed PRA and lowered the concentration of plasma aldosterone in WKY rats but not in SHR. The plasma aldosterone in WKY rats and SHR were 0.9 +/- 0.4 and 0.6 +/- 0.2 fmol/mL, respectively. 4. These findings demonstrate that chronically infused hAM has a hypotensive effect in both WKY rats and SHR without an increase in urinary volume or Na+ excretion at a plasma AM concentration within the physiological limit.

Adrenomedullin receptor expression in human lung and in pulmonary tumors

Adrenomedullin (AM) is a multifunctional regulatory peptide that stimulates cyclic AMP production in many target tissues and is highly expressed in the lung. Analysis of the distribution of the recently cloned AM receptor (AM-R) by non-radioactive in situ hybridization revealed abundant expression in the basal cells of the airway epithelium and Type II pneumocytes. The expression of AM-R in the two cell types involved in epithelial regeneration of the lung suggests that AM may be relevant in such functions as organ development, wound repair, and epithelial turnover. AM-Rs are also synthesized in vivo and in vitro by a variety of tumor cells that also express the ligand, suggesting the existence of an autocrine loop that may be involved in tumor growth stimulation. The present findings suggest that the AM/AM-R regulatory system plays a major role in respiratory physiology and lung carcinogenesis and that new functions for AM remain to be identified.

Adrenomedullin: a new modulator of vascular tone

We have discovered a novel hypotensive peptide, designated "adrenomedullin", in human pheochromocytoma extracts. It has potent and long-lasting vasodilatory effects in several vascular systems. In addition to adrenomedullin, another hypotensive peptide, termed PAMP, is also produced from the adrenomedullin precursor. Although initially isolated from human pheochromocytoma and porcine adrenal medullary tissue, adrenomedullin mRNA is highly expressed in several peripheral organs including cardiovascular tissues. Taken together with the presence of adrenomedullin-specific receptors on VSMCs and the significant increase in plasma immunoreactive adrenomedullin levels in patients with hypertension, renal failure and congestive heart failure, adrenomedullin may participate in the pathogenesis of these diseases as a factor regulating blood pressure and circulation.

Adrenomedullin-sensitive receptors are preferentially expressed in cultured rat mesangial cells

By using cultured rat mesangial cells, we compared the effects on cyclic nucleotide levels of adrenomedullin with those of the structurally related peptides, calcitonin gene-related peptide (CGRP) and amylin. Adrenomedullin potently increased cAMP levels 7-fold in a time- and concentration-dependent manner. Its EC50 was 3 x 10(-9) M. CGRP was less potent (2-fold) with an EC50 of 10(-7) M, and amylin had no effect on cAMP levels. All three peptides failed to increase cGMP levels. Treatment of cells with near maximal concentrations of adrenomedullin (10(-7) M) and CGRP (10(-6) M) had no additive effect on cAMP levels. Human adrenomedullin-(22-52)-NH2, a putative adrenomedullin receptor antagonist, inhibited the production of cAMP elicited by adrenomedullin (IC50: 7 x 10(-8) M) and CGRP (IC50: 5 x 10(-8) M). Human CGRP-(8-37), a CGRP receptor antagonist, conversely, reduced the cAMP elevation caused by these peptides with a lower potency (IC50: 10(-6) M for both peptides). This demonstrated that human adrenomedullin-(22-52)-NH2 was a more effective antagonist for adrenomedullin- and CGRP-specific receptors than human CGRP-(8-37). Results suggest that receptors sensitive to adrenomedullin are preferentially expressed in cultured rat mesangial cells. Immunohistochemical study showed almost no immunoreactive adrenomedullin and CGRP, if any, in the cells. Adrenomedullin may regulate mesangial function as either a paracrine or circulating hormone via a cAMP- but not a cGMP-dependent mechanism.

Adrenomedullin, a novel vasoactive hormone, binds to mouse astrocytes and stimulates cyclic AMP production

We have examined the effects of adrenomedullin (AM), a novel hypotensive peptide first isolated from human pheochromocytoma, on receptor binding and cyclic AMP (cAMP) generation in primary cultures of mouse astrocytes. Competition binding studies showed that rat adrenomedullin (rAM) displaced the specific binding of [125I]rAM in a dose-dependent manner, with an estimated IC50 of 33 nM. Rat calcitonin gene-related peptide (rCGRP), which interacts with AM receptors in some vascular tissues, did not produce significant displacement of [125I]rAM at concentrations up to 3.3 microM. rAM stimulated cAMP production in mouse astrocytes in a dose-dependent manner, with an EC50 of 74 nM and a maximal stimulatory concentration of 1 microM. CGRP8-37, a CGRP receptor antagonist, failed to inhibit the cAMP response to rAM, although it attenuated CGRP-stimulated cAMP production. These data indicate that cultured mouse astrocytes possess specific AM receptors which are coupled to adenylate cyclase but do not interact with CGRP. AM may function as a neuropeptide and may play a role in the central regulation of blood pressure and body fluid balance.

Detection of adrenomedullin, a hypotensive peptide, in amniotic fluid and fetal membranes

OBJECTIVE

Our purpose was to determine whether adrenomedullin, a multifunctional regulatory peptide involved in blood flow regulation and growth stimulation and with antimicrobial activity, was a component of amniotic fluid from second-trimester human fetus and to determine the source of this peptide.

STUDY DESIGN

A prospective descriptive study was performed on 134 patients undergoing amniocentesis after genetic counseling, ultrasonography, and informed consent. Adrenomedullin expression was determined by immunocytochemical analysis, Western blot analysis, reverse transcriptase-polymerase chain reaction, and in situ reverse transcriptase-polymerase chain reaction in fetal membranes and with radioimmunoassay in amniotic fluids.

RESULTS

Radioimmunoassay of the 134 amniotic fluid specimens revealed adrenomedullin-like immunoreactivity in all of them, ranging in concentration from 10 to 300 fmol/25 microliters (170 +/- 62 fmol/25 microliters). Immunocytochemical analysis, Western blot analysis, reverse transcriptase-polymerase chain reaction, and in situ reverse transcriptase-polymerase chain reaction further established the expression of adrenomedullin protein and messenger ribonucleic acid in fetal amniotic membranes, suggesting that this organ is the source of amniotic adrenomedullin.

CONCLUSIONS

Our results clearly demonstrate the presence of adrenomedullin in second-trimester human amniotic fluid and adrenomedullin messenger ribonucleic acid and protein in amniotic membranes, suggesting that adrenomedullin is a hormone involved in the maintenance of normal pregnancy. Further studies with these molecular tools are in progress to determine the precise role of this hormone and whether adrenomedullin plays a role in the pathogenesis of various disorders of pregnancy.

Adrenomedullin expression in human tumor cell lines. Its potential role as an autocrine growth factor

Although adrenomedullin (AM) previously has been identified in human tumors, its role has remained elusive. Analysis by reverse transcriptase-polymerase chain reaction (RT-PCR) revealed AM mRNA in 18 of 20 human normal tissues representing major organs, and 55 of 58 (95%) malignant cell lines. Western blot and high performance liquid chromatography analysis showed immunoreactive AM species of 18, 14, and 6 kDa that are consistent with the precursor, intermediate product, and active peptide, respectively. Immunohistochemistry and in situ RT-PCR performed on paraffin-embedded tumor cell lines of various tissue origins exhibited AM cytoplasmic staining. Neutralizing monoclonal antibody to AM inhibits tumor cell growth in a concentration-dependent manner, an effect that was reversed with the addition of exogenous AM. Responding tumor cells were shown to have approximately 50,000 AM receptors per cell by Scatchard analysis with 125I-AM and expressed AM receptor mRNA by RT-PCR. Our data showed 36 of 48 (75%) tumor cell lines expressed AM receptor mRNA by RT-PCR assessment, all of them also expressed AM. In the presence of AM, cAMP levels were shown to increase in tumor cells. Our collective data demonstrate that AM and AM receptor are expressed in numerous human cancer cell lines of diverse origin and constitute a potential autocrine growth mechanism that could drive neoplastic proliferation.

Atrial natriuretic peptide increases adrenomedullin in the circulation of healthy humans

Adrenomedullin (ADM) is a new 52 amino acid peptide originally isolated from extracts of human pheochromocytoma. ADM's biologic properties are nearly identical to those of atrial natriuretic peptides. Thus, the 4 peptide hormones originating from amino acids 1-30 [long acting natriuretic peptide], 31-67 [vessel dilator], 79-98 [kaliuretic peptide] and 99-126 [atrial natriuretic factor; ANF] of the 126 amino acid ANF prohormone as well as ADM have blood pressure lowering and diuretic properties. The present investigation was designed to determine if one or more of these 4 atrial natriuretic peptides increase adrenomedullin within the circulation of healthy humans. Infusion of 100 ng/kg body weight/minute for 60 minutes of the respective atrial peptides resulted in a 4-fold (P < 0.001) increase in the circulating concentration of adrenomedullin secondary to the ANF infusion but no increase in adrenomedullin with the long acting natriuretic peptide, vessel dilator, or kaliuretic peptide infusions. The four-fold increase of adrenomedullin in the circulation persisted throughout the infusion of ANF, but returned to pre-infusion levels within 30 minutes of stopping the ANF infusion. Infusion of 10 pg/kg body weight/minute for 60 minutes of ANF resulted in a 2 1/2-fold increase (P < 0.05) in the circulating concentration of adrenomedullin. There was a significant (P < 0.01) diuresis and blood pressure lowering effect with each of the atrial natriuretic peptides in the present investigation. This investigation suggests that 1) atrial natriuretic factor increases the release of adrenomedullin and 2) that the diuretic and blood pressure lowering effects previously attributed to atrial natriuretic factor may be partially due to adrenomedullin since both increased during the ANF infusion and both have similar biologic effects. As opposed to atrial natriuretic factor, adrenomedullin was not increased by long acting natriuretic peptide, vessel dilator, or kaliuretic peptide suggesting that their biologic effects do not involve adrenomedullin.

Increased plasma adrenomedullin in acute myocardial infarction

Adrenomedullin has a potent vasodilating effect comparable to that of calcitonin gene-related peptide. To investigate the pathophysiologic role of endogenous adrenomedullin, we determined sequentially the plasma adrenomedullin level in 15 consecutive patients with acute myocardial infarction (AMI). Plasma adrenomedullin was higher immediately after the onset of AMI and decreased gradually; plasma levels during the 3-week period after the AMI were higher than plasma levels in 15 healthy control subjects (p < 0.001), with higher levels in patients with congestive heart failure than in patients without congestive heart failure throughout the period of the study (p < 0.05). Plasma adrenomedullin was positively correlated with pulmonary capillary wedge pressure, pulmonary arterial pressure, right atrial pressure, and heart rate in the early stage of AMI. These findings suggest that the elevation of plasma adrenomedullin is related to the retention of body fluid volume, the enhancement of sympathetic activity, and/or the elevation of pressure in pulmonary vascular beds. Adrenomedullin may act against excessive vasoconstrictors increased in AMI.

Adrenomedullin: A newly discovered hormone controlling fluid and electrolyte homeostasis

Neural and humoral mechanisms controlling fluid and electrolyte homeostasis employ a diverse array of physiologic mechanisms that often, when aberrant, are the underlying cause of disease. Behavioral, hormonal, renal, and vascular responses to volume and osmotic challenges must be coordinated to achieve the goal of homeostasis. In recent years, it has become apparent that there exist a number of hormonal factors produced throughout the body that can coordinate these multiple regulatory mechanisms by complementary effects in several tissues. Thus, in addition to their vasoactive properties, recently characterized hormones such as the natriuretic peptides and the endothelins, as well as the better established renin-angiotensin system, exert central nervous, renal, cardiac, and pituitary effects that regulate normal fluid and electrolyte balance. Now a new player, adrenomedullin, has been added to the cast, and the interplay of multiple hormonal factors involved in the physiology and pathophysiology of volume and osmotic status continues to be elucidated.

ACTH-suppressive and vasodilator actions of adrenomedullin in conscious sheep

Adrenomedullin (ADM) is a 52 amino-acid peptide which is a potent vasodilator in rats, and suppresses basal and CRF-induced ACTH release from cultured pituitary cells. The present study examines the hemodynamic and hormonal actions of human ADM (1-52) infusion in conscious, chronically instrumented sheep. Five sheep were infused intravenously (IV) or intracerebroventricularly (ICV) with ADM at 100 micrograms/h for 60 min, and mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), stroke volume (SV), total peripheral conductance (TPC), coronary blood flow (CF), coronary conductance (CC), peak aortic flow (Fmax), and left ventricular dF/dt were monitored by a computer-based data collection system every 2 min. Plasma concentrations of adrenocorticotropin (ACTH), arginine vasopressin (AVP) and renin were measured after 60 min of infusion. IV ADM produced a small fall in MAP of 3 +/- 1 mmHg, associated with a reflex increase in HR of 14 +/- 3 b/min. CO increased by 1.3 +/- 0.3 l/min, whereas SV remained unchanged. TPC was markedly increased by 20 +/- 3 ml/min/mmHg. Changes in CF were also seen with an increase of 10 +/- 2 ml/min, and CC increased in parallel by 0.15 +/- 0.02 ml/min/mmHg. Fmax and dF/dt showed small increases of 2.1 +/- 0.5 l/min and 85 +/- 20 l/min/sec respectively. Plasma concentrations of ACTH and cortisol were reduced by 58% and 55% respectively, whereas plasma renin concentration increased by 106%. There was no change in plasma levels of AVP. ICV infusion of ADM had no effect on any parameter measured. These data suggest that systemic ADM produces a sustained vasodilator action to lower blood pressure in sheep, and this is the first study to report the ACTH-suppressor action of ADM in conscious animals. ADM may therefore be an important hormone involved in the regulation of pituitary/adrenal function, in addition to its cardiovascular and fluid regulatory actions in mammals.

Increased plasma levels of adrenomedullin in patients with heart failure

OBJECTIVES

To investigate the role of adrenomedullin in the pathophysiology of heart failure, we measured plasma levels of adrenomedullin in patients with heart failure.

BACKGROUND

Adrenomedullin is a potent hypotensive peptide newly discovered in pheochromocytoma tissue by monitoring its elevating activity on platelet adenosine 3',5'-cyclic monophosphate (cAMP). A significant level of adrenomedullin has been identified in human plasma. These findings suggest the possibility of adrenomedullin as a new circulating hormone that participates in the regulation of the cardiovascular system.

METHODS

Venous blood samples at rest were obtained from patients with heart failure in New York Heart Association functional classes I (n = 15), II (n = 25), III (n = 16) and i.v. (n = 10) and from normal subjects (n = 27). Plasma adrenomedullin levels were determined by our newly developed radioimmunoassay. Other humoral factor levels measured simultaneously included norepinephrine, atrial natriuretic peptide, brain natriuretic peptide, plasma renin activity, aldosterone and cAMP. Left ventricular ejection fraction was measured by echocardiography. In eight patients with severe heart failure, plasma adrenomedullin levels were measured before and after treatment.

RESULTS

The mean (+/- SD) plasma level of adrenomedullin in control subjects was 2.52 +/- 0.75 pmol/liter. Plasma levels of adrenomedullin in patients with heart failure were unaffected in those in functional class I (2.85 +/- 0.62 pmol/liter) but tended to be increased in those in class II (3.54 +/- 0.82 pmol/liter) and were significantly increased in those in classes III and i.v. (4.78 +/- 1.218 and 8.74 +/- 3.43 pmol/liter, respectively). There was a significant correlation between plasma levels of adrenomedullin and norepinephrine (r = 0.618, p < 0.001), atrial natriuretic peptide (r = 0.696, p < 0.001) and brain natriuretic peptide (r = 0.692, p < 0.001). Left ventricular ejection fraction inversely correlated with plasma adrenomedullin levels (r = 0.485, p < 0.001). Plasma adrenomedullin levels significantly decreased after treatment (from 7.40 +/- 3.40 to 3.98 +/- 1.00 pmol/liter, p < 0.05).

CONCLUSIONS

These results suggest that plasma level of adrenomedullin are elevated in heart failure and that an increased plasma volume and an activated sympathetic nervous system in this condition may be related to its synthesis or secretion. Given that adrenomedullin exerts potent cardiovascular effects, increased adrenomedullin may be involved in the defense mechanism against further peripheral vascular resistance elevation in heart failure.

Effects of calcitonin gene-related peptide receptor antagonists on renal actions of adrenomedullin

1. Adrenomedullin is a novel vasoactive peptide which is produced in the lungs, ventricle, kidneys, heart and adrenal medulla. Adrenomedullin shows homology to calcitonin gene-related peptide (CGRP) and has similar pharmacological actions to CGRP. 2. This study examined the dose-response effects of adrenomedullin (rat, 11-50) on mean arterial pressure (MAP), heart rate (HR), renal blood flow (RBF), glomerular filtration rate (GFR) and renal tubular electrolyte excretion in Inactin-anaesthetized Sprague Dawley rats. The possible involvement of CGRP receptors in actions of adrenomedullin was also examined via renal arterial injection of a CGRP receptor antagonist, CGRP (8-37) (1 or 10 nmol kg-1) or [Tyr0]CGRP(28-37) (3 or 30 nmol kg-1), starting 15 min prior to the administration of adrenomedullin. 3. Renal arterial infusion (0.001 to 1 nmol kg-1) of adrenomedullin did not alter MAP, HR and renal K+ excretion but dose-dependently increased RBF and arterial conductance, GFR, urine flow and Na+ excretion. 4. The renal actions of adrenomedullin were not blocked by either the low or the high dose of CGRP(8-37) or [Tyr0]CGRP(28-37). 5. The results show that adrenomedullin causes renal vasodilatation, increments in GFR, diuresis and natriuresis. The renal actions of adrenomedullin are not mediated via the activation of CGRP1 receptors.

Attenuation of adrenomedullin-induced renal vasodilatation by NG-nitro L-arginine but not glibenclamide

1. The present study was conducted in order to elucidate the in vivo contribution of nitric oxide (NO) and the glibenclamide-sensitive potassium channel in the renal action of adrenomedullin in anaesthetized dogs. 2. Intrarenal arterial infusion of adrenomedullin (20 ng kg-1 min-1) elicited a pronounced increase in renal blood flow with no changes in systemic blood pressure. The renal vasodilator action of adrenomedullin was markedly attenuated by pretreatment with NG-nitro L-arginine (L-NOARG), but this was reversed by continuous infusion of L-arginine. 3. Pretreatment with glibenclamide almost completely blocked the renal vasodilatation induced by lemakalim, but had no effect on the renal vasodilator and diuretic action of adrenomedullin. 4. Intrarenal arterial infusion of adrenomedullin induced diuresis and natriuresis. Diuretic and natriuretic action of adrenomedullin was also attenuated by L-NOARG. L-Arginine partly reversed the effect of L-NOARG and adrenomedullin-induced diuresis and natriuresis. 5. These data indicate that the in vivo renal vasodilator action of adrenomedullin is mediated by the release of NO. The glibenclamide-sensitive potassium channel is not involved in the renal action of adrenomedullin, at least, not in anaesthetized dogs. Since the inhibition of L-NOARG of adrenomedullin-induced diuresis occurred concomitantly with the attenuation of the renal vasodilator action of adrenomedullin, direct involvement of NO in adrenomedullin-induced diuresis remains to be established.

Adrenomedullin increases cyclic AMP more potently than CGRP and amylin in rat renal tubular basolateral membranes

In rat renal tubular basolateral membranes, the potency to increase cAMP of adrenomedullin (AM), a novel vasorelaxant peptide originally isolated from human pheochromocytoma, was compared with those of calcitonin gene-related peptide (CGRP) and amylin. Although all three peptides raised cAMP in a time- and concentration-dependent manner with a 4-fold increase at 10(-6)-10(-5) M, the EC50 value (10(-9) M) of AM was 100-fold smaller than those of CGRP and amylin. CGRP[8-37], an antagonist for CGRP receptors, attenuated cAMP elevation induced by these peptides with the essentially similar concentration-inhibition curves. These results suggest that the receptors for AM, CGRP and amylin share a common structural homology, and that the receptors sensitive to AM are preferentially expressed in renal tubular basolateral membranes.