Intracerebroventricular injection of adrenomedullin inhibits vasopressin release in conscious rats

Cerebellar Adrenomedullinergic System. Role in Cardiovascular Regulation

Adrenomedullin (AM) is a multifunctional peptide which exerts numerous biological activities through the activation of AM1 (CRLR + RAMP2) and AM2 (CRLR + RAMP3) receptors. AM immunoreactivity, AM binding sites and CRLR, RAMP1, RAMP2 and RAMP3 are expressed in rat cerebellar vermis. AM binding sites are discretely and differentially distributed in the rat cerebellar cortex with higher levels detected in SHR when compared with WKY rats. In addition, there is an up-regulation of cerebellar CGRP1 (CRLR + RAMP1) and AM2 (CRLR + RAMP3) receptors and a down-regulation of AM1 (CRLR + RAMP2) receptor during hypertension associated with a decreased AM expression. These changes may constitute a mechanism which contributes to the development of hypertension, and supports the notion that cerebellar AM is involved in the regulation of blood pressure. Cerebellar AM activates ERK, increases cAMP, cGMP and nitric oxide, and decreases antioxidant enzyme activity. These effects are mediated through AM₁ receptor since they are blunted by AM(22-52). AM-stimulated cAMP production is mediated through AM2 and CGRP receptors. In vivo administration of AM into the cerebellar vermis caused a profound, specific and dose-dependent hypotensive effect in SHR, but not in normotensive WKY rats. This effect was mediated through AM1 receptor since it was abolished by AM(22-52). In addition, AM injected into the cerebellar vermis reduced vasopressor response to footshock stress. These findings demonstrate dysregulation of cerebellar AM system during hypertension, and suggest that cerebellar AM plays an important role in the regulation of blood pressure. Likewise, they constitute a novel mechanism of blood pressure control which has not been described so far.

Role of cerebellar adrenomedullin in blood pressure regulation

Adrenomedullin (AM) and their receptor components, calcitonin-receptor-like receptor (CRLR) and receptor activity-modifying protein (RAMP1, RMP2 and RAMP3) are widely expressed in the central nervous system, including cerebellum. We have shown that AM binding sites are altered in cerebellum during hypertension, suggesting a role for cerebellar adrenomedullinergic system in blood pressure regulation. To further evaluate the role of AM in cerebellum, we assessed the expression of AM, RAMP1, RAMP2, RAMP3 and CRLR in the cerebellar vermis of 8 and 16week old spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats. In addition, the effect of microinjection of AM into rat cerebellar vermis on arterial blood pressure (BP) was determined. Animals were sacrificed by decapitation and cerebellar vermis was dissected for quantification of AM, CRLR, RAMP1, RAMP2 and RAMP3 expression using western blot analysis. Another group of male, 16week old SHR and WKY rats was anesthetized, and a cannula was implanted in the cerebellar vermis. Following recovery AM (0.02 to 200pmol/5μL) or vehicle was injected into cerebellar vermis. BP was determined, before and after treatments, by non-invasive plethysmography. In addition, to establish the receptor subtype involved in AM action in vivo, animals received microinjections of AM22-52 (200pmol/5μL), an AM1 receptor antagonist, or the CGRP1 receptor antagonist, CGRP8-37 (200pmol/5μL) into the cerebellar vermis, administered simultaneously with AM or vehicle microinjection. Cannulation was verified post mortem with the in situ injection of a dye solution. Our findings demonstrated that the expression of CRLR, RAMP1 and RAMP3 was higher in cerebellum of SHR rats, while AM and RAMP2 expression was lower than those of WKY rats, both in 8 and 16week old rats. In vivo microinjection of AM into the cerebellar vermis caused a profound, dose dependent, hypotensive effect in SHR but not in normotensive WKY rats. Coinjections of a putative AM receptor antagonist, AM22-52 abolished the decreases in mean arterial pressure (MAP) evoked by AM, showing that AM acts through its AM1 receptor in the vermis to reduce MAP. These findings demonstrate a dysregulation of cerebellar AM-system during hypertension, and suggest that cerebellar AM plays an important role in the regulation of BP. Likewise; they constitute a novel mechanism of BP control which has not been described so far.

Cardiovascular effects of the intracerebroventricular injection of adrenomedullin: roles of the peripheral vasopressin and central cholinergic systems

Our objective was to investigate in conscious Sprague-Dawley (6-8 weeks, 250-300 g) female rats (N = 7 in each group) the effects of intracerebroventricularly (icv) injected adrenomedullin (ADM) on blood pressure and heart rate (HR), and to determine if ADM and calcitonin gene-related peptide (CGRP) receptors, peripheral V₁ receptors or the central cholinergic system play roles in these cardiovascular effects. Blood pressure and HR were observed before and for 30 min following drug injections. The following results were obtained: 1) icv ADM (750 ng/10 µL) caused an increase in both blood pressure and HR (ΔMAP = 11.8 ± 2.3 mmHg and ΔHR = 39.7 ± 4.8 bpm). 2) Pretreatment with a CGRP receptor antagonist (CGRP_8-37) and ADM receptor antagonist (ADM_22-52) blocked the effect of central ADM on blood pressure and HR. 3) The nicotinic receptor antagonist mecamylamine (25 µg/10 µL, icv) and the muscarinic receptor antagonist atropine (5 µg/10 µL, icv) prevented the stimulating effect of ADM on blood pressure. The effect of ADM on HR was blocked only by atropine (5 µg/10 µL, icv). 4) The V₁ receptor antagonist [β-mercapto-β-β-cyclopentamethylenepropionyl¹, O-me-Tyr²,Arg⁸]-vasopressin (V2255; 10 µg/kg), that was applied intravenously, prevented the effect of ADM on blood pressure and HR. This is the first study reporting the role of specific ADM and CGRP receptors, especially the role of nicotinic and muscarinic central cholinergic receptors and the role of peripheral V₁ receptors in the increasing effects of icv ADM on blood pressure and HR.

Adrenomedullin 2 (AM2)/intermedin is a more potent activator of hypothalamic oxytocin-secreting neurons than AM possibly through an unidentified receptor in rats

Central administration of either adrenomedullin 2 (AM2) or adrenomedullin (AM) activates hypothalamic oxytocin (OXT)-secreting neurons in rats. We compared AM2 with AM, given intracerebroventricularly (icv), across multiple measures: (1) plasma OXT levels in conscious rats; (2) blood pressure, heart rate and circulating catecholamine levels in urethane-anesthetized rats; and (3) the expression of the c-fos gene in the supraoptic (SON) and the paraventricular nuclei (PVN). We also tested the effects of the AM receptor antagonist, AM(22-52) and calcitonin gene-related peptide (CGRP) antagonist, CGRP(8-37) on these measures. Plasma OXT levels at 10 min after icv injection of AM (1 nmol/rat) were increased (compared with vehicle), but OXT levels after AM2 (1 nmol/rat) were nearly double the levels seen after AM injection. OXT levels remained elevated at 30 min. Pretreatment with AM(22-52) (27 nmol/rat) and CGRP(8-37) (3 nmol/rat), nearly abolished the increase in plasma OXT level after AM injection, but partially blocked OXT level changes due to AM2. Increases in blood pressure, heart rate and circulating catecholamines were all greater in response to central AM2 than to AM at the same dose. In situ hybridization histochemistry showed that both AM2 and AM induced expression of the c-fos gene in the SON and the PVN, but AM(22-52)+CGRP(8-37) could only nearly abolish the effects of centrally administered AM. These results suggest that the more potent central effects of AM2 and only partial blockade by AM/CGRP receptor antagonists may result from its action on an additional, as yet unidentified, specific receptor in the central nervous system.

Pathophysiologic and therapeutic implications of adrenomedullin in cardiovascular disorders

Adrenomedullin (AM) is a vasodilator peptide that originally isolated from pheochromocytoma tissue. However, the mRNA is expressed in the normal adrenal gland, heart, kidney and blood vessels. The human AM gene is located in the short arm of chromosome 11 and is composed of 4 exons. There are 2 single nucleotide polymorphisms in introns 1 and 3, and the 3'-end of the AM gene is flanked by a microsatellite marker of cytosine-adenine repeats that is associated with an increased risk of developing hypertension and diabetic nephropathy. AM gene expression is promoted by various stimuli, including inflammation, hypoxia, oxidative stress, mechanical stress and activation of the renin-angiotensin and sympathetic nervous systems. The AM gene promoter region possessed binding site for several transcription factors, including nuclear factor for interleukin-6 expression (NF-IL6) and activator protein 2 (AP-2). Further, plasma AM levels are increased in patients with various cardiovascular diseases, including hypertension, heart failure and renal failure. These findings suggest that AM plays a role in the development of or response to cardiovascular disease. Indeed, experimental and clinical studies have demonstrated that systemic infusion of AM may have a therapeutic effect on myocardial infarction, heart failure and renal failure. Further, vasopeptidase inhibitors which augment the bioactivity of endogenous AM may benefit patients with hypertension and arteriosclerosis. Finally, the angiogenic and cytoprotective properties of AM may have utility in revascularization and infarcted myocardium and ischemic limbs. Because of the potential clinical benefits of AM, indications for use and optimal dosing strategies should be established.

Centrally administered adrenomedullin 2 activates hypothalamic oxytocin-secreting neurons, causing elevated plasma oxytocin level in rats

We examined the effects of intracerebroventricular (i.c.v.) administration of adrenomedullin 2 (AM2) on plasma oxytocin (OXT) and arginine vasopressin (AVP) levels in conscious rats. Plasma OXT levels were markedly increased 5 min after i.c.v. administration of AM2 (1 nmol/rat) compared with vehicle and remained elevated in samples taken at 10, 15, 30, and 60 min. By contrast, plasma AVP levels were not significantly elevated in samples taken between 5 and 180 min after i.c.v. administration of AM2 except at the 30-min time point. Fos-like immunoreactivity (Fos-LI) was observed in various brain areas, including the paraventricular (PVN) and the supraoptic nuclei (SON) after i.c.v. administration of AM2 (2 nmol/rat) in conscious rats (measured at 90 min post-AM2 infusion). Dual immunostaining for OXT/Fos and AVP/Fos showed that OXT-LI neurons predominantly exhibited nuclear Fos-LI compared with AVP-LI neurons in the PVN and the SON. In situ hybridization histochemistry showed that i.c.v. administration of AM2 (0.2, 1, and 2 nmol/rat) caused marked induction of the expression of the c-fos gene in the PVN and the SON. This induction was significantly reduced by pretreatment with both the calcitonin gene-related peptide (CGRP) antagonist CGRP-(8-37) (3 nmol/rat) and the AM receptor antagonist AM-(22-52) (27 nmol/rat). These results suggest that centrally administered AM2 mainly activates OXT-secreting neurons in the PVN and the SON, at least in part through the CGRP and/or AM receptors with marked elevation of plasma OXT levels in conscious rats.

Adrenomedullin: a new target for the design of small molecule modulators with promising pharmacological activities

Adrenomedullin (AM) is a 52-amino acid peptide with a pluripotential activity. AM is expressed in many tissues throughout the body, and plays a critical role in several diseases such as cancer, diabetes, cardiovascular and renal disorders, among others. While AM is a protective agent against cardiovascular disorders, it behaves as a stimulating factor in other pathologies such as cancer and diabetes. Therefore, AM is a new and promising target for the development of molecules which, through their ability to regulate AM levels, could be used in the treatment of these pathologies.

Sympatho-Inhibitory Action of Endogenous Adrenomedullin Through Inhibition of Oxidative Stress in the Brain

Central sympathetic activation is one of the possible mechanisms underlying hypertension, in which reactive oxygen species may play a role. Thus, we examined whether adrenomedullin, an antioxidant peptide, is involved in the central regulation of arterial pressure through sympatho-modulatory action. Adrenomedullin knockout mice were fed with high-salt diet for 4 weeks to stimulate adrenomedullin production. In the wild-type littermates, brain adrenomedullin content was significantly increased with salt loading, but not in the knockout mice. Intracerebroventricular hyperosmotic saline increased arterial pressure and sympathetic nerve activity in a dose-dependent fashion. With the normal salt diet, the hyperosmotic saline-induced response did not significantly differ between the knockout and wild-type mice; with the high-salt diet, however, the response was significantly greater in the knockout mice than in wild-type littermates (arterial pressure: 35.3±5.7% versus 20.1±2.1%, P <0.05; sympathetic nerve activity: 30.3±4.8% versus 15.9±1.5%, P <0.05; respectively). Moreover, pretreatment with 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol), a membrane-permeable superoxide dismutase mimetic, inhibited the augmented response to central hyperosmotic saline in salt-loaded knockout mice. Consistently, the hyperosmotic saline-induced production of reactive oxygen species, measured by the lucigenin chemiluminescence method, was significantly greater in the isolated hypothalamus of salt-loaded knockout mice than in that of salt-loaded wild-type ones. In conclusion, endogenous adrenomedullin in the brain may inhibit sympathetic activation through its antioxidant action.

Brain-derived adrenomedullin controls blood volume through the regulation of arginine vasopressin production and release

Central nervous system-derived adrenomedullin (AM) has been shown to be a physiological regulator of thirst. Administration of AM into the lateral ventricle of the brain attenuated water intake, whereas a decrease in endogenous AM, induced by an AM-specific ribozyme, led to exaggerated water intake. We hypothesized that central AM may control fluid homeostasis, in part by regulating plasma arginine vasopressin (AVP) levels. To test this hypothesis, AM or a ribozyme specific to AM was administered intracerebroventricularly, and alterations in plasma AVP concentrations were examined under basal and stimulated (hypovolemic) conditions. Additionally, we examined changes in blood volume, kidney function, and plasma electrolyte and protein levels, as well as changes in plasma aldosterone concentrations. Intracerebroventricular administration of AM increased plasma AVP levels, whereas AM ribozyme treatment led to decreased plasma AVP levels under stimulated conditions. During hypovolemic challenges, AM ribozyme treatment led to an increased loss of plasma volume compared with control animals. Although overall plasma osmolality did not differ between treatment groups during hypovolemia, aldosterone levels were significantly higher and, consequently, plasma potassium concentrations were lower in AM ribozyme-treated rats than in controls. These data suggest that brain-derived AM is a physiological regulator of vasopressin secretion and, thereby, fluid homeostasis.

Stress hormone secretion is altered by central administration of intermedin/adrenomedullin-2

Intermedin/Adrenomedullin-2 (IMD), a newly described peptide with structural homology to adrenomedullin (AM), is present in brain and pituitary gland and binds to the same receptors as AM and calcitonin gene-related peptide (CGRP). We hypothesized that IMD would exert actions similar to AM and CGRP and previously have demonstrated that indeed IMD, like AM and CGRP, increases sympathetic tone and inhibits feeding and drinking when administered centrally. Here, we extend those observations by demonstrating that like AM, IMD acts in brain to stimulate the secretions of prolactin (PRL) and adrenocorticotropin (ACTH) and to inhibit the secretion of growth hormone (GH) in conscious rats. In addition, in conscious rats, central administration of IMD results in increased plasma levels of oxytocin (OT) and vasopressin (AVP). The ability of IMD to activate the hypothalamo-pituitary-adrenal (HPA) axis can be blocked by intravenous pretreatment with the corticotropin releasing factor (CRF) antagonist, astressin. These results suggest that multiple members of the AM family of peptides may be involved in the cardiovascular, behavioral and neuroendocrine responses to stress.

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.

Novel fish-derived adrenomedullin in mammals: structure and possible function

Adrenomedullin (AM) has been recognized as a member of the calcitonin (CT)/CT gene-related peptide (CGRP) family. However, an independent AM family consisting of five paralogous peptides exists in teleost fish. Among them, the peptide named AM1 is an ortholog of mammalian AM as determined by the linkage analysis of orthologous genes and the presence of proAM N-terminal 20 peptide (PAMP)-like sequence in the prosegment. Since the peptides named AM2 and 3 are distinct from other members with respect to the precursor sequence, tissue distribution of the transcripts, and exon-intron organization, we searched for their mammalian orthologs from genome databases, which resulted in an identification of AM2 in human, rat, and mouse. AM2 was expressed abundantly in the submaxillary gland, kidney, and some vascular and digestive tissues of mice. AM2 injected in vivo induced potent cardiovascular and renal effects in mice. In the heart and kidney of mice, AM2 was localized in endothelial cells of the coronary vessels and in glomeruli and vasa recta, respectively. AM2 increased cAMP accumulation in cells expressing human CT receptor-like receptor (CRLR) and one of receptor activity-modifying proteins (RAMPs), but it was no more potent than CGRP and AM. AM2 was also less potent than CT in cells expressing CT receptor and RAMP. There remains a possibility that a new AM2-specific receptor or an additional RAMP that enables CRLR to be an AM2-specific receptor, exists in mammals.

Role of brain dopaminergic system in the adrenomedullin-induced diuresis and natriuresis

Intracerebroventricular (IVT) administration of adrenomedullin (AM) to conscious male hydrated rats increases urinary volume and sodium excretion. The possible involvement of brain dopamine (DA) system on the renal action of IVT-AM was investigated. AM-induced diuretic and natriuretic action was prevented following selective central dopaminergic denervation with 6-hydroxydopamine (6OHDA) in combination with desmethylimipramine (DMI). Selective D(2) DA receptor antagonism with haloperidol, sulpiride, and remoxipride; or with the D(1) DA receptor antagonist, SCH 23390, blunted the increase in urinary volume and sodium excretion induced by IVT-AM. The present results suggest that AM acts centrally, at least in part, via an interaction with endogenous DA through the activation of both DA D(1)/D(2) receptor subtype.

Receptor activity modifying protein 2 distribution in the rat central nervous system and regulation by changes in blood pressure

The neuropeptide, adrenomedullin, acts in the central nervous system (CNS) to regulate blood pressure, at least partly through an adrenomedullin receptor which is composed of receptor activity modifying protein 2 (RAMP-2) and calcitonin receptor-like receptor (CRLR). We used in situ hybridization to localize RAMP-2 mRNA throughout the brain, and we performed reverse transcription-polymerase chain reaction to detect CRLR mRNA in the brain. We found that RAMP-2 mRNA is expressed in numerous areas, including autonomic nuclei such as the paraventricular, supraoptic, arcuate and ventromedial nuclei, as well as the nucleus of the solitary tract (NTS), area postrema and dorsal motor nucleus of the vagus. Many regions expressing RAMP-2 mRNA also express low levels of CRLR mRNA. We examined changes in the mRNA expression of RAMP-2 and preproadrenomedullin in the brain in response to blood pressure manipulations. Rats received intravenous infusions of nitroprusside or phenylephrine to decrease or increase blood pressure, respectively. Decreased blood pressure elicited an increase in RAMP-2 mRNA expression in the NTS and a decrease in preproadrenomedullin mRNA expression in the paraventricular nucleus (PVN). Increased blood pressure elicited a decrease in RAMP-2 mRNA expression in the PVN and NTS. The CNS distribution and modulation of adrenomedullin signalling components by changes in blood pressure provide anatomical and physiological evidence for a homeostatic role for adrenomedullin in the brain.

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.

Autonomic and neuroendocrine actions of adrenomedullin in the brain: mechanisms for homeostasis

In addition to its role as a potent vasodilator, adrenomedullin (ADM) affects an animal's physiological status through its effects in the brain. We have shown that circulating ADM activates neurons, including nitric oxide (NO)-producing neurons, in autonomic centers of the brain such as the hypothalamic paraventricular nucleus (PVN). Systemic ADM gains access to the brain through the area postrema (AP), a brainstem circumventricular organ, and the PVN is a major target of these ADM-sensitive AP neurons. Neurons expressing the preproADM (ppADM) gene are distributed throughout the brain, with high levels in autonomic centers. Lipopolysaccharide (LPS, immune stress), restraint (psychological stress), and 24 h dehydration all down-regulate ppADM gene expression in different subsets of autonomic centers. Receptor-activity-modifying protein (RAMP) 2 and RAMP3, ADM receptor subunits, are expressed in autonomic centers including the PVN and supraoptic nucleus. Intracerebroventricular injections of ADM increase arterial pressure, heart rate, tyrosine hydroxylase mRNA levels in the locus coeruleus, plasma levels of ACTH, and NO production in the hypothalamus. ADM excites putative GABAergic and cholinergic neurons in dissociated cells from a basal forebrain integrative center, the diagonal band of Broca. These results demonstrate that the signalling components necessary for ADM to influence physiological systems are present in the brain and that ADM is an important transmitter of brain autonomic pathways which are involved in regulating homeostatic balance.

Adrenomedullin influences magnocellular and parvocellular neurons of paraventricular nucleus via separate mechanisms

We previously reported that adrenomedullin (AM) decreases blood pressure following microinjection into the paraventricular nucleus of the hypothalamus (PVN) of the rat. With the use of whole cell recordings in rat hypothalamic slice preparations, we characterized the effects of AM on electrophysiologically identified PVN neurons and described the membrane events underlying such actions. AM hyperpolarized magnocellular (type I) neurons in a dose-dependent manner, a response associated with an increase in the frequency and amplitude of inhibitory postsynaptic potentials. Blockade of action potentials with tetrodotoxin (TTX) abolished AM effects on membrane potential and synaptic activity in magnocellular neurons, suggesting direct actions on inhibitory interneurons. Furthermore, blockade of inhibitory synaptic transmission with the GABA(A) receptor antagonist bicuculline methiodide also abolished AM effects on membrane potential in magnocellular neurons. In contrast, parvocellular (type II) neurons depolarized following AM receptor activation. AM effects on parvocellular neurons were dose dependent and were maintained in the presence of TTX, indicating direct effects on this population of neurons. Voltage-clamp recordings from parvocellular neurons showed AM enhances a nonselective cationic conductance, suggesting a potential mechanism through which AM influences membrane potential. These observations show clear population-specific actions of AM on separate identified groups of PVN neurons. Such effects on magnocellular neurons likely contribute to the hypotensive actions of this peptide in PVN. Although the effects on parvocellular neurons may also contribute to such cardiovascular effects of AM, it is more likely that actions on this population of PVN neurons underlie the previously demonstrated activational effects of AM on the hypothalamic-pituitary-adrenal axis.

Adrenomedullin and the integrative physiology of fluid and electrolyte balance

Adrenomedullin (AM) is hypothesized to be a physiologically relevant regulator in fluid and electrolyte homeostasis. AM acts within the central nervous system to inhibit both water and salt intake. The peptide has direct actions in the hypothalamus to decrease vasopressin secretion and in the pituitary gland to inhibit ACTH release. AM decreases production and release of aldosterone from the adrenal glands and acts directly in the kidneys to increase renal blood flow and cause diuresis and natriuresis. Whether or not these complementary actions in brain, pituitary, adrenal gland, and kidney reflect coordinated regulatory mechanisms is currently unknown. Development of molecular tools to determine the physiologic role of endogenous AM will greatly enhance our understanding of AM and its regulation of fluid and electrolyte homeostasis.

Intrapericardial procaine affects volume expansion-induced fos immunoreactivity in unanesthetized rats

Acute volume expansion is associated with a specific pattern of Fos expression and the goal of the present study was to evaluate the contribution of cardiac receptors to this response. Adult male rats were instrumented with pericardial catheters introduced at the level of the thymus. Rats were also catheterized for measuring blood pressure, heart rate, central venous pressure, and intravenous infusion. Each rat received a 200-microl intrapericardial (i.p.c) injection of 2% procaine or 0.9% NaCl. Rats were then volume expanded with isotonic saline (10% body weight in 10 min) or given a control infusion (0.01 ml/min for 10 min). Ninety minutes after the start of the infusion, the rats were anesthetized and perfused transcardially. Their brains were sectioned and processed for Fos, dopamine-beta-hydroxylase, and oxytocin immunocytochemistry. Volume expansion plus i.p.c. saline produced a significant increase in Fos expression in the nucleus of the solitary tract, the ventrolateral medulla, the area postrema, the locus coeruleus, the paraventricular nucleus of the hypothalamus, the perinuclear zone of the supraoptic nucleus, and oxytocin neurons in the supraoptic nucleus. The i.p.c. procaine significantly blocked Fos expression produced by the volume expansion in the all of the regions examined except for the area postrema and the SON oxytocin neurons.

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.

Effects of adrenomedullin on adrenocorticotropic hormone (ACTH) release in pituitary cell cultures and on ACTH and oxytocin responses to shaker stress in conscious rat

Adrenomedullin (AM) is a potent vasodilator peptide, which is initially isolated from tissue of human pheochromocytoma. In addition to the effect on cardiovascular system, previous studies suggest that AM plays some roles as a neuropeptide in the brain. In the present study, we examined the effect of AM on in vitro adrenocorticotropic hormone (ACTH) secretion stimulated by corticotropin-releasing hormone (CRH), vasopressin (VP) or oxytocin (OT) in cultured rat corticotrophs and on the response of plasma ACTH, corticosterone (B) and OT to shaker stress in vivo. In contrast to the previous report, basal or CRH (10(-9) M)-stimulated ACTH secretion was not affected by coincubation with AM. Either of VP (10(-8) M) or OT (10(-8) M) significantly increased ACTH secretion in cultured rat anterior pituitary cells (156.7+/-24.9 in basal incubation vs. 267.8+/-15.0 in VP-stimulation, P<0.05, and 308.6+/-41.3 pg/ml in OT-stimulation, P<0.05). AM (10(-10) M) significantly inhibited OT-stimulated ACTH secretion. AM tended to inhibit VP-stimulated ACTH secretion, although the inhibitory effect was not statistically significant. Thus, it is likely that AM attenuates OT-stimulated ACTH secretion in corticotrophs. In vivo study, male Wistar rats were prepared with a guide cannula in the lateral ventricle and a catheter in femoral artery for blood sampling. AM (0.5, 1.0 microg in 5 microl) or normal saline (5 microl, control) was intracerebroventricularly (i.c.v.) injected in conscious rats. Shaker stress (110 cycles/min for 5 min) produced a significant increase of plasma ACTH (baseline: 106.4+/-48.6; vs. just after stress: 388.9+/-56.1 pg/ml, P<0.05) and B (baseline: 198.6+/-46.8 vs. 15 min after stress: 378.5+/-13.6 ng/ml, P<0.05) in the control group. Plasma OT tended to increase after stress, although the change was not significantly different (baseline: 29.8+/-6.5; just after stress: 65.6+/-18.2 pg/ml). I.c.v. injection of AM at 3 min before the stress did not significantly affect stress-induced changes of plasma ACTH, B and OT. These results suggest that AM has an inhibitory effect on OT-induced ACTH release in vitro and the inhibitory effect may be overwhelmed in ACTH and B response to shaker stress.

Action sites of adrenomedullin in the rat brain: functional mapping by Fos expression

The effects of intracerebroventricular (icv) administration of adrenomedullin (AM) and proadrenomedullin NH2-terminal 20 peptide (PAMP) on the expression of Fos in the central nervous system (CNS) were examined in conscious rats, using immunohistochemistry. Fos-like immunoreactivity (LI) was detected in various brain areas of the rats, including the supraoptic nucleus, the paraventricular nucleus, the locus coeruleus, the area postrema and the nucleus of the tractus solitarius 90 min after icv administration of AM. Few cells with Fos-LI were found in the CNS 90 min after icv administration of saline. Fos-LI was also detected in the various hypothalamic areas after icv administration of PAMP. These results suggest that centrally administered AM and PAMP may cause physiological responses through the activation of a neural network in the hypothalamus and the brainstem.

Adrenomedullin and central cardiovascular regulation

Adrenomedullin gene products have been localized to neurons in brain that innervate sites known to be important in the regulation of cardiovascular function. Those sites also have been demonstrated to possess receptors for the peptide and central administrations of adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP) elevate blood pressure and heart rate in both conscious and anesthetized animals. The accumulated evidence points to a role of the sympathetic nervous system in these cardiovascular effects. These sympathostimulatory actions of AM and PAMP have been hypothesized to be cardioprotective in nature and to reflect the central nervous system (CNS) equivalent of the direct cardiostimulatory effects of the peptides in the periphery. This review summarizes the most recent data on the CNS actions of the adrenomedullin gene-derived peptides and suggests future strategies for the elucidation of the physiologic relevance of the already demonstrated, pharmacologic actions of these peptides.

Adrenomedullin and diabetes mellitus

Adrenomedullin (AM) is a novel 52 amino acid peptide hormone, originally isolated from human pheochromocytoma. AM acts as a local autocrine and/or paracrine vasoactive hormone and has vasodilator and blood pressure lowering properties. AM as a vasodilative molecule protects the vascular wall but its exact role is still uncertain. AM is considered to play an important endocrine role in various tissues in maintaining electrolyte and fluid homeostasis. Its plasma concentration in healthy conditions is low. In hypertension, chronic renal failure and congestive heart failure its plasma concentration increases in a parallel manner with the severity of the disease. It is assumed that this peptide plays an important role in physiological and pathological conditions compensating the effects of vasoconstrictive molecules. Investigations have proven that in diabetic angiopathies the levels and production of vasoconstrictive factors and AM are increased, while other relaxing substances such as nitric oxide (NO) are decreased. It is still uncertain whether the increased release of AM is a compensatory mechanism or a coincidental event. Although the precise role of AM in the pathogenesis of diabetic complications is still to be elucidated, the altered concentration of AM in diabetes could indicate a certain interaction between AM induction and vascular function. Hence, the induction of vascular AM can be a new target of therapeutic approach to diabetic complications.

Effects of adrenomedullin on angiotensin II stimulated atrial natriuretic peptide and arginine vasopressin secretion in healthy humans

AIMS

Adrenomedullin is a newly described peptide that has widespread tissue distribution. Its presence in cardiovascular (including vascular endothelial cells, smooth muscle cells, and cardiac atria and ventricles) and renal tissues, together with its vasodilatory and natriuretic properties, suggest a role in blood pressure regulation and fluid and electrolyte balance.

METHODS

Nine normal volunteers were studied to determine whether or not adrenomedullin influenced plasma atrial natriuretic peptide and arginine vasopressin concentrations during systemic angiotensin II infusion.

RESULTS

A significant (P = 0.02) augmentation of atrial natriuretic peptide concentrations, but no suppression of arginine vasopressin concentrations, was found with coinfusion of adrenomedullin and angiotensin II when compared with vehicle and angiotensin II.

CONCLUSIONS

Despite its vasodilator and natriuretic action, adrenomedullin significantly augmented angiotensin II-stimulated plasma atrial natriuretic peptide concentrations in healthy humans. This provides further evidence of a synergistic interaction between adrenomedullin and atrial natriuretic peptide and suggests that adrenomedullin may have a role in fluid and electrolyte balance and blood pressure regulation.

Effect of adrenomedullin receptor and calcitonin gene-related peptide receptor antagonists on centrally mediated adrenomedullin renal action

To determine pharmacological specificity of intracerebroventricular (IVT) administration of adrenomedullin (AM) on water and sodium excretion, studies were performed in rats pretreated with AM (22-52), a putative AM-receptor antagonist or CGRP(8-37), a ligand that preferentially antagonizes the CGRP(1)-receptor subtype. In addition, the effects of IVT injection of calcitonin-gene-related peptide (CGRP) on urinary water and electrolyte excretion was assessed. Intracerebroventricular administration of rat adrenomedullin to conscious hydrated rats resulted in a significant increase in urinary volume and sodium excretion during the 6-h period of urine collection and was most effective at 3 and 6 h. Although less effective than AM, central administration of CGRP induced diuresis and natriuresis. Pretreatment with AM (22-52) or CGRP(8-37) significantly suppressed the diuretic and natriuretic effect of IVT-AM. These data suggest that both CGRP(1) and AM receptors are involved in the centrally mediated diuretic and natriuretic action of the AM. Our results provide evidence supporting the hypothesis that endogenous AM plays a role in the central nervous control of fluid and electrolyte homeostasis.

Diuretic and natriuretic action of adrenomedullin administered intracerebroventricularly in conscious rats

Intracerebroventricular administration of rat adrenomedullin (AM) to conscious hydrated or salt-loaded rats, resulted in a significant increase in urinary volume. The diuretic effect of adrenomedullin occurred during the 6-h period of urine collection and was most effective during the 3 and 6 h. Most remarkably, AM given IVT induced a dose-related increase in urinary sodium excretion at all periods of urine collection. In addition, AM induced significant kaliuresis. Our results strongly suggest that AM may play a significant role in the central regulation of fluid and electrolyte homeostasis, and that its diuretic and natriuretic effect may be, at least in part, centrally mediated.

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.

Adrenomedullin: potential in physiology and pathophysiology

Adrenomedullin (ADM), a 52-amino acid ringed-structure peptide with C-terminal amidation, was originally isolated from human pheochromocytoma. ADM mediates vasodilatory and natriuretic properties through the second messenger cyclic adenosine 3',5'-monophosphate (cAMP), nitric oxide and the renal prostaglandin system. ADM immunoreactivity and its gene are widely distributed in cardiovascular, pulmonary, renal, gastrointestinal, cerebral and endocrine tissues. ADM is also synthesized and secreted from vascular endothelial and smooth muscle cells. When injected intravenously, ADM increases flow rates predominantly in organs in which the ADM gene is highly expressed, suggesting that ADM acts as a local autocrine and/or paracrine vasoactive hormone. In addition, ADM is a circulating hormone and its plasma concentration is increased in various cardiorenal diseases such as hypertension, chronic renal failure and congestive heart failure. Current evidence suggests that ADM plays an important role in fluid and electrolyte homeostasis and cardiorenal regulation, however further investigations are required to address the importance of ADM under various physiological and pathophysiological conditions.

A physiological role for adrenomedullin in rats; a potent hypotensive peptide in the hypothalamo-neurohypophysial system

Adrenomedullin, a potent hypotensive peptide, was originally isolated from human phaeochromocytoma. Adrenomedullin immunoreactivity and gene expression are found not only in peripheral organs but also in the central nervous system. Adrenomedullin labelled cells were localised in the hypothalamus, including in the paraventricular and supraoptic nuclei, in rats. Abundant adrenomedullin-immunoreactive fibres and varicosities were found in the hypothalamo-neurohypophysial tract and the internal zone of the median eminence in colchicine-treated and hypophysectomized rats, whereas in control rats few adrenomedullin-labelled fibres were observed. We examined the effects of intracerebroventricular administration of adrenomedullin on neurosecretory cells in the paraventricular and supraoptic nuclei of rats, using immunohistochemistry for Fos protein and in situ hybridisation histochemistry for c-fos mRNA. Intracerebroventricular administration of adrenomedullin caused a marked induction of Fos-like immunoreactivity in the paraventricular nucleus and the dorsal part of the supraoptic nucleus. In the paraventricular and supraoptic nuclei, nuclear Fos-like immunoreactivity was predominantly in oxytocin-immunoreactive cells rather than vasopressin-immunoreactive cells. The induction of c-fos mRNA in the paraventricular and supraoptic nuclei was increased in a dose-related manner 30 min after intracerebroventricular administration of adrenomedullin. This induction was reduced by pre-treatment with the adrenomedullin receptor antagonist, human adrenomedullin-(22-52)-NH2. Intracerebroventricular administration of adrenomedullin also caused a marked increase in the plasma concentration of oxytocin. Extracellular recordings from magnocellular neurosecretory cells in the paraventricular nucleus revealed that putative oxytocin-secreting cells were activated by intracerebroventricular administration of adrenomedullin. These results suggest that central adrenomedullin preferentially stimulates the secretion of oxytocin by activating hypothalamic oxytocin-secreting cells and may have an important role in salt appetite and body fluid homeostasis in rats.

Centrally administered adrenomedullin increases plasma oxytocin level with induction of c-fos messenger ribonucleic acid in the paraventricular and supraoptic nuclei of the rat

The effects of intracerebroventricular (i.c.v.) administration of adrenomedullin (AM) on plasma oxytocin (OXT), c-Fos protein (Fos), and c-fos messenger RNA (mRNA) in the paraventricular (PVN) and supraoptic nuclei (SON) of the rat were investigated using RIA for OXT, immunohistochemistry for Fos, and in situ hybridization histochemistry for c-Fos mRNA. Central administration of AM caused a significant increase in the plasma OXT level. Intracerebroventricular administration of AM caused a marked induction of Fos-like immunoreactivity (LI) in the PVN and in the dorsal parts of the SON. In the PVN and SON, OXT-LI cells predominantly exhibited nuclear Fos-LI in comparison with arginine vasopressin-LI cells. In situ hybridization histochemistry revealed that the induction of c-fos mRNA in the PVN and SON was increased in a dose-related manner 30 min after i.c.v. administration of AM. This induction was reduced by pretreatment with the AM receptor antagonist, human AM-(22-52)-NH2. These results suggest that central AM is responsible for activating the neurosecretory cells in the PVN and SON via selective AM receptors, and that AM stimulates the secretion of OXT by activating hypothalamic OXT-producing cells.

Adrenomedullin-like immunoreactivity in the rat hypothalamo-neurohypophysial tract

Adrenomedullin-like immunoreactivity in the hypothalamo-neurohypophysial tract in colchicine-treated and hypophysectomized rats was examined by immunohistochemistry. Adrenomedullin-like immunoreactive (AM-LI) neurons were localized in the hypothalamic areas, including the paraventricular nuclei and the supraoptic nuclei. Abundant AM-LI fibers and varicosities were found in the hypothalamoneurohypophysial tract and the internal zone of the median eminence in the colchicine-treated and hypophysectomized rats, whereas in control rats few AM-LI fibers were observed. These results suggest that the axons of the AM-LI neurons in the hypothalamus may terminate in the neurohypophysis.

Central administration of urocortin inhibits vasopressin release in conscious rats

Urocortin (UCN) is a new mammalian member of the corticotropin releasing factor (CRF) family and supposed to be an endogenous ligand for type 2 CRF receptors. Previous studies have revealed that UCN mRNA exists in the supraoptic nucleus (SON), and that water deprivation increases UCN immunoreactivity in SON. In this study, we examined the effect of centrally-administered UCN on arginine vasopressin (AVP) release in conscious rats. Intracerebroventricular (i.c.v.) injection of UCN (5.0 microg/rat) significantly attenuated AVP release induced by hyperosmolality at 30 min after the injection. In contrast, CRF (5.0 microg/rat) injected i.c.v. had no significant effect on AVP release. These results suggest that central UCN play an inhibitory role in osmoregulation of AVP release.

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.

Hemodynamic, hormonal, and renal effects of intracerebroventricular adrenomedullin in conscious sheep

Adrenomedullin, the recently described vasodilator that exhibits potent hypotensive actions when administered systemically, is also found in the central nervous system, suggesting a role for adrenomedullin as a neurohormone. However, only a limited number of studies have examined the central effects of adrenomedullin. Therefore, we have examined the integrative hemodynamic, renal, and hormonal effects of intracerebroventricular (I.C.V.) adrenomedullin in conscious sheep. Eight surgically prepared sheep received I.C.V. infusions of adrenomedullin at two doses (2 ng/kg x min followed immediately by 20 ng/kg x min each for 90 min) in a vehicle-controlled study. Water deprivation for 48 h before control infusion resulted in sheep drinking 2617 +/- 583 ml in the 90-min period following reintroduction of water. On the adrenomedullin day, drinking was halved to 1392 +/- 361 ml (P < 0.05). Adrenomedullin had no significant effect on urinary volume and sodium excretion. Plasma adrenomedullin levels remained unchanged during control infusions but were elevated by the end of I.C.V. adrenomedullin infusions (P < 0.001). Plasma ANP levels were also increased approximately 50% (P < 0.05). Plasma levels of both ACTH and cortisol were also increased 3- to 4-fold in response to I.C.V. adrenomedullin (P < 0.05). There was no significant difference in arterial pressure, heart rate, or cardiac output between study days. In conclusion, adrenomedullin within the central nervous system may have at least two roles: modulation of the hypothalamo-pituitary-adrenal axis and protection against fluid overload.

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.

Specific adrenomedullin binding sites in the human brain

Binding sites for adrenomedullin in human brain were investigated and characterized by radioligand binding. Specific binding sites for adrenomedullin were present in every region of human brain (cerebral cortex, cerebellum, thalamus, hypothalamus, pons and medulla oblongata) obtained at autopsy. Despite the homology with calcitonin gene-related peptide (CGRP), CGRP was a poor inhibitor of [125I]adrenomedullin binding (IC50 > 1 microM) compared with adrenomedullin(1-52) (IC50 = 1.2 +/- 0.5 nM, mean +/- SEM, n = 3). Three adrenomedullin fragments, adrenomedullin(1-12), adrenomedullin(22-52), and adrenomedullin(13-52), were also poor inhibitors of the binding (IC50 = 0.3 microM), suggesting that the whole molecule of adrenomedullin(1-52) is required for binding to the receptor. Scatchard plots of [125I]adrenomedullin binding in human brain (cerebral cortex) gave a dissociation constant of 0.17 +/- 0.03 nM and maximal binding of 99.3 +/- 1.9 fmol/mg protein (n = 5). These findings suggest that specific adrenomedullin binding sites that differ from the CGRP receptors exist in human brain. This indicates a possible novel neurotransmitter/neuromodulator role for adrenomedullin in human brain.

Expression of adrenomedullin mRNA in the human brain and pituitary

We have recently reported the presence of immunoreactive (IR) adrenomedullin (ADM) in the human brain. In the present study, the expression of ADM mRNA was studied by Northern blot analysis in the human brain and pituitary, and the presence of IR-ADM in the human pituitary was studied by radioimmunoassay. ADM mRNA was clearly detected in every region of the brain examined and in the pituitary. High concentrations of IR-ADM were present in the whole pituitary (16.7 +/- 2.0 pmol/g wet weight, mean +/- SEM, n = 4). Reverse phase high performance liquid chromatography of the pituitary showed a peak eluting in the position of human ADM(1-52). These findings suggest that ADM acts as a neuromodulator or a neurotransmitter in the brain, and as an autocrine factor, a paracrine factor, or a neurohormone in the pituitary.

Role of adrenomedullin and related peptides in the regulation of the hypothalamo-pituitary-adrenal axis

Adrenomedullin (ADM) is a hypotensive peptide, originally isolated from human pheochromocytomas, and then found to be widely distributed in the various body systems. ADM derives from preproadrenomedullin, a 185-amino acid residue prohormone, containing at its N-terminal a 20-amino acid sequence, named proadrenomedullin N-terminal 20 peptide (PAMP). ADM and PAMP immunoreactivities have been detected in the hypothalamo-pituitary-adrenal (HPA) axis of humans, rats, and pigs. Adrenal glands possess binding sites for both ADM and PAMP, the former being mainly of the subtype 1 of calcitonin gene-related peptide (CGRP) receptors. ADM exerts a direct inhibitory action on angiotensin II- or potassium-stimulated aldosterone secretion of zona glomerulosa cells. This effect is mediated by the CGRP1 receptor and its mechanism probably involves the blockade of Ca2+ influx. In contrast, ADM enhances aldosterone production by in situ perfused rat adrenals and human adrenal slices (containing medullary chromaffin cells), again through the activation of CGRP1 receptors. This aldosterone secretagogue effect of ADM is blocked by the beta-adrenoceptor antagonist l-alprenolol, thereby suggesting that it is indirectly mediated by the release of catecholamines by chromaffin cells. The effects of ADM on adrenal glucocorticoid release are doubtful and probably mediated by the increase in adrenal blood flow rate and the inhibition of ACTH release by pituitary corticotropes. The concentrations reached by ADM and PAMP in the blood rule out the possibility that they act on the HPA axis as circulating hormones. Conversely, their content in both adrenal and hypothalamo-pituitary complex is consistent with a paracrine mechanism of action, which may play a potentially important role in the regulation of fluid and electrolyte homeostasis.