Activity of amylin at CGRP1-preferring receptors coupled to positive contractile response in rat ventricular cardiomyocytes

Systemic and Central Amylin, Amylin Receptor Signaling, and Their Physiological and Pathophysiological Roles in Metabolism

This article in the Neural and Endocrine Section of Comprehensive Physiology discusses the physiology and pathophysiology of the pancreatic hormone amylin. Shortly after its discovery in 1986, amylin has been shown to reduce food intake as a satiation signal to limit meal size. Amylin also affects food reward, sensitizes the brain to the catabolic actions of leptin, and may also play a prominent role in the development of certain brain areas that are involved in metabolic control. Amylin may act at different sites in the brain in addition to the area postrema (AP) in the caudal hindbrain. In particular, the sensitizing effect of amylin on leptin action may depend on a direct interaction in the hypothalamus. The concept of central pathways mediating amylin action became more complex after the discovery that amylin is also synthesized in certain hypothalamic areas but the interaction between central and peripheral amylin signaling remains currently unexplored. Amylin may also play a dominant pathophysiological role that is associated with the aggregation of monomeric amylin into larger, cytotoxic molecular entities. This aggregation in certain species may contribute to the development of type 2 diabetes mellitus but also cardiovascular disease. Amylin receptor pharmacology is complex because several distinct amylin receptor subtypes have been described, because other neuropeptides [e.g., calcitonin gene-related peptide (CGRP)] can also bind to amylin receptors, and because some components of the functional amylin receptor are also used for other G-protein coupled receptor (GPCR) systems. © 2020 American Physiological Society. Compr Physiol 10:811-837, 2020.

The Cardiopulmonary Effects of the Calcitonin Gene-related Peptide Family

Cardiopulmonary diseases are very common among the population. They are high-cost diseases and there are still no definitive treatments. The roles of members of the calcitonin-gene related-peptide (CGRP) family in treating cardiopulmonary diseases have been studied for many years and promising results obtained. Especially in recent years, two important members of the family, adrenomedullin and adrenomedullin2/intermedin, have been considered new treatment targets in cardiopulmonary diseases. In this review, the roles of CGRP family members in cardiopulmonary diseases are investigated based on the studies performed to date.

Amylin: Pharmacology, Physiology, and Clinical Potential

Amylin is a pancreatic β-cell hormone that produces effects in several different organ systems. Here, we review the literature in rodents and in humans on amylin research since its discovery as a hormone about 25 years ago. Amylin is a 37-amino-acid peptide that activates its specific receptors, which are multisubunit G protein-coupled receptors resulting from the coexpression of a core receptor protein with receptor activity-modifying proteins, resulting in multiple receptor subtypes. Amylin's major role is as a glucoregulatory hormone, and it is an important regulator of energy metabolism in health and disease. Other amylin actions have also been reported, such as on the cardiovascular system or on bone. Amylin acts principally in the circumventricular organs of the central nervous system and functionally interacts with other metabolically active hormones such as cholecystokinin, leptin, and estradiol. The amylin-based peptide, pramlintide, is used clinically to treat type 1 and type 2 diabetes. Clinical studies in obesity have shown that amylin agonists could also be useful for weight loss, especially in combination with other agents.

Differential Effects of an Anti-Oxidant Intervention on Cardiomyocyte Expression of Adrenomedullin and Intermedin and their Receptor Components in Chronic Nitric Oxide Deficiency

BACKGROUND

Chronic inhibition of nitric oxide (NO) synthesis is associated with hypertension, myocardial oxidative stress and hypertrophic remodeling. Up-regulation of the cardiomyocyte adrenomedullin (AM) / intermedin (IMD) receptor signaling cascade is also apparent in NO-deficient cardiomyocytes: augmented expression of AM and receptor activity modifying proteins RAMP2 and RAMP3 is prevented by blood pressure normalization while that of RAMP1 and intermedin (IMD) is not, indicating that the latter is regulated by a pressure-independent mechanism.

AIMS

to verify the ability of an anti-oxidant intervention to normalize cardiomyocyte oxidant status and to investigate the influence of such an intervention on expression of AM, IMD and their receptor components in NO-deficient cardiomyocytes.

METHODS

NO synthesis inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME, 35 mg/kg/day) was given to rats for 8 weeks, with/without con-current administration of antioxidants (Vitamin C (25mg/kg/day) and Tempol (25mg/kg/day)).

RESULTS

In left ventricular cardiomyocytes isolated from L-NAME treated rats, increased oxidative stress was indicated by augmented (3.6 fold) membrane protein oxidation, enhanced expression of catalytic and regulatory subunits of pro-oxidant NADPH oxidases (NOX1, NOX2) and compensatory increases in expression of anti-oxidant glutathione peroxidase and Cu/Zn superoxide dismutases (SOD1, SOD3). Vitamin C plus Tempol did not reduce systolic blood pressure but normalized augmented plasma levels of IMD, but not of AM, and in cardiomyocytes: (i) abolished increased membrane protein oxidation; (ii) normalized augmented expression of prepro-IMD and RAMP1, but not prepro-AM, RAMP2 and RAMP3; (iii) attenuated (by 42%) increased width and normalized expression of hypertrophic markers, skeletal-alpha-actin and prepro-endothelin-1 similarly to blood pressure normalization but in contrast to blood pressure normalization did not attenuate augmented brain natriuretic peptide (BNP) expression.

CONCLUSION

normalization specifically of augmented IMD/RAMP1 expression in NO-deficient cardiomyocytes by antioxidant intervention in the absence of blood pressure reduction indicates that these genes are likely to be induced directly by myocardial oxidative stress. Although oxidative stress contributed to cardiomyocyte hypertrophy, induction of IMD and RAMP1 is unlikely to be secondary to cardiomyocyte hypertrophy.

Cardiovascular effects

Amylin can lower blood pressure in anesthetized animals (in which reflex bradycardia is absent), or evoke reflex bradycardia. This effect is likely in response to vasodilatation mediated via calcitonin gene-related peptide (CGRP) receptors, and only occurs at concentrations two to three orders of magnitude higher than physiological amylin concentrations. There is suggestive, but not fully established, evidence for an amylin-like pharmacology with cardiotropic effects, consisting of inotropy (stimulation of contractility) and suppression of secretion of atrial natriuretic peptide (ANP).

Amylin and the integrated control of nutrient influx

The most potent actions of amylin that occur at physiological plasma concentrations include inhibition of food intake, gastric emptying, acid and digestive enzyme secretion, and glucagon secretion. These actions share a common outcome; they each help regulate the rate at which nutrients (including glucose) appear in the blood (Ra). Amylin physiologically orchestrates, via several parallel processes, the rate of entry of nutrient into the circulation, as shown schematically in Fig. 1. In this way, amylin's function may be viewed as complementary to that of insulin (secreted from the same pancreatic beta-cells), which orchestrates the exit of nutrient from blood and its storage in peripheral tissues. The following discussion addresses the emerging picture that, although amylin is co-secreted with an endocrine hormone from endocrine tissue (the pancreatic islets), the target for its most potent and physiologically relevant effects appears to be the central nervous system. Amylin thus may be primarily regarded as a neuroendocrine hormone (Young et al., 2000).

The effect of adrenomedullin, amylin fragment 8-37 and calcitonin gene-related peptide on contractile force, heart rate and coronary perfusion pressure in isolated rat hearts

The effect of human adrenomedullin, human amylin fragment 8-37 (amylin 8-37) and rat calcitonin gene-related peptide (CGRP) on contractile force, heart rate and coronary perfusion pressure has been investigated in the isolated perfused rat hearts. Adrenomedullin (2x10(-10), 2x10(-9) and 2x10(-8) M) produced a significant decrease in contractile force and perfusion pressure, but only the peptide caused a decline in heart rate at the highest dose. Amylin (10(-9), 10(-8) and 10(-7) M) significantly increased and then decreased contractile force. Two doses of amylin (10(-8) and 10(-7) M) induced a significant increase in heart rate, however amylin did not change perfusion pressure in all the doses used. Rat alpha CGRP (10(-8), 10(-7) and 10(-6) M) evoked a slight decline in contractile force following a significant increase in contractile force induced by the peptide. CGRP in all the doses raised heart rate and lowered perfusion pressure. Our results suggest that adrenomedullin has negative inotropic, negative chronotropic and coronary vasodilator actions. Amylin produces a biphasic inotropic effect and evokes a positive chronotropy. CGRP causes positive inotropic, positive chronotropic and vasodilatory effects in isolated rat hearts.

Amylin-induced suppression of ANP secretion through receptors for CGRP1 and salmon calcitonin

Amylin cosecretes with insulin from pancreatic beta-cells and shows high sequence homology with CGRP, adrenomedullin, and salmon calcitonin. This study aimed to investigate the effect of amylin on the atrial hemodynamics and ANP release from rat atria and to identify its receptor subtypes. Isolated perfused left atria from either control or streptozotocin-treated rats were paced at 1.3 Hz. The concentration of ANP was measured by radioimmunoassay and the translocation of ECF was measured by [3H]-inulin clearance. Rat amylin increased atrial contractility and suppressed the release of ANP. Rat CGRP showed similar effects but was approximately 300-fold more potent than amylin. Pretreatment with receptor antagonist for CGRP1 [rat alpha-CGRP (8-37)] or salmon calcitonin [acetyl-(Asn30, Tyr32)-calcitonin(8-32), (AC 187)] blocked the suppressive effect of ANP release and the positive inotropic effect by rat amylin. However, receptor antagonists for amylin [amylin (8-37), acetyl-amylin] did not block those effects. Amylin (8-37), acetyl-amylin, or rat alpha-CGRP (8-37) alone accentuated the release of ANP with no changes in atrial contractility. The effect of rat amylin and rat amylin (8-37) on the ANP release was attenuated in streptozotocin-treated rats. We suggest that amylin suppressed ANP release with increased atrial contractility through receptors for CGRP1 and salmon calcitonin and the attenuation of amylin and its antagonist on ANP release from streptozotocin-treated rat atria may be due to the downregulation of amylin receptor.

Receptor autoradiography as mean to explore the possible functional relevance of neuropeptides: focus on new agonists and antagonists to study natriuretic peptides, neuropeptide Y and calcitonin gene-related peptides

Over the past 20 years, receptor autoradiography has proven most useful to provide clues as to the role of various families of peptides expressed in the brain. Early on, we used this method to investigate the possible roles of various brain peptides. Natriuretic peptide (NP), neuropeptide Y (NPY) and calcitonin (CT) peptide families are widely distributed in the peripheral and central nervous system and induced multiple biological effects by activating plasma membrane receptor proteins. The NP family includes atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The NPY family is composed of at least three peptides NPY, peptide YY (PYY) and the pancreatic polypeptides (PPs). The CT family includes CT, calcitonin gene-related peptide (CGRP), amylin (AMY), adrenomedullin (AM) and two newly isolated peptides, intermedin and calcitonin receptor-stimulating peptide (CRSP). Using quantitative receptor autoradiography as well as selective agonists and antagonists for each peptide family, in vivo and in vitro assays revealed complex pharmacological responses and radioligand binding profile. The existence of heterogeneous populations of NP, NPY and CT/CGRP receptors has been confirmed by cloning. Three NP receptors have been cloned. One is a single-transmembrane clearance receptor (NPR-C) while the other two known as CG-A (or NPR-A) and CG-B (or NPR-B) are coupled to guanylate cyclase. Five NPY receptors have been cloned designated as Y(1), Y(2), Y(4), Y(5) and y(6). All NPY receptors belong to the seven-transmembrane G-protein coupled receptors family (GPCRs; subfamily type I). CGRP, AMY and AM receptors are complexes which include a GPCR (the CT receptor or CTR and calcitonin receptor-like receptor or CRLR) and a single-transmembrane domain protein known as receptor-activity-modifying-proteins (RAMPs) as well as an intracellular protein named receptor-component-protein (RCP). We review here tools that are currently available in order to target each NP, NPY and CT/CGRP receptor subtype and establish their respective pathophysiological relevance.

Calcitonin gene-related peptide-induced suppression of atrial natriuretic peptide release through receptors for CGRP1 but not for calcitonin and amylin

Calcitonin gene-related peptide (CGRP), a 37-amino acid neuropeptide, is found in the central nervous system as well as in the heart. CGRP shows high sequence homology with amylin, salmon calcitonin, and adrenomedullin. This study aimed to investigate the effect of CGRP on atrial hemodynamics and atrial natriuretic peptide (ANP) release by using isolated perfused beating left atria and to identify its receptor subtypes. Rat alpha-CGRP (0.1, 1, 10, or 100 nM) increased atrial contractility and suppressed the release of ANP in a concentration-dependent manner. However, cys-CGRP (1 microM), a CGRP(2) receptor agonist, slightly decreased ANP release without positive inotropism. Human alpha-CGRP (1 nM) showed an effect on ANP release similar to that of rat alpha-CGRP with potent positive inotropism. However, salmon and rat calcitonin (1 microM) caused a slight decrease or no change in ANP release. Pretreatment with a receptor antagonist for CGRP(1) [rat alpha-CGRP-(8-37)] blocked rat alpha-CGRP-induced suppression of ANP release and positive inotropism, whereas the antagonists for salmon or amylin did not. Therefore, we suggest that rat alpha-CGRP causes a suppression of ANP release with positive inotropism through the receptor for CGRP(1) but not that for calcitonin and amylin.

The molecular pharmacology of CGRP and related peptide receptor subtypes

Calcitonin gene-related peptides (alpha and beta isoforms), better known as CGRPalpha and CGRPbeta, were isolated twenty years ago. In fact, these were the first peptides to be characterized using a molecular cloning strategy, which is not the traditional approach of biochemical extraction and purification. Paradoxically, progress in the characterization of CGRP receptor subtypes has been extremely slow as a result of difficulties in their cloning and the lack of selective receptor subtype agonists and antagonists. However, exciting progress has been made overthe pasttwo years and is briefly reviewed here.

Positive inotropy mediated via CGRP receptors in isolated human myocardial trabeculae

Isometric contractile force were studied on isolated human myocardial trabeculae that were paced at 1.0 Hz in tissue baths. Alpha calcitonin gene-related peptide (alpha-CGRP) had a potent positive inotropic effect in most trabeculae from both the right atrium and left ventricle, and this effect was partially antagonized by the CGRP(1) receptor antagonist alpha-CGRP-(8-37) (10(-6) M). Amylin and the CGRP(2) receptor agonist [Cys(acetylmethoxy)(2, 7)]CGRP had a positive inotropic effect in some trabeculae, whereas adrenomedullin had no inotropic effect. Using reverse transcriptase-polymerase chain reaction (PCR) mRNAs encoding the human calcitonin receptor-like receptor and the receptor associated modifying proteins (RAMPs) RAMP1, RAMP2, and RAMP3 were detected in human myocardial trabeculae from both the right atrium and left ventricle. In conclusion, functional CGRP(1) and CGRP(2) receptors may mediate a positive inotropic effect at both the atrial and ventricular level of the human heart. mRNAs for calcitonin receptor-like receptor and specific RAMPs further support the presence of CGRP receptors.

Positive inotropic effects of adrenomedullin on rat papillary muscle

Adrenomedullin is a peptide recently isolated from pheochromocytoma that has vasorelaxant and long-lasting hypotensive activities. Plasma levels of adrenomedullin are elevated in patients with congestive heart failure, but the effects of adrenomedullin on the cardiac function are unclear. We, thus, investigated the effects of adrenomedullin on the contraction of rat papillary muscles. We measured the isometric tension and cAMP contents of isolated rat papillary muscles. Adrenomedullin exhibited concentration-dependent inotropic effects. Adrenomedullin also significantly increased intracellular contents of cAMP. Addition of the calcitonin gene-related peptide (CGRP) receptor antagonist inhibited both contractile force and cAMP generation of papillary muscles stimulated by adrenomedullin. The adrenomedullin-induced inotropic effect was further increased in the presence of the phosphodiesterase inhibitor, 3-isobutyl-1-methyl-xanthine (IBMX), while the effect was significantly suppressed by KT5720 and Rp-8-bromoadenosine-3',5'-cyclic monophosphorothioate (Rp-8-Br-cAMPS), protein kinase A inhibitors. These results indicate that adrenomedullin has positive inotropic effects on the heart, at least partially through a cAMP-dependent pathway.

Positive inotropy of calcitonin gene-related peptide and amylin on porcine isolated myocardium

Isolated porcine myocardial trabeculae from right atria and left ventricles were paced at 1.5 Hz in tissue baths, and changes in isometric contractile force upon exposure to agonist were studied. Alpha calcitonin gene-related peptide (alpha-CGRP) increased contractile force in nearly half of the trabeculae, whereas the selective CGRP(2) receptor agonist [Cys(acetylmethoxy)(2,7)]-CGRP had effect in only a few. Preincubation with the CGRP(1) receptor antagonist alpha-CGRP-(8-37) (10(-6) M) almost completely blocked positive inotropic responses to alpha-CGRP. Amylin had weak positive inotropic effects in some atrial, but not in ventricular trabeculae. Adrenomedullin did not affect contractility in either atrial or ventricular trabeculae. In conclusion, these results suggest that alpha-CGRP has a positive inotropic effect that can be mediated by both CGRP(1) and CGRP(2) receptors. Amylin seems to have a potential positive inotropic effect on atrial tissue, whereas no direct effect of adrenomedullin could be measured.

Proadrenomedullin N-terminal 20 peptide (PAMP) inhibits proliferation of human neuroblastoma TGW cells

We investigated the effects of proadrenomedullin N-terminal 20 peptide (PAMP) and adrenomedullin (AM) on the growth of human neuroblastoma TGW cells. Both PAMP and AM inhibited growth and DNA synthesis in neuroblastoma cells. Calcitonin gene-related peptide (CGRP)(8-37), an antagonist to CGRP, abolished the inhibitory effect of AM on growth and DNA synthesis of neuroblastoma cells but did not affect that of PAMP. AM(22-52), an antagonist to AM, also reversed the effect of AM. On the other hand, pertussis toxin (PTX) and omega-conotoxin GIVA blocked the effect of PAMP alone. Thus, PAMP inhibits the growth of neuroblastoma cells by inhibiting N-type Ca2+ channels through PTX-sensitive G protein-coupled receptors, which is different mechanism of AM-induced inhibition of the cell growth.