Intermedin (adrenomedullin-2): a novel counter-regulatory peptide in the cardiovascular and renal systems

Abstract

Intermedin (IMD) is a novel peptide related to calcitonin gene-related peptide (CGRP) and adrenomedullin (AM). Proteolytic processing of a larger precursor yields a series of biologically active C-terminal fragments, IMD(1-53), IMD(1-47) and IMD(8-47). IMD shares a family of receptors with AM and CGRP composed of a calcitonin-receptor like receptor (CALCRL) associated with one of three receptor activity modifying proteins (RAMP). Compared to CGRP, IMD is less potent at CGRP(1) receptors but more potent at AM(1) receptors and AM(2) receptors; compared to AM, IMD is more potent at CGRP(1) receptors but less potent at AM(1) and AM(2) receptors. The cellular and tissue distribution of IMD overlaps in some aspects with that of CGRP and AM but is distinct from both. IMD is present in neonatal but absent or expressed sparsely, in adult heart and vasculature and present at low levels in plasma. The prominent localization of IMD in hypothalamus and pituitary and in kidney is consistent with a physiological role in the central and peripheral regulation of the circulation and water-electrolyte homeostasis. IMD is a potent systemic and pulmonary vasodilator, influences regional blood flow and augments cardiac contractility. IMD protects myocardium from the deleterious effects of oxidative stress associated with ischaemia-reperfusion injury and exerts an anti-growth effect directly on cardiomyocytes to oppose the influence of hypertrophic stimuli. The robust increase in expression of the peptide in hypertrophied and ischaemic myocardium indicates an important protective role for IMD as an endogenous counter-regulatory peptide in the heart.

Figure 1

Schematic diagram (a) of human prepro-IMD showing the fragments derived from processing of the 148 amino-acid prepro-peptide precursor at the putative cleavage sites indicated; (b) comparison of the primary sequences of adrenomedullin_13–52, intermedin_8–47 (IMDS), αCGRP_1–37 and AMY_1–37 in human; note the cysteine residues common to all of these peptides which contribute to the disulphide bridge giving rise to the characteristic intramolecular loop structure (adapted from Chang et al., 2004; Roh et al., 2004). AMY, amylin.

Figure 2

CGRP, AM and IMD share a common family of G-protein (Gαβγ)-coupled receptors formed by the association of the calcitonin receptor-like receptor (CL) with one of three receptor –activity-modifying proteins (RAMPs). A receptor component protein (RCP) is also required for optimum activation of signal transduction. Pharmacological characteristics and putative signalling mechanisms associated with each of the three recognized receptor subtypes (CGRP₁, AM₁ and AM₂) are indicated (based on Poyner et al., 2002; Roh et al., 2004). CGRP, calcitonin gene-related peptide; AM, adrenomedullin; IMD, intermedin.

Figure 3

Schematic diagram outlining the proposed endothelium-dependent and -independent signalling pathways by which IMD might evoke vasodilatation. The relative contribution of such processes is likely to vary significantly with respect to vessel and species. Endothelial cell (EC); vascular smooth muscle cell (VSMC); adenylate cyclase (AC); nitric oxide synthase (NOS); protein kinase A (PKA); protein kinase G (PKG); guanylate cyclase (GC). The endothelium-dependent effect of IMD is attributed to cyclic AMP-dependent activation of constitutive endothelial NOS and synthesis of nitric oxide which then diffuses to VSMC to stimulate GC leading to accumulation of cyclic GMP and activation of PKG. Accumulation of cyclic AMP in EC is stimulated mainly by activation of endothelial AM receptors although a possible contribution of CGRP₁ receptors, at least in some vessels, cannot be discounted. Stimulation of CGRP₁ receptors, or alternatively in some vessels of AM receptors, on VSMC directly is associated with accumulation of cyclic AMP and activation of PKA. PKA and PKG trigger processes in VSMC which lead to reduced intracellular calcium ion concentration resulting in vasodilatation; such processes may involve activation of outward potassium currents leading to membrane hyperpolarization and reduced calcium entry. CGRP, calcitonin gene-related peptide; IMD, intermedin; NOS, nitric oxide synthase.

Figure 4

Schematic diagram depicting the combination of direct and indirect effects exerted upon the heart by IMD, which influence cardiac output. IMD exerts chronotropic and inotropic effects directly on myocardium. Coronary vasodilatation enhances myocardial perfusion, which augments contractility indirectly. Systemic vasodilatation reduces afterload and activates the baroreceptor mechanism to increase central sympathetic outflow, which also enhances cardiac contractility. The actions of IMD centrally and locally within the kidney influence blood volume which in turn influences venous return and cardiac output. IMD, intermedin.

Figure 5

Schematic diagram outlining the proposed signalling pathways by which IMD might influence (a) contraction; (b) hypertrophic remodelling; (c) protection against the deleterious consequences of oxidative stress directly in ventricular cardiomyocytes. Adenylate cyclase (AC); protein kinase A (PKA); protein kinase C (PKC); diacylglycerol (DAG); mitogen-activated protein kinase (MAPK); receptor tyrosine kinase (rTK); phospholipase C-β (PLC-β); extracellular signal-regulated kinase (ERK); phosphoinositide 3-kinase (PI3K); receptor component protein (RCP). IMD, intermedin.

Figure 6

Expression of prepro-IMD and prepro-AM mRNAs in left ventricular cardiomyocytes of (a) SHR and WKY rat at 20 weeks of age (mean+s.e., n=8), data taken from McDermott et al., 2007; (b) 8-week-old SD rat given L-NAME (35 mg kg⁻¹ day⁻¹) in drinking water for 8 weeks and age-matched untreated rat for comparison, (mean+s.e., n=10), data taken from Bell et al. (2007). Data were expressed relative to glyceraldehyde 3-phosphate dehydrogenase mRNA levels. ^*Denotes significant difference between treatment groups, ^*P<0.05. AM, adrenomedullin; IMD, intermedin; SHR, spontaneously hypertensive rat; WKY, Wistar–Kyoto; L-NAME, N^G-nitro-L-arginine-methyl-ester.

Figure 7

Effect of IMD (10 pM–100 nM) on (a) protein synthesis; (b) protein mass in ventricular cardiomyocytes (combined population obtained from left and right ventricles of 12-week-old normotensive SD control rat) and maintained in short-term (24 h) culture under basal conditions and in the presence of the hypertrophic stimulus, phorbol 12-myristate 13-acetate (PMA) (100 nM). Data are the mean values+s.e. of 4–6 heart cell preparations. ^*Denotes significant difference between PMA response in the absence and presence of IMD. ⁺Denotes significant difference between basal value and PMA-stimulated response (P<0.05). Unpublished observation; methodology employed as described for AM in Bell et al. (2006). IMD, intermedin.