Regional distribution and regulation of [125I]calcitonin gene-related peptide binding sites in coronary arteries

Evolution of the CT/CGRP family: comparative study with new data from models of teleosts, the eel, and cephalopod molluscs, the cuttlefish and the nautilus

In mammals, alternative splicing of the calcitonin gene generates two distinct peptides: calcitonin (CT), synthesised in the thyroid C cells and involved in the regulation of calcium metabolism, and calcitonin gene-related peptide (CGRP), brain neuromediator synthesised in the peripheral and central nerves. CGRP is well represented and molecularly conserved during evolution whereas CT has not been detected in any of the invertebrates analysed so far. In order to better understand the evolution of this CT/CGRP peptide family we reviewed the major data concerning its evolution from the literature and our recent data obtained in models of teleosts and cephalopod molluscs. The presence of both CGRP-like molecules and its specific bindings sites in the central nervous system of eel, cuttlefish and nautilus, suggests that the brain neurotransmitter role of CGRP could represent an ancient role in metazoa, already present in cephalopods and conserved among vertebrates, as still observed in mammals. In contrast, the presence of CGRP specific binding sites, and not the peptide itself, in the gills suggests an endocrine role for CGRP, in cephalopods and teleosts, that may have been lost during the evolution of the tetrapod lineage. These data, and the absence of CT-like molecules that we observed in cephalopods, support the hypothesis that CGRP represents the ancestral molecule of the CT/CGRP family, appeared in metazoa before the vertebrate emergence. The distinction between CT and CGRP receptors appears to be an event posterior to the emergence of ecdysozoan and lophotrochozoan protostomes, probably in relation to the CT appearance. The evolution of the CT/CGRP peptide family is probably similar to the evolution of the CT/CGRP receptor family. In fact, the genic duplication that induced the appearance of the two separate molecules, CT and CGRP, may constitute an event close to that, which induced the appearance of the two specific receptors. These events remain to be further studied in order to better understand the peptide and receptor evolution of the CT/CGRP family.

Regional localization and abundance of calcitonin gene-related peptide receptors in guinea pig heart

Calcitonin gene-related peptide (CGRP) is a neurotransmitter that is released within the heart during myocardial ischemia. The present study was done to determine the regional localization and abundance of CGRP receptors in the guinea pig heart. CGRP binding sites in 20 microm frozen sections of heart were labeled using [125I]CGRP. Non-specific binding was determined in the presence of 1 microm unlabeled CGRP or CGRP(8-37). Significant amounts of specific CGRP binding were identified in atrial and ventricular myocardium, all portions of the conducting system, coronary arteries, the aorta and pulmonary trunk and intracardiac ganglia. Specific binding of CGRP to the left atrium was two-fold higher than binding to the right atrium (0.667+/-0.052 v 0.340+/-0.029 fmol/mg tissue, n=5, CGRP(8-37)group). In contrast to the atria, a lower and uniform density of CGRP receptors occurred in contractile tissue of the ventricular myocardium (e.g. 0.239+/-0.013 fmol/mg left ventricle, n=5). The highest concentration of CGRP receptors in guinea pig cardiac tissue occurred at the bundle of His and the bundle branches (0.752+/-0.087 and 0.713+/-0.138 fmol/mg tissue, respectively, n=5). CGRP receptors were localized to coronary vessels throughout the heart and to the ascending aorta and pulmonary trunk. Lastly, intracardiac ganglia exhibited moderate levels of specific [125I]CGRP binding (0.475+/-0.043 fmol/mg, n=5). These findings support the concept that CGRP can have direct effects on atrial and ventricular function as well as coronary flow. The high density of CGRP receptors in the distal conducting system and the presence of CGRP receptors in intracardiac ganglia further suggest that CGRP could have important effects on cardiac conduction velocity and parasympathetic regulation of the heart.

Immunohistochemical detection of calcitonin gene-related peptide receptor (CGRPR)-1 in the endothelium of human coronary artery and bronchial blood vessels

The potent vasodilatory peptide, calcitonin gene-related peptide (CGRP) is present in the innervation of vascular tissue. The actions of CGRP occur via a receptor, CGRP receptor(R)-1, which is also a target for the cardioprotective peptide adrenomedullin. The human version of the pharmacologically-defined CGRPR-1 has been cloned but its distribution and cellular location is unknown. A rabbit antibody was generated to a synthetic peptide that corresponds to the C-terminus of human CGRPR-1 Immunochemical analysis of the human cell-line, SK-N-MC, which exhibits functional expression of the CGRPR-1 confirmed the antibody's specificity. The antiserum revealed specific staining in the endothelium of human coronary arteries. The vascular smooth muscle and ventricular myocardium were not immunoreactive. In bronchial blood vessels CGRPR-1-immunoreactivity was detected in the endothelium of the venules and not in the arterioles, which is particularly relevant for elucidating the putative role of CGRP in inflammation in this tissue.

Endogenous tachykinins cause bradycardia by stimulating cholinergic neurons in the isolated guinea pig heart

The purpose of this study was to determine if endogenous tachykinins can cause bradycardia in the isolated perfused guinea pig heart through stimulation of cholinergic neurons. Capsaicin was used to stimulate release of tachykinins and calcitonin gene-related peptide (CGRP) from cardiac afferents. A bolus injection of 100 nmol capsaicin increased heart rate by 26 +/- 7% from a baseline of 257 +/- 14 beats/min (n = 6, P < 0.01). This positive chronotropic response was converted to a minor bradycardic effect in hearts with 1 microM CGRP-(8-37) present to block CGRP receptors. The negative chronotropic response to capsaicin was markedly potentiated in another group of hearts with the further addition of 0.5 microM neostigmine to inhibit cholinesterases. In this group, capsaicin decreased heart rate by 30 +/- 10% from a baseline of 214 +/- 6 beats/min (n = 8, P < 0.05). This large bradycardic response to capsaicin was inhibited by 1) infusion of neurokinin A to desensitize tachykinin receptors or 2) treatment with 1 microM atropine to block muscarinic receptors. The latter observations implicate tachykinins and acetylcholine, respectively, as mediators of the bradycardia. These findings support the hypothesis that endogenous tachykinins could mediate axon reflexes to stimulate cholinergic neurons of the intrinsic cardiac ganglia.

Non-competitive antagonism of amylin on CGRP(1)-receptors in rat coronary small arteries

We examined the interaction between rat-amylin and relaxations induced by rat-alphaCGRP and isoprenaline in rat isolated coronary small arteries. Amylin, 0.1 - 100 nM, had a concentration dependent non-competitive antagonistic effect on rat-alphaCGRP-induced responses with an EC(50) of approximately 1 nM. Amylin did not affect the relaxations induced by isoprenaline at a concentration of 10 nM. The apparent equilibrium dissociation constant, K(A), for CGRP(1)-receptors in the rat coronary small arteries was approximately 2 nM. Analysis of the relationship between receptor occupancy and response to rat-alphaCGRP indicates that the receptor reserve is small. Our results show that amylin in low concentrations acts as a selective non-competitive inhibitor at CGRP(1)-receptors in rat isolated coronary small arteries.

Pharmacologic characterization of receptor types mediating coronary vasodilator actions of sensory neuropeptides in the guinea pig

The receptor types mediating sensory neuropeptide-induced coronary vasodilatation were elucidated on isolated guinea pig hearts perfused with isotonic buffer containing 20 mM KCl. Substance P and the selective neurokinin-1 (NK1) receptor agonist [Sar9, Met(O2)11]-substance P produced dose-dependent reductions in perfusion pressure, but the selective NK2 receptor agonist [Nle10]-neurokinin A4-10 and the selective NK3 receptor agonist [MePhe7]-neurokinin B produced no change. The vasorelaxant effects of substance P and the NK1 receptor agonist were abolished by the selective NK1 receptor antagonist FK888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-prolyl]-N-methy l-N-phenylmethyl-3-(2-naphthyl)-L-alaninamide), whereas the selective NK2 receptor antagonist SR48968 ((S)-N-methyl-N-[4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophenyl )-butyl] benzamide) and the selective NK3 receptor antagonist SR142801 ((S)-(N)-( 1-(3-(1-benzoyl-3-(3,4-dichlorophenyl)piperidin-3-yl)propyl)4-p henylpiperidin-4-yl)-N-methylacetamide) produced partial inhibition on their responses. Calcitonin gene-related peptide (CGRP) produced dose-dependent vasodilatation on the guinea pig coronary blood vessels, which was significantly (p = 0.0067) inhibited by the selective CGRP1 receptor antagonist hCGRP8-37. The selective CGRP2 receptor agonist [Cys(acetomethoxy)2,7]CGRP had no effect on perfusion pressure. These results demonstrate that the sensory neuropeptides substance P and CGRP are effective vasodilators of the guinea pig coronary vascular bed. The receptor types mediating their vasorelaxant effects were identified to be the NK1 receptors and CGRP1 receptors, respectively.

Caliber dependent calcitonin gene-related peptide-induced relaxation in rat coronary arteries: effect of K+ on the tachyphylaxis

The influence of vessel caliber on rat calcitonin gene-related peptide (rat-alphaCGRP)-induced responses and the reproducibility of rat-alphaCGRP concentration-response curves were investigated in the left intramural coronary artery of Sprague-Dawley rats. Rat-alphaCGRP (10(-11)-10(-7) M) induced concentration-dependent relaxations with a pD2-value equal to 8.43 +/- 0.05 (n = 44) and maximal relaxation equal to 52 +/- 3% (n = 44). Both the maximal relaxation and the sensitivity to rat-alphaCGRP were significantly and inversely correlated with vessel lumen diameter. The coronary arteries developed tachyphylaxis in response to rat-alphaCGRP, which was concentration dependently decreased by activating the vessels twice with a buffer containing 36 or 125 mM K+. The rat-alphaCGRP-curve became fully reproducible after activation of the arteries twice with 125 mM K+. These results indicate a caliber-related dependency of both the effect of and sensitivity to rat-alphaCGRP in intramural rat coronary arteries because the arteries become more sensitive and reactive to rat-alphaCGRP with decreasing caliber. Tachyphylaxis can be avoided by repeated activation with 125 mM K+.

Developmental changes in binding sites and reactivity for CGRP and VIP in porcine pulmonary arteries

During postnatal adaptation pulmonary arteries dilate. CGRP and VIP are pulmonary vasodilators. In this report, porcine lungs from newborn to adult were studied. Radiolabeled ligand binding and autoradiography showed CGRP binding sites on the endothelium of pulmonary arteries and veins, which increased postnatally, and VIP binding sites on smooth muscle, which decreased. Isolated conduit arteries relaxed normally (initially endothelium dependent) in response to CGRP from birth. VIP first caused relaxation at 10 days and was endothelium dependent. Age-related changes in receptor binding density were not always reflected in an appropriate alteration in pharmacological response.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Effect of parasympathetic and sensory transmitters on human epicardial coronary arteries and veins

Vasomotor effects of various agonists were tested on isolated human epicardial coronary arteries and veins at resting tension and after precontraction with U46619. Acetylcholine relaxed all arteries with intact endothelium but only some endothelium-denuded arteries. Most veins did not relax to acetylcholine. Higher concentrations of acetylcholine induced powerful contractions of all arteries and veins. Preincubation with atropine significantly lowered the pD(2) values but not E(max) values for contractile responses to acetylcholine in arteries and veins (pA(2) value for atropine 9.1 arteries and 9.6 veins). Vasoactive intestinal peptide, human alpha-calcitonin gene-related peptide and substance P potently relaxed all arteries with intact endothelium and all veins. Removal of the arterial endothelium abolished relaxation to substance P in most arteries whereas responses to vasoactive intestinal peptide were unaffected, and for alpha-calcitonin gene-related peptide the pD(2) value but not the E(max) value was significantly lowered. In both arteries and veins, the antagonists alpha-calcitonin gene-related peptide (8-37) and spantide lowered significantly the potency for alpha-calcitonin gene-related peptide and substance P, respectively, without significant changes in E(max) values (pA(2) value for alpha-calcitonin gene-related peptide (8-37) 7.9 arteries and 7.9 veins, for spantide 7.6 arteries and 8.1 veins).

Nonadrenergic noncholinergic nerves regulate basal coronary flow via release of capsaicin-sensitive neuropeptides in the rat heart

Nonadrenergic noncholinergic nerve fibers supposedly modulate basal coronary flow by releasing capsaicin-sensitive neuropeptides, but the physiological effects of this intrinsic action have not been clarified. We investigated the intrinsic function of nonadrenergic noncholinergic innervation in modulating basal coronary flow in rats. We administered capsaicin to 44 rats to deplete neuropeptides such as calcitonin gene-related peptide (CGRP) and substance P and administered inert vehicle to 60 control rats. Four days later, we measured the coronary pressure-flow relation in the basal state and during maximal coronary vasodilation induced by intracoronary adenosine administration using Langendorff's method. Changes in basal coronary flow prompted by intracoronary infusion of CGRP or substance P and their antagonists were measured in 54 and 30 rats, respectively. Capsaicin-treated rats showed a 31.5 +/- 0.9% (mean +/- SEM) reduction (P < .01) of basal coronary flow in the range of perfusion pressures between 60 and 140 mm Hg compared with untreated control rats, but the maximal coronary flow after adenosine was similar between the two groups. Although basal coronary flow was reduced in capsaicin-treated hearts, left ventricular contractile force and myocardial oxygen consumption did not fall significantly. CGRP increased the coronary flow, but substance P did not. CGRP(8-37), a CGRP receptor antagonist, reduced basal coronary flow by 24.5 +/- 2.1% (P < .01), but FK888, a substance P antagonist, had little effect on it. Thus, capsaicin-sensitive neuropeptides in the rat heart modulate basal coronary flow, providing approximately 30% of it.(ABSTRACT TRUNCATED AT 250 WORDS)

HIV-related neuronal injury. Potential therapeutic intervention with calcium channel antagonists and NMDA antagonists

Perhaps as many as 25-50% of adult patients and children with acquired immunodeficiency syndrome (AIDS) eventually suffer from neurological manifestations, including dysfunction of cognition, movement, and sensation. How can human immunodeficiency virus type 1 (HIV-1) result in neuronal damage if neurons themselves are for all intents and purposes not infected by the virus? This article reviews a series of experiments leading to a hypothesis that accounts at least in part for the neurotoxicity observed in the brains of AIDS patients. There is growing support for the existence of HIV- or immune-related toxins that lead indirectly to the injury or demise of neurons via a potentially complex web of interactions among macrophages (or microglia), astrocytes, and neurons. HIV-infected monocytoid cells (macrophages, microglia, or monocytes), after interacting with astrocytes, secrete eicosanoids, i.e., arachidonic acid and its metabolites, including platelet-activating factor. Macrophages activated by HIV-1 envelope protein gp120 also appear to release arachidonic acid and its metabolites. In addition, interferon-gamma (IFN-gamma) stimulation of macrophages induces release of the glutamate-like agonist, quinolinate. Furthermore, HIV-infected macrophage production of cytokines, including TNF-alpha and IL1-beta, contributes to astrogliosis. A final common pathway for neuronal susceptibility appears to be operative, similar to that observed in stroke, trauma, epilepsy, neuropathic pain, and several neurodegenerative diseases, possibly including Huntington's disease, Parkinson's disease, and amyotrophic lateral sclerosis. This mechanism involves the activation of voltage-dependent Ca2+ channels and N-methyl-D-aspartate (NMDA) receptor-operated channels, and, therefore, offers hope for future pharmacological intervention. This article focuses on clinically tolerated calcium channel antagonists and NMDA antagonists with the potential for trials in humans with AIDS dementia in the near future.

Peptidergic innervation of human epicardial coronary arteries

The peptidergic innervation of proximal (internal diameter, > 0.8 mm) and distal (internal diameter, < 0.8 mm) regions of human epicardial coronary arteries was investigated by means of immunohistochemical, chromatographic, radioimmunological, and in vitro pharmacological techniques. The use of an antiserum to the general neuronal marker protein gene product 9.5 revealed that the proximal part of epicardial arteries possessed a relatively sparse supply of nerve fibers forming a loose network in the adventitia. The perivascular network increased in density as the vessels were followed distally. In both proximal and distal regions, the majority of nerve fibers possessed neuropeptide Y and tyrosine hydroxylase immunoreactivity. Calcitonin gene-related peptide (CGRP)- and substance P-immunoreactive nerve fibers were very sparse in the proximal region of the arteries and increased in number distally. Only a few scattered vasoactive intestinal peptide (VIP)-immunoreactive nerve fibers were detected in both arterial regions. The use of high-performance liquid chromatography and radioimmunoassay revealed that the immunoreactive material present in coronary artery extracts closely resembled synthetic peptides. An in vitro pharmacological method demonstrated that neuropeptide Y elicited no detectable response in either proximal or distal arterial segments. In contrast, CGRP, substance P, and VIP all produced a concentration-dependent relaxation of both arterial regions. CGRP and substance P were stronger and more potent than VIP. CGRP and substance P induced a more potent response in distal compared with proximal regions of the arteries. These results suggest that the peptidergic nerves supplying human large epicardial coronary arteries may be predominantly involved in mediating vasodilation.

Capsaicin induced afferent denervation and receptor-linked responses to CGRP in the rat

We have examined the effect of neonatal capsaicin-induced destruction of primary afferent nerves in rats on the response to calcitonin gene-related peptide (CGRP) in vitro and in vivo. Denervation was confirmed by immunohistochemistry. Rat alpha-CGRP (rCGRP) activated adenylate cyclase in homogenates of rat spleen (basal activity 45 +/- 8.27, delta Vmax 75 +/- 16 pmol cyclic AMP min-1 protein; K(act) 2.04 +/- 0.44 nM). A single specific binding site for [125I]hCGRP was demonstrated in homogenates of spleen (Kd = 4.84 +/- 0.66 nM, Bmax = 1.43 +/- 0.35 pmol mg-1 protein). Neither adenylate cyclase activation nor binding site characteristics were affected by capsaicin-induced denervation. In addition, hypotensive responses to intravenous boluses of rCGRP were examined in anaesthetized rats. Neither the basal blood pressure nor the blood pressure fall in response to rCGRP were altered by neonatal denervation by capsaicin. In conclusion, there is no evidence of denervation hypersensitivity of receptor mediated responses to CGRP in vivo or in vitro, following capsaicin-induced denervation in the rat. This suggests that CGRP is unlikely to exert a sustained tonic influence on cardiovascular regulation in the rat.