Receptor crosstalk. Implications for cardiovascular function, disease and therapy

Co-Evolution of Opioid and Adrenergic Ligands and Receptors: Shared, Complementary Modules Explain Evolution of Functional Interactions and Suggest Novel Engineering Possibilities

Cross-talk between opioid and adrenergic receptors is well-characterized and involves second messenger systems, the formation of receptor heterodimers, and the presence of extracellular allosteric binding regions for the complementary ligand; however, the evolutionary origins of these interactions have not been investigated. We propose that opioid and adrenergic ligands and receptors co-evolved from a common set of modular precursors so that they share binding functions. We demonstrate the plausibility of this hypothesis through a review of experimental evidence for molecularly complementary modules and report unexpected homologies between the two receptor types. Briefly, opioids form homodimers also bind adrenergic compounds; opioids bind to conserved extracellular regions of adrenergic receptors while adrenergic compounds bind to conserved extracellular regions of opioid receptors; opioid-like modules appear in both sets of receptors within key ligand-binding regions. Transmembrane regions associated with homodimerization of each class of receptors are also highly conserved across receptor types and implicated in heterodimerization. This conservation of multiple functional modules suggests opioid–adrenergic ligand and receptor co-evolution and provides mechanisms for explaining the evolution of their crosstalk. These modules also suggest the structure of a primordial receptor, providing clues for engineering receptor functions.

Cardiovascular Diseases and Pharmacomicrobiomics: A Perspective on Possible Treatment Relevance

Cardiovascular diseases (CVDs), the most common cause of mortality in rich countries, include a wide variety of pathologies of the heart muscle and vascular system that compromise the proper functioning of the heart. Most of the risk factors for cardiovascular diseases are well-known: lipid disorders, high serum LDL cholesterol, hypertension, smoking, obesity, diabetes, male sex and physical inactivity. Currently, much evidence shows that: (i) the human microbiota plays a crucial role in maintaining the organism’s healthy status; and (ii) a link exists between microbiota and cardiovascular function that, if dysregulated, could potentially correlate with CVDs. This scenario led the scientific community to carefully analyze the role of the microbiota in response to drugs, considering this the right path to improve the effectiveness of disease treatment. In this review, we examine heart diseases and highlight how the microbiota actually plays a preponderant role in their development. Finally, we investigate pharmacomicrobiomics—a new interesting field—and the microbiota’s role in modulating the response to drugs, to improve their effectiveness by making their action targeted, focusing particular attention on cardiovascular diseases and on innovative potential treatments.

Intra- and intercellular signaling pathways associated with drug-induced cardiac pathophysiology

Cardiac physiology and homeostasis are maintained by the interaction of multiple cell types, via both intra- and intercellular signaling pathways. Perturbations in these signaling pathways induced by oncology therapies can reduce cardiac function, ultimately leading to heart failure. As cancer survival increases, related cardiovascular complications are becoming increasingly prevalent, thus identifying the perturbations and cell signaling drivers of cardiotoxicity is increasingly important. Here, we discuss the homotypic and heterotypic cellular interactions that form the basis of intra- and intercellular cardiac signaling pathways, and how oncological agents disrupt these pathways, leading to heart failure. We also highlight the emerging systems biology techniques that can be applied, enabling a deeper understanding of the intra- and intercellular signaling pathways across multiple cell types associated with cardiovascular toxicity.

Unpuzzling COVID-19: tissue-related signaling pathways associated with SARS-CoV-2 infection and transmission

The highly infective coronavirus disease 19 (COVID-19) is caused by a novel strain of coronaviruses - the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) - discovered in December 2019 in the city of Wuhan (Hubei Province, China). Remarkably, COVID-19 has rapidly spread across all continents and turned into a public health emergency, which was ultimately declared as a pandemic by the World Health Organization (WHO) in early 2020. SARS-CoV-2 presents similar aspects to other members of the coronavirus family, mainly regarding its genome, protein structure and intracellular mechanisms, that may translate into mild (or even asymptomatic) to severe infectious conditions. Although the mechanistic features underlying the COVID-19 progression have not been fully clarified, current evidence have suggested that SARS-CoV-2 may primarily behave as other β-coronavirus members. To better understand the development and transmission of COVID-19, unveiling the signaling pathways that may be impacted by SARS-CoV-2 infection, at the molecular and cellular levels, is of crucial importance. In this review, we present the main aspects related to the origin, classification, etiology and clinical impact of SARS-CoV-2. Specifically, here we describe the potential mechanisms of cellular interaction and signaling pathways, elicited by functional receptors, in major targeted tissues/organs from the respiratory, gastrointestinal (GI), cardiovascular, renal, and nervous systems. Furthermore, the potential involvement of these signaling pathways in evoking the onset and progression of COVID-19 symptoms in these organ systems are presently discussed. A brief description of future perspectives related to potential COVID-19 treatments is also highlighted.

β-Adrenergic Receptor Signaling and Heart Failure: From Bench to Bedside

Despite improvements in management and therapeutic approach in the last decades, heart failure is still associated with high mortality rates. The sustained enhancement in the sympathetic nervous system tone, observed in patients with heart failure, causes alteration in β-adrenergic receptor signaling and function. This latter phenomenon is the result of several heart failure-related molecular abnormalities involving adrenergic receptors, G-protein-coupled receptor kinases, and β-arrestins. This article summarizes novel encouraging preclinical strategies to reactivate β-adrenergic receptor signaling in heart failure, including pharmacologic and gene therapy approaches, and attempts to translate acquired notions into the clinical setting.

Heteromerization of chemokine (C-X-C motif) receptor 4 with α1A/B-adrenergic receptors controls α1-adrenergic receptor function

Recent evidence suggests that chemokine (C-X-C motif) receptor 4 (CXCR4) contributes to the regulation of blood pressure through interactions with α1-adrenergic receptors (ARs) in vascular smooth muscle. The underlying molecular mechanisms, however, are unknown. Using proximity ligation assays to visualize single-molecule interactions, we detected that α1A/B-ARs associate with CXCR4 on the cell surface of rat and human vascular smooth muscle cells (VSMC). Furthermore, α1A/B-AR could be coimmunoprecipitated with CXCR4 in a HeLa expression system and in human VSMC. A peptide derived from the second transmembrane helix of CXCR4 induced chemical shift changes in the NMR spectrum of CXCR4 in membranes, disturbed the association between α1A/B-AR and CXCR4, and inhibited Ca(2+) mobilization, myosin light chain (MLC) 2 phosphorylation, and contraction of VSMC upon α1-AR activation. CXCR4 silencing reduced α1A/B-AR:CXCR4 heteromeric complexes in VSMC and abolished phenylephrine-induced Ca(2+) fluxes and MLC2 phosphorylation. Treatment of rats with CXCR4 agonists (CXCL12, ubiquitin) reduced the EC50 of the phenylephrine-induced blood pressure response three- to fourfold. These observations suggest that disruption of the quaternary structure of α1A/B-AR:CXCR4 heteromeric complexes by targeting transmembrane helix 2 of CXCR4 and depletion of the heteromeric receptor complexes by CXCR4 knockdown inhibit α1-AR-mediated function in VSMC and that activation of CXCR4 enhances the potency of α1-AR agonists. Our findings extend the current understanding of the molecular mechanisms regulating α1-AR and provide an example of the importance of G protein-coupled receptor (GPCR) heteromerization for GPCR function. Compounds targeting the α1A/B-AR:CXCR4 interaction could provide an alternative pharmacological approach to modulate blood pressure.

Chemokine (C-X-C motif) receptor 4 and atypical chemokine receptor 3 regulate vascular α₁-adrenergic receptor function

Chemokine (C-X-C motif) receptor (CXCR) 4 and atypical chemokine receptor (ACKR) 3 ligands have been reported to modulate cardiovascular function in various disease models. The underlying mechanisms, however, remain unknown. Thus, it was the aim of the present study to determine how pharmacological modulation of CXCR4 and ACKR3 regulate cardiovascular function. In vivo administration of TC14012, a CXCR4 antagonist and ACKR3 agonist, caused cardiovascular collapse in normal animals. During the cardiovascular stress response to hemorrhagic shock, ubiquitin, a CXCR4 agonist, stabilized blood pressure, whereas coactivation of CXCR4 and ACKR3 with CXC chemokine ligand 12 (CXCL12), or blockade of CXCR4 with AMD3100 showed opposite effects. While CXCR4 and ACKR3 ligands did not affect myocardial function, they selectively altered vascular reactivity upon α1-adrenergic receptor (AR) activation in pressure myography experiments. CXCR4 activation with ubiquitin enhanced α1-AR-mediated vasoconstriction, whereas ACKR3 activation with various natural and synthetic ligands antagonized α1-AR-mediated vasoconstriction. The opposing effects of CXCR4 and ACKR3 activation by CXCL12 could be dissected pharmacologically. CXCR4 and ACKR3 ligands did not affect vasoconstriction upon activation of voltage-operated Ca(2+) channels or endothelin receptors. Effects of CXCR4 and ACKR3 agonists on vascular α1-AR responsiveness were independent of the endothelium. These findings suggest that CXCR4 and ACKR3 modulate α1-AR reactivity in vascular smooth muscle and regulate hemodynamics in normal and pathological conditions. Our observations point toward CXCR4 and ACKR3 as new pharmacological targets to control vasoreactivity and blood pressure.

Arg389Gly β1-adrenergic receptor polymorphism and susceptibility to syncope during tilt test

INTRODUCTION

Numerous hormones, neurotransmitters, and other stimuli exert their biological effect on cellular functioning through heptahelical receptors coupled to G proteins (GPCR - G protein-coupled receptors). Adrenergic receptors that belong to this superfamily of receptors are components of the sympathetic nervous system. They play a pivotal role in blood pressure regulation and myocardial contractility. Alterations of the adrenergic receptor pathway have been suggested to be involved in the pathophysiology of vasovagal syncope (VVS). The aim of the present study was to evaluate the distribution of Arg389Gly polymorphism within the ADRB1 gene among patients with recurrent syncope.

MATERIAL AND METHODS

Arg389Gly single nucleotide polymorphism was analyzed in 205 patients with recurrent syncope. Ninety-five patients (46%) had a positive head-up tilt test (HUT) result. The control group comprised 143 non-fainting subjects. Genotyping was performed by restriction fragment length polymorphism (RFLP) with BstNI enzyme.

RESULTS

Both analyzed groups had similar distribution of the 389Gly allele. Sixty percent of polymorphic 389Gly carriers belong to the group of syncopal patients, while 40% belong to the control group of healthy subjects.

CONCLUSIONS

An association between syncopal incidence and Arg389Gly polymorphism within the ADRB1 gene was not found. The analyzed polymorphism affecting sympathetic activity does not influence vasovagal syncope in Polish patients.

Role of endogenous opioid peptides in the infarct size-limiting effect of adaptation to chronic continuous hypoxia

AIMS

The objective of this study was to examine the involvement of endogenous opioid peptides and opioid receptor (OR) subtypes in the cardioprotective effect of adaptation to chronic hypoxia in rats.

MAIN METHODS

Rats were exposed to continuous normobaric hypoxia (CNH; 12% oxygen) for 3 weeks. Myocardial ischemia was induced by 20-min coronary artery occlusion followed by 3-h reperfusion in anesthetized open-chest animals. Various OR antagonists were administered to rats prior to ischemia. The size of myocardial infarction and the incidence of ischemic ventricular arrhythmias were assessed. Myocardial and plasma concentrations of opioid peptides (met-enkephalin, β-endorphin, and endomorphins) were determined.

KEY FINDINGS

Adaptation to CNH significantly increased myocardial and plasma concentrations of opioids, potentiated their further elevation by ischemia/reperfusion, and reduced myocardial infarct size, but it did not affect the incidence of ischemic arrhythmias. The infarct size-limiting effect of CNH was abolished by OR antagonists naltrexone (non-selective), naloxone methiodide (non-selective peripherally acting), TIPP[ψ] (δ-OR), naltriben (δ2-OR), or CTAP (μ-OR), while BNTX (δ1-OR) and nor-binaltorphimine (κ-OR) had no effect.

SIGNIFICANCE

The results suggest that the infarct size-limiting effect afforded by adaptation to CNH is mediated by activation of peripheral δ2- and μ-ORs by elevated levels of endogenous opioid peptides.

β₁-adrenergic receptor up-regulation induced by nadolol is mediated via signal transduction pathway coupled to α₁-adrenergic receptors

Although up-regulation of β-adrenergic receptors (β-ARs) occurs after long-term use of their antagonists in various tissues, the available data are little on mechanisms of β-AR up-regulation induced by their continuous blockade. The present study attempted to clarify mechanisms of β-AR up-regulation using mouse cerebral cortical neurons continuously exposed to nadolol (10 nM), a non-selective β-AR antagonist, for 24 h. Nadolol dose-dependently induced both subtypes of β-ARs, β₁- and β₂-ARs, which were not suppressed by protein A kinase inhibition with KT5720. On the other hand, blockade of α₁-ARs, which are immunohistochemically confirmed to be co-localized with β-ARs in the same neurons, significantly inhibited only β₁-AR up-regulation and the expression of β₂-ARs did not alter. In addition, phenylephrine, an agonist specific to α₁-ARs up-regulated β₁-ARs, but not β₂-ARs. Under the conditions with β-AR up-regulation, the level of phosphorylated protein kinase Cα (pPKCα) increased, which is significantly suppressed by prazosin, an α1-AR antagonist. Furthermore, nadolol decreased the degradation of mRNA of β₁-ARs, but not β₂-ARs. These results indicate that the nadolol-induced β₁-AR up-regulation is mediated via PKC-relating pathway via α₁-AR activation with stabilizing β₁-AR mRNA and that the increased expression of β₂-ARs is regulated by pathways different from those for β₁-AR expression.

Functional crosstalk of prejunctional receptors on the modulation of noradrenaline release in mesenteric vessels: A differential study of artery and vein

The role of angiotensin II receptors, bradykinin receptors and β-adrenoceptors in the modulation of noradrenaline release and the influence of α(2)-autoinhibition in these effects was investigated in the mesenteric artery and vein. Rings of mesenteric vessels of male Wistar rats were labelled with [(3)H]-noradrenaline and the effects of modulators on tritium overflow evoked by 100 pulses at 2Hz (marked α(2)-autoinhibition) and by 20 pulses at 50Hz or 100 pulses at 2Hz plus yohimbine (1μM; reduced α(2)-autoinhibition) were evaluated. Angiotensin II and bradykinin enhanced noradrenaline release evoked by 100 pulses at 2Hz, in a concentration-dependent manner, in both vessels. These effects were attenuated under conditions of reduced α(2)-autoinhibition. The attenuation was partially reversed by activation of adenosine A(1) receptors in both vessels and by activation of P2Y receptors in the vein. Isoprenaline and the selective β(2)-adrenoceptor agonist formoterol enhanced tritium overflow independently of α(2)-autoinhibition in the vein. In the artery, the enhancement by formoterol was only observed under reduced α(2)-autoinhibition. Pharmacological characterization of the β-adrenoceptors indicated that in the artery the effect of isoprenaline was mediated by the β(1)-subtype under marked α(2)-autoinhibition and by the β(2)-subtype under reduced α(2)-autoinhibition whereas in the vein the effect was independent of α(2)-autoinhibition. The results indicate that α(2)-autoinhibition is a key determinant of the magnitude of facilitation caused by angiotensin II and bradykinin in both types of mesenteric vessels and regulates the effects mediated by β(1)-and β(2)-adrenoceptors which co-exist in the artery.

The antioxidant effect of angiotensin II receptor blocker, losartan, in streptozotocin-induced diabetic rats

We determined the effect of a short-term angiotensin II signaling blockade on vascular endothelial growth factor (VEGF), soluble intercellular adhesion molecule-1 (sICAM-1), nitric oxide (NO), and malondialdehyde (MDA) (index of lipid peroxidation) levels in the systemic circulation and on peroxynitrite generation and insulitis development in the streptozotocin (STZ) diabetic rats' pancreas. Diabetes was induced in Wistar rats by intraperitoneal STZ injection. Diabetic rats were treated for 1 week with losartan (20 mg/kg/body weight/day in the drinking water), and pancreas and blood were collected for histochemical, immunohistochemical, and biochemical studies. Diabetic rats showed greater VEGF, sICAM-1, NO, and MDA levels, a high score of insulitis, increased nitrotyrosine staining, and markedly reduced pancreatic insulin content when compared with controls. Losartan treatment suppressed the excessive NO and lipid peroxidation production systemically without restoring them to that of healthy subjects and reduced VEGF levels while leaving sICAM-1 levels unchanged. The insulitis score and nitrotyrosine staining were reduced, whereas the pancreatic islets and the beta-cell area were increased significantly in the treated group, indicating the reduction of inflammation and nitrosative stress and an early regeneration of beta-cell mass in the pancreas. Conclusively, in the STZ diabetic rat model, even a short-term losartan treatment improves oxidative and nitrosative stress systemically and locally, improving the islets' environment and accelerating beta-cell regeneration.

The relationship between birthweight and longitudinal changes of blood pressure is modulated by beta-adrenergic receptor genes: the Bogalusa Heart Study

This study examines the genetic influence of β-adrenergic receptor gene polymorphisms (β₂-AR Arg16Gly and β₃-AR Trp64Arg) on the relationship of birthweight to longitudinal changes of blood pressure (BP) from childhood to adulthood in 224 black and 515 white adults, aged 21–47 years, enrolled in the Bogalusa Heart Study. Blacks showed significantly lower birthweight and frequencies of β₂-AR Gly16 and β₃-AR Trp64 alleles and higher BP levels and age-related trends than whites. In multivariable regression analyses using race-adjusted BP and birthweight, low birthweight was associated with greater increase in age-related trend of systolic BP (standardized regression coefficient β = −0.09, P = .002) and diastolic BP (β = −0.07, P = .037) in the combined sample of blacks and whites, adjusting for the first BP measurement in childhood, sex, age, and gestational age. Adjustment for the current body mass index strengthened the birthweight-BP association. Importantly, the strength of the association, measured as regression coefficients, was modulated by the combination of β₂-AR and β₃-AR genotypes for systolic (P = .042 for interaction) and diastolic BP age-related trend (P = .039 for interaction), with blacks and whites showing a similar trend in the interaction. These findings indicate that the intrauterine programming of BP regulation later in life depends on β-AR genotypes.

Modelling of alpha1-adrenoceptor-mediated temporal dynamics of inotropic response in rat heart to assess ligand binding and signal transduction parameters

BACKGROUND AND PURPOSE

In order to use the transient response to an antagonist (prazosin) to evaluate properties of agonist interactions with the alpha(1)-adrenoceptor system, an integrative mechanistic model of cardiac uptake of prazosin and its competitive interaction with phenylephrine at the receptor site was developed. Based on the operational model of agonism, the aim was to evaluate both the receptor binding and signal transduction process as determinants of the inotropic effect of phenylephrine.

EXPERIMENTAL APPROACH

In Langendorff-perfused rat hearts, prazosin outflow concentration and left ventricular developed pressure were measured, first in the presence of 12.3 micromol x L(-1) phenylephrine following a 1 min infusion of 1.27 nmol [(3)H]-prazosin, and second, when after 30 min the phenylephrine concentration in perfusate was reduced to 6.1 micromol x L(-1), the 1 min infusion of 1.27 nmol [(3)H]-prazosin was repeated.

KEY RESULTS

The kinetic model accounted for cardiac uptake and receptor binding kinetics of prazosin (dissociation constant, mean +/- SD: 0.057 +/- 0.012 nmol.L(-1)), assuming that the competitive displacement of phenylephrine (dissociation constant: 101 +/- 13 nmol x L(-1)) reduced the receptor occupation by the agonist and, consequently, contractility. This competitive binding process appeared to be the rate-determining step in response generation. The relationship between receptor occupancy and inotropic response was described by an efficacy parameter (tau, ratio of receptor density and coupling efficiency) of 4.9.

CONCLUSIONS AND IMPLICATIONS

Mechanistic pharmacodynamic modelling of the kinetics of antagonism by prazosin allows quantitative assessment of the alpha(1)-adrenoceptor system both at the receptor and post-receptor levels.

Facilitatory interplay in alpha 1a and beta 2 adrenoceptor function reveals a non-Gq signaling mode: implications for diversification of intracellular signal transduction

Agonist occupied alpha(1)-adrenoceptors (alpha(1)-ARs) engage several signaling pathways, including phosphatidylinositol hydrolysis, calcium mobilization, arachidonic acid release, mitogen-activated protein (MAP) kinase activation, and cAMP accumulation. The natural agonist norepinephrine (NE) activates with variable affinity and intrinsic efficacy all adrenoceptors, and in cells that coexpress alpha(1)- and beta-AR subtypes, such as cardiomyocytes, this leads to coactivation of multiple downstream pathways. This may result in pathway cross-talk with significant consequences to heart physiology and pathologic state. To dissect signaling components involved specifically in alpha(1A)- and beta(2)-AR signal interplay, we have developed a recombinant model system that mimics the levels of receptor expression observed in native cells. We followed intracellular Ca(2+) mobilization to monitor in real time the activation of both G(q) and G(s) pathways. We found that coactivation of alpha(1A)- and beta(2)-AR by the nonselective agonist NE or via a combination of the highly selective alpha(1A)-AR agonist A61603 and the beta-selective agonist isoproterenol led to increases in Ca(2+) influx from the extracellular compartment relative to stimulation with A61603 alone, with no effect on the associated transient release of Ca(2+) from intracellular stores. This effect became more evident upon examination of an alpha(1A)-AR variant exhibiting a partial defect in coupling to G(q), and we attribute it to potentiation of a non G(q)-pathway, uncovered by application of a combination of xestospongin C, an endoplasmic reticulum inositol 1,4,5-triphosphate receptor blocker, and 2-aminoethoxydiphenyl borate, a nonselective storeoperated Ca(2+) entry channel blocker. We also found that stimulation with A61603 of a second alpha(1A)-AR variant entirely unable to signal induced no Ca(2+) unless beta(2)-AR was concomitantly activated. These results may be accounted for by the presence of alpha(1A)/beta(2)-AR heterodimers or alternatively by specific adrenoceptor signal cross-talk resulting in distinct pharmacological behavior. Finally, our findings provide a new conceptual framework to rationalize outcomes from clinical studies targeting alpha- and beta-adrenoceptors.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

Hypothalamic angiotensinergic–noradrenergic systems interaction in fructose induced hypertension

OBJECTIVE

Several studies suggest the importance of the interaction between the renin angiotensin and sympathetic nervous systems in blood pressure control, especially in clinical situations such as the metabolic syndrome. Previously, we have demonstrated changes in noradrenergic hypothalamic control of blood pressure in an animal model of insulin resistance and hypertension. The aim of the present study was to evaluate the effects of the interaction between the noradrenergic and angiotensinergic systems on hypothalamic blood pressure regulation in fructose hypertensive rats.

METHODS

In control (C) and fructose-fed hypertensive (F) rats, we studied: 1) the effects of hypothalamic perfusion of irbesartan (AT(1) angiotensin receptor antagonist, 50 and 500 microg ml(-1)) and metoprolol (beta(1) adrenergic receptor antagonist, 10 and 100 microg ml(-1)) on blood pressure, heart rate and noradrenaline intrahypothalamic levels, by means of the microdialysis technique; and 2) the effects of intrahypothalamic microinjection of angiotensin II alone or after metoprolol pre-administration, on blood pressure and heart rate.

RESULTS

Meanwhile irbesartan perfusion did not modify neither mean arterial pressure (MAP) nor heart rate or noradrenaline hypothalamic levels in the C group, its highest dose diminished MAP (DeltaMAP: F: - 16.3+/-1 mm Hg, p<0.05) and noradrenaline levels (% of basal levels: 58+/-7%, p<0.05) in the F group, without affecting heart rate. Intrahypothalamic perfusion of metoprolol diminished MAP only in the F group (DeltaMAP: F: -12.1+/-1.1 mm Hg, p<0.05), but did not modify heart rate in both groups. On the other hand, it diminished noradrenaline hypothalamic levels in C (% of basal levels: 53+/-6%, p<0.05) but not in the F group. The pressor response to angiotensin II microinjection was increased in F rats (DeltaMAP: F: 13.3+/-1.5 mm Hg vs. C: 6.9+/-1.8 mm Hg; p<0.05). Previous administration of metoprolol markedly abolished this increment.

CONCLUSIONS

Our results suggest the existence of an increase in AT(1) and beta(1) adrenergic receptors tone in the hypothalamus of F rats, which could be related to the increase in blood pressure present in this experimental model. On the other hand, considering that the enhanced pressor response to angiotensin II intrahypothalamic injection in F rats was abolished by previous administration of a beta(1) adrenergic receptor antagonist, these results would indicate that beta(1) adrenergic receptors activation participates in the pressor response to angiotensin II in this experimental model of insulin resistance and hypertension.

G-protein coupled receptor signaling in myocardium: not for the faint of heart

Catecholamines, endothelin-1 and angiotensin II are among a diverse group of diffusible extracellular signals that regulate pump function of the heart by binding to G-protein coupled receptors (GPCR). When the body demands a temporary boost of power output or if temporary budgeting of resources is required, these signals can adjust heart rate and contractile strength to maintain continuous perfusion of all vascular beds with nutrient- and oxygen-rich blood. Given adequate time in the face of prolonged challenges, activation of GPCRs can also promote "remodeling of the heart" by increasing cell size, organ size, and chamber dimensions, or by varying tissue composition and altering the expression of protein isoforms controlling excitability and contractility. A common feature of heart disease is the state of chronic activation of GPCR signaling systems. Paradoxically, whereas acute activation is beneficial, chronic activation often contributes to further deterioration of cardiac performance. A better understanding of how chronic GPCR activation contributes to the development of heart disease is needed so that it can be translated into better prevention and therapeutic strategies in the clinic.

Synergistic activation of salmon cardiac function by endothelin and beta-adrenergic stimulation

The aim was to find out the effects of endothelin-1 (ET-1) in salmon (Salmo salar) cardiac contractile and endocrine function and its possible interaction with beta-adrenergic regulation. We found that ET-1 has a positive inotropic effect in salmon heart. ET-1 (30 nM) increased the contraction amplitude 17+/-4.7% compared with the basal level. beta-Adrenergic activation (isoprenaline, 100 nM) increased contraction amplitude 30+/-13.1%, but it did not affect the contractile response to ET-1. ET-1 (10 nM) stimulated the secretion of salmon cardiac natriuretic peptide (sCP) from isolated salmon ventricle (3.3+/-0.14-fold compared with control) but did not have any effect on ventricular sCP mRNA. Isoprenaline alone (0.1-1,000 nM) did not stimulate sCP release, but ET-1 (10 nM) together with isoprenaline (0.1 nM) caused a significantly greater increase of sCP release than ET-1 alone (5.4+/-0.07 vs. 3.3+/-0.14 times increase compared with control). The effects on the contractile and secretory function could be inhibited by a selective ETA-receptor antagonist BQ-610 (1 microM), whereas ETB-receptor blockage (by 100 nM BQ-788) enhanced the secretory response. Thus ET-1 is a phylogenetically conserved regulator of cardiac function, which has synergistic action with beta-adrenergic stimulation. The modulatory effects of ET-1 may therefore be especially important in situations with high beta-adrenergic tone.

A comprehensive pathway map of epidermal growth factor receptor signaling

The epidermal growth factor receptor (EGFR) signaling pathway is one of the most important pathways that regulate growth, survival, proliferation, and differentiation in mammalian cells. Reflecting this importance, it is one of the best-investigated signaling systems, both experimentally and computationally, and several computational models have been developed for dynamic analysis. A map of molecular interactions of the EGFR signaling system is a valuable resource for research in this area. In this paper, we present a comprehensive pathway map of EGFR signaling and other related pathways. The map reveals that the overall architecture of the pathway is a bow-tie (or hourglass) structure with several feedback loops. The map is created using CellDesigner software that enables us to graphically represent interactions using a well-defined and consistent graphical notation, and to store it in Systems Biology Markup Language (SBML).

Heterodimerization with β2-Adrenergic Receptors Promotes Surface Expression and Functional Activity of α1D-Adrenergic Receptors

The alpha1D-adrenergic receptor (alpha1D-AR) is a G protein-coupled receptor (GPCR) that is poorly trafficked to the cell surface and largely nonfunctional when heterologously expressed by itself in a variety of cell types. We screened a library of approximately 30 other group I GPCRs in a quantitative luminometer assay for the ability to promote alpha1D-AR cell surface expression. Strikingly, these screens revealed only two receptors capable of inducing robust increases in the amount of alpha1D-AR at the cell surface: alpha1B-AR and beta2-AR. Confocal imaging confirmed that coexpression with beta2-AR resulted in translocation of alpha1D-AR from intracellular sites to the plasma membrane. Additionally, coimmunoprecipitation studies demonstrated that alpha1D-AR and beta2-AR specifically interact to form heterodimers when coexpressed in HEK-293 cells. Ligand binding studies revealed an increase in total alpha1D-AR binding sites upon coexpression with beta2-AR, but no apparent effect on the pharmacological properties of the receptors. In functional studies, coexpression with beta2-AR significantly enhanced the coupling of alpha1D-AR to norepinephrine-stimulated Ca2+ mobilization. Heterodimerization of beta2-AR with alpha1D-AR also conferred the ability of alpha1D-AR to cointernalize upon beta2-AR agonist stimulation, revealing a novel mechanism by which these different adrenergic receptor subtypes may regulate each other's activity. These findings demonstrate that the selective association of alpha1D-AR with other receptors is crucial for receptor surface expression and function and also shed light on a novel mechanism of cross talk between alpha1- and beta2-ARs that is mediated through heterodimerization and cross-internalization.

Covalent and noncovalent modification of thin filament action: the essential role of troponin in cardiac muscle regulation

Troponin is essential for the regulation of cardiac contraction. Troponin is a sarcomeric molecular switch, directly regulating the contractile event in concert with intracellular calcium signals. Troponin isoform switching, missense mutations, proteolytic cleavage, and posttranslational modifications are known to directly affect sarcomeric regulation. This review focuses on physiologically relevant covalent and noncovalent modifications in troponin as part of a thematic series on cardiac thin filament function in health and disease.

Cross Talk between Angiotensin II and Alpha 1 Adrenergic Receptors in Rabbit Aorta: Role of Endothelium

Interaction between the renin-angiotensin system and the sympathetic nervous system has been proposed to be like a physiological regulation mechanism. The present work was designed to study the cross talk between angiotensin II and adrenergic receptors on the smooth muscle contractile response and the endothelium influence in this phenomenon. Homologous and endothelium independent desensitization of angiotensin II-contractile response was observed. Treatment with noradrenaline between two cumulative doses response curves (CDRC) to angiotensin II caused a rightward shift of the second CDRC in unrubbed arteries and increased the maximal response in rubbed arteries. Prazosin blocked these effects. No homologous desensitization of noradrenaline contractile response was found. Treatment with angiotensin II between two CDRC to noradrenaline caused a loss of affinity in the second CDRC in unrubbed arteries. Losartan was able to avoid this phenomenon. Maximal response was enhanced both in arteries with and without endothelium treated or not with angiotensin II. Results demonstrate homologous and endothelium-independent desensitization of the contractile response to angiotensin II but not to noradrenaline. In addition, heterologous and endothelium-dependent desensitization induced by noradrenaline and angiotensin II on the contractile response to each other was found. Furthermore, results provided the first evidence that there is an endothelium-dependent cross talk between alpha1-adrenergic and angiotensin II receptors in smooth muscle of rabbit aorta.

G Protein–Coupled Receptor Oligomerization

The cardiovascular system is richly endowed with G protein–coupled receptors (GPCRs), members of the largest family of plasma membrane-localized receptors. During the last 10 years, it has become increasingly clear that many, if not all, GPCRs function in oligomeric complexes, as either homo- or hetero-oligomers. This review explores the mechanistic implications of GPCR dimerization and/or oligomerization on receptor activation and interactions with G proteins. The effects of GPCR oligomerization on receptor pharmacology, GPCR-mediated signaling, and potential contributions to GPCR crosstalk will be considered in the context of receptors important in the cardiovascular system. Our evolving understanding of the structural and functional consequences of GPCR oligomerization may provide novel and more selective sites for pharmacological tuning of cardiovascular function.