Cloning of the rat alpha 1C-adrenergic receptor from cardiac myocytes. alpha 1C, alpha 1B, and alpha 1D mRNAs are present in cardiac myocytes but not in cardiac fibroblasts

The alpha-1A adrenergic receptor regulates mitochondrial oxidative metabolism in the mouse heart

AIMS

The sympathetic nervous system regulates numerous critical aspects of mitochondrial function in the heart through activation of adrenergic receptors (ARs) on cardiomyocytes. Mounting evidence suggests that α1-ARs, particularly the α1A subtype, are cardioprotective and may mitigate the deleterious effects of chronic β-AR activation by shared ligands. The mechanisms underlying these adaptive effects remain unclear. Here, we tested the hypothesis that α1A-ARs adaptively regulate cardiomyocyte oxidative metabolism in both the uninjured and infarcted heart.

METHODS

We used high resolution respirometry, fatty acid oxidation (FAO) enzyme assays, substrate-specific electron transport chain (ETC) enzyme assays, transmission electron microscopy (TEM) and proteomics to characterize mitochondrial function comprehensively in the uninjured hearts of wild type and α1A-AR knockout mice and defined the effects of chronic β-AR activation and myocardial infarction on selected mitochondrial functions.

RESULTS

We found that isolated cardiac mitochondria from α1A-KO mice had deficits in fatty acid-dependent respiration, FAO, and ETC enzyme activity. TEM revealed abnormalities of mitochondrial morphology characteristic of these functional deficits. The selective α1A-AR agonist A61603 enhanced fatty-acid dependent respiration, fatty acid oxidation, and ETC enzyme activity in isolated cardiac mitochondria. The β-AR agonist isoproterenol enhanced oxidative stress in vitro and this adverse effect was mitigated by A61603. A61603 enhanced ETC Complex I activity and protected contractile function following myocardial infarction.

CONCLUSIONS

Collectively, these novel findings position α1A-ARs as critical regulators of cardiomyocyte metabolism in the basal state and suggest that metabolic mechanisms may underlie the protective effects of α1A-AR activation in the failing heart.

G-Protein-Coupled Receptors in Heart Disease

GPCRs (G-protein [guanine nucleotide-binding protein]-coupled receptors) play a central physiological role in the regulation of cardiac function in both health and disease and thus represent one of the largest class of surface receptors targeted by drugs. Several antagonists of GPCRs, such as βARs (β-adrenergic receptors) and Ang II (angiotensin II) receptors, are now considered standard of therapy for a wide range of cardiovascular disease, such as hypertension, coronary artery disease, and heart failure. Although the mechanism of action for GPCRs was thought to be largely worked out in the 80s and 90s, recent discoveries have brought to the fore new and previously unappreciated mechanisms for GPCR activation and subsequent downstream signaling. In this review, we focus on GPCRs most relevant to the cardiovascular system and discuss traditional components of GPCR signaling and highlight evolving concepts in the field, such as ligand bias, β-arrestin-mediated signaling, and conformational heterogeneity.

A high-throughput ratiometric method for imaging hypertrophic growth in cultured primary cardiac myocytes

Maladaptive hypertrophy of cardiac myocytes increases the risk of heart failure. The underlying signaling can be triggered and interrogated in cultured neonatal ventricular myocytes (NRVMs) using sophisticated pharmacological and genetic techniques. However, the methods for quantifying cell growth are, by comparison, inadequate. The lack of quantitative, calibratable and computationally-inexpensive high-throughput technology has limited the scope for using cultured myocytes in large-scale analyses. We present a ratiometric method for quantifying the hypertrophic growth of cultured myocytes, compatible with high-throughput imaging platforms. Protein biomass was assayed from sulforhodamine B (SRB) fluorescence, and image analysis calculated the quotient of signal from extra-nuclear and nuclear regions. The former readout relates to hypertrophic growth, whereas the latter is a reference for correcting protein-independent (e.g. equipment-related) variables. This ratiometric measure, when normalized to the number of cells, provides a robust quantification of cellular hypertrophy. The method was tested by comparing the efficacy of various chemical agonists to evoke hypertrophy, and verified using independent assays (myocyte area, transcripts of markers). The method's high resolving power and wide dynamic range were confirmed by the ability to generate concentration-response curves, track the time-course of hypertrophic responses with fine temporal resolution, describe drug/agonist interactions, and screen for novel anti-hypertrophic agents. The method can be implemented as an end-point in protocols investigating hypertrophy, and is compatible with automated plate-reader platforms for generating high-throughput data, thereby reducing investigator-bias. Finally, the computationally-minimal workflow required for obtaining measurements makes the method simple to implement in most laboratories.

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Maladaptive hypertrophy of myocytes can lead to heart failure.

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Common methods for tracking growth in cultured myocytes are inadequate.

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We design and test a method for tracking myocyte hypertrophy in vitro.

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The method provides a ratiometric index of growth for high throughput analyses.

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Using the method, we characterize further details of (anti)hypertrophic responses.

Adrenergic Receptors in Individual Ventricular Myocytes: The Beta-1 and Alpha-1B Are in All Cells, the Alpha-1A Is in a Subpopulation, and the Beta-2 and Beta-3 Are Mostly Absent

RATIONALE

It is unknown whether every ventricular myocyte expresses all 5 of the cardiac adrenergic receptors (ARs), β1, β2, β3, α1A, and α1B. The β1 and β2 are thought to be the dominant myocyte ARs.

OBJECTIVE

Quantify the 5 cardiac ARs in individual ventricular myocytes.

METHODS AND RESULTS

We studied ventricular myocytes from wild-type mice, mice with α1A and α1B knockin reporters, and β1 and β2 knockout mice. Using individual isolated cells, we measured knockin reporters, mRNAs, signaling (phosphorylation of extracellular signal-regulated kinase and phospholamban), and contraction. We found that the β1 and α1B were present in all myocytes. The α1A was present in 60%, with high levels in 20%. The β2 and β3 were detected in only ≈5% of myocytes, mostly in different cells. In intact heart, 30% of total β-ARs were β2 and 20% were β3, both mainly in nonmyocytes.

CONCLUSION

The dominant ventricular myocyte ARs present in all cells are the β1 and α1B. The β2 and β3 are mostly absent in myocytes but are abundant in nonmyocytes. The α1A is in just over half of cells, but only 20% have high levels. Four distinct myocyte AR phenotypes are defined: 30% of cells with β1 and α1B only; 60% that also have the α1A; and 5% each that also have the β2 or β3. The results raise cautions in experimental design, such as receptor overexpression in myocytes that do not express the AR normally. The data suggest new paradigms in cardiac adrenergic signaling mechanisms.

An Oral Selective Alpha-1A Adrenergic Receptor Agonist Prevents Doxorubicin Cardiotoxicity

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There are 2 α1-ARs on cardiac myocytes: α1A and α1B. α1A adrenergic receptors serve important cardioprotective roles and do not mediate cardiac hypertrophy.

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Dabuzalgron, an oral α1A-AR agonist developed for the treatment of urinary incontinence and tolerated well in Phase 2 clinical trials, protects against doxorubicin-induced cardiotoxicity in vivo. Dabuzalgron enhances contractile function, regulates transcription of genes related to energy production and mitochondrial function, and preserves myocardial ATP content after doxorubicin.

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Activation of α1A-ARs on cardiomyocytes protects against doxorubicin cytotoxicity and enhances mitochondrial function in vitro. These cytoprotective effects likely are mediated by activation of ERK 1/2.

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Future studies will explore whether dabuzalgron, a well-tolerated oral α1A-AR agonist, might be repurposed to treat heart failure.

Alpha-1 adrenergic receptors (α1-ARs) play adaptive and protective roles in the heart. Dabuzalgron is an oral selective α1A-AR agonist that was well tolerated in multiple clinical trials of treatment for urinary incontinence, but has never been used to treat heart disease in humans or animal models. In this study, the authors administered dabuzalgron to mice treated with doxorubicin (DOX), a widely used chemotherapeutic agent with dose-limiting cardiotoxicity that can lead to heart failure (HF). Dabuzalgron protected against DOX-induced cardiotoxicity, likely by preserving mitochondrial function. These results suggest that activating cardiac α1A-ARs with dabuzalgron, a well-tolerated oral agent, might represent a novel approach to treating HF.

Nuclear compartmentalization of α1-adrenergic receptor signaling in adult cardiac myocytes

Although convention dictates that G protein-coupled receptors localize to and signal at the plasma membrane, accumulating evidence suggests that G protein-coupled receptors localize to and signal at intracellular membranes, most notably the nucleus. In fact, there is now significant evidence indicating that endogenous alpha-1 adrenergic receptors (α1-ARs) localize to and signal at the nuclei in adult cardiac myocytes. Cumulatively, the data suggest that α1-ARs localize to the inner nuclear membrane, activate intranuclear signaling, and regulate physiologic function in adult cardiac myocytes. Although α1-ARs signal through Gαq, unlike other Gq-coupled receptors, α1-ARs mediate important cardioprotective functions including adaptive/physiologic hypertrophy, protection from cell death (survival signaling), positive inotropy, and preconditioning. Also unlike other Gq-coupled receptors, most, if not all, functional α1-ARs localize to the nuclei in adult cardiac myocytes, as opposed to the sarcolemma. Together, α1-AR nuclear localization and cardioprotection might suggest a novel model for compartmentalization of Gq-coupled receptor signaling in which nuclear Gq-coupled receptor signaling is cardioprotective.

The alpha1-adrenergic receptors in cardiac hypertrophy: signaling mechanisms and functional implications

Cardiac hypertrophy is a complex remodeling process of the heart induced by physiological or pathological stimuli resulting in increased cardiomyocyte size and myocardial mass. Whereas cardiac hypertrophy can be an adaptive mechanism to stressful conditions of the heart, prolonged hypertrophy can lead to heart failure which represents the primary cause of human morbidity and mortality. Among G protein-coupled receptors, the α1-adrenergic receptors (α1-ARs) play an important role in the development of cardiac hypertrophy as demonstrated by numerous studies in the past decades, both in primary cardiomyocyte cultures and genetically modified mice. The results of these studies have provided evidence of a large variety of α1-AR-induced signaling events contributing to the defining molecular and cellular features of cardiac hypertrophy. Recently, novel signaling mechanisms have been identified and new hypotheses have emerged concerning the functional role of the α1-adrenergic receptors in the heart. This review will summarize the main signaling pathways activated by the α1-AR in the heart and their functional implications in cardiac hypertrophy.

Alpha-1-adrenergic receptors in heart failure: the adaptive arm of the cardiac response to chronic catecholamine stimulation

Alpha-1-adrenergic receptors (ARs) are G protein-coupled receptors activated by catecholamines. The alpha-1A and alpha-1B subtypes are expressed in mouse and human myocardium, whereas the alpha-1D protein is found only in coronary arteries. There are far fewer alpha-1-ARs than beta-ARs in the nonfailing heart, but their abundance is maintained or increased in the setting of heart failure, which is characterized by pronounced chronic elevation of catecholamines and beta-AR dysfunction. Decades of evidence from gain and loss-of-function studies in isolated cardiac myocytes and numerous animal models demonstrate important adaptive functions for cardiac alpha-1-ARs to include physiological hypertrophy, positive inotropy, ischemic preconditioning, and protection from cell death. Clinical trial data indicate that blocking alpha-1-ARs is associated with incident heart failure in patients with hypertension. Collectively, these findings suggest that alpha-1-AR activation might mitigate the well-recognized toxic effects of beta-ARs in the hyperadrenergic setting of chronic heart failure. Thus, exogenous cardioselective activation of alpha-1-ARs might represent a novel and viable approach to the treatment of heart failure.

Aberrant α-adrenergic hypertrophic response in cardiomyocytes from human induced pluripotent cells

Cardiomyocytes from human embryonic stem cells (hESC-CMs) and induced pluripotent stem cells (hiPSC-CMs) represent new models for drug discovery. Although hypertrophy is a high-priority target, we found that hiPSC-CMs were systematically unresponsive to hypertrophic signals such as the α-adrenoceptor (αAR) agonist phenylephrine (PE) compared to hESC-CMs. We investigated signaling at multiple levels to understand the underlying mechanism of this differential responsiveness. The expression of the normal α1AR gene, ADRA1A, was reversibly silenced during differentiation, accompanied by ADRA1B upregulation in either cell type. ADRA1B signaling was intact in hESC-CMs, but not in hiPSC-CMs. We observed an increased tonic activity of inhibitory kinase pathways in hiPSC-CMs, and inhibition of antihypertrophic kinases revealed hypertrophic increases. There is tonic suppression of cell growth in hiPSC-CMs, but not hESC-CMs, limiting their use in investigation of hypertrophic signaling. These data raise questions regarding the hiPSC-CM as a valid model for certain aspects of cardiac disease.

Cardiac alpha1-adrenergic receptors: novel aspects of expression, signaling mechanisms, physiologic function, and clinical importance

Adrenergic receptors (AR) are G-protein-coupled receptors (GPCRs) that have a crucial role in cardiac physiology in health and disease. Alpha1-ARs signal through Gαq, and signaling through Gq, for example, by endothelin and angiotensin receptors, is thought to be detrimental to the heart. In contrast, cardiac alpha1-ARs mediate important protective and adaptive functions in the heart, although alpha1-ARs are only a minor fraction of total cardiac ARs. Cardiac alpha1-ARs activate pleiotropic downstream signaling to prevent pathologic remodeling in heart failure. Mechanisms defined in animal and cell models include activation of adaptive hypertrophy, prevention of cardiac myocyte death, augmentation of contractility, and induction of ischemic preconditioning. Surprisingly, at the molecular level, alpha1-ARs localize to and signal at the nucleus in cardiac myocytes, and, unlike most GPCRs, activate "inside-out" signaling to cause cardioprotection. Contrary to past opinion, human cardiac alpha1-AR expression is similar to that in the mouse, where alpha1-AR effects are seen most convincingly in knockout models. Human clinical studies show that alpha1-blockade worsens heart failure in hypertension and does not improve outcomes in heart failure, implying a cardioprotective role for human alpha1-ARs. In summary, these findings identify novel functional and mechanistic aspects of cardiac alpha1-AR function and suggest that activation of cardiac alpha1-AR might be a viable therapeutic strategy in heart failure.

Alpha-1 adrenoceptor-angiotensin II type 1 receptor cross-talk and its relevance in clinical medicine

The two most relevant clinical trials investigating the efficacy of multiple neurohormonal drug combinations in the treatment of chronic congestive heart failure are the Valsartan Heart Failure Trial and the Candesartan in Heart Failure Assessment of Reduction in Mortality and Morbidity-added studies. The Valsartan Heart Failure Trial study randomized patients with congestive heart failure to the angiotensin receptor blocker (ARB) valsartan versus placebo, in addition to baseline angiotensin-converting enzyme inhibitor (ACE-I) therapy. Overall, valsartan was found to significantly reduce the combined morbidity and mortality end point compared with placebo, mainly due to a reduction in heart failure admissions. However, a subgroup analysis showed that patients receiving triple therapy with valsartan, an ACE-I and a β-adrenoceptor blocker, appeared to do worse. These findings led to speculation that "triple therapy" with ARB, ACE-I, and nonselective β-blocker might be harmful, possibly due to excessive neurohormonal inhibition. In contrast, in the Candesartan in Heart Failure Assessment of Reduction in Mortality and Morbidity-added study, the "triple therapy" combination of ARB, ACE-I, and β-adrenoceptor blocker was proven safe and beneficial. We propose that the discrepancy in outcomes observed in these two trials is related to the interaction between the α1-adrenoceptor and the angiotensin II type-1 receptor, and it is not just an inherent adverse event related to "triple therapy."

Alpha-1-adrenergic receptors: targets for agonist drugs to treat heart failure

Evidence from cell, animal, and human studies demonstrates that α1-adrenergic receptors mediate adaptive and protective effects in the heart. These effects may be particularly important in chronic heart failure, when catecholamine levels are elevated and β-adrenergic receptors are down-regulated and dysfunctional. This review summarizes these data and proposes that selectively activating α1-adrenergic receptors in the heart might represent a novel and effective way to treat heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Regeneration of cardiomyocytes from bone marrow: Use of mesenchymal stem cell for cardiovascular tissue engineering

We have isolated a cardiomyogenic cell line (CMG cell) from murine bone marrow mesenchymal stem cells. The cells showed a fibroblast-like morphology, but the morphology changed after 5-azacytidine exposure. They began spontaneous beating after 2 weeks, and expressed ANP and BNP. Electron microscopy revealed a cardiomyocyte-like ultrastructure. These cells had several types of action potentials; sinus node-like and ventricular cell-like action potentials. The isoform of contractile protein genes indicated that their muscle phenotype was similar to fetal ventricular cardiomyocytes. They expressed alpha(1A), alpha(1B), alpha(1D), beta(1), and beta(2) adrenergic and M(1) and M(2) muscarinic receptors. Stimulation with phenylephrine, isoproterenol and carbachol increased ERK phosphorylation and second messengers. Isoproterenol increased the beating rate, which was blocked with CGP20712A (beta(1)-selective blocker). These findings indicated that cell transplantation therapy for the patients with heart failure might possibly be achieved using the regenerated cardiomyocytes from autologous bone marrow cells in the near future.

Bone marrow-derived cells contribute to fibrosis in the chronically failing heart

Cardiac fibrosis contributes significantly to the phenotype of the chronically failing heart. It is not clear whether in this setting the fibrosis is contributed by native cardiac fibroblasts or alternatively by recruitment of cells arising from the bone marrow. We aimed to determine the contribution of bone marrow-derived cells to cardiac fibrosis in the failing heart and to investigate potentially contributing cytokines. Bone marrow-derived fibrocyte recruitment to the failing heart was studied in a transgenic (Mst1 mice) model of dilated cardiomyopathy. In conjunction, we examined the role of stromal-derived factor-1 (SDF-1), a key chemoattractant, by assessing myocardial expression and secretion by cardiomyocytes and in clinical samples. Bone marrow-derived cells were recruited in significantly greater numbers in Mst1 versus control mice (P < 0.001), contributing 17 +/- 4% of the total fibroblast load in heart failure. Patients with heart failure had higher plasma levels of SDF-1 than healthy control subjects (P < 0.01). We found that cardiomyocytes constitutively secrete SDF-1, which is significantly up-regulated by angiotensin II. SDF-1 was shown to increases cardiac fibroblast migration by 59% (P < 0.05). Taken together, our data suggest that recruitment of bone marrow-derived cells under the influence of factors, including SDF-1, may play an important role in the pathogenesis of cardiac fibrosis in heart failure.

The alpha-1D Is the predominant alpha-1-adrenergic receptor subtype in human epicardial coronary arteries

OBJECTIVES

The goal was to identify alpha-1-adrenergic receptor (AR) subtypes in human coronary arteries.

BACKGROUND

The alpha1-ARs regulate human coronary blood flow. The alpha1-ARs exist as 3 molecular subtypes, alpha1A, alpha1B, and alpha1D, and the alpha1D subtype mediates coronary vasoconstriction in the mouse. However, the alpha1A is thought to be the only subtype in human coronary arteries.

METHODS

We obtained human epicardial coronary arteries and left ventricular (LV) myocardium from 19 transplant recipients and 6 unused donors (age 19 to 70 years; 68% male; 32% with coronary artery disease). We cultured coronary rings and human coronary smooth muscle cells. We assayed alpha1- and beta-AR subtype messenger ribonucleic acid (mRNA) by quantitative real-time reverse transcription polymerase chain reaction and subtype proteins by radioligand binding and extracellular signal-regulated kinase (ERK) activation.

RESULTS

The alpha1D subtype was 85% of total coronary alpha1-AR mRNA and 75% of total alpha1-AR protein, and alpha1D stimulation activated ERK. In contrast, the alpha1D was low in LV myocardium. Total coronary alpha1-AR levels were one-third of beta-ARs, which were 99% the beta2 subtype.

CONCLUSIONS

The alpha1D subtype is predominant and functional in human epicardial coronary arteries, whereas the alpha1A and alpha1B are present at very low levels. This distribution is similar to the mouse, where myocardial alpha1A- and alpha1B-ARs mediate beneficial functional responses and coronary alpha1Ds mediate vasoconstriction. Thus, alpha1D-selective antagonists might mediate coronary vasodilation, without the negative cardiac effects of nonselective alpha1-AR antagonists in current use. Furthermore, it could be possible to selectively activate beneficial myocardial alpha1A- and/or alpha1B-AR signaling without causing coronary vasoconstriction.

Light and electron microscopic localization of alpha-1 adrenergic receptor immunoreactivity in the rat striatum and ventral midbrain

Electrophysiological and pharmacological studies have demonstrated that alpha-1 adrenergic receptor (alpha1AR) activation facilitates dopamine (DA) transmission in the striatum and ventral midbrain. However, because little is known about the localization of alpha1ARs in dopaminergic regions, the substrate(s) and mechanism(s) underlying this facilitation of DA signaling are poorly understood. To address this issue, we used light and electron microscopy immunoperoxidase labeling to examine the cellular and ultrastructural distribution of alpha1ARs in the caudate putamen, nucleus accumbens, ventral tegmental area, and substantia nigra in the rat. Analysis at the light microscopic level revealed alpha1AR immunoreactivity mainly in neuropil, with occasional staining in cell bodies. At the electron microscopic level, alpha1AR immunoreactivity was found primarily in presynaptic elements, with scarce postsynaptic labeling. Unmyelinated axons and about 30-50% terminals forming asymmetric synapses contained the majority of presynaptic labeling in the striatum and midbrain, while in the midbrain a subset of terminals forming symmetric synapses also displayed immunoreactivity. Postsynaptic labeling was scarce in both striatal and ventral midbrain regions. On the other hand, only 3-6% of spines displayed alpha1AR immunoreactivity in the caudate putamen and nucleus accumbens. These data suggest that the facilitation of dopaminergic transmission by alpha1ARs in the mesostriatal system is probably achieved primarily by pre-synaptic regulation of glutamate and GABA release.

Stromal cell derived factor-1 alpha confers protection against myocardial ischemia/reperfusion injury: role of the cardiac stromal cell derived factor-1 alpha CXCR4 axis

BACKGROUND

Stromal cell-derived factor-1alpha (SDF-1alpha) binding to its cognate receptor, CXCR4, regulates a variety of cellular functions such as stem cell homing, trafficking, and differentiation. However, the role of the SDF-1alpha-CXCR4 axis in modulating myocardial ischemia/reperfusion injury is unknown.

METHODS AND RESULTS

In mice subjected to ischemic preconditioning, myocardial SDF-1alpha mRNA was found to be increased 3 hours later (P<0.05). Myocardial SDF-1alpha and CXCR4 mRNA and protein were found to be expressed in both cardiac myocytes and fibroblasts. SDF-1alpha production increased significantly after 1 or 4 hours of hypoxia and 18 hours of reoxygenation in cultured myocytes (P<0.05) but did not change in fibroblast cultures. In isolated myocytes, CXCR4 activation by SDF-1alpha resulted in increased phosphorylation of both ERK 1/2 and AKT and decreased phosphorylation of JNK and p38. Cultured myocytes pretreated with SDF-1alpha were resistant to hypoxia/reoxygenation damage, exhibiting less lactate dehydrogenase release, trypan blue uptake, and apoptotic cell death (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay) (P<0.05). This protective effect was blocked by the CXCR4 selective antagonist AMD3100. In vivo, administration of SDF-1alpha before 30 minutes of coronary occlusion followed by 4 hours of reperfusion decreased infarct size (P<0.05). The decrease in infarct size with SDF-1alpha administration also was blocked by AMD3100.

CONCLUSIONS

We conclude that SDF-1alpha and its receptor, CXCR4, constitute a paracrine or autocrine axis in cardiac myocytes that is activated in response to preconditioning and hypoxic stimuli, recruiting the antiapoptotic kinases ERK and AKT and promoting an antiapoptotic program that confers protection against ischemia/reperfusion damage.

Receptor subtype involved in alpha 1-adrenergic receptor-mediated Ca2+ signaling in cardiomyocytes

AIM

The enhancement of intracellular Ca2+ signaling in response to alpha 1-adrenergic receptor (alpha 1-AR) stimulation is an essential signal transduction event in the regulation of cardiac functions, such as cardiac growth, cardiac contraction, and cardiac adaptation to various situations. The present study was intended to determine the role(s) of the alpha 1-AR subtype(s) in mediating this response.

METHODS

We evaluated the effects of subtype-specific agonists and antagonists of the alpha 1- AR on the intracellular Ca2+ signaling of neonatal rat ventricular myocytes using a confocal microscope.

RESULTS

After being cultured for 48 h, the myocytes exhibited spontaneous local Ca2+ release, sparks, and global Ca2+ transients. The activation of the alpha 1-AR with phenylephrine, a selective agonist of the alpha 1-AR, dose-dependently increased the frequency of Ca2+ transients with an EC50 value of 2.3 micromol/L. Blocking the alpha 1A-AR subtype with 5-methylurapidil (5-Mu) inhibited the stimulatory effect of phenylephrine with an IC(50) value of 6.7 nmol/L. In contrast, blockade of the alpha 1B-AR and alpha 1D-AR subtypes with chloroethylclonidine and BMY 7378, respectively, did not affect the phenylephrine effect. Similarly, the local Ca2+ spark numbers were also increased by the activation of the alpha 1-AR, and this effect could be abolished selectively by 5-Mu. More importantly, A61603, a novel selective alpha 1A-AR agonist, mimicked the effects of phenylephrine, but with more potency (EC(50) value =6.9 nmol/L) in the potentiation of Ca2+ transients, and blockade of the alpha 1A-AR by 5-Mu caused abolishment of its effects.

CONCLUSION

These results indicate that alpha 1-adrenergic stimulation of intracellular Ca2+ activity is mediated selectively by the alpha 1A-AR.

Epigenetic regulation of human alpha1d-adrenergic receptor gene expression: a role for DNA methylation in Sp1-dependent regulation

A growing body of evidence implicates alpha1-adrenergic receptors (alpha1ARs) as potent regulators of growth pathways. The three alpha1AR subtypes (alpha1aAR, alpha1bAR, alpha1dAR) display highly restricted tissue expression that undergoes subtype switching with many pathological stimuli, the mechanistic basis of which remains unknown. To gain insight into transcriptional pathways governing cell-specific regulation of the human alpha1dAR subtype, we cloned and characterized the alpha1dAR promoter region in two human cellular models that display disparate levels of endogenous alpha1dAR expression (SK-N-MC and DU145). Results reveal that alpha1dAR basal expression is regulated by Sp1-dependent binding of two promoter-proximal GC boxes, the mutation of which attenuates alpha1dAR promoter activity 10-fold. Mechanistically, chromatin immunoprecipitation data demonstrate that Sp1 binding correlates with expression of the endogenous gene in vivo, correlating highly with alpha1dAR promoter methylation-dependent silencing of both episomally expressed reporter constructs and the endogenous gene. Further, analysis of methylation status of proximal GC boxes using sodium bisulfite sequencing reveals differential methylation of proximal GC boxes in the two cell lines examined. Together, the data support a mechanism of methylation-dependent disruption of Sp1 binding in a cell-specific manner resulting in repression of basal alpha1dAR expression.

Adapter molecule DOC-2 is differentially expressed in pressure and volume overload hypertrophy and inhibits collagen synthesis in cardiac fibroblasts

DOC-2 (differentially expressed in ovarian carcinoma) is involved in Ras-, β-integrin-, PKC-, and transforming growth factor-β-mediated cell signaling. These pathways are implicated in the accumulation of extracellular matrix proteins during progression of hypertrophy to heart failure; however, the role of DOC-2 in cardiac pathophysiology has never been examined. This study was undertaken to 1) analyze DOC-2 expression in primary cultures of cardiac fibroblasts and cardiac myocytes and in the heart following different types of hemodynamic overloads and 2) examine its role in growth factor-mediated ERK activation and collagen production. Pressure overload and volume overload were induced for 10 wk in Sprague-Dawley rats by aortic constriction and by aortocaval shunt, respectively. ANG II (0.3 mg·kg−1·day−1) was infused for 2 wk. Results showed that, compared with myocytes, DOC-2 was found abundantly expressed in cardiac fibroblasts. Treatment of cardiac fibroblasts with ANG II and TPA resulted in increased expression of DOC-2. Overexpression of DOC-2 in cardiac fibroblasts led to inhibition of hypertrophy agonist-stimulated ERK activation and collagen expression. An inverse correlation between collagen and DOC-2 was observed in in vivo models of cardiac hypertrophy; in pressure overload and after ANG II infusion, increased collagen mRNA correlated with reduced DOC-2 levels, whereas in volume overload increased DOC-2 levels were accompanied by unchanged collagen mRNA. These data for the first time describe expression of DOC-2 in the heart and demonstrate its modulation by growth-promoting agents in cultured cardiac fibroblasts and in in vivo models of heart hypertrophy. Results suggest a role of DOC-2 in cardiac remodeling involving collagen expression during chronic hemodynamic overload.

Alpha1-adrenergic receptors prevent a maladaptive cardiac response to pressure overload

An alpha1-adrenergic receptor (alpha1-AR) antagonist increased heart failure in the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT), but it is unknown whether this adverse result was due to alpha1-AR inhibition or a nonspecific drug effect. We studied cardiac pressure overload in mice with double KO of the 2 main alpha1-AR subtypes in the heart, alpha 1A (Adra1a) and alpha 1B (Adra1b). At 2 weeks after transverse aortic constriction (TAC), KO mouse survival was only 60% of WT, and surviving KO mice had lower ejection fractions and larger end-diastolic volumes than WT mice. Mechanistically, final heart weight and myocyte cross-sectional area were the same after TAC in KO and WT mice. However, KO hearts after TAC had increased interstitial fibrosis, increased apoptosis, and failed induction of the fetal hypertrophic genes. Before TAC, isolated KO myocytes were more susceptible to apoptosis after oxidative and beta-AR stimulation, and beta-ARs were desensitized. Thus, alpha1-AR deletion worsens dilated cardiomyopathy after pressure overload, by multiple mechanisms, indicating that alpha1-signaling is required for cardiac adaptation. These results suggest that the adverse cardiac effects of alpha1-antagonists in clinical trials are due to loss of alpha1-signaling in myocytes, emphasizing concern about clinical use of alpha1-antagonists, and point to a revised perspective on sympathetic activation in heart failure.

Cellular and subcellular localization of alpha-1 adrenoceptors in the rat visual cortex

Noradrenaline is thought to play modulatory roles in a number of physiological, behavioral, and cellular processes. Although many of these modulatory effects are mediated through alpha-1 adrenoceptors, basic knowledge of the cellular and subcellular distributions of these receptors is limited. We investigated the laminar distribution pattern of alpha-1 adrenoceptors in rat visual cortex, using immunohistochemistry at both light and electron microscopic levels. Affinity-purified anti-alpha-1 antibody was confirmed to react only with a single band of about 70-80 kDa in total proteins prepared from rat visual cortex. Alpha-1 adrenoceptors were widely distributed though all cortical layers, but relatively high in density in layers I, II/III, and V. Immunoreactivity was observed in both neuronal perikarya and processes including apical dendrites. In double-labeling experiments with anti-microtubule-associated protein 2, anti-neurofilament, anti-glial fibrillary acidic protein, anti-glutamic acid decarboxylase 65/67, anti-2-3-cyclic nucleotide 3-phosphodiesterase, and anti-tyrosine hydroxylase antibodies, alpha-1 adrenoceptors were found mainly in dendrites and somata of microtubule-associated protein 2-immunopositive neurons. About 20% of alpha-1 adrenoceptors were in GABAergic neurons. A small number of alpha-1 adrenoceptors were also distributed in axons of excitatory neurons, astrocytes, oligodendrocytes and noradrenergic fibers. Using an immunoelectron microscopic technique, numerous regions of alpha-1 adrenoceptor immunoreactivity were found in cell somata, on membranes of dendrites, and in postsynaptic regions. Moreover, a small number of immunoreaction products were also detected in axons and presynaptic sites. These findings provide the first quantitative evidence regarding the cellular and subcellular localization of alpha-1 adrenoceptor immunoreactivity in visual cortex. Moreover, the ultrastructural distribution of alpha-1 adrenoceptor immunoreactivity suggests that alpha-1 adrenoceptors are transported mainly into dendrites and that they exert effects at postsynaptic sites of neurons.

Cardiac-specific transgenic overexpression of alpha1B-adrenergic receptors induce chronic activation of ERK MAPK signalling

Cardiomyocyte-specific overexpression of the wild-type alpha(1B)-adrenergic receptor (alpha(1B)-AR) produces a slowly progressing cardiomyopathy associated with clinical signs of heart failure and premature death around middle age (Lemire et al. 2001). In the heart, alpha(1)-AR activate the extracellular signal-regulated kinase (ERK) MAPK cascade. The aim of this project was to determine if cardiac-specific overexpression of the wild-type alpha(1B)-AR results in sustained activation of the ERK pathway. At 3 and 9 months, ERK activity was increased in alpha(1B)-AR overexpressing hearts relative to non-transgenic animals. Similarly, phosphorylation of MEK and p90(rsk) were also elevated. MAP kinase phosphatases (MKPs), which inactivate MAP kinases, are transcriptionally regulated. MKP2 mRNA levels were reduced at 3 months in alpha(1B)-AR overexpressing hearts. Interestingly, there was a general trend for reduced expression of MKP-1, -2, and -3 with increased age. In addition, expression of the modulatory calcineurin-interacting protein (MCIP) 1, an indicator of calcineurin activity, was elevated 3-fold in alpha(1B)-AR overexpressing hearts at both 3 and 9 months. These results indicate that the overexpression of the wild-type alpha(1B)-AR leads to chronic changes in the activation of signalling pathways previously shown to be associated with the hypertrophic response.

Different roles of alpha1-adrenoceptor subtypes in mediating cardiomyocyte protein synthesis in neonatal rats

1. Three different alpha1-adrenoceptor subtypes, designated alpha1A, alpha1B and alpha1D, have been cloned and identified pharmacologically in cardiomyocytes. In vitro studies have suggested that alpha1-adrenoceptors play an important role in facilitating cardiac hypertrophy. However, it remains controversial as to which subtype of alpha1-adrenoceptors is involved in this response. In the present study, we investigated the different role of each alpha1-adrenoceptor subtype in mediating cardiomyocyte protein synthesis, which is a most important characteristic of cardiac hypertrophy in cultured neonatal rat cardiomyocytes. 2. Cardiomyocyte hypertrophy was monitored by the following characteristic phenotypic changes: (i) an increase in protein synthesis; (ii) an increase in total protein content; and (iii) an increase in cardiomyocyte size. 3. The role of each alpha1-adrenoceptor subtype in mediating cardiomyocyte protein synthesis was investigated by the effect of specific alpha1-adrenoceptor subtype-selective antagonists on noradrenaline-induced [3H]-leucine incorporation. In addition, pKB values for alpha1-adrenoceptor subtype-selective antagonists were calculated and compared with the corresponding pKi values to further identify their effects. 4. Activation of alpha1-adrenoceptors by phenylephrine or noradrenaline in the presence of propranolol significantly increased [3H]-leucine incorporation, protein content and cell size. 5. Pre-incubating cardiomyocytes with 5-methyl-urapidil, RS 17053 or WB 4101 significantly inhibited noradrenaline-induced [3H]-leucine incorporation. However, there was no effect when cardiomyocytes were pre-incubated with BMY 7378. The correlation coefficients between pKB values for alpha1-adrenoceptor subtype-selective antagonists and pKi values obtained from cloned alpha1A-, alpha1B- or alpha1D-adrenoceptors were 0.92 (P <0.01), 0.66 (P >0.05) and 0.24 (P >0.05), respectively. 6. Our results suggest that the alpha1-adrenoceptor is dominantly responsible for adrenergic hypertrophy of cultured cardiomyocytes in neonatal rats. The efficiency in mediating cardiomyocyte protein synthesis is alpha1A > alpha1B >> alpha1D.

Upregulation of Phospholipase D Expression and Activation in Ventricular Pressure-Overload Hypertrophy

Evidence for a role of phospholipase D (PLD) in cellular proliferation and differentiation is accumulating. We studied PLD activity and expression in normal and hypertrophic rat and human hearts. In rat heart, abdominal aortic banding (constriction to 50% of original lumen) caused hypertrophy in the left ventricle (as shown by weight index and ANP expression) by about 15% after 30 days without histological evidence of fibrosis or signs of decompensation and in the right ventricle after 100 days. The hypertrophy was accompanied by small increases of basal PLD activity and strong potentiation of stimulated PLD activity caused by 4beta-phorbol-12beta,13alpha-dibutyrate (PDB) and by phenylephrine. The mRNA expressions of both PLD1 and PLD2 determined by semiquantitative competitive RT-PCR were markedly enhanced after aortic banding. In the caveolar fraction of the rat heart, PLD2 protein determined by Western blot analysis was upregulated in parallel with the expression of caveolin-3. A similar induction of PLD mRNA and protein expression was observed in hypertrophied human hearts of individuals (39-45-year-old) who had died from non-cardiac causes. In conclusion, PLD1 and PLD2 expressions were strongly enhanced both in rat and human heart hypertrophy, which may be responsible for the coincident potentiation of the PLD activation by alpha-adrenoceptor and protein kinase C stimulation. These results are compatible with a significant role of PLD activation in cell signaling of ventricular pressure-overload hypertrophy.

Activation of alpha1A-adrenergic receptor promotes differentiation of rat-1 fibroblasts to a smooth muscle-like phenotype

Background

Fibroblasts, as connective tissue cells, are able to transform into another cell type including smooth muscle cells. α_1A-adrenergic receptor (α_1A-AR) stimulation in rat-1 fibroblasts is coupled to cAMP production. However, the significance of an increase in cAMP produced by α_1A-AR stimulation on proliferation, hypertrophy and differentiation in these cells is not known.

Results

Activation of the α_1A-AR in rat-1 fibroblasts by phenylephrine (PE) inhibited DNA synthesis by 67% and blocked the re-entry of 81% of the cells into S phase of the cell cycle. This cell cycle blockage was associated with hypertrophy characterized by an increase in protein synthesis (64%) and cell size. Elevation of cAMP levels decreased both DNA and protein synthesis. Inhibition of adenylyl cyclase or protein kinase A reversed the antiproliferative effect of cAMP analogs but not PE; the hypertrophic effect of PE was also not altered. The functional response of rat-1 cells to PE was accompanied by increased expression of cyclin-dependent kinase (Cdk) inhibitors p27^kip1 and p21^cip1/waf1, which function as negative regulators of the cell cycle. Stimulation of α_1A-AR also upregulated the cell cycle regulatory proteins pRb, cyclin D1, Cdk 2, Cdk 4, and proliferating cell nuclear antigen. The antiproliferative effect of PE was blocked by p27^kip1 antisense but not sense oligonucleotide. PE also promoted expression of smooth muscle cell differentiation markers (smooth muscle alpha actin, caldesmon, and myosin heavy chain) as well as the muscle development marker MyoD.

Conclusions

Stimulation of α_1A-AR promotes cell cycle arrest, hypertrophy and differentiation of rat-1 fibroblasts into smooth muscle-like cells and expression of negative cell cycle regulators by a mechanism independent of the cAMP/PKA signaling pathway.

Application of mesenchymal stem cells for the regeneration of cardiomyocyte and its use for cell transplantation therapy

We have isolated a cardiomyogenic cell line (CMG cell) from murine bone marrow mesenchymal stem cells. The cells showed a fibroblast-like morphology, but the morphology changed after 5-azacytidine exposure. They began spontaneous beating after 2 weeks, and expressed ANP and BNP. Electron microscopy revealed a cardiomyocyte-like ultrastructure. These cells had several types of action potentials; sinus node-like and ventricular cell-like action potentials. The isoform of contractile protein genes indicated that their muscle phenotype was similar to fetal ventricular cardiomyocytes. They expressed alpha1A, alpha1B, alpha1D, beta1, and beta2 adrenergic and M1 and M2 muscarinic receptors. Stimulation with phenylephrine, isoproterenol and carbachol increased ERK phosphorylation and second messengers. Isoproterenol increased the beating rate, which was blocked with CGP20712A (beta1-selective blocker). These findings indicated that cell transplantation therapy for the patients with heart failure might possibly be achieved using the regenerated cardiomyocytes from autologous bone marrow cells in the near future.

The alpha(1A/C)- and alpha(1B)-adrenergic receptors are required for physiological cardiac hypertrophy in the double-knockout mouse

Catecholamines and alpha(1)-adrenergic receptors (alpha(1)-ARs) cause cardiac hypertrophy in cultured myocytes and transgenic mice, but heart size is normal in single KOs of the main alpha(1)-AR subtypes, alpha(1A/C) and alpha(1B). Here we tested whether alpha(1)-ARs are required for developmental cardiac hypertrophy by generating alpha(1A/C) and alpha(1B) double KO (ABKO) mice, which had no cardiac alpha(1)-AR binding. In male ABKO mice, heart growth after weaning was 40% less than in WT, and the smaller heart was due to smaller myocytes. Body and other organ weights were unchanged, indicating a specific effect on the heart. Blood pressure in ABKO mice was the same as in WT, showing that the smaller heart was not due to decreased load. Contractile function was normal by echocardiography in awake mice, but the smaller heart and a slower heart rate reduced cardiac output. alpha(1)-AR stimulation did not activate extracellular signal-regulated kinase (Erk) and downstream kinases in ABKO myocytes, and basal Erk activity was lower in the intact ABKO heart. In female ABKO mice, heart size was normal, even after ovariectomy. Male ABKO mice had reduced exercise capacity and increased mortality with pressure overload. Thus, alpha(1)-ARs in male mice are required for the physiological hypertrophy of normal postnatal cardiac development and for an adaptive response to cardiac stress.

Cloning and characterization of the rat alpha 1a-adrenergic receptor gene promoter. Demonstration of cell specificity and regulation by hypoxia

Recent studies reveal important and distinct roles for cardiac alpha(1a) adrenergic receptors (alpha(1a)ARs). Surprisingly, given their importance in myocardial ischemia/reperfusion, hypoxia, and hypertrophy as well as frequent use of rat cardiomyocyte model systems, the rat alpha(1a)AR gene promoter has never been characterized. Therefore, we isolated 3.9 kb of rat alpha(1a)AR 5'-untranslated region and 5'-regulatory sequences and identified multiple transcription initiation sites. One proximal (P1) and several clustered upstream distal promoters (P2, P3, and P4) were delineated. Sequences surrounding both proximal and distal promoters lack typical TATA or CCAAT boxes but contain cis-elements for multiple myocardium-relevant nuclear regulators including Sp1, GATA, and CREB, findings consistent with enhanced cardiac basal alpha(1a)AR expression seen in Northern blots and reporter constructs. Promoter analysis using deletion reporter constructs reveals, in addition to a powerful upstream enhancer, a key region (-558/-542) important in regulating all alpha(1a)AR promoters with hypoxic stress. Gel shift analysis of this 14-bp region confirms a hypoxia-induced shift independent of direct hypoxia-inducible factor binding. Mutational analysis of this sequence identifies a novel 9-bp hypoxia response element, the loss of which severely attenuates hypoxia-mediated repression of alpha(1a)AR transcription. These findings for the alpha(1a) gene should facilitate elucidation of alpha(1)AR-mediated mechanisms involved in distinct myocardial pathologies.

Cytokines down-regulate alpha1-adrenergic receptor expression during endotoxemia

OBJECTIVE

The reduced pressure response to norepinephrine in septic patients has directed our interest to the regulation of alpha1-adrenergic receptors in vitro and in vivo during conditions mimicking acute sepsis.

DESIGN

Prospective animal trial followed by a controlled cell culture study.

SETTING

Laboratory of the Department of Anesthesiology.

SUBJECTS

Male Sprague-Dawley rats weighing 200 to 250 g and a mesangial cell line.

INTERVENTIONS

Experimental endotoxemia was induced in rats with lipopolysaccharide, and blood pressure dose-response studies with norepinephrine were performed. Alpha1-receptor gene expression was determined in various organs by a specific RNase protection assay, and tissue concentrations of the proinflammatory cytokines interleukin-1beta and tumor necrosis factor-alpha were measured. Rat renal mesangial cells were incubated with these cytokines or with nitric oxide donors to investigate the regulation of alpha1-adrenergic receptors during severe inflammation on a cellular level.

MEASUREMENTS AND MAIN RESULTS

The pressor effect of norepinephrine was markedly diminished during endotoxemia. The animals showed down-regulated mRNA levels of alpha1A-, alpha1B- and alpha1D-receptors in all organs investigated, and the tissue concentrations of interleukin-1beta and tumor necrosis factor-alpha were highly increased during experimental endotoxemia. Incubation of cultured rat renal mesangial cells with the cytokines resulted in diminished alpha -receptor gene expression and [3H]prazosin binding capacity, whereas incubation of the cells with nitric oxide donors did not affect alpha1B-receptor expression. In line, blocking of cytokine-induced nitric oxide synthesis by coincubation of mesangial cells with N(G)-nitro-L-arginine methyl ester did not influence cytokine-induced down-regulation of alpha1B-receptors.

CONCLUSIONS

Our data show that endotoxemia causes a systemic down-regulation of alpha1-receptors on the level of gene expression and suggest that this effect is likely mediated by proinflammatory cytokines in a synergistic but nitric oxide-independent fashion. We propose that this down-regulation of alpha1-adrenergic receptors contributes to the attenuated blood pressure response to norepinephrine and, therefore, to septic circulatory failure in patients.

Reprogramming of bone marrow mesenchymal stem cells into cardiomyocytes

We have isolated a cardiomyogenic cell line (CMG cell) from murine bone marrow mesenchymal stem cells. The cells showed a fibroblast-like morphology, but the morphology changed after 5-azacytidine exposure. They began spontaneous beating after 2 weeks, and expressed ANP and BNP. Electron microscopy revealed a cardiomyocyte-like ultrastructure. These cells had several types of action potentials: sinus-node-like and ventricular-cell-like action potentials. The isoform of contractile protein genes indicated that their muscle phenotype was similar to fetal ventricular cardiomyocytes. They expressed alpha 1A, alpha 1B, alpha 1D, beta 1, and beta 2 adrenergic and M1 and M2 muscarinic receptors. Stimulation with phenylephrine, isoproterenol and carbachol increased ERK phosphorylation and second messengers. Isoproterenol increased the beating rate, which was blocked with CGP20712A (beta 1-selective blocker). These findings indicated that cell transplantation therapy for the patients with heart failure might possibly be achieved using the regenerated cardiomyocytes from autologous bone marrow cells in the near future.

Alpha(1)-adrenoceptor subtypes mediate negative inotropy in myocardium from alpha(1A/C)-knockout and wild type mice

Cardiac alpha(1)-adrenoceptors (AR) have two predominant subtypes (alpha(1A)-AR and alpha(1B)-AR) however, their roles in regulating contraction are unclear. We determined the effects of stimulating alpha(1A)-AR (using the subtype-selective agonist A61603) and alpha(1B)-AR (using a gene knockout mouse lacking alpha(1A)-AR) separately, and together (using phenylephrine) on Ca(2+) transients, intracellular pH, and contraction of mouse cardiac trabeculae. Stimulation of alpha(1)-AR subtypes separately or together caused a triphasic contractile response. After a transient ( approximately 3%) force rise (phase 1), force declined markedly (phase 2), then partially recovered (phase 3). In phase 2, the force decline (% of initial) with combined alpha(1A)-AR plus alpha(1B)-AR stimulation (50+/-3%) was more than with separate subtype stimulation (P<0.01), suggesting alpha(1A)-AR and alpha(1B)-AR mediate additive effects during phase 2. Force decline in phase 2 paralleled decreases of Ca(2+) transients that were reduced more with combined vs. separate subtype stimulation. During phase 3 the final force reduction was similar with stimulation of alpha(1A)-AR (20+/-5%), or alpha(1B)-AR (20+/-3%), or both (26+/-4%) suggesting alpha(1A)-AR and alpha(1B)-AR mediate non-additive effects during phase 3. In contrast, Ca(2+) transients recovered fully in phase 3 suggesting reduced force in phase 3 involved decreased myofilament Ca(2+)-sensitivity. Decreased Ca(2+)-sensitivity was not mediated by changes of intracellular pH since this was not affected by alpha(1)-AR stimulation. In contrast to mouse trabeculae, rat trabeculae demonstrated a positive inotropic response to alpha(1)-AR stimulation. In conclusion, for mouse myocardium in vitro both alpha(1)-adrenoceptor subtypes mediate negative inotropy involving decreased Ca(2+) transients and a decreased Ca(2+) sensitivity that does not involve altered intracellular pH.

Identification of alpha1-adrenergic receptors and their involvement in phosphoinositide hydrolysis in the frog heart

The aim of this study was to characterize alpha(1)-adrenergic receptors in frog heart and to examine their related signal transduction pathway. alpha(1)-Adrenergic binding sites were studied in purified heart membranes using the specific alpha(1)-adrenergic antagonist [(3)H]prazosin. Analysis of the binding data indicated one class of binding sites displaying a K(d) of 4.19 +/- 0.56 nM and a B(max) of 14.66 +/- 1.61 fmol/mg original wet weight. Adrenaline, noradrenaline, or phenylephrine, in the presence of propranolol, competed with [(3)H]prazosin binding with a similar potency and a K(i) value of about 10 microM. The kinetics of adrenaline binding was closely related to its biological effect. Adrenaline concentration dependently increased the production of inositol phosphates in the heart in the presence or absence of propranolol. Maximal stimulation was about 8.5-fold, and the half-maximum effective concentration was 30 and 21 microM in the absence and presence of propranolol, respectively. These data clearly show that alpha(1)-adrenergic receptors are coupled to the phosphoinositide hydrolysis in frog heart. To our knowledge, this is the first direct evidence supporting the presence of functional alpha(1)-adrenergic receptors in the frog heart.

alpha(1)-adrenergic stimulation mediates Ca(2+)-dependent inositol phosphate formation through the alpha(1B)-like adrenoceptor subtype in adult rat cardiac myocytes

We studied the effects of increased Ca(2+) influx on alpha(1)-adrenoceptor-stimulated InsP formation in adult rat cardiac myocytes. We further examined if such effects could be mediated through a specific alpha(1)-adrenoceptor subtype. [(3)H]InsP responses to adrenaline were dependent on extracellular Ca(2+) concentration, from 0.1 microM to 2 mM, and were completely blocked by Ca(2+) removal. However, in cardiac myocytes preloaded with BAPTA, a highly selective calcium chelating agent, Ca(2+) concentrations higher than 1 microM had no effect on adrenaline-stimulated [(3)H]InsP formation. Taken together these results suggest that [(3)H]InsP formation induced by alpha(1)-adrenergic stimulation is in part mediated by increased Ca(2+) influx. Consistent with this, ionomycin, a calcium ionophore, stimulated [(3)H]InsP formation. This response was additive with the response to adrenaline stimulation implying that different signaling mechanisms may be involved. In cardiac myocytes treated with the alpha(1B)-adrenoceptor alkylating agent, CEC, [(3)H]InsP formation remained unaffected by increased Ca(2+) concentrations, a pattern similar to that observed when intracellular Ca(2+) was chelated with BAPTA. In contrast, addition of the alpha(1A)-subtype antagonist, 5'-methyl urapidil, did not affect the Ca(2+) dependence of [(3)H]InsP formation. Neither nifedipine, a voltage-dependent Ca(2+) channel blocker nor the inorganic Ca(2+) channel blockers, Ni(2+) and Co(2+), had any effect on adrenaline stimulated [(3)H]InsP, at concentrations that inhibit Ca(2+) channels. The results suggest that in adult rat cardiac myocytes, in addition to G protein-mediated response, alpha(1)-adrenergic-stimulated [(3)H]InsP formation is activated by increased Ca(2+) influx mediated by the alpha(1B)-subtype.

Molecular characterization of regenerated cardiomyocytes derived from adult mesenchymal stem cells

We recently isolated a cardiomyogenic (CMG) cell line from murine bone marrow stroma, and in this paper characterize regenerated cardiomyocytes derived from adult mesenchymal stem cells at the molecular level. Stromal cells were immortalized, exposed to 5-azacytidine, and repeatedly screened for spontaneously beating cells. CMG cells began to beat spontaneously after 2 weeks, and beat synchronously after 3 weeks. They exhibited sinus-node-like or ventricular-cell-like action potentials. Analysis of the isoforms of contractile protein genes, such as of myosin and alpha-actin, indicated that their phenotype was similar to that of fetal ventricular cardiomyocytes. The cells expressed Nkx2.5, GATA4, TEF-1, and MEF2-C mRNA before 5-azacytidine exposure, and MEF2-A and MEF2-D after exposure. CMG cells expressed alpha1A, alpha1B, and alpha1D-adrenergic receptor mRNA prior to differentiation, and beta1, beta2-adrenergic and M1, M2-muscarinic receptors after acquiring the cardiomyocyte phenotype. Phenylephrine induced phosphorylation of ERK1/2, and the phosphorylation was inhibited by prazosin. Isoproterenol increased the cAMP level 38-fold and beating rate, cell motion, %shortening, and contractile velocity by 48%, 38%, 27%, and 51%, respectively, and the increases were blocked by CGP20712A (beta1-selective blocker). Carbachol increased IP3 32-fold, and the increase was inhibited by AFDX116 (M2-selective blocker). These findings demonstrated that the regenerated cardiomyocytes were capable of responding to adrenergic and muscarinic stimulation. This new cell line provides a model for the study of cardiomyocyte transplantation.

Expression of alpha1-adrenergic receptor subtypes in heart cell culture

Three alpha1-AR subtypes have been cloned so far and are designated as alpha1a, alpha1b,, and alpha1d. Organ-specific distribution pattern and subtype-specific effects are known but not fully understood. To address a cell-type specific expression pattern in the heart we investigated expression pattern of alpha1-AR subtypes on RNA- and protein-level in heart tissue, cultured cardiomyocytes and non-myocytes of the rat. Each alpha1AR-subtype mRNA was present in neonatal and adult rat heart culture but the relative distribution pattern was significantly different. While the alpha1a-AR subtype is preferentially expressed in adult cardiomyocytes, the alpha1b-AR subtype was preferentially expressed in the non-myocyte cell fraction. The RT-PCR results were confirmed by Western-blotting (alpha1b) and immunocytochemical studies. Incubation with an alpha1-agonist (phenylephrine) for 72 h led to a significant reduction of the alpha1b-AR in neonatal heart cell culture on both mRNA and protein level. In contrast, incubation with an alpha1-antagonist (prazosin) induced a 1.6 fold upregulation of the alpha1a-AR mRNA without significant effects on radioligand binding and functional assay. The results indicate a distribution pattern of the alpha1-AR subtype which is specific for cell type and ontogeny of the rat heart and may be regulated by adrenergic agents.

alpha(1D)-Adrenoceptors do not contribute to phosphoinositide hydrolysis in adult rat cardiac myocytes

We have used the alpha(1D)-adrenoceptor selective antagonist, BMY 7378, to investigate the presence of alpha(1D)-adrenoceptor subtype in adult rat heart by radioligand binding assays. We also determined the role of this subtype in stimulating phosphoinositide (PI) hydrolysis in adult rat cardiac myocytes. BMY 7378 inhibited [(3)H]prazosin binding to cardiac membranes in a biphasic mode with a pK(i) of 9.19 +/- 0.26 for high affinity sites and 6.64 +/- 0.09 for low affinity sites. The inhibition of the adrenaline-induced stimulation of PI hydrolysis by BMY 7378 fitted a one-site model and the calculated pK(b) value (6.92 +/- 0.28) was consistent with the involvement of alpha(1A) and alpha(1B) adrenoceptors. In addition, BMY 7378, at concentrations up to 100 nM, did not significantly affect the concentration-response curves for the adrenaline-induced stimulation of PI hydrolysis. Taken together, these data suggest that alpha(1D)-adrenoceptors are expressed in adult rat heart but this subtype is not involved in the adrenaline-induced stimulation of PI hydrolysis.

Cloning and characterization of the mouse alpha1C/A-adrenergic receptor gene and analysis of an alpha1C promoter in cardiac myocytes: role of an MCAT element that binds transcriptional enhancer factor-1 (TEF-1)

alpha1-Adrenergic receptor (AR) subtypes in the heart are expressed by myocytes but not by fibroblasts, a feature that distinguishes alpha1-ARs from beta-ARs. Here we studied myocyte-specific expression of alpha1-ARs, focusing on the subtype alpha1C (also called alpha1A), a subtype implicated in cardiac hypertrophic signaling in rat models. We first cloned the mouse alpha1C-AR gene, which consisted of two exons with an 18 kb intron, similar to the alpha1B-AR gene. The receptor coding sequence was >90% homologous to that of rat and human. alpha1C-AR transcription in mouse heart was initiated from a single Inr consensus sequence at -588 from the ATG; this and a putative polyadenylation sequence 8.5 kb 3' could account for the predominant 11 kb alpha1C mRNA in mouse heart. A 5'-nontranscribed fragment of 4.4 kb was active as a promoter in cardiac myocytes but not in fibroblasts. Promoter activity in myocytes required a single muscle CAT (MCAT) element, and this MCAT bound in vitro to recombinant and endogenous transcriptional enhancer factor-1. Thus, alpha1C-AR transcription in cardiac myocytes shares MCAT dependence with other cardiac-specific genes, including the alpha- and beta-myosin heavy chains, skeletal alpha-actin, and brain natriuretic peptide. However, the mouse alpha1C gene was not transcribed in the neonatal heart and was not activated by alpha1-AR and other hypertrophic agonists in rat myocytes, and thus differed from other MCAT-dependent genes and the rat alpha1C gene.

Alpha 1-adrenergic receptor regulation: basic science and clinical implications

Adrenergic receptors (ARs) are members of the G-protein-coupled receptor family, which includes alpha 1ARs, alpha 2ARs, beta 1ARs, beta 2ARs, beta 3ARs, adenosine, muscarinic, angiotensin, endothelin receptors, and many others that are responsible for a large variety of physiologic effects through G-protein coupling. This review focuses on alpha 1ARs and their regulation at both the mRNA and protein levels. Currently, three alpha 1AR subtypes have been characterized both pharmacologically and at the gene level: alpha 1aAR, alpha 1bAR, and alpha 1dAR. These are expressed in a species- and tissue-dependent manner. Mutagenesis approaches have been extremely valuable in the identification of key residues that govern alpha 1AR ligand binding and signaling. These studies reveal that alpha 1ARs have evolved an exquisitely sensitive regulation of their activity in which any disruption of the native structure has profound effects on subsequent function and effector coupling. Significant advances have also been made in the elucidation of signaling pathway components, resulting in the identification of novel pathways that can lead to pathologic conditions. Specific topics include mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and G-protein-coupled receptor cross-talk pathways. Within this context, recent studies identifying underlying transcriptional mechanisms involved in the regulation of the alpha 1AR subtypes are also discussed. Finally, given the potentially important role of alpha 1ARs in the vasculature, as well as in the pathology of many diseases, such as myocardial hypertrophy and benign prostatic hyperplasia, the clinical relevance of alpha 1AR distribution, pharmacology, and therapeutic intervention is reviewed.

The alpha(1)-adrenoceptor subtype- and protein kinase C isoform-dependence of Norepinephrine's actions in cardiomyocytes

Catecholamines modulate cardiac function at least in part through alpha(1)-adrenergic receptors linked to the activation of protein kinase C (PKC). This study examines the molecular forms of the alpha(1)-receptor and PKC that mediate norepinephrine's actions in cardiomyocytes; distinct approaches (activation-dependent down-regulation of PKC isoforms) and novel reagents (A61603, an alpha(1A/c)-receptor agonist) are used to resolve this issue which has been the focus of dispute in previous studies. Norepinephrine (NE) induces a rise in diacylglycerol levels which is sustained for 24 h and is associated with the translocation (at 5 min) and down-regulation (at 24 h) of PKC delta and PKC xi (but not PKC alpha). The selective targeting of the alpha(1)-adrenergic receptor to activate novel PKC isoforms is remarkable, given an 8-fold greater abundance of PKC alpha relative to PKC xi in this preparation. NE activates the extracellular signal-regulated protein kinase (ERK) subfamily of mitogen-activated protein kinases through a PKC delta/PKC xi -dependent pathway. WB-4101 (alpha(1A/c)- and alpha(1D)-receptor antagonist) and 5-methylurapidil (alpha(1A/c)-receptor antagonist) inhibit norepinephrine-dependent accumulation of inositol phosphate and diacylglycerol, down-regulation of PKC delta and PKC xi, and activation of ERK. Each of these responses is stimulated by A61603, but not attenuated by high concentrations of chloroethylclonidine (which irreversibly inactivates the alpha(1B)-, and to a lesser extent, the alpha(1D)-receptor) or BMY 7378 (selective alpha(1D)-receptor antagonist). A61603 also activates p38-MAPK and induces hypertrophy. These studies establish that NE's actions in cardiomyocytes can be attributed to the alpha(1A/c)-adrenergic receptor subtype and nPKC isoforms, thereby identifying specific targets for the development of pharmaceuticals to influence cardiac contractile function and/or growth responses.

Expression and activity of protein kinase D/protein kinase C mu in myocardium: evidence for alpha1-adrenergic receptor- and protein kinase C-mediated regulation

Protein kinase D (PKD), which is also known as protein kinase C (PKC) mu, is a novel serine/threonine kinase that can be activated in parallel with or downstream of PKC in various cell types, but its expression and regulation in myocardium have not been characterized. In the present study, two proteins of 110 and 115 kDa were detected in rat ventricular myocardium using antibodies directed at the extreme N- or C-terminus of PKD. Both proteins were highly expressed in the fetal heart but showed a developmental decline in abundance. Fractionation studies showed that PKD was distributed between myocyte and non-myocyte fractions in the neonatal heart, but was found predominantly in the non-myocyte fraction in the adult heart. In cultured neonatal rat ventricular myocytes, an in vitro kinase assay revealed increased autophosphorylation of PKD (EC50 2.8 nM) in response to phorbol-12-myristate-13-acetate (PMA). Exposure to norepinephrine also induced a dose-dependent increase in PKD autophosphorylation (EC50 0.6 microM). Pretreatment with the alpha1-adrenergic receptor (AR) antagonist prazosin blocked norepinephrine-induced PKD autophosphorylation, while the beta1-AR antagonist atenolol had no effect, indicating that activation of PKD by norepinephrine occurred via the alpha1-AR. Involvement of the alpha1-AR was confirmed by exposure of myocytes to the alpha1-AR agonist phenylephrine, which induced a similar profile of PKD autophosphorylation to norepinephrine (EC50 0.6 microM). The effects of both alpha1-AR stimulation and PMA on PKD autophosphorylation were mediated by PKC, since these effects could be attenuated by pretreatment of myocytes with the PKC inhibitor bisindolylmaleimide. These data show that PKD is expressed in rat ventricular myocardium, where its expression is subject to developmental control, and that PKD activity in ventricular myocytes is regulated through alpha1-AR- and PKC-mediated pathways.

Distribution of α1-adrenoceptor subtype proteins in different tissues of neonatal and adult rats

Distribution of α1-adrenoceptor (α1AR) subtype (α1A, α1B, α1D) proteins in brain, heart, kidney, and liver of 1-week-old rats and in brain, heart, aorta, kidney, liver, vas deferens, prostate, and adrenal glands of adult rats was investigated by Western analysis, using receptor subtype specific polyclonal antibodies. High levels of immunoreactive α1AAR and α1DAR in brain and heart and of α1BAR in liver and heart of neonatal rats were detected. In adult rat tissues, the abundance of α1AAR protein was most marked in the brain, intermediate in heart, aorta, liver, vas deferens, and adrenals, and minimal in the kidney and prostate; relative to other tissues, the expression of α1BAR was higher in brain and heart and that of α1DAR in brain. All the three receptor subtypes increased with age in the brain cortex, whereas the abundance of α1BAR increased in the heart but decreased in the liver; α1AAR and α1DAR in liver, kidney, and heart were not affected by age. It is concluded that α1AR subtypes are widely expressed in different neonatal and adult rat tissues.Key words: α1A-adrenoceptors, α1B-adrenoceptors, α1D-adrenoceptors, α1-adrenoceptor proteins.

Cardiovascular α1-adrenoceptor subtypes: functions and signaling

α1-Adrenoceptors (α1AR) are G protein-coupled receptors and include α1A, α1B, and α1D subtypes corresponding to cloned α1a, α1b, and α1d, respectively. α1AR mediate several cardiovascular actions of sympathomimetic amines such as vasoconstriction and cardiac inotropy, hypertrophy, metabolism, and remodeling. α1AR subtypes are products of separate genes and differ in structure, G protein-coupling, tissue distribution, signaling, regulation, and functions. Both α1AAR and α1BAR mediate positive inotropic responses. On the other hand, cardiac hypertrophy is primarily mediated by α1AAR. The only demonstrated major function of α1DAR is vasoconstriction. α1AR are coupled to phospholipase C, phospholipase D, and phospholipase A2; they increase intracellular Ca2+ and myofibrillar sensitivity to Ca2+ and cause translocation of specific phosphokinase C isoforms to the particulate fraction. Cardiac hypertrophic responses to α1AR agonists might involve activation of phosphokinase C and mitogen-activated protein kinase via Gq. α1AR subtypes might interact with each other and with other receptors and signaling mechanisms.Key words: cardiac hypertrophy, inotropic responses, central α1-adrenoreceptors, arrythmias.

Both alpha(1A)- and alpha(1B)-adrenergic receptor subtypes couple to the transient outward current (I(To)) in rat ventricular myocytes

1. Regulation of transient outward current (I(To)) by alpha(1)-adrenergic (alpha(1)AR) plays a key role in cardiac repolarization. alpha(1)ARs comprise a heterogeneous family; two natively expressed subtypes (alpha(1A) and alpha(1B)) and three cloned subtypes (alpha(1a), alpha(1b) and alpha(1d)) can be distinguished. We have examined the electrophysiological role of each alpha(1)AR subtype in regulating I(To) in isolated rat ventricular myocytes. 2. Reverse transcription-PCR study revealed the presence of three subtype mRNAs (alpha(1a), alpha(1b) and alpha(1d)) in rat myocytes. 3. Radioligand binding assay using [(125)I]-HEAT showed that the inhibition curves for alpha(1A)AR-selective antagonists (WB4101, 5-methylurapidil, (+)-niguldipine and KMD-3213) in rat ventricles best fit a two-site model, with 30% high and 70% low affinity binding sites. The high affinity sites were resistant to 100 microM chloroethylclonidine (CEC), while the low affinity sites were highly inactivated by CEC. 4. Whole cell voltage clamp study revealed that methoxamine reduced a 4-aminopyridine(4-AP)-sensitive component of I(To) in the isolated rat ventricle myocytes. Lower concentrations of KMD-3213 (1 nM) or 5-MU (10 nM) did not affect the methoxamine-induced reduction of I(To). On the other hand, CEC treatment (100 microM) of isolated myocytes reduced the methoxamine-induced reduction of I(To) by 46%, and the remaining response was abolished by lower concentrations of KMD-3213 or 5-MU. 5. The results indicate that rat ventricular myocytes express transcripts of the three alpha(1)AR subtypes (alpha(1a), alpha(1b) and alpha(1d)); however, two pharmacologically distinct alpha(1)AR subtypes (alpha(1A) and alpha(1B)) are predominating in receptor populations, with approximately 30% alpha(1A)AR and 70% alpha(1B)AR. Although both alpha(1A) and alpha(1B)AR subtypes are coupled to the cardiac I(To), alpha(1B)ARs predominantly mediate alpha(1)AR-induced effect.

Constitutively active mutants of the alpha(1a)- and the alpha(1b)-adrenergic receptor subtypes reveal coupling to different signaling pathways and physiological responses in rat cardiac myocytes

Activation of alpha(1)-adrenergic receptors influences both the contractile activity and the growth potential of cardiac myocytes. However, the signaling pathways linking activation of specific alpha(1)-adrenergic receptor (AR) subtypes to these physiological responses remain controversial. In the present study, a molecular approach was used to identify conclusively the signaling pathways activated in response to the individual alpha(1A)- and alpha(1B)-AR subtypes in cardiac myocytes. For this purpose, a mutant alpha(1a)-AR subtype (alpha(1a)-S(290/293)-AR) was constructed based on analogy to the previously described constitutively active mutant alpha(1b)-AR subtype (alpha(1b)-S(288-294)-AR). The mutant alpha(1a)-S(290/293)-AR subtype displayed constitutive activity based on four criteria. To introduce the constitutively active alpha(1)-AR subtypes into cardiac myocytes, recombinant Sindbis viruses encoding either the alpha(1a)-S(290/293)-AR or alpha(1b)-S(288-294)-AR subtype were used to infect the whole cell population with >90% efficiency, thereby allowing the biochemical activities of the various signaling pathways to be measured. When expressed at comparable levels, the alpha(1a)-S(290/293)-AR subtype exhibited a significantly elevated basal level as well as agonist-stimulated level of inositol phosphate accumulation, coincident with activation of atrial natriuretic factor-luciferase gene expression. By contrast, the alpha(1b)-S(288-294)-AR subtype displayed a markedly increased serum response element-luciferase gene expression but no activation of atrial natriuretic factor-luciferase gene expression. Taken together, this study provides the first molecular evidence for coupling of the alpha(1a)-AR and the alpha(1b)-AR subtypes to different signaling pathways in cardiac myocytes.

Characterization of alpha1-adrenoceptor subtypes mediating noradrenaline-induced contraction of rat epididymal vas deferens in calcium-free medium

The alpha1-adrenoceptor subtype mediating noradrenaline (NA)-induced contractions of rat epididymal vas deferens in Ca2+-free/EGTA (1 mM) medium was studied using competitive antagonists. The effects of chloroethylclonidine (CEC) was investigated in Ca2+-free and normal Krebs' medium and RT-PCR was used to identify alpha1-adrenoceptor specific mRNA in epididymal vas deferens. In Ca2+-free medium, NA evoked sustained contractions but was less potent (pD2, 5.9) than in normal Krebs' medium (pD2, 7.3). The contractions in Ca2+-free medium were inhibited by prazosin (pA2, 9.3), 5-methylurapidil (pA2, 8.4), spiperone (pA2, 7.6) and BMY 7378 (pK(B), 6.8) consistent with activation of alpha1A-subtype. Repeated pretreatment with CEC (100 microM) reduced the potency of NA and maximum contractions in normal and Ca2+-free media. CEC-sensitivity in normal Krebs' medium was enhanced by prior treatment with phenoxybenzamine. mRNA for alpha1a- and alpha1d- but not alpha1b-adrenoceptors were detected in epididymal vas deferens. These results suggest that NA contracts the tissue in Ca2+-free medium by the stimulation of alpha1A-adrenoceptors. Two factors affecting CEC-sensitivity of NA-induced contractions in this tissue are discussed.

Catecholamines in cardiac hypertrophy

There has been intense interest in the roles catecholamines may play in compensatory myocardial hypertrophy. This article reviews the following: (1) chronic infusions of catecholamines in experimental animals result in cardiac hypertrophy, but in many of the studies mechanical factors have played a role; (2) experiments using isolated papillary muscles and isolated hearts, stretched isolated myocytes, and denervated hearts in vivo demonstrate that mechanical activity is sufficient to cause increased protein synthesis and cell growth; (3) in neonatal myocyte cell cultures, alpha-adrenergic agonists are powerful stimulants for protein synthesis and cell growth. Beta-adrenergic stimulation of nonmyocyte myocardial cells causes release of a factor that promotes protein synthesis in neonatal myocytes. Either alpha or beta stimulation, probably through different mechanisms, appears to have growth-promoting effects on isolated adult myocytes in culture; (4) alpha stimulation is transduced through the Gq pathway and its activation of phospholipase C, cleavage of phosphatidylinositol (4,5)-bisphosphate, and then further through the ras/raf, mitogen-activated protein (MAP) kinase system; (5) transgenic mice with upregulation of catecholamine-related systems have not clarified the independent role of either the alpha- or beta-adrenergic pathway; and (6) observations in humans suggest that mechanical factors predominate in the development and regression of cardiac hypertrophy. Humoral mechanisms, including catecholamines, may play a role, but their quantitative importance has not been determined. It is hypothesized that catecholamines may play a role in transition from the adaptive to the maladaptive state.