Beta-adrenergic receptors and calcium cycling proteins in non-failing, hypertrophied and failing human hearts: transition from hypertrophy to failure

Stage-dependent benefits and risks of pimobendan in mice with genetic dilated cardiomyopathy and progressive heart failure

BACKGROUND AND PURPOSE

The Ca(2+) sensitizer pimobendan is a unique inotropic agent that improves cardiac contractility with less of an increase in oxygen consumption and potentially fewer adverse effects on myocardial remodelling and arrhythmia, compared with traditional inotropes. However, clinical trials report contradictory effects of pimobendan in patients with heart failure (HF). We provide mechanistic experimental evidence of the efficacy of pimobendan using a novel mouse model of progressive HF.

EXPERIMENTAL APPROACH

A knock-in mouse model of human genetic dilated cardiomyopathy, which shows a clear transition from compensatory to end-stage HF at a fixed time during growth, was used to evaluate the efficacy of pimobendan and explore the underlying molecular and cellular mechanisms.

KEY RESULTS

Pimobendan prevented myocardial remodelling in compensated HF and significantly extended life span in both compensated and end-stage HF, but dose-dependently increased sudden death in end-stage HF. In cardiomyocytes isolated from end-stage HF mice, pimobendan induced triggered activity probably because of early or delayed afterdepolarizations. The L-type Ca(2+) channel blocker verapamil decreased the incidence of triggered activity, suggesting that this was from over-elevated cytoplasmic Ca(2+) through increased Ca(2+) entry by PDE3 inhibition under diminished sarcoplasmic reticulum Ca(2+) reuptake and increased Ca(2+) leakage from sarcoplasmic reticulum in end-stage HF.

CONCLUSIONS AND IMPLICATIONS

Pimobendan was beneficial regardless of HF stage, but increased sudden cardiac death in end-stage HF with extensive remodelling of Ca(2+) handling. Reduction of cytoplasmic Ca(2+) elevated by PDE3 inhibition might decrease this risk of sudden cardiac death.

Experimental models of inherited cardiomyopathy and its therapeutics

Cardiomyopathy is a disease of myocardium categorized into three major forms, hypertrophic (HCM), dilated (DCM) and restrictive cardiomyopathy (RCM), which has recently been demonstrated to be a monogenic disease due to mutations in various proteins expressed in cardiomyocytes. Mutations in HCM and RCM typically increase the myofilament sensitivity to cytoplasmic Ca²⁺, leading to systolic hyperfunction and diastolic dysfunction. In contrast, mutations in DCM typically decrease the myofilament sensitivity to cytoplasmic Ca²⁺ and/or force generation/transmission, leading to systolic dysfunction. Creation of genetically-manipulated transgenic and knock-in animals expressing mutant proteins exogenously and endogenously, respectively, in their hearts provides valuable animal models to discover the molecular and cellular mechanisms for pathogenesis and promising therapeutic strategy in vivo. Recently, cardiomyocytes have been differentiated from patient’s induced pluripotent stem cells as a model of inherited cardiomyopathies in vitro. In this review, we provide overview of experimental models of cardiomyopathies with a focus on revealed molecular and cellular pathogenic mechanisms and potential therapeutics.

Expression of sarco (endo) plasmic reticulum calcium ATPase (SERCA) system in normal mouse cardiovascular tissues, heart failure and atherosclerosis

The sarco(endo)plasmic reticulum Ca(2+)ATPases (SERCA) system, a key regulator of calcium cycling and signaling, is composed of several isoforms. We aimed to characterize the expression of SERCA isoforms in mouse cardiovascular tissues and their modulation in cardiovascular pathologies (heart failure and/or atherosclerosis). Five isoforms (SERCA2a, 2b, 3a, 3b and 3c) were detected in the mouse heart and thoracic aorta. Absolute mRNA quantification revealed SERCA2a as the dominant isoform in the heart (~99%). Both SERCA2 isoforms co-localized in cardiomyocytes (CM) longitudinal sarcoplasmic reticulum (SR), SERCA3b was located at the junctional SR. In the aorta, SERCA2a accounted for ~91% of total SERCA and SERCA2b for ~5%. Among SERCA3, SERCA3b was the most expressed (~3.3%), mainly found in vascular smooth muscle cells (VSMC), along with SERCA2a and 2b. In failing CM, SERCA2a was down-regulated by 2-fold and re-localized from longitudinal to junctional SR. A strong down-regulation of SERCA2a was also observed in atherosclerotic vessels containing mainly synthetic VSMCs. The proportion of both SERCA2b and SERCA3b increased to 9.5% and 8.3%, respectively.

IN CONCLUSION

1) SERCA2a is the major isoform in both cardiac and vascular myocytes; 2) the expression of SERCA2a mRNA is ~30 fold higher in the heart compared to vascular tissues; and 3) nearly half the amount of SERCA2a mRNA is measured in both failing cardiomyocytes and synthetic VSMCs compared to healthy tissues, with a relocation of SERCA2a in failing cardiomyocytes. Thus, SERCA2a is the principal regulator of excitation-contraction coupling in both CMs and contractile VSMCs.

S100A1 in human heart failure: lack of recovery following left ventricular assist device support

BACKGROUND

We hypothesized that S100A1 is regulated during human hypertrophy and heart failure and that it may be implicated in remodeling after left ventricular assist device. S100A1 is decreased in animal and human heart failure, and restoration produces functional recovery in animal models and in failing human myocytes. With the potential for gene therapy, it is important to carefully explore human cardiac S100A1 regulation and its role in remodeling.

METHODS AND RESULTS

We measured S100A1, the sarcoplasmic endoplasmic reticulum Ca(2+)ATPase, phospholamban, and ryanodine receptor proteins, as well as β-adrenergic receptor density in nonfailing, hypertrophied (left ventricular hypertrophy), failing, and failing left ventricular assist device-supported hearts. We determined functional consequences of protein alterations in isolated contracting muscles from the same hearts. S100A1, sarcoplasmic endoplasmic reticulum Ca(2+)ATPase and phospholamban were normal in left ventricular hypertrophy, but decreased in failing hearts, while ryanodine receptor was unchanged in either group. Baseline muscle contraction was not altered in left ventricular hypertrophy or failing hearts. β-Adrenergic receptor and inotropic response were decreased in failing hearts. In failing left ventricular assist device-supported hearts, S100A1 and sarcoplasmic endoplasmic reticulum Ca(2+)ATPase showed no recovery, while phospholamban, β-adrenergic receptor, and the inotropic response fully recovered.

CONCLUSIONS

S100A1 and sarcoplasmic endoplasmic reticulum Ca(2+)ATPase, both key Ca(2+)-regulatory proteins, are decreased in human heart failure, and these changes are not reversed after left ventricular assist device. The clinical significance of these findings for cardiac recovery remains to be addressed.

Irregular rhythm adversely influences calcium handling in ventricular myocardium: implications for the interaction between heart failure and atrial fibrillation

BACKGROUND

Despite adequate rate control, the combination of atrial fibrillation with heart failure (HF) has been shown, in a number of studies, to hasten HF progression. In this context, we aimed to test the hypothesis that an irregular ventricular rhythm causes an alteration in ventricular cardiomyocyte excitation-contraction coupling which contributes to the progression of HF.

METHODS AND RESULTS

We investigated the effects of electrical field stimulation (average frequency 2 Hz) in an irregular versus regular drive train pattern on the expression of calcium-handling genes and proteins in rat ventricular myocytes. The effect of rhythm on intracellular calcium transients was examined using Fura-2AM fluorescence spectroscopy. In conjunction, calcium-handling protein expression was examined in left ventricular samples obtained from end-stage HF patients, in patients with either persistent atrial fibrillation or sinus rhythm. Compared with regularly paced ventricular cardiomyocytes, in cells paced irregularly for 24 hours, there was a significant reduction in the expression of sarcoplasmic reticulum calcium (Ca(2+)) ATPase together with reduced serine-16 phosphorylation of phospholamban. These findings were accompanied by a 59% reduction (P<0.01) in the peak Ca2+ transient in irregulary paced myocytes compared with those with regular pacing. Consistent with these observations, we observed a 54% (P<0.05) decrease in sarcoplasmic reticulum Ca(2+)ATPase protein expression and an 85% (P<0.01) reduction in the extent of phosphorylation of phospholamban in the left ventricular myocardium of HF patients in atrial fibrillation compared with those in sinus rhythm.

CONCLUSIONS

Together, these data demonstrate that ventricular rhythmicity contributes significantly to excitation-contraction coupling by altering the expression and activity of key calcium-handling proteins. These data suggest that control of rhythm may be of benefit in patients with HF.

Role of brain serotonin dysfunction in the pathophysiology of congestive heart failure

Inherited or non-inherited dilated cardiomyopathy (DCM) patients develop varied disease phenotypes leading to death after developing congestive heart failure (HF) or sudden death with mild or no overt HF symptoms, suggesting that environmental and/or genetic factors may modify the disease phenotype of DCM. In this study, we sought to explore unknown genetic factors affecting the disease phenotype of monogenic inherited human DCM. Knock-in mice bearing a sarcomeric protein mutation that causes DCM were created on different genetic backgrounds; BALB/c and C57Bl/6. DCM mice on the BALB/c background showed cardiac enlargement and systolic dysfunction and developed congestive HF before died. In contrast, DCM mice on the C57Bl/6 background developed no overt HF symptoms and died suddenly, although they showed considerable cardiac enlargement and systolic dysfunction. BALB/c mice have brain serotonin dysfunction due to a single nucleotide polymorphism (SNP) in tryptophan hydroxylase 2 (TPH2). Brain serotonin dysfunction plays a critical role in depression and anxiety and BALB/c mice exhibit depression- and anxiety-related behaviors. Since depression is common and associated with poor prognosis in HF patients, we examined therapeutic effects of anti-depression drug paroxetine and anti-anxiety drug buspirone that could improve the brain serotonin function in mice. Both drugs reduced cardiac enlargement and improved systolic dysfunction and symptoms of severe congestive HF in DCM mice on the BALB/c background. These results strongly suggest that genetic backgrounds involving brain serotonin dysfunction, such as TPH2 gene SNP, may play an important role in the development of congestive HF in DCM.

Ca²+-regulatory proteins in cardiomyocytes from the right ventricle in children with congenital heart disease

BACKGROUND

Hypoxia and hypertrophy are the most frequent pathophysiological consequence of congenital heart disease (CHD) which can induce the alteration of Ca2+-regulatory proteins and inhibit cardiac contractility. Few studies have been performed to examine Ca2+-regulatory proteins in human cardiomyocytes from the hypertrophic right ventricle with or without hypoxia.

METHODS

Right ventricle tissues were collected from children with tetralogy of Fallot [n = 25, hypoxia and hypertrophy group (HH group)], pulmonary stenosis [n = 25, hypertrophy group (H group)], or small isolated ventricular septal defect [n = 25, control group (C group)] during open-heart surgery. Paraffin sections of tissues were stained with 3,3'-dioctadecyloxacarbocyanine perchlorate to measure cardiomyocyte size. Expression levels of Ca2+-regulatory proteins [sarcoplasmic reticulum Ca2+-ATPase (SERCA2a), ryanodine receptor (RyR2), sodiumcalcium exchanger (NCX), sarcolipin (SLN) and phospholamban (PLN)] were analysed by means of real-time PCR, western blot, or immunofluorescence. Additionally, phosphorylation level of RyR and PLN and activity of protein phosphatase (PP1) were evaluated using western blot.

RESULTS

Mild cardiomyocyte hypertrophy of the right ventricle in H and HH groups was confirmed by comparing cardiomyocyte size. A significant reduction of SERCA2a in mRNA (P<0.01) was observed in the HH group compared with the C group. The level of Ser16-phosphorylated PLN was down-regulated (P<0.01) and PP1 was increased (P<0.01) in the HH group compared to that in the C group.

CONCLUSIONS

The decreased SERCA2a mRNA may be a biomarker of the pathological process in the early stage of cyanotic CHD with the hypertrophic right ventricle. A combination of hypoxia and hypertrophy can induce the adverse effect of PLN-Ser16 dephosphorylation. Increased PP1 could result in the decreased PLN-Ser16 and inhibition of PP1 is a potential therapeutic target for heart dysfunction in pediatrics.

Impaired β-adrenergic responsiveness accentuates dysfunctional excitation-contraction coupling in an ovine model of tachypacing-induced heart failure

Reduced inotropic responsiveness is characteristic of heart failure (HF). This study determined the cellular Ca2+ homeostatic and molecular mechanisms causing the blunted β-adrenergic (β-AR) response in HF.We induced HF by tachypacing in sheep; intracellular Ca2+ concentration was measured in voltage-clamped ventricular myocytes. In HF, Ca2+ transient amplitude and peak L-type Ca2+ current (ICa-L) were reduced (to 70 ± 11% and 50 ± 3.7% of control, respectively, P <0.05) whereas sarcoplasmic reticulum (SR) Ca2+ content was unchanged. β-AR stimulation with isoprenaline (ISO) increased Ca2+ transient amplitude, ICa-L and SRCa2+ content in both cell types; however, the response of HF cells was markedly diminished (P <0.05).Western blotting revealed an increase in protein phosphatase levels (PP1, 158 ± 17% and PP2A, 188 ± 34% of control, P <0.05) and reduced phosphorylation of phospholamban in HF (Ser16, 30 ± 10% and Thr17, 41 ± 15% of control, P <0.05). The β-AR receptor kinase GRK-2 was also increased in HF (173 ± 38% of control, P <0.05). In HF, activation of adenylyl cyclase with forskolin rescued the Ca2+ transient, SR Ca2+ content and SR Ca2+ uptake rate to the same levels as control cells in ISO. In conclusion, the reduced responsiveness of the myocardium to β-AR agonists in HF probably arises as a consequence of impaired phosphorylation of key intracellular proteins responsible for regulating the SR Ca2+ content and therefore failure of the systolic Ca2+ transient to increase appropriately during β-AR stimulation.

Computational models reduce complexity and accelerate insight into cardiac signaling networks

Cardiac signaling networks exhibit considerable complexity in size and connectivity. The intrinsic complexity of these networks complicates the interpretation of experimental findings. This motivates new methods for investigating the mechanisms regulating cardiac signaling networks and the consequences these networks have on cardiac physiology and disease. Next-generation experimental techniques are also generating a wealth of genomic and proteomic data that can be difficult to analyze or interpret. Computational models are poised to play a key role in addressing these challenges. Computational models have a long history in contributing to the understanding of cardiac physiology and are useful for identifying biological mechanisms, inferring multiscale consequences to cell signaling activities and reducing the complexity of large data sets. Models also integrate well with experimental studies to explain experimental observations and generate new hypotheses. Here, we review the contributions computational modeling approaches have made to the analysis of cardiac signaling networks and forecast opportunities for computational models to accelerate cardiac signaling research.

What we know and do not know about sex and cardiac disease

Cardiovascular disease (CVD) remains the single leading cause of death in both men and women. A large proportion of the population with CVD will die with a diagnosis of congestive heart failure (CHF). It is becoming increasingly recognized that sex differences exist in the etiology, development, and outcome of CHF. For example, compared to male counterparts, women that present with CHF are typically older and have systolic cardiac function that is not impaired. Despite a growing body of literature addressing the underlying mechanisms of sex dimorphisms in cardiac disease, there remain significant inconsistencies reported in these studies. Given that the development of CHF results from the complex integration of genetic and nongenetic cues, it is not surprising that the elucidation and subsequent identification of molecular mechanisms remains unclear. In this review, key aspects of sex differences in CVD and CHF will be highlighted with an emphasis on some of the unanswered questions regarding these differences. The contention is presented that it becomes critical to reference cellular mechanisms within the context of each sex to better understand these sex dimorphisms.

Regulation of myocardial SERCA2a expression in ventricular hypertrophy and heart failure

Diminished contractility of the hypertrophic cardiomyocyte is a principal determinant of ventricular dysfunction in chronic heart failure. Reduction of activity of the sarcoplasmic/endoplasmic reticulum calcium ion (Ca2+)-ATPase (SERCA2a), underlies many of the effects of overload-induced hypertrophy on cardiomyocyte performance, and it may be critical in the progression of compensatory hypertrophy to heart failure. This review shall focus on the transcriptional regulation of SERCA2a expression as the primary cause of decreased SERCA2a activity in heart failure. Furthermore, the relevance for SERCA2a expression of signal transduction routes involved in pathologic hypertrophy and the possible therapeutic implications, shall be addressed.

Gene and cell therapy for heart failure

Cardiac gene and cell therapy have both entered clinical trials aimed at ameliorating ventricular dysfunction in patients with chronic congestive heart failure. The transduction of myocardial cells with viral constructs encoding a specific cardiomyocyte Ca(2+) pump in the sarcoplasmic reticulum (SR), SRCa(2+)-ATPase has been shown to correct deficient Ca(2+) handling in cardiomyocytes and improvements in contractility in preclinical studies, thus leading to the first clinical trial of gene therapy for heart failure. In cell therapy, it is not clear whether beneficial effects are cell-type specific and how improvements in contractility are brought about. Despite these uncertainties, a number of clinical trials are under way, supported by safety and efficacy data from trials of cell therapy in the setting of myocardial infarction. Safety concerns for gene therapy center on inflammatory and immune responses triggered by viral constructs, and for cell therapy with myoblast cells, the major concern is increased incidence of ventricular arrhythmia after cell transplantation. Principles and mechanisms of action of gene and cell therapy for heart failure are discussed, together with the potential influence of reactive oxygen species on the efficacy of these treatments and the status of myocardial-delivery techniques for viral constructs and cells.

Location and density of alpha- and beta-adrenoreceptor sub-types in myocardium after mechanical left ventricular unloading

BACKGROUND

We hypothesized that not all subtypes of alpha- and beta-adrenoreceptors undergo similar upregulation and redistribution in human myocardium after mechanical unloading with an assist device.

METHODS

We obtained core biopsy samples of the left ventricle in 19 patients before and after removal of a Jarvik or Thoratec left ventricular assist device (LVAD) to study the effect of mechanical unloading on the distribution of alpha- and beta-adrenoreceptors. Fresh, embedded tissue sections were incubated with receptor blockers and antibodies before the fluorescent labeling of receptors. Images were obtained by fluorescence deconvolution microscopy, and composite tissue renditions were made from the stacked images. Multiple adrenoreceptor subtypes were studied.

RESULTS

We saw a reversal of myocyte hypertrophy in all patients, but the upregulation of receptors was not seen in all post-LVAD tissue samples. Furthermore, we noted receptor relocalization from an initial punctate/clumped pattern to a normal homogeneous distribution in many patients. Significant differences were seen in the distribution of beta(2)- and alpha(1)-receptors and in alpha(1A) subtypes.

CONCLUSIONS

In this study we show not only the expected reversal of myocyte hypertrophy and the increase in adrenoreceptors after ventricular unloading, but also the relocalization of specific receptor subtypes.

Multiple alterations in Ca2+ handling determine the negative staircase in a cellular heart failure model

BACKGROUND

The flat or negative force frequency relationship (FFR) is a hallmark of the failing heart. Either decreases in SERCA2a expression, increases in Na(+)/Ca(2+) exchanger (NCX) expression or elevated Na(+)(i) have been independently proposed as mediators of the negative FFR.

METHODS AND RESULTS

To determine whether each one of these mechanisms is sufficient to account for the negative FFR of the failing heart or on the contrary, various mechanisms, acting in concert are required. SERCA2a was pharmacologically inhibited with thapsigargin (TG) or cyclopiazonic acid (CPA) or by using siRNA technology; Na(+)(i) was increased with either ouabain (Oua) or monensin and NCX protein was overexpressed by gene transfer (Ad.NCX), to mimic in nonfailing cat myocytes the phenotype of the failing heart and examine their effect on the FFR. The positive FFR of healthy myocytes remained unaffected after either SERCA2a inhibition, Na(+)(i) elevation, or NCX overexpression. However, the combination of TG + Oua, Oua + Ad.NCX, or TG + Ad.NCX, converted the positive FFR to negative. Moreover, the FFR became negative at lower frequencies, when the 3 interventions were combined.

CONCLUSIONS

Ca(2+) handling has to be altered at several levels to explain the negative FFR of the failing heart. These anomalies in Ca(2+) homeostasis acting in synergy have additive effects.

Left Ventricular Unloading with an Assist Device Results in Receptor Relocalization as well as Increased Beta-Adrenergic Receptor Numbers: Are These Changes Indications for Outcome?

BACKGROUND

The use of left ventricular (LV) assist devices (LVADs) can improve performance and recovery of failing human hearts.

AIM

Following our alpha-adrenergic receptor work, we hypothesized that mechanical unloading in patients with low output syndrome and LV failure would yield similar results with beta-adrenergic receptors ((beta)AR), that being increased numbers and intra-myocytic relocalization.

METHODS

(beta)AR density and localization were investigated by fluorescence deconvolution microscopy and compared at LVAD insertion and removal in 13 heart failure patients, the patients therefore acting as their own control. (beta)AR densities and distribution were determined in snap frozen sections of human core biopsy left ventricular apical tissue. Samples were probed with tagged CGP 12177 for visualization of (beta)AR and challenged with cold agonists and antagonists. (beta)AR density was measured by two independent methods. Localization of receptors was examined in reconstructed, deconvoluted, stacked section images.

RESULTS

There was an increase in (beta)AR density following ventricular unloading in most of the patients, and also significant normalization in the location of the receptors in the myocardium comparing pre- and post-LVAD tissue.

CONCLUSIONS

These findings suggest that supporting an ailing heart via unloading initiates mechanisms and pathways responsible for myocardial recovery and repair. With appropriate pharmacological support, patients with LVAD might recover to the point where they no longer depend on eventual organ transplantation, and (beta)AR number, type, and distribution in pre-LVAD myocardial tissue, could predict outcome with regard to recovery, repair, and improvement in cardiac function.

Low-intensity exercise training delays onset of decompensated heart failure in spontaneously hypertensive heart failure rats

Data regarding the effectiveness of chronic exercise training in improving survival in patients with congestive heart failure (CHF) are inconclusive. Therefore, we conducted a study to determine the effect of exercise training on survival in a well-defined animal model of heart failure (HF), using the lean male spontaneously hypertensive HF (SHHF) rat. In this model, animals typically present with decompensated, dilated HF between approximately 18 and 23 mo of age. SHHF rats were assigned to sedentary or exercise-trained groups at 9 and 16 mo of age. Exercise training consisted of 6 mo of low-intensity treadmill running. Exercise training delayed the onset of overt HF and improved survival (P < 0.01), independent of any effects on the hypertensive status of the rats. Training delayed the myosin heavy chain (MyHC) isoform shift from alpha- to beta-MyHC that was seen in sedentary animals that developed HF. Exercise was associated with a concurrent increase in cardiomyocyte length (approximately 6%), width, and area and prevented the increase in the length-to-width ratio seen in sedentary animals in HF. The increases in proteinuria, plasma atrial natriuretic peptide, and serum leptin levels observed in rats with HF were suppressed by low-intensity exercise training. No significant alterations in sarco(endo)plasmic reticulum Ca2+ ATPase, phospholamban, or Na+/Ca2+ exchanger protein expression were found in response to training. Our results indicate that 6 mo of low-intensity exercise training delays the onset of decompensated HF and improves survival in the male SHHF rat. Similarly, exercise intervention prevented or suppressed alterations in several key variables that normally occur with the development of overt CHF. These data support the idea that exercise may be a useful and inexpensive intervention in the treatment of HF.

Gene therapy for the treatment of heart failure--calcium signaling

The knowledge of molecular mechanisms indicated in cardiac dysfunction has increased dramatically over the last decade and yields considerable potential for new treatment options in heart failure. Alterations in intracellular calcium signaling play a crucial role in the pathophysiology of heart failure, and in recent years, somatic gene transfer has been identified as an important tool to help understand the relative contribution of specific calcium-handling proteins in heart failure. This article reviews recent advances in gene delivery techniques aimed at global myocardial transfection and discusses molecular therapeutic targets identified within intracellular calcium signaling pathways in heart failure.

Is depressed myocyte contractility centrally involved in heart failure?

This review examines the evidence for and against the hypothesis that abnormalities in cardiac contractility initiate the heart failure syndrome and drive its progression. There is substantial evidence that the contractility of failing human hearts is depressed and that abnormalities of basal Ca2+ regulation and adrenergic regulation of Ca2+ signaling are responsible. The cellular and molecular defects that cause depressed myocyte contractility are not well established but seem to culminate in abnormal sarcoplasmic reticulum uptake, storage, and release. There are also strong links between Ca2+ regulation, Ca2+ signaling pathways, hypertrophy, and heart failure that need to be more clearly delineated. There is not substantial direct evidence for a causative role for depressed contractility in the initiation and progression of human heart failure, and some studies show that heart failure can occur without depressed myocyte contractility. Stronger support for a causal role for depressed contractility in the initiation of heart failure comes from animal studies where maintaining or improving contractility can prevent heart failure. Recent clinical studies in humans also support the idea that beneficial heart failure treatments, such as beta-adrenergic antagonists, involve improved contractility. Current or previously used heart failure treatments that increase contractility, primarily by increasing cAMP, have generally increased mortality. Novel heart failure therapies that increase or maintain contractility or adrenergic signaling by selectively modulating specific molecules have produced promising results in animal experiments. How to reliably implement these potentially beneficial inotropic therapies in humans without introducing negative side effects is the major unanswered question in this field.

Upregulation of beta 1-adrenergic receptors in ovariectomized rat hearts

Changes in cardiac myofilament Ca(2+) activation have been demonstrated in ovariectomized rats. The underlying mechanisms responsible for these changes, however, are unknown. Accordingly, we measured both density and binding affinity of cardiac beta(1)-adrenergic receptors in sarcolemmal preparations from 10-week ovariectomized rats, pair-fed ovariectomized rats, and sham-operated control rats. Receptor protein content was also measured by immunoblotting. Deprivation of ovarian sex hormones for 10 weeks induced a significant upregulation of beta(1)-adrenergic receptors without affecting binding affinity. The same magnitude of receptor upregulation was also detected in pair-fed ovariectomized hearts. To determine which hormone is responsible for the observed increase in beta(1)-adrenergic receptor density, various sex hormone supplemental regimens were administered to ovariectomized rats. Subcutaneous injection of estrogen (5 microg/rat), progesterone (1 mg/rat), or estrogen plus progesterone three times a week all effectively prevented the upregulation of the beta(1)-adrenoceptors. Western blot analyses using polyclonal antibody of beta(1)-adrenergic receptors revealed the same pattern of changes in the protein content of the receptors in these various groups of experimental hearts as those obtained from the receptor binding assay. These results suggest a possible direct suppressive effect of ovarian sex hormones on the expression of cardiac beta(1)-adrenergic receptors.

Quantitation and distribution of beta-tubulin in human cardiac myocytes

Increasing evidence suggests that derangements of cytoskeletal proteins contribute to alterations in intracellular signaling, myocyte function, and the coupling of myocytes to the extracellular matrix during cardiac hypertrophy and failure. Data from animal studies have shown an increased density of beta-tubulin protein in the right or left ventricle subjected to pressure overload, and have demonstrated that interfering with excess polymerization of beta-tubulin improves contractility. We tested the hypothesis that beta-tubulin is increased in human left ventricular hypertrophy and end-stage heart failure. Confocal microscopy of fluorescently labeled beta-tubulin protein revealed an increased density of the beta-tubulin network in cardiomyocytes from both hypertrophied and failing human hearts as compared to cells from nonfailing hearts. Western blot analysis on total heart homogenate showed no change in beta-tubulin when data were normalized to either actin or calsequestrin, although there was a significant increase in failing human hearts when data were normalized only for a constant amount of protein per heart. The mRNA for beta-tubulin was not changed in hypertrophied hearts, but was significantly decreased in failing human hearts. Thus, similar to animal models, we have shown that the density of the microtubular network within the cardiomyocyte is increased in end-stage failing human hearts. We have also shown for the first time that beta-tubulin density is increased in cells from hypertrophied human hearts. Although the functional implications of this finding in the human heart remain to be explored, data from animal studies suggest that increased beta-tubulin protein contributes to cardiac dysfunction.