Phosphatidylinositol metabolism in hypertrophic rat heart

Role of Vasoactive Hormone-Induced Signal Transduction in Cardiac Hypertrophy and Heart Failure

Heart failure is the common concluding pathway for a majority of cardiovascular diseases and is associated with cardiac dysfunction. Since heart failure is invariably preceded by adaptive or maladaptive cardiac hypertrophy, several biochemical mechanisms have been proposed to explain the development of cardiac hypertrophy and progression to heart failure. One of these includes the activation of different neuroendocrine systems for elevating the circulating levels of different vasoactive hormones such as catecholamines, angiotensin II, vasopressin, serotonin and endothelins. All these hormones are released in the circulation and stimulate different signal transduction systems by acting on their respective receptors on the cell membrane to promote protein synthesis in cardiomyocytes and induce cardiac hypertrophy. The elevated levels of these vasoactive hormones induce hemodynamic overload, increase ventricular wall tension, increase protein synthesis and the occurrence of cardiac remodeling. In addition, there occurs an increase in proinflammatory cytokines and collagen synthesis for the induction of myocardial fibrosis and the transition of adaptive to maladaptive hypertrophy. The prolonged exposure of the hypertrophied heart to these vasoactive hormones has been reported to result in the oxidation of catecholamines and serotonin via monoamine oxidase as well as the activation of NADPH oxidase via angiotensin II and endothelins to promote oxidative stress. The development of oxidative stress produces subcellular defects, Ca2+-handling abnormalities, mitochondrial Ca2+-overload and cardiac dysfunction by activating different proteases and depressing cardiac gene expression, in addition to destabilizing the extracellular matrix upon activating some metalloproteinases. These observations support the view that elevated levels of various vasoactive hormones, by producing hemodynamic overload and activating their respective receptor-mediated signal transduction mechanisms, induce cardiac hypertrophy. Furthermore, the occurrence of oxidative stress due to the prolonged exposure of the hypertrophied heart to these hormones plays a critical role in the progression of heart failure.

Upregulation of Phospholipase C Gene Expression Due to Norepinephrine-Induced Hypertrophic Response

The activation of phospholipase C (PLC) is thought to have a key role in the cardiomyocyte response to several different hypertrophic agents such as norepinephrine, angiotensin II and endothelin-1. PLC activity results in the generation of diacylglycerol and inositol trisphosphate, which are downstream signal transducers for the expression of fetal genes, increased protein synthesis, and subsequent cardiomyocyte growth. In this article, we describe the signal transduction elements that regulate PLC gene expression. The discussion is focused on the norepinephrine- α₁-adrenoceptor signaling pathway and downstream signaling processes that mediate an upregulation of PLC isozyme gene expression. Evidence is also indicated to demonstrate that PLC activities self-regulate the expression of PLC isozymes with the suggestion that PLC activities may be part of a coordinated signaling process for the perpetuation of cardiac hypertrophy. Accordingly, from the information provided, it is plausible that specific PLC isozymes could be targeted for the mitigation of cardiac hypertrophy.

Role of Phospholipase C in Catecholamine-induced Increase in Myocardial Protein Synthesis

The activation of the α1-adrenoceptor-(α1-AR) by norepinephrine results in the G-protein (Gqα) mediated increase in the phosphoinositide-specific phospholipase C (PLC) activity. The byproducts of PLC hydrolytic activity, namely, 1,2-diacylglycerol and inositol-1,4,5-trisphosphate, are important downstream signal transducers for increased protein synthesis in the cardiomyocyte and the subsequent hypertrophic response. In this article, evidence is outlined to demonstrate the role of cardiomyocyte PLC isozymes in the catecholamine-induced increase in protein synthesis by using a blocker of α1-AR and an inhibitor of PLC. The discussion will be focused on the α1-AR-Gqα-PLC-mediated hypertrophic signaling pathway from the viewpoint that it may compliment the other β1-AR-Gs protein-adenylyl cyclase signal transduction mechanisms in the early stages of cardiac hypertrophy development, but may become more relevant at the late stage of cardiac hypertrophy. From the information provided here, it is suggested that some specific PLC isozymes may potentially serve as important targets for the attenuation of cardiac hypertrophy in the vulnerable patient population at-risk for heart failure.

Phosphatidylinositol-4,5-Bisphosphate Binding to Amphiphysin-II Modulates T-Tubule Remodeling: Implications for Heart Failure

BIN1 (amphyphysin-II) is a structural protein involved in T-tubule (TT) formation and phosphatidylinositol-4,5-bisphosphate (PIP2) is responsible for localization of BIN1 to sarcolemma. The goal of this study was to determine if PIP2-mediated targeting of BIN1 to sarcolemma is compromised during the development of heart failure (HF) and is responsible for TT remodeling. Immunohistochemistry showed co-localization of BIN1, Cav1.2, PIP2, and phospholipase-Cβ1 (PLCβ1) in TTs in normal rat and human ventricular myocytes. PIP2 levels were reduced in spontaneously hypertensive rats during HF progression compared to age-matched controls. A PIP Strip assay of two native mouse cardiac-specific isoforms of BIN1 including the longest (cardiac BIN1 #4) and shortest (cardiac BIN1 #1) isoforms as well human skeletal BIN1 showed that all bound PIP2. In addition, overexpression of all three BIN1 isoforms caused tubule formation in HL-1 cells. A triple-lysine motif in a short loop segment between two helices was mutated and replaced by negative charges which abolished tubule formation, suggesting a possible location for PIP2 interaction aside from known consensus binding sites. Pharmacological PIP2 depletion in rat ventricular myocytes caused TT loss and was associated with changes in Ca²⁺ release typically found in myocytes during HF, including a higher variability in release along the cell length and a slowing in rise time, time to peak, and decay time in treated myocytes. These results demonstrate that depletion of PIP2 can lead to TT disruption and suggest that PIP2 interaction with cardiac BIN1 is required for TT maintenance and function.

Involvement of phospholipase C in the norepinephrine-induced hypertrophic response in Cardiomyocytes

Norepinephrine (NE) is known to mediate cardiomyocyte hypertrophy through the G protein coupled a1 -adrenoceptor (a1 -AR) and the activation of the phosphoinositide-specific phospholipase C (PLC). Since the by-products of PLC activity are important downstream signal transducers for cardiac hypertrophy, the role of and the regulatory mechanisms involved in the activation of PLC isozymes in cardiac hypertrophy are highlighted in this review. The discussion is focused to underscore PLC in different experimental models of cardiac hypertrophy, as well as in isolated adult and neonatal cardiomyocytes treated with NE. Particular emphasis is laid concerning the a1 -AR-PLC-mediated hypertrophic signalling pathway. From the information provided, it is evident that the specific activation of PLC isozymes is a primary signalling event in the a1 -AR mediated response to NE as well as initiation and progression of cardiac hypertrophy. Furthermore, the possibility of PLC involvement in the perpetuation of cardiac hypertrophy is also described. It is suggested that specific PLC isozymes may serve as viable targets for the prevention of cardiac hypertrophy in patient population at-risk for the development of heart failure.

Adipose tissue ATGL modifies the cardiac lipidome in pressure-overload-induced left ventricular failure

Adipose tissue lipolysis occurs during the development of heart failure as a consequence of chronic adrenergic stimulation. However, the impact of enhanced adipose triacylglycerol hydrolysis mediated by adipose triglyceride lipase (ATGL) on cardiac function is unclear. To investigate the role of adipose tissue lipolysis during heart failure, we generated mice with tissue-specific deletion of ATGL (atATGL-KO). atATGL-KO mice were subjected to transverse aortic constriction (TAC) to induce pressure-mediated cardiac failure. The cardiac mouse lipidome and the human plasma lipidome from healthy controls (n = 10) and patients with systolic heart failure (HFrEF, n = 13) were analyzed by MS-based shotgun lipidomics. TAC-induced increases in left ventricular mass (LVM) and diastolic LV inner diameter were significantly attenuated in atATGL-KO mice compared to wild type (wt) -mice. More importantly, atATGL-KO mice were protected against TAC-induced systolic LV failure. Perturbation of lipolysis in the adipose tissue of atATGL-KO mice resulted in the prevention of the major cardiac lipidome changes observed after TAC in wt-mice. Profound changes occurred in the lipid class of phosphatidylethanolamines (PE) in which multiple PE-species were markedly induced in failing wt-hearts, which was attenuated in atATGL-KO hearts. Moreover, selected heart failure-induced PE species in mouse hearts were also induced in plasma samples from patients with chronic heart failure. TAC-induced cardiac PE induction resulted in decreased PC/ PE-species ratios associated with increased apoptotic marker expression in failing wt-hearts, a process absent in atATGL-KO hearts. Perturbation of adipose tissue lipolysis by ATGL-deficiency ameliorated pressure-induced heart failure and the potentially deleterious cardiac lipidome changes that accompany this pathological process, namely the induction of specific PE species. Non-cardiac ATGL-mediated modulation of the cardiac lipidome may play an important role in the pathogenesis of chronic heart failure.

Knockdown of Inhibitory Guanine Nucleotide Binding Protein Giα-2 by Antisense Oligodeoxynucleotides Attenuates the Development of Hypertension and Tachycardia in Spontaneously Hypertensive Rats

BACKGROUND

We previously showed that the levels of both Giα-2 and Giα-3 proteins were augmented in spontaneously hypertensive rats (SHRs) before the onset of hypertension. In addition, intraperitoneal injection of pertussis toxin, which inactivates both Giα proteins, prevented the development of hypertension in SHRs. The aim of the present study was to determine the specific contributions of Giα-2 and Giα-3 proteins to the development of hypertension.

METHODS AND RESULTS

Antisense oligodeoxynucleotide of Giα-2 and Giα-3 encapsulated in PEG/DOTAP/DOPE cationic liposomes were administrated intravenously into 3-week-old prehypertensive SHRs and Wistar Kyoto rats, whereas the control Wistar Kyoto rats and SHRs received PBS, empty liposomes, or sense. The knockdown of Giα-2 but not Giα-3 protein attenuated tachycardia and prevented the development of hypertension up to age 6 weeks; thereafter, blood pressure started increasing and reached the same level as that of untreated SHRs at 9 weeks. Furthermore, Giα-2 and Giα-3 antisense oligodeoxynucleotide treatments significantly decreased the enhanced levels of Giα-2 and Giα-3 proteins, respectively, and enhanced levels of superoxide anion and NADPH oxidase activity in heart, aorta, and kidney and hyperproliferation of vascular smooth muscle cells from SHRs aged 6 weeks. In addition, antisense oligodeoxynucleotide treatment with Giα-2 but not Giα-3 restored enhanced inhibition of adenylyl cyclase by oxotremorine to WKY levels.

CONCLUSIONS

These results suggested that the enhanced expression of Giα-2 but not Giα-3 protein plays an important role in the pathogenesis of hypertension and tachycardia in SHRs.

Cytosolic and nuclear calcium signaling in atrial myocytes: IP3-mediated calcium release and the role of mitochondria

In rabbit atrial myocytes Ca signaling has unique features due to the lack of transverse (t) tubules, the spatial arrangement of mitochondria and the contribution of inositol-1,4,5-trisphosphate (IP3) receptor-induced Ca release (IICR). During excitation-contraction coupling action potential-induced elevation of cytosolic [Ca] originates in the cell periphery from Ca released from the junctional sarcoplasmic reticulum (j-SR) and then propagates by Ca-induced Ca release from non-junctional (nj-) SR toward the cell center. The subsarcolemmal region between j-SR and the first array of nj-SR Ca release sites is devoid of mitochondria which results in a rapid propagation of activation through this domain, whereas the subsequent propagation through the nj-SR network occurs at a velocity typical for a propagating Ca wave. Inhibition of mitochondrial Ca uptake with the Ca uniporter blocker Ru360 accelerates propagation and increases the amplitude of Ca transients (CaTs) originating from nj-SR. Elevation of cytosolic IP3 levels by rapid photolysis of caged IP3 has profound effects on the magnitude of subcellular CaTs with increased Ca release from nj-SR and enhanced CaTs in the nuclear compartment. IP3 uncaging restricted to the nucleus elicites 'mini'-Ca waves that remain confined to this compartment. Elementary IICR events (Ca puffs) preferentially originate in the nucleus in close physical association with membrane structures of the nuclear envelope and the nucleoplasmic reticulum. The data suggest that in atrial myocytes the nucleus is an autonomous Ca signaling domain where Ca dynamics are primarily governed by IICR.

Inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ signaling mediates delayed myogenesis in Duchenne muscular dystrophy fetal muscle

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disorder characterized by muscle wasting and premature death. The defective gene is dystrophin, a structural protein, absence of which causes membrane fragility and myofiber necrosis. Several lines of evidence showed that in adult DMD patients dystrophin is involved in signaling pathways that regulate calcium homeostasis and differentiation programs. However, secondary aspects of the disease, such as inflammation and fibrosis development, might represent a bias in the analysis. Because fetal muscle is not influenced by gravity and does not suffer from mechanical load and/or inflammation, we investigated 12-week-old fetal DMD skeletal muscles, highlighting for the first time early alterations in signaling pathways mediated by the absence of dystrophin itself. We found that PLC/IP3/IP3R/Ryr1/Ca2+ signaling is widely active in fetal DMD skeletal muscles and, through the calcium-dependent PKCα protein, exerts a fundamental regulatory role in delaying myogenesis and in myofiber commitment. These data provide new insights into the origin of DMD pathology during muscle development.

Metabolomic approach to profile functional and metabolic changes in heart failure

Background

Heart failure (HF) is characterized by a series of adaptive changes in energy metabolism. The use of metabolomics enables the parallel assessment of a wide range of metabolites. In this study, we appraised whether metabolic changes correlate with HF severity, assessed as an impairment of functional contractility, and attempted to interpret the role of metabolic changes in determining systolic dysfunction.

Methods

A 500 MHz proton nuclear magnetic resonance (¹H-NMR)-based analysis was performed on blood samples from three groups of individuals: 9 control subjects (Group A), 9 HF patients with mild to moderate impairment of left ventricle ejection fraction (LVEF: 41.9 ± 4.0 %; Group B), and 15 HF patients with severe LVEF impairment (25.3 ± 10.3 %; Group C). In order to create a descriptive model of HF, a supervised orthogonal projection on latent structures discriminant analysis (OPLS-DA) was applied using speckle tracking-derived longitudinal strain rate as the Y-variable in the multivariate analysis.

Results

OPLS-DA identified three metabolic clusters related to the studied groups achieving good values for R² [R²(X) = 0.64; R²(Y) = 0.59] and Q² (0.39). The most important metabolites implicated in the clustering were 2-hydroxybutyrate, glycine, methylmalonate, and myo-inositol.

Conclusions

The results demonstrate the suitability of metabolomics in combination with functional evaluation techniques in HF staging. This innovative tool should facilitate investigation of perturbed metabolic pathways in HF and their correlation with the impairment of myocardial function.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0661-3) contains supplementary material, which is available to authorized users.

Enhanced expression of Gqα and PLC-β1 proteins contributes to vascular smooth muscle cell hypertrophy in SHR: role of endogenous angiotensin II and endothelin-1

Vascular Gqα signaling has been shown to contribute to cardiac hypertrophy. In addition, angiotensin II (ANG II) was shown to induce vascular smooth muscle cell (VSMC) hypertrophy through Gqα signaling; however, the studies on the role of Gqα and PLC-β1 proteins in VSMC hypertrophy in animal model are lacking. The present study was therefore undertaken to examine the role of Gqα/PLC-β1 proteins and the signaling pathways in VSMC hypertrophy using spontaneously hypertensive rats (SHR). VSMC from 16-wk-old SHR and not from 12-wk-old SHR exhibited enhanced levels of Gqα/PLC-β1 proteins compared with age-matched Wistar-Kyoto (WKY) rats as determined by Western blotting. However, protein synthesis as determined by [3H]leucine incorporation was significantly enhanced in VSMC from both 12- and 16-wk-old SHR compared with VSMC from age-matched WKY rats. Furthermore, the knockdown of Gqα/PLC-β1 in VSMC from 16-wk-old SHR by antisense and small interfering RNA resulted in attenuation of protein synthesis. In addition, the enhanced expression of Gqα/PLC-β1 proteins, enhanced phosphorylation of ERK1/2, and enhanced protein synthesis in VSMC from SHR were attenuated by the ANG II AT1 and endothelin-1 (ET-1) ETA receptor antagonists losartan and BQ123, respectively, but not by the ETB receptor antagonist BQ788. In addition, PD98059 decreased the enhanced expression of Gqα/PLC-β1 and protein synthesis in VSMC from SHR. These results suggest that the enhanced levels of endogenous ANG II and ET-1 through the activation of AT1 and ETA receptors, respectively, and MAP kinase signaling, enhanced the expression of Gqα/PLC-β1 proteins in VSMC from 16-wk-old SHR and result in VSMC hypertrophy.

Nuclear inositol 1,4,5-trisphosphate is a necessary and conserved signal for the induction of both pathological and physiological cardiomyocyte hypertrophy

It is well established that inositol 1,4,5-trisphosphate (IP3) dependent Ca(2+) signaling plays a crucial role in cardiomyocyte hypertrophy. However, it is not yet known whether nuclear IP3 represents a Ca(2+) mobilizing pathway involved in this process. The goal of the current work was to investigate the specific role of nuclear IP3 in cardiomyocyte hypertrophic response. In this work, we used an adenovirus construct that selectively buffers IP3 in the nuclear region of neonatal cardiomyocytes. We showed for the first time that nuclear IP3 mediates endothelin-1 (ET-1) induced hypertrophy. We also found that both calcineurin (Cn)/nuclear factor of activated T Cells (NFAT) and histone deacetylase-5 (HDAC5) pathways require nuclear IP3 to mediate pathological cardiomyocyte growth. Additionally, we found that nuclear IP3 buffering inhibited insulin-like growth factor-1 (IGF-1) induced hypertrophy and prevented reexpression of fetal gene program. Together, these results demonstrated that nuclear IP3 is an essential and a conserved signal for both pathological and physiological forms of cardiomyocyte hypertrophy.

The IP3 receptor regulates cardiac hypertrophy in response to select stimuli

RATIONALE

Inositol 1,4,5-trisphosphate (IP(3)) is a second messenger that regulates intracellular Ca(2+) release through IP(3) receptors located in the sarco(endo)plasmic reticulum of cardiac myocytes. Many prohypertrophic G protein-coupled receptor (GPCR) signaling events lead to IP(3) liberation, although its importance in transducing the hypertrophic response has not been established in vivo.

OBJECTIVE

Here, we generated conditional, heart-specific transgenic mice with both gain- and loss-of-function for IP(3) receptor signaling to examine its hypertrophic growth effects following pathological and physiological stimulation.

METHODS AND RESULTS

Overexpression of the mouse type-2 IP(3) receptor (IP(3)R2) in the heart generated mild baseline cardiac hypertrophy at 3 months of age. Isolated myocytes from overexpressing lines showed increased Ca(2+) transients and arrhythmias in response to endothelin-1 stimulation. Although low levels of IP(3)R2 overexpression failed to augment/synergize cardiac hypertrophy following 2 weeks of pressure-overload stimulation, such levels did enhance hypertrophy following 2 weeks of isoproterenol infusion, in response to Galphaq overexpression, and/or in response to exercise stimulation. To inhibit IP(3) signaling in vivo, we generated transgenic mice expressing an IP(3) chelating protein (IP(3)-sponge). IP(3)-sponge transgenic mice abrogated cardiac hypertrophy in response to isoproterenol and angiotensin II infusion but not pressure-overload stimulation. Mechanistically, IP(3)R2-enhanced cardiac hypertrophy following isoproterenol infusion was significantly reduced in the calcineurin-Abeta-null background.

CONCLUSION

These results indicate that IP(3)-mediated Ca(2+) release plays a central role in regulating cardiac hypertrophy downstream of GPCR signaling, in part, through a calcineurin-dependent mechanism.

Phospholipase Cbeta4 isozyme is expressed in human, rat, and murine heart left ventricles and in HL-1 cardiomyocytes

Phospholipase C-beta (PLCbeta) isozymes (PLCbeta(1) and PLCbeta(3)) have been extensively characterized in cardiac tissue, but no data are available for the PLCbeta(4) isozyme. In this study, PLCbeta((1-4)) isozymes mRNA relative expression was studied by real-time PCR (RT-PCR) in human, rat, and murine left ventricle and the presence of PLCbeta(4) isozyme at the protein level was confirmed by Western blotting in all species studied. Confocal microscopy experiments carried out in HL-1 cardiomyocytes revealed a sarcoplasmic subcellular distribution of PLCbeta(4). Although there were unexpected significant interspecies differences in the PLCbeta((1-4)) mRNA expression, PLCbeta(4) mRNA was the main transcript expressed in all left ventricles studied. Thus, whereas in human and rat left ventricles PLCbeta(4) > PLCbeta(3) > PLCbeta(2) > PLCbeta(1) mRNA pattern of expression was found, in murine left ventricle the pattern of expression was different, i.e., PLCbeta(4) > PLCbeta(1) > PLCbeta(3) > PLCbeta(2). However, results obtained in mouse HL-1 cardiomyocytes showed PLCbeta(3) approximately PLCbeta(4) > PLCbeta(1) > PLCbeta(2) pattern of mRNA expression indicating a probable cell type specific expression of the different PLCbeta isozymes in cardiomyocytes. Finally, RT-PCR experiments showed a trend, even though not significant (P = 0.067), to increase PLCbeta(4) mRNA levels in HL-1 cardiomyocytes after angiotensin II treatment. These results demonstrate the presence of PLCbeta(4) in the heart and in HL-1 cardiomyocytes showing a different species-dependent pattern of expression of the PLCbeta((1-4)) transcripts. We discuss the relevance of these findings in relation to the development of cardiac hypertrophy.

Phospholipid-mediated signaling and heart disease

Cardiac hypertrophy, congestive heart failure, diabetic cardiomyopathy and myocardial ischemia-reperfusion injury are associated with a disturbance in cardiac sarcolemmal membrane phospholipid homeostasis. The contribution of the different phospholipases and their related signaling mechanisms to altered function of the diseased myocardium is not completely understood. Resolution of this issue is essential for both the understanding of the pathophysiology of heart disease and for determining if components of the phospholipid signaling pathways could serve as appropriate therapeutic targets. This review provides an outline of the role of phospholipase A2, C and D and subsequent signal transduction mechanisms in different cardiac pathologies with a discussion of their potential as targets for drug development for the prevention/treatment of heart disease.

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

Inositol 1,4,5-trisphosphate (IP(3)) is a ubiquitous intracellular messenger regulating diverse functions in almost all mammalian cell types. It is generated by membrane receptors that couple to phospholipase C (PLC), an enzyme which liberates IP(3) from phosphatidylinositol 4,5-bisphosphate (PIP(2)). The major action of IP(3), which is hydrophilic and thus translocates from the membrane into the cytoplasm, is to induce Ca(2+) release from endogenous stores through IP(3) receptors (IP(3)Rs). Cardiac excitation-contraction coupling relies largely on ryanodine receptor (RyR)-induced Ca(2+) release from the sarcoplasmic reticulum. Myocytes express a significantly larger number of RyRs compared to IP(3)Rs (~100:1), and furthermore they experience substantial fluxes of Ca(2+) with each heartbeat. Therefore, the role of IP(3) and IP(3)-mediated Ca(2+) signaling in cardiac myocytes has long been enigmatic. Recent evidence, however, indicates that despite their paucity cardiac IP(3)Rs may play crucial roles in regulating diverse cardiac functions. Strategic localization of IP(3)Rs in cytoplasmic compartments and the nucleus enables them to participate in subsarcolemmal, bulk cytoplasmic and nuclear Ca(2+) signaling in embryonic stem cell-derived and neonatal cardiomyocytes, and in adult cardiac myocytes from the atria and ventricles. Intriguingly, expression of both IP(3)Rs and membrane receptors that couple to PLC/IP(3) signaling is altered in cardiac disease such as atrial fibrillation or heart failure, suggesting the involvement of IP(3) signaling in the pathology of these diseases. Thus, IP(3) exerts important physiological and pathological functions in the heart, ranging from the regulation of pacemaking, excitation-contraction and excitation-transcription coupling to the initiation and/or progression of arrhythmias, hypertrophy and heart failure.

Impaired activation of ATP-sensitive K+ channels in endocardial myocytes from left ventricular hypertrophy

ATP-sensitive K(+) (K(ATP)) channels are essential for maintaining the cellular homeostasis against metabolic stress. Myocardial remodeling in various pathologies may alter this adaptive response to such stress. It was reported that transmural electrophysiological heterogeneity exists in ventricular myocardium. Therefore, we hypothesized that the K(ATP) channel properties might be altered in hypertrophied myocytes from endocardium. To test this hypothesis, we determined the K(ATP) channel currents using the perforated patch-clamp technique, open cell-attached patches, and excised inside-out patches in both endocardial and epicardial myocytes isolated from hypertrophied [spontaneous hypertensive rats (SHR)] vs. normal [Wistar-Kyoto rats (WKY)] left ventricle. In endocardial cells, K(ATP) channel currents (I(K,ATP)), produced by 2 mM CN(-) and no glucose at 0 mV, were significantly smaller (P < 0.01), and time required to reach peak currents after onset of K(ATP) channel opening (Time(onset to peak)) was significantly longer (319 +/- 46 vs. 177 +/- 37 s, P = 0.01) in the SHR group (n = 9) than the WKY group (n = 13). However, in epicardial cells, there were no differences in I(K,ATP) and Time(onset to peak) between the groups (SHR, n = 12; WKY, n = 12). The concentration-open probability-response curves obtained during the exposure of open cells and excised patches to exogenous ATP revealed the impaired K(ATP) channel activation in endocardial myocytes from SHR. In conclusion, K(ATP) channel activation under metabolic stress was impaired in endocardial cells from rat hypertrophied left ventricle. The deficit of endocardial K(ATP) channels to decreased intracellular ATP might contribute to the maladaptive response of hypertrophied hearts to ischemia.

Phospholipid-mediated signaling systems as novel targets for treatment of heart disease

The phospholipases associated with the cardiac sarcolemmal (SL) membrane hydrolyze specific membrane phospholipids to generate important lipid signaling molecules, which are known to influence normal cardiac function. However, impairment of the phospholipases and their related signaling events may be contributory factors in altering cardiac function of the diseased myocardium. The identification of the changes in such signaling systems as well as understanding the contribution of phospholipid-signaling pathways to the pathophysiology of heart disease are rapidly emerging areas of research in this field. In this paper, I provide an overview of the role of phospholipid-mediated signal transduction processes in cardiac hypertrophy and congestive heart failure, diabetic cardiomyopathy, as well as in ischemia-reperfusion. From the cumulative evidence presented, it is suggested that phospholipid-mediated signal transduction processes could serve as novel targets for the treatment of the different types of heart disease.

Losartan attenuates phospholipase C isozyme gene expression in hypertrophied hearts due to volume overload

Because the left ventricular (LV) hypertrophy due to volume overload induced by arteriovenous (AV) shunt was associated with an increase in phospholipase C (PLC) isozyme mRNA levels, PLC is considered to be involved in the development of cardiac hypertrophy. Since the renin-angiotensin system (RAS) is activated in cardiac hypertrophy, the role of RAS in the stimulation of PLC isozyme gene expression in hypertrophied heart was investigated by inducing AV shunt in Sprague-Dawley rats. The animals were treated with or without losartan (20 mg/kg, daily) for 3 days as well as 1, 2 and 4 weeks, and atria, right ventricle (RV) and LV were used for analysis. The increased muscle mass as well as the mRNA levels for PLC beta1 and beta3 in atria and RV, unlike PLC beta3 gene expression in LV, at 3 days of AVshunt were attenuated by losartan. The increased gene expression for PLC beta1 at 2 weeks in atria, at 1 and 4 weeks in RV, and at 2 and 4 weeks in LV was also depressed by losartan treatment. Likewise, the elevated mRNA levels for PLC beta3 in RV at 1 week and in LVat 4 weeks of cardiac hypertrophy were decreased by losartan. On the other hand, the increased levels of mRNA for PLC gamma1 in RV and LV at 2 and 4 weeks of inducing hypertrophy, unlike in atria at 4 weeks were not attenuated by losartan treatment. While the increased mRNA level for PLC delta1 in LV was reduced by losartan, gene expression for PLC delta1 was unaltered in atria and decreased in RV at 3 days of inducing AV shunt. These results suggest that changes in PLC isozyme gene expression were chamber specific and time-dependent upon inducing cardiac hypertrophy due to AV shunt. Furthermore, partial attenuation of the increased gene expression for some of the PLC isozymes and no effect of losartan on others indicate that both RAS dependent and independent mechanisms may be involved in hypertrophied hearts due to volume overload.

Improvement of defective sarcoplasmic reticulum Ca2+transport in diabetic heart of transgenic rats expressing the human kallikrein‐1 gene

The bradykinin-forming enzyme kallikrein-1 is expressed in the heart. To examine whether contractile performance and sarcoplasmic reticulum Ca2+ transport of the diabetic heart can be rescued by targeting the kallikrein-kinin system, we studied left ventricular function and sarcoplasmic reticular Ca2+ uptake after induction of streptozotocin-induced diabetes mellitus in transgenic rats expressing the human tissue kallikrein-1 gene. Six weeks after a single injection of either streptozotocin (70 mg/kg ip) or vehicle, left ventricular performance was determined using a Millar-Tip catheter system. The Ca2+-transporting activity of reticulum-derived membrane vesicles was determined in left ventricular homogenates as oxalate-supported 45Ca2+ uptake. Western blot analysis was used to quantify the reticular Ca2+-ATPase SERCA2a, phospholamban, and the phosphorylation status of the latter. Contractile performance and Ca2+ uptake activity were similar in nondiabetic wild-type and transgenic rats. Severely diabetic wild-type animals exhibited impaired left ventricular performance and decreased reticular Ca2+ uptake (-39% vs. wild-type rats, P<0.05, respectively). These changes were attenuated in diabetic transgenic rats that, in addition, exhibited a markedly increased phospholamban phosphorylation at the Ca2+/calmodulin kinase-specific site threonine17 (2.2-fold vs. diabetic wild-type rats, P<0.05). These transgene-related effects were abolished after treatment with the bradykinin B2 receptor antagonist icatibant (Hoe 140). The SERCA2-to-phospholamban ratio, phosphoserine16-phospholamban levels, and the apparent affinity for Ca2+ of the uptake reaction did not differ between the groups. Increasing the activity of the kallikrein-kinin system by expressing a human kallikrein-1 transgene protects rat heart against diabetes-induced contractile and reticular Ca2+ transport dysfunctions. An increased phosphorylation of the SERCA2 regulatory protein phospholamban at threonine17 via a B2 receptor-mediated mechanism is thereby involved.

Phospholipase C gene expression, protein content, and activities in cardiac hypertrophy and heart failure due to volume overload

Volume overload due to arteriovenous (AV) shunt results in cardiac hypertrophy followed by the progression to heart failure. The phosphoinositide phospholipase C (PLC) converts phosphatidylinositol 4,5-bisphosphate (PIP(2)) to 1,2-diacylglycerol (DAG) and inositol (1,4,5)-trisphosphate (IP(3)), which are known to influence cardiac function. Therefore, we examined the time course of changes in DAG and IP(3) as well as PLC isozyme gene expression, protein content, and activities in cardiac hypertrophy and heart failure induced by AV shunt in Sprague-Dawley rats by the needle technique. An increase in the left ventricle (LV)-to-body weight ratio demonstrated that LV hypertrophy was established at 4 wk after the induction of the shunt. PLC-beta(1) activity was increased two- and sevenfold at 3 days and 1 and 2 wk after the induction of volume overload, respectively. These changes were associated with increases in the mRNA and sarcolemmal (SL) protein content; however, no changes in PLC-beta(1) were detected at 4 wk. On the other hand, a significant increase in PLC-gamma(1) activity as well as mRNA and SL protein was seen at 3 days and 4 wk. A progressive decrease in PLC-delta(1) activity with concomitant reductions in the gene expression and SL protein abundance was detected during 1 to 4 wk. Activity of gamma(1)- and delta(1)-isozymes was significantly depressed during the 8- and 16-wk time points, whereas beta(1)-isozyme was increased significantly during these time points. A progressive decrease in the SL PIP(2) content was observed during cardiac hypertrophy and heart failure. Our findings indicate that PLC isozyme signaling processes are increased in hypertrophy and decreased in heart failure due to volume overload.

Temporal characteristics of cardiomyocyte hypertrophy in the spontaneously hypertensive rat

BACKGROUND

The spontaneously hypertensive rat (SHR) is frequently used as model of cardiovascular disease, with considerable disparity in reported parameters of hypertrophy. The aim of this study was to assess the temporal changes occurring during the development and progression of cardiomyocyte hypertrophy in SHR, subsequent to pressure overload, compared to changes associated with normal aging using the normotensive Wistar-Kyoto (WKY) rat.

METHODS

Ventricular cardiomyocytes were isolated from rats at 8, 12, 16, 20 and 24 weeks, and parameters of hypertrophy (cell dimensions, protein mass, de novo protein synthesis, and gene expression) and function (contraction and hypertrophic responsiveness in vitro) were assessed.

RESULTS

Hypertension was evident at > or =7 weeks in SHRs. Heart:body mass ratio, cardiomyocyte protein mass and width were elevated (P<.05) in SHRs at 16-20 weeks compared to WKYs. In SHRs compared to WKYs at 16 weeks, there was a transient increase (P<.05) in protein synthesis, enhanced hypertrophic responsiveness to phorbol-12-myristate-13-acetate, and induced hypertrophic responsiveness to isoprenaline. Skeletal-alpha-actin mRNA was detected in SHR but not WKY cells at all ages. ANP mRNA was lower in SHR than in WKY cells at 8-20, but progressively increased (P<.05) from 12 to 24 weeks within SHRs. Contractile function increased (P<.05) at 20 weeks in SHR compared to WKY rats.

CONCLUSION

Structural and functional changes occurring at the cellular level in the myocardium of SHR follow a distinct pattern, such that pressure overload was initially accompanied by expressional changes (8-12 weeks), followed by active hypertrophic growth and enhanced function (16-20 weeks), which subsequently decelerated as stable compensation was attained.

Characterization of myocardium, isolated cardiomyocytes, and blood pressure in WKHA and WKY rats

We previously reported that the left ventricular (LV) mass of Wistar-Kyoto (WKY)-derived hyperactive (WKHA) rats was higher than that of WKY rats in the absence of a difference in systolic blood pressure. To extend these earlier observations, we conducted a series of functional and morphological investigations on both strains. Analysis of tissue sections revealed that the surface of ventricular tissue from WKHA rats was higher than that of WKY rats, without any enlargement of the cavity area. Analysis of isolated adult cells showed that cell width (as well as cell volume) of ventricular cardiomyocytes was significantly higher in WKHA than WKY rats. However, LV of WKHA rats contained approximately 33% less cardiomyocytes than those from WKY rats. Mean intracellular free calcium concentration of cardiomyocytes was also higher in WKHA than WKY rats. Hemodynamic measurements revealed that the values of the maximum rates of pressure change (dP/dt) were higher in LV from WKHA rats. However, these differences were reduced (-dP/dt) or abolished (+dP/dt) when the values were normalized for both the number and mean cross-sectional area of ventricular cardiomyocytes. Mean levels of systolic and diastolic blood pressure (corresponding to the 24-h average of measurements obtained continuously in conscious unrestrained animals using radiotelemetric implants) were not different between strains. However, circadian rhythm was more evident in WKY rats, because the difference between morning and night values of systolic and diastolic blood pressure was greater (by 3 mmHg) in WKY rats. Altogether, our data validate the use of WKHA rats as models of predominantly concentric LV hypertrophy developing in the absence of increased mean levels of hemodynamic cardiac load and show that the hypertrophy phenotype is more pronounced in isolated cardiomyocytes than at the level of the whole ventricle.

[18F] labeled diacylglycerol analogue as a potential agent to trace myocardial phosphoinositide metabolism

Phosphoinositide metabolism plays an important role in cardiac pathophysiology. To investigate whether [18F]diacylglycerol could be used to trace myocardial phosphoinositide metabolism, lipids were extracted from rat myocardium after the injection. 1-[8-[18F]fluorooctanoyl]-2-palmitoylglycerol and 1-[8-[18F]fluoropalmitoyl]-2-palmitoylglycerol were predominantly metabolized to phosphatidylethanolamine and triacylglycerol, respectively. The radioactivity incorporated into phosphoinositide metabolism was 51, 44, 32, and 30% 3, 5, 10, and 30 minutes after the injection of 1-[4-[18F]fluorobutyryl]-2-palmitoylglycerol, respectively. 1-[4-[18F]fluorobutyryl]-2-palmitoylglycerol might be a potential tracer to evaluate myocardial phosphoinositide metabolism early after the injection.

Abnormal intracellular ca(2+)homeostasis and disease

A whole range of cell functions are regulated by the free cytosolic Ca(2+)concentration. Activator Ca(2+)from the extracellular space enters the cell through various types of Ca(2+)channels and sometimes the Na(+)/Ca(2+)-exchanger, and is actively extruded from the cell by Ca(2+)pumps and Na(+)/Ca(2+)-exchangers. Activator Ca(2+)can also be released from internal Ca(2+)stores through inositol trisphosphate or ryanodine receptors and is taken up into these organelles by means of Ca(2+)pumps. The resulting Ca(2+)signal is highly organized in space, frequency and amplitude because the localization and the integrated free cytosolic Ca(2+)concentration over time contain specific information. Mutations or functional abnormalities in the various Ca(2+)transporters, which in vitro seem to induce trivial functional alterations, therefore, often lead to a plethora of diseases. Skeletal-muscle pathology can be caused by mutations in ryanodine receptors (malignant hyperthermia, porcine stress syndrome, central-core disease), dihydropyridine receptors (familial hypokalemic periodic paralysis, malignant hyperthermia, muscular dysgenesis) or Ca(2+)pumps (Brody disease). Ca(2+)-pump mutations in cutaneous epidermal keratinocytes and cochlear hair cells lead to, skin diseases (Darier and Hailey-Hailey) and hearing/vestibular problems respectively. Mutated Ca(2+)channels in the photoreceptor plasma membrane cause vision problems. Hemiplegic migraine, spinocerebellar ataxia type-6, one form of episodic ataxia and some forms of epilepsy can be due to mutations in plasma-membrane Ca(2+)channels, while antibodies against these channels play a pathogenic role in all patients with the Lambert-Eaton myasthenic syndrome and may be of significance in sporadic amyotrophic lateral sclerosis. Brain inositol trisphosphate receptors have been hypothesized to contribute to the pathology in opisthotonos mice, manic-depressive illness and perhaps Alzheimer's disease. Various abnormalities in Ca(2+)-handling proteins have been described in heart during aging, hypertrophy, heart failure and during treatment with immunosuppressive drugs and in diabetes mellitus. In some instances, disease-causing mutations or abnormalities provide us with new insights into the cell biology of the various Ca(2+)transporters.

Vasopressin accelerates protein synthesis in neonatal rat cardiomyocytes

Arginine vasopressin (AVP) has been shown to promote vascular smooth muscle cell hypertrophy and hyperplasia of fibroblasts. The present study examines the effect of AVP and endothelin-1 (ET-1) on protein, DNA, and RNA synthesis in primary cultures of serum deprived neonatal rat cardiomyocytes (RC) as assessed by changes in [3H] phenylalanine, [3H] thymidine, and [14C] uridine incorporation respectively. Both AVP and ET-1 evoked significant increases in protein synthesis in RC of 36 +/- 12% (p < 0.05) and 53 +/- 22% (p < 0.01) respectively. The stimulating action of AVP on [3H] phenylalanine incorporation was abolished by pretreatment with 2-nitro-4carboxyphenyl-N, N-diphenylcarbamate (NCDC), a phospholipase C (PLC) inhibitor. [14C] uridine incorporation was significantly higher in cells incubated with ET-1 (95 +/- 12%) but not AVP (9 +/- 11%). Neither AVP nor ET-1 significantly affected cell number or [3H]thymidine incorporation, suggesting a lack of a hyperplastic effect. AVP evoked an increase in [Ca2+]i levels (162 +/- 12 nmol/L from a basal value of 77 +/- 6 nmol/L) which was completely abolished by pretreatment with either NCDC or cyclopiazonic acid (sarcoplasmic reticulum (SR) Ca2+ pump inhibitor) but unaffected by ryanodine (ryanodine sensitive SR Ca2+ store depletor). Taken together, these data suggest that AVP, in a PLC dependent manner, both stimulates protein synthesis and augments [Ca2+]i release in RC from ryanodine insensitive (IP3 sensitive) Ca2+ stores. Thus, AVP may promote cardiac hypertrophy via direct effects on cardiomyocyte protein synthesis secondary to IP3 mediated [Ca2+]i release.

Ischaemic preconditioning protects against myocardial dysfunction caused by ischaemia in isolated hypertrophied rat hearts

Although ischaemic preconditioning (PC) has been shown to protect normal hearts from a subsequent ischaemic insult, its protective effect on the hypertrophied myocardium has not been widely studied. This study was designed to investigate whether ischaemic preconditioning protects hearts with hypertrophy (HYP). Cardiac HYP was produced in rats by suprarenal abdominal aortic constriction of 5 weeks' duration, and was defined as left ventricular weight: body weight [LVW: BW (mg/g)] ratio over 3.0. Isolated rat hearts were perfused with a modified Krebs-Henseleit buffer at 37 degrees C in a Langendorff preparation. Hearts from sham-operated animals (NORM) and those with HYP underwent a PC protocol consisting of 3 min of global zero flow ischaemia, 5 min of reperfusion followed by 5 min of ischaemia and 5 min of reperfusion. This was followed by 20 min ischaemia and 45 min reperfusion. Control hearts in the HYP and NORM groups were not subjected to the PC protocol. There were, thus, four experimental groups: NORM control (n = 9), NORM, PC (n = 9), HYP control (n = 9), HYP, PC (n = 11). The recovery of function after ischaemia was evaluated by recovery of left ventricular developed pressure (LVDP) expressed as % of the initial value (LVDP%). The LVW: BW ratio for the HYP groups was 3.4 (SEM 0.08). LVDP% was higher (p < 0.01) in preconditioned groups as compared with controls. In NORM control recovery was 49.3 (6.1), NORM, PC 76.5 (3.4), HYP control 39.8 (4.6) HYP, PC 70.1 (4.1). These data indicate that the ability of preconditioning to protect against ischaemic ventricular dysfunction is preserved in this model of cardiac hypertrophy.

Heterotrimeric G proteins in heart disease

Heterotrimeric G proteins couple many types of cell surface receptors to intracellular effectors such as enzymes or ion channels. In the mammalian heart, G protein-mediated signalling pathways are involved in the regulation of contractile force, heart rate, conduction velocity, and relaxation. In the first part of this review we summarize some important structural and functional features of receptors, G proteins, and effectors with special focus on the heart. In the second part, we review the current knowledge about alterations of G protein-mediated signalling in heart disease such as myocardial hypertrophy and heart failure.

Alteration of extracellular matrix in dilated cardiomyopathic hamster heart

The purpose of this study was to characterize the collagen in hereditary dilated cardiomyopathic hamster hearts, and to examine the participation of the collagen in the occurrence and progression of cardiomyopathy. BIO 53.58 hamsters (5, 10, 20 weeks old) were used as the model of dilated cardiomyopathy. Flb hamsters were used as controls. The collagen content was almost constant at any age in the Flb hamsters, but increased with age in BIO 53.58 hamsters. Type III collagen increased significantly in BIO 53.58 hamsters at 10 weeks. The acetic acid solubility of collagen decreased in BIO 53.58 hamsters as the fibrosis progressed, but was unchanged in controls. Reducible crosslinks showed a tendency to decrease progressively in BIO 53.58 hamsters. There were no differences between Flb and BIO 53.58 hamsters at 5 weeks, but its expression in BIO 53.58 hamsters at 10 and 20 weeks of age increased compared to Flb controls. These findings indicate that in the early phase of cardiomyopathy the extracellular matrix of the myocardium is rich in type III collagen. In the later phase, the matrix resembles that of hard tissues, whose collagen is mainly of type I collagen and is insoluble. These data suggest that the increased collagen synthesis may impair the cardiac function in the development of cardiomyopathy.

Renin-angiotensin system in failing heart

Extrahepatic synthesis and localization of angiotensinogen have been described in animals, thus establishing the tissue renin-angiotensin system. We examined angiotensinogen messenger RNA synthesis by northern blotting. It was detected not only in the liver, but also in both the atrial and ventricular heart tissues, suggesting that angiotensinogen is synthesized in the human heart. Immunohistochemical studies using a specific antibody to angiotensinogen revealed a stronger reaction in the endocardial layer of the human left ventricle, than in the epicardial layer, and intense immunoreactivity in the conduction system and right atrium. Furthermore, our experiments revealed a widespread immunopositive reaction for angiotensinogen in the left ventricle of diseased hearts. We examined the participation of the collagen in the occurrence and progression of cardiomyopathy. The acetic acid solubility of collagen and reducible crosslink decreased in cardiomyopathic hamsters as the fibrosis progressed, but was unchanged in controls. These findings indicate that in the early phase of cardiomyopathy the extracellular matrix of the myocardium is similar to immature tissues. In the later phase, the matrix resembles that of hard tissues, and is insoluble. Furthermore, we examined the relationship between angiotensin II and collagen synthesis. Basal collagen synthesis in cardiac fibroblasts from spontaneously hypertensive rats was 1.6-fold greater than that in Wistar-Kyoto rats. The responsiveness of collagen production to Ang II was significantly enhanced in SHR. This effect was angiotensin receptor-specific, because it was blocked by the competitive inhibitor. These results indicate angiotensin II may play an important role in collagen accumulation in hypertensive cardiac hypertrophy.

Distribution of angiotensinogen in diseased human hearts

Extrahepatic synthesis and localization of angiotensinogen (ATN) have been described in animals, thus establishing the tissue renin-angiotensin (RA) system. However, there had been no reports of tissue RA systems in human organs, including the heart. In earlier, we have reported the possibility of ATN synthesis in the human heart using ribonuclease protection assay system. ATN mRNA was detected not only in the liver, but also in both the atrial and ventricular heart tissues, suggesting that ATN is synthesized in the human heart. In this report, we looked for the distribution of ATN in diseased human heart. Northern blot hybridization of cDNA with total RNA extracted from human liver, brain, kidney, atrial and ventricular tissues revealed that ATN mRNA exists in cardiac ventricule. Immunohistochemical studies using a specific antibody to ATN revealed a stronger reaction in the endocardial layer of the human left ventricle, than in the epicardial layer, and intense immunoreactivity in the conduction system and right atrium. This distribution pattern was similar to that of human atrial natriuretic peptide (hANP), which functions a smooth muscle relaxant. Double immunostaining of ATN and hANP demonstrated that all myocytes in the right atrium had immunopositive reactions to ATN, hANP or both of ATN and hANP. Double immunoelectron staining enabled us to show more detailed localization of ATN and hANP; hANP only existed in the specific granules and ATN existed in the myofibril, but not in the granule. Furthermore, our experiments provide evidence of ATN in healthy human hearts and also reveal a widespread immunopositive reaction for ATN in the left ventricle of diseased hearts.