Changes in sarcolemmal PLC isoenzymes in postinfarct congestive heart failure: partial correction by imidapril

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.

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.

Mechanisms for the development of heart failure and improvement of cardiac function by angiotensin-converting enzyme inhibitors

Angiotensin-converting enzyme (ACE) inhibitors, which prevent the conversion of angiotensin I to angiotensin II, are well-known for the treatments of cardiovascular diseases, such as heart failure, hypertension and acute coronary syndrome. Several of these inhibitors including captopril, enalapril, ramipril, zofenopril and imidapril attenuate vasoconstriction, cardiac hypertrophy and adverse cardiac remodeling, improve clinical outcomes in patients with cardiac dysfunction and decrease mortality. Extensive experimental and clinical research over the past 35 years has revealed that the beneficial effects of ACE inhibitors in heart failure are associated with full or partial prevention of adverse cardiac remodeling. Since cardiac function is mainly determined by coordinated activities of different subcellular organelles, including sarcolemma, sarcoplasmic reticulum, mitochondria and myofibrils, for regulating the intracellular concentration of Ca2+ and myocardial metabolism, there is ample evidence to suggest that adverse cardiac remodelling and cardiac dysfunction in the failing heart are the consequence of subcellular defects. In fact, the improvement of cardiac function by different ACE inhibitors has been demonstrated to be related to the attenuation of abnormalities in subcellular organelles for Ca2+-handling, metabolic alterations, signal transduction defects and gene expression changes in failing cardiomyocytes. Various ACE inhibitors have also been shown to delay the progression of heart failure by reducing the formation of angiotensin II, the development of oxidative stress, the level of inflammatory cytokines and the occurrence of subcellular defects. These observations support the view that ACE inhibitors improve cardiac function in the failing heart by multiple mechanisms including the reduction of oxidative stress, myocardial inflammation and Ca2+-handling abnormalities in cardiomyocytes.

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.

Role of angiotensin II in the development of subcellular remodeling in heart failure

The development of heart failure under various pathological conditions such as myocardial infarction (MI), hypertension and diabetes are accompanied by adverse cardiac remodeling and cardiac dysfunction. Since heart function is mainly determined by coordinated activities of different subcellular organelles including sarcolemma, sarcoplasmic reticulum, mitochondria and myofibrils for regulating the intracellular concentration of Ca2+, it has been suggested that the occurrence of heart failure is a consequence of subcellular remodeling, metabolic alterations and Ca2+-handling abnormalities in cardiomyocytes. Because of the elevated plasma levels of angiotensin II (ANG II) due to activation of the renin-angiotensin system (RAS) in heart failure, we have evaluated the effectiveness of treatments with angiotensin converting enzyme (ACE) inhibitors and ANG II type 1 receptor (AT1R) antagonists in different experimental models of heart failure. Attenuation of marked alterations in subcellular activities, protein content and gene expression were associated with improvement in cardiac function in MI-induced heart failure by treatment with enalapril (an ACE inhibitor) or losartan (an AT1R antagonist). Similar beneficial effects of ANG II blockade on subcellular remodeling and cardiac performance were also observed in failing hearts due to pressure overload, volume overload or chronic diabetes. Treatments with enalapril and losartan were seen to reduce the degree of RAS activation as well as the level of oxidative stress in failing hearts. These observations provide evidence which further substantiate to support the view that activation of RAS and high level of plasma ANG II play a critical role in inducing subcellular defects and cardiac dys-function during the progression of heart failure.

An Arrhythmic Mutation E7K Facilitates TRPM4 Channel Activation via Enhanced PIP2 Interaction

A Ca²⁺-activated monovalent cation-selective TRPM4 channel is abundantly expressed in the heart. Recently, a single gain-of-function mutation identified in the distal N-terminus of the human TRPM4 channel (Glu⁵ to Lys⁵; E7K) was found to be arrhythmogenic because of enhanced cell membrane expression. In this study, we conducted detailed analyses of this mutant channel from more functional aspects, in comparison with its wild type (WT). In an expression system, intracellular application of a short soluble PIP₂ (diC₈PIP₂) restored the single-channel activities of both WT and E7K, which had quickly faded after membrane excision. The potency (K_d) of diC₈PIP₂ for this recovery was stronger in E7K than its WT (1.44 vs. 2.40 μM). FRET-based PIP₂ measurements combined with the Danio rerio voltage-sensing phosphatase (DrVSP) and patch clamping revealed that lowering the endogenous PIP₂ level by DrVSP activation reduced the TRPM4 channel activity. This effect was less prominent in E7K than its WT (apparent K_d values estimated from DrVSP-mediated PIP₂ depletion: 0.97 and 1.06 μM, respectively), being associated with the differential PIP₂-mediated modulation of voltage dependence. Moreover, intracellular perfusion of short N-terminal polypeptides containing either the ‘WT’ or ‘E7K’ sequences respectively attenuated the TRPM4 channel activation at whole-cell and single-channel levels, but in both configurations, the E7K polypeptide exerted greater inhibitory effects. These results collectively suggest that N-terminal interaction with endogenous PIP₂ is essential for the TRPM4 channel to function, the extent of which may be abnormally strengthened by the E7K mutation through modulating voltage-dependent activation. The altered PIP₂ interaction may account for the arrhythmogenic potential of this mutation.

Prevention of remodeling in congestive heart failure due to myocardial infarction by blockade of the renin–angiotensin system

Ventricular remodeling subsequent to myocardial infarction (MI) is a complex process and is considered to be a major determinant of the clinical course of congestive heart failure (CHF). Emerging evidence suggests that activation of the renin-angiotensin system (RAS) plays an important role in post-MI ventricular remodeling; however, it is becoming clear that this is one of several neurohumoral systems that are activated in CHF. Blockade of RAS by angiotensin-converting enzyme inhibitors or angiotensin II type 1 receptor antagonists attenuates the ventricular dysfunction, but the effects of individual drugs in reducing the morbidity and mortality in CHF patients are variable. Furthermore, there is a difference of opinion as to the time of initiation of therapy with RAS blockers after the onset of MI. Since blockade of RAS partially improves cardiac function, it is suggested that a combination therapy involving RAS blockers (angiotensin-converting enzyme inhibitors or angiotensin II type 1 receptor antagonists) and agents that affect other neurohumoral systems may prove useful for improved treatment of CHF. Although activation of RAS has been shown to promote oxidative stress in experimental studies, the use of antioxidant therapy in CHF patients is controversial. Recent experimental studies have shown that ventricular remodeling in CHF is associated with remodeling of subcellular organelles such as sarcolemma, sarcoplasmic reticulum, myofibrils and extracellular matrix in terms of their molecular structure and composition. Since attenuation of remodeling in one and/or more subcellular organelles by different agents may prevent the progression of CHF, it is a challenge to develop specific drugs affecting molecular mechanisms associated with subcellular remodeling for the improved therapy of CHF.

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.

Phosphatidylinositol-bisphosphate regulates intercellular coupling in cardiac myocytes

Changes in the lipid composition of cardiac myocytes have been reported during cardiac hypertrophy, cardiomyopathy, and infarction. Because a recent study indicates a relation between low phosphatidylinositol-bisphosphate (PIP(2)) levels and reduced intercellular coupling, we tested the hypothesis that agonist-induced changes in PIP(2) can result in a reduction of the functional coupling of cardiomyocytes and, consequently, in changes in conduction velocity. Intercellular coupling was measured by Lucifer Yellow dye transfer in cultured neonatal rat cardiomyocytes. Conduction velocity was measured in cardiomyocytes grown on microelectrode arrays. Intercellular coupling was reduced by angiotensin II (43.7 +/- 9.3%, N = 11) and noradrenaline (58.0 +/- 10.7%, N = 11). To test if reduced intercellular coupling after agonist stimulation was caused by PIP(2)-depletion, myocytes were stimulated by angiotensin II (57.3 +/- 5.7%, N = 14) and then allowed to recover in medium with or without wortmannin (an inhibitor of PIP(2) synthesis). Intercellular coupling fully recovered in control medium (102.1 +/- 8.9%, N = 10), whereas no recovery occurred in the presence of wortmannin (69.3 +/- 7.8%, N = 12). Inhibition of PKC, calmodulin, or arachidonic acid production did not affect the response to either angiotensin II or noradrenaline. Furthermore, decreasing or increasing PIP(2) also decreased and increased intercellular coupling, respectively. This supports the role of PIP(2) in the regulation of intercellular coupling. In beating myocytes, conduction velocity was reduced by angiotensin II stimulation, and recovery after wash out was prevented by inhibition of PIP(2) production. Reductions in PIP(2) inhibit intercellular coupling in cardiomyocytes, and stimulation by physiologically relevant agonists reduces intercellular coupling by this mechanism. The reduction in intercellular coupling lowered conduction velocity.

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.

Inhibition of phospholipase C-γ1augments the decrease in cardiomyocyte viability by H2O2

The present study was conducted to examine the role of a major cardiac phospholipase C (PLC) isozyme, PLC-γ1, in cardiomyocytes during oxidative stress. Left ventricular cardiomyocytes were isolated by collagenase digestion from adult male Sprague-Dawley rats (250–300 g) and treated with 20, 50, and 100 μM H2O2for 15 min. A concentration-dependent (up to 50 μM) increase in the mRNA level and membrane protein content of PLC-γ1was observed with H2O2treatment. Furthermore, PLC-γ1was activated in response to H2O2, as revealed by an increase in the phosphorylation of its tyrosine residues. There was a marked increase in the phosphorylation of the antiapoptotic protein Bcl-2 by H2O2; this change was attenuated by a PLC inhibitor, U-73122. Although both protein kinase C (PKC)-δ and -ε protein contents were increased in the cardiomyocyte membrane fraction in response to H2O2, PKC-ε activation, unlike PKC-δ, was attenuated by U-73122 (2 μM). Inhibition of PKC-ε with inhibitory peptide (0.1 μM) prevented Bcl-2 phosphorylation. Moreover, different concentrations (0.05, 0.1, and 0.2 μM) of this peptide augmented the decrease in cardiomyocyte viability in response to H2O2. In addition, a decrease in cardiomyocyte viability, as assessed by trypan blue exclusion, due to H2O2was also seen when cells were pretreated with U-73122 and was as a result of increased apoptosis. It is therefore suggested that PLC-γ1may play a role in cardiomyocyte survival during oxidative stress via PKC-ε and phosphorylation of Bcl-2.

Sarcoplasmic reticulum Ca2+ transport and gene expression in congestive heart failure are modified by imidapril treatment

This study was designed to test the hypothesis that blockade of the renin-angiotensin system improves cardiac function in congestive heart failure by preventing changes in gene expression of sarcoplasmic reticulum (SR) proteins. We employed rats with myocardial infarction (MI) to examine effects of an angiotensin-converting enzyme inhibitor, imidapril, on SR Ca2+ transport, protein content, and gene expression. Imidapril (1 mg·kg−1·day−1) was given for 4 wk starting 3 wk after coronary artery occlusion. Infarcted rats exhibited a fourfold increase in left ventricular end-diastolic pressure, whereas rates of pressure development and decay were decreased by 60 and 55%, respectively. SR Ca2+ uptake and Ca2+ pump ATPase, as well as Ca2+ release and ryanodine receptor binding activities, were depressed in the failing hearts; protein content and mRNA levels for Ca2+ pump ATPase, phospholamban, and ryanodine receptor were also decreased by ∼55–65%. Imidapril treatment of infarcted animals improved cardiac performance and attenuated alterations in SR Ca2+ pump and Ca2+ release activities. Changes in protein content and mRNA levels for SR Ca2+ pump ATPase, phospholamban, and ryanodine receptor were also prevented by imidapril treatment. Beneficial effects of imidapril on cardiac function and SR Ca2+ transport were not only seen at different intervals of MI but were also simulated by another angiotensin-converting enzyme inhibitor, enalapril, and an ANG II receptor antagonist, losartan. These results suggest that blockade of the renin-angiotensin system may increase the abundance of mRNA for SR proteins and, thus, may prevent the depression in SR Ca2+ transport and improve cardiac function in congestive heart failure due to MI.

Defective sarcolemmal phospholipase C signaling in diabetic cardiomyopathy

Phospholipase C (PLC) activity is known to influence cardiac function. This study was undertaken to examine the status of PLC beta3 in the cardiac cell plasma membrane (sarcolemma, SL) in an experimental model of chronic diabetes. SL membrane was isolated from diabetic rat hearts at 8 weeks after a single i.v. injection of streptozotocin (65 mg/kg body weight). The total SL PLC was decreased in diabetes and was associated with a decrease in SL PLC beta3 activity, which immunofluorescence in frozen diabetic left ventricular tissue sections revealed to be due to a decrease in PLC beta3 protein abundance. In contrast, the SL abundance of Gqalpha was significantly increased during diabetes. These changes were associated with a loss of contractile function (+/- dP/dt). A 2-week insulin treatment of 6-week diabetic animals partially normalized all of these parameters. These findings suggest a defect in PLC beta3-mediated signaling processes may contribute to the cardiac dysfunction seen during diabetes.

Expression of phospholipase D isozymes in scar and viable tissue in congestive heart failure due to myocardial infarction

The phospholipase D (PLD) associated with the cardiac sarcolemmal (SL) membrane hydrolyses phosphatidylcholine to produce phosphatidic acid, an important phospholipid signaling molecule known to influence cardiac function. The present study was undertaken to examine PLD isozyme mRNA expression, protein contents and activities in congestive heart failure (CHF) subsequent to myocardial infarction (MI). MI was induced in rats by occlusion of the left anterior descending coronary artery. At 8 weeks after the surgical procedure, hemodynamic assessment revealed that these experimental rats were at a moderate stage of CHF. Semi-quantitative reverse transcriptase-polymerase chain reaction revealed that PLD1 and PLD2 mRNA amounts were unchanged in viable left ventricular (LV) tissue of the failing heart. Furthermore, this technique demonstrated the presence of PLD1 and PLD2 mRNA in the scar tissue. While SL PLD1 and PLD2 protein contents were elevated in the viable LV tissue of the failing heart, SL PLD1 activity was significantly decreased, whereas SL PLD2 activity was significantly increased. On the other hand, although PLD1 protein was undetectable, PLD2 protein and activity were detected in the scar tissue. Our findings suggest that differential changes in PLD isozymes may contribute to the pathophysiology of CHF and may also be involved in the processes of scar remodeling.

Imidapril treatment improves the attenuated inotropic and intracellular calcium responses to ATP in heart failure due to myocardial infarction

1. Adenosine 5'-triphosphate (ATP) is known to augment cardiac contractile activity and cause an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in isolated cardiomyocytes. However, no information regarding the ATP-mediated signal transduction in the myocardium in congestive heart failure (CHF) is available. 2. CHF due to myocardial infarction (MI) in rats was induced by the occlusion of the left coronary artery for 8 weeks. The positive inotropy due to ATP was depressed in failing hearts. Treatment of 3 weeks infarcted animals with imidapril (1 mg kg(-1) day(-1)) for a period of 5 weeks improved the left ventricle function and decreased the attenuation of inotropic response to ATP. 3. ATP-induced increase in [Ca(2+)](i) was significantly depressed in cardiomyocytes isolated from the failing heart and this change was partially attenuated by imidapril treatment. However, the binding characteristics of (35)S-labeled adenosine 5'-(gamma-thio) triphosphate in sarcolemma isolated from the failing heart remained unaltered. 4. ATP-induced increase in [Ca(2+)](i) was depressed by verapamil and cibacron blue in both control and failing heart cardiomyocytes; however, the ATP response in the failing hearts, unlike the control preparations, was not decreased by ryanodine. This insensitivity to ryanodine was attenuated by imidapril treatment. 5. Treatment of infarcted rats with enalapril and losartan produced effects similar to imidapril. 6. These findings indicate that the positive inotropic response to ATP and ATP-induced increase in [Ca(2+)](i) in cardiomyocytes are impaired in heart failure. Furthermore, blockade of renin angiotensin system prevented the impairment of the ATP-mediated inotropic and [Ca(2+)](i) responses in the failing heart.

Inhibition of PLC improves postischemic recovery in isolated rat heart

The Ca2+-dependent PLC converts phosphatidylinositol 4,5-bisphosphate to diacylglycerol (DAG) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. Because these products modulate Ca2+ movements in the myocardium, PLC may also contribute to a self-perpetuating cycle that exacerbates cardiomyocyte Ca2+-overload and subsequent cardiac dysfunction in ischemia-reperfusion (I/R). Although we have reported that I/R-induced changes in PLC isozymes might contribute to cardiac dysfunction, the present study was undertaken to examine the beneficial effects of the PLC inhibitor, U-73122, as well as determining the role of Ca2+ on the I/R-induced changes in PLC isozymes. Isolated rat hearts were subjected to global ischemia 30 min, followed by 5 or 30 min of reperfusion. Pretreatment of hearts with U-73122 (0.5 μM) significantly inhibited DAG and Ins(1,4,5)P3 production in I/R and was associated with enhanced recovery of cardiac function as indicated by measurement of left ventricular (LV) end-diastolic pressure (EDP), LV diastolic pressure (LVDP), maximum rate of pressure development (+dP/d tmax), and maximum rate of LV pressure decay (−dP/d tmax). Verapamil (0.1 μM) partially prevented the increase in sarcolemmal (SL) PLC-β1 activity in ischemia and the decrease in its activity during the reperfusion phase as well as elicited a partial protection of the depression in SL PLC-δ1 and PLC-γ1 activities during the ischemic phase and attenuated the increase during the reperfusion period. Although these changes were associated with an improved myocardial recovery after I/R, verapamil was less effective than U-73122. Perfusion with high Ca2+ resulted in the activation of the PLC isozymes studied and was associated with a markedly increased LVEDP and reduced LVDP, +dP/d tmax, and −dP/d tmax. These results suggest that inhibition of PLC improves myocardial recovery after I/R.

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.

Attenuation of changes in G(i)-proteins and adenylyl cyclase in heart failure by an ACE inhibitor, imidapril

Cardiac dysfunction in animals with congestive heart failure due to myocardial infarction (MI) is known to be associated with a wide variety of defects in receptor and post-receptor mechanisms. Since the heart function have been shown to be improved by treatment with different angiotensin converting enzyme (ACE) inhibitors, we examined the effects of imidapril, an ACE inhibitor, on changes in post-receptor mechanisms involving adenylyl cyclase (AC) and G proteins in the failing heart. Heart failure in rats was induced by occluding the coronary artery and 3 weeks later the animals were treated daily with 1 mg/kg (orally) imidapril for 5 weeks. The animals were assessed for their left ventricular function and crude membranes were isolated from the viable left ventricle and examined for AC activities as well as G-protein activities and expression. Animals with heart failure exhibited depressions in ventricular function and AC activities in the absence or presence of forskolin, NaF and Gpp(NH)p. The AC activity in the presence of pertussis toxin was increased whereas that in the presence of cholera toxin was decreased in the failing heart. Protein contents and mRNA levels for G(i)-proteins were increased whereas those for G(s)-proteins were unaltered in the infarcted heart. All these changes due to MI were prevented by imidapril treatment. The results indicate that the depressed cardiac function in the failing heart may partly be due to the direct effects of changes in AC and G(i) proteins.

Increased expression of protein kinase C isoforms in heart failure due to myocardial infarction

The activities of cardiac protein kinase C (PKC) were examined in hemodynamically assessed rats subsequent to myocardial infarction (MI). Both Ca(2+)-dependent and Ca(2+)-independent PKC activities increased significantly in left ventricular (LV) and right ventricular (RV) homogenates at 1, 2, 4, and 8 wk after MI was induced. PKC activities were also increased in both LV and RV cytosolic and particulate fractions from 8-wk infarcted rats. The relative protein contents of PKC-alpha, -beta, -epsilon, and -zeta isozymes were significantly increased in LV homogenate, cytosolic (except PKC-alpha), and particulate fractions from the failing rats. On the other hand, the protein contents of PKC-alpha, -beta, and -epsilon isozymes, unlike the PKC-zeta isozyme, were increased in RV homogenate and cytosolic fractions, whereas the RV particulate fraction showed an increase in the PKC-alpha isozyme only. These changes in the LV and RV PKC activities and protein contents in the 8-wk infarcted animals were partially corrected by treatment with the angiotensin-converting enzyme inhibitor imidapril. No changes in protein kinase A activity and its protein content were seen in the 8-wk infarcted hearts. The results suggest that the increased PKC activity in cardiac dysfunction due to MI may be associated with an increase in the expression of PKC-alpha, -beta, and -epsilon isozymes, and the improvement of heart function in the infarcted animals by imidapril may be due to partial prevention of changes in PKC activity and isozyme contents.

Interruption of signal transduction between G protein and PKC-epsilon underlies the impaired myocardial response to ischemic preconditioning in postinfarct remodeled hearts

We have recently shown that the protective mechanism of ischemic preconditioning (PC) is impaired in the myocardium that survived infarction and underwent postinfarct ventricular remodeling. In this study, we examined the hypothesis that failure of PC to activate PKC-epsilon underlies the refractoriness of the remodeling heart to PC. Circumflex coronary arteries were ligated in rabbits to induce infarction and subsequent ventricular remodeling, and only sham operations were performed in controls. Hearts were isolated before (i.e. 4 days later) or after (i.e. 2 weeks later) remodeling of the left ventricle and used for isolated buffer-perfused heart experiments. Myocardial infarction was induced in isolated hearts by 30 min global ischemia/2 h reperfusion, and its size was measured by tetrazolium staining. Using separate groups of hearts, tissue biopsies were taken before and after PC, and PKC translocation was assessed by Western blotting. Areas infarcted in vivo by coronary ligation (CL) were excluded from subsequent infarct size/PKC analyses. In the hearts 4 days after CL, PC with 2 cycles of 5 min ischemia/5 min reperfusion induced PKC-epsilon translocation from cytosol to particulate fractions and limited infarct size to 40% of control value. In the hearts remodeled 2 weeks after CL, PC failed to induce PKC-epsilon translocation and infarct size limitation. In this group, PKC activity and hemodynamic responses to adenosine were similar to those in sham-operated controls. When remodeling after CL was prevented by valsartan infusion (10 mg/kg/day), an angiotensin II type 1 (AT1) receptor blocker, PC could induce both infarct limitation and PKC-epsilon translocation. The present results suggest that persistent activation of AT1 receptors during remodeling disturbed the PC signaling between G proteins and PKC-epsilon, which underlies the refractoriness of the remodeled myocardium to PC.

Modification of myosin gene expression by imidapril in failing heart due to myocardial infarction

The beneficial effects of imidapril, an angiotensin converting enzyme (ACE) inhibitor were investigated on changes in myofibrillar ATPase as well as myosin heavy chain (MHC) content and gene expression due to myocardial infarction (MI). Three weeks after occluding the left coronary artery, rats were treated with or without imidapril (1 mg/kg/day), for 4 weeks. The infarcted hearts exhibited depressed rates of left ventricular (LV) pressure development (57+/-2.4% reduction) and pressure decay (55.5+/-1.6% reduction). LV myofibrillar Ca(2+) ATPase activity, unlike that in the right ventricle (RV), was decreased in the infarcted animals compared with controls (6.8+/-0.4 vs 10.3+/-0.6 micromol Pi/mg/hr). MHC alpha-isoform contents were decreased by 47 and 41% whereas those of MHC beta-isoform were increased by 823 and 1200% in the LV and RV due to MI, respectively. MHC alpha-isoform mRNA levels were decreased by 55 and 35% whereas those for MHC beta-isoform were increased by 50 and 30% in the infarcted LV and RV, respectively. Imidapril treatment partially prevented the changes due to MI in LV function (rate of pressure development, 24+/-2.3% reduction and rate of pressure decay, 14+/-1.8% reduction), myofibrillar Ca(2+) ATPase activity (8.2+/-0.7 micromol Pi/mg/hr), MHC protein content (alpha-MHC, 24% reduction and beta-MHC, 525% increase) and MHC gene expression (alpha-MHC, 18% reduction and beta-MHC, 15% increase). The results suggest that the beneficial effects of ACE inhibition on the failing heart are associated with improvements in myofibrillar ATPase activities as well as prevention of changes in MHC isozyme protein contents and their gene expression.

Redistribution and abnormal activity of phospholipase A(2) isoenzymes in postinfarct congestive heart failure

Cardiac sarcolemmal (SL) cis-unsaturated fatty acid sensitive phospholipase D (cis-UFA PLD) is modulated by SL Ca(2+)-independent phospholipase A(2) (iPLA(2)) activity via intramembrane release of cis-UFA. As PLD-derived phosphatidic acid influences intracellular Ca(2+) concentration and contractile performance of the cardiomyocyte, changes in iPLA(2) activity may contribute to abnormal function of the failing heart. We examined PLA(2) immunoprotein expression and activity in the SL and cytosol from noninfarcted left ventricular (LV) tissue of rats in an overt stage of congestive heart failure (CHF). Hemodynamic assessment of CHF animals showed an increase of the LV end-diastolic pressure with loss of contractile function. In normal hearts, immunoblot analysis revealed the presence of cytosolic PLA(2) (cPLA(2)) and secretory PLA(2) (sPLA(2)) in the cytosol, with cPLA(2) and iPLA(2) in the SL. Intracellular PLA(2) activity was predominantly Ca(2+) independent, with minimal sPLA(2) activity. CHF increased cPLA(2) immunoprotein and PLA(2) activity in the cytosol and decreased SL iPLA(2) and cPLA(2) immunoprotein and SL PLA(2) activity. sPLA(2) activity and abundance decreased in the cytosol and increased in SL in CHF. The results show that intrinsic to the pathophysiology of post-myocardial infarction CHF are abnormalities of SL PLA(2) isoenzymes, suggesting that PLA(2)-mediated bioprocesses are altered in CHF.