Toward an understanding of the molecular basis of cardiomyopathies

Beneficial Effect of Eplerenone on Cardiac Remodelling and Electrical Properties of the Failing Heart

Introduction. The effect of chronic administration of eplerenone on cardiac remodelling and electrical properties was investigated in the failing heart of cardiomyopathic hamsters (TO-2) at five months of age.

Materials and Methods. Two-month-old hamsters were treated with eplerenone (200 mg/kg/day) administered into the chow for a period of three months. Measurements of membrane potential were performed with intracellular microelectrodes connected to a high impedance DC amplifier. The thickness of the ventricular wall as well as the area of fibrosis were measured. To investigate the influence of eplerenone on the electrogenic sodium pump myocytes were isolated from the ventricle and the pump current density was whole cell clamp configuration. measured in voltage clamped cells using the whole cell clamp configuration

Results. The results indicated that: 1) the width of the left and right ventricular wall was significantly reduced; 2) the heart weight/body weight ratio was decreased by 38±2.4% (n=24 P ) (p<0.05); 3) the fibrotic area in the left ventricle (LV) was reduced by 12.6±2% (n=25) (p<0.05); 4) the incidence of cardiac arrhythmias was decreased from 58±3.8% (n=20) in the control to 40±4.1% (n=20) (p<0.05) in animals treated with eplerenone. Moreover, a significant reduction in the dispersion of the QT interval was found with the drug; 5) eplerenone increased the resting potential of ventricular fibres from 64.3±1.5 mV to 73.4±1.4 mV (n=30) (p<0.05), an effect related to the activation of an electrogenic sodium pump. The conduction velocity, in longitudinal direction, was enhanced from 50±2.2 cm/s (n=10) in the controls to 59±2.4 cm/s (n=13) (p<0.05) in animals treated with eplerenone.

Conclusions. Eplerenone reduces cardiac remodelling, the incidence of cardiac arrhythmias and improves impulse propagation, an effect in part related to the antifibrotic effect of the drug but also to the activation of the electrogenic sodium pump.

Calcium channel blockers and modulation of innate immunity

PURPOSE OF REVIEW

Researchers and physicians are gaining more understanding of the utility of calcium channel blockers (CCBs) especially in modulation of innate immunity, and choose suitable ones in clinical practice. This review summarizes the recent related research findings.

RECENT FINDINGS

Sustained and/or dysregulated expression of pro-inflammatory cytokines is sufficient to produce tissue injury and provoke overt cardiac decompensation. The important question that remains to be addressed is whether or not it will be possible to modulate the inappropriate or maladaptive consequences of innate immune activation and pro-inflammatory cytokine expression in the mammalian heart. CCBs, such as nifedipine, amlodipine, diltiazem, and verapamil, promote the relaxation of cardiac and smooth muscle cells by inhibiting calcium influx through calcium channels and calcium release from intracellular stores, and are commonly used in the treatment of cardiovascular disorders. Recently, several in-vitro studies have shown that, besides the effects they exert on muscle cells, CCBs also suppress the activation of various participants in immune reactions, including T cells, mast cells and macrophages, suggesting that they can be immunosuppressant.

SUMMARY

CCBs maybe suppress the activation of various participants in immune reactions.

Renin increments the inward calcium current in the failing heart

BACKGROUND

Evidence is available that activation of the renin-angiotensin system is involved in cardiac remodeling. It is unknown whether renin can change the inward calcium current (ICa) in the failing heart. This problem was investigated in the present study.

METHODS

Cardiomyocytes were isolated from the ventricle of 4-month-old cardiomyopathic hamsters and measurements of the L-type ICa were performed using the patch-clamp technique in a whole-cell configuration.

RESULTS

Extracellular renin (128 pmol Ang I/ml per min) plus angiotensinogen (110 pmol angiotensin I generated by renin to exhaustion) incremented the peak ICa density significantly, an effect suppressed by enalapril maleate (10 mol/l) or by losartan (10 mol/l) added to the bath, indicating that the effect of renin plus angiotensinogen was related to the formation of angiotensin I and its conversion to angiotensin II at the surface cell membrane. Renin internalization seems to increment the ICa because intracellular dialysis of renin (128 pmol Ang I/ml per min) plus angiotensinogen (110 pmol angiotensin I generated by renin to exhaustion) also increased the peak ICa density significantly, an effect suppressed by intracellular losartan (10 mol/l) but not by extracellular losartan (10 mol/l).

CONCLUSIONS

Extracellular renin plus angiotensinogen increases the ICa in isolated myocytes from the failing heart of cardiomyopathic hamsters through the formation of angiotensin II and the activation of angiotensin type 1 receptors at the surface cell membrane. A similar increment of ICa was found with intracellular administration of renin plus angiotensinogen. This finding might indicate that renin internalization is involved in control of inward calcium current in the failing heart.

Intracellular and extracellular angiotensin II enhance the L-type calcium current in the failing heart

The influence of intracellular and extracellular angiotensin II (Ang II) on the L-type calcium current of cardiomyocytes isolated from cardiomyopathic hamsters was investigated. The results indicated that Ang II (10(-8) mmol/L), added to the bath, increased the peak inward calcium current (I(Ca)) density by 37+/-3.4% (P<0.05), an effect that depends on the activation of protein kinase C. Intracellular administration of the same dose of Ang II (10(-8) mmol/L) also elicited an increase of peak I(Ca) density but enhanced the rate of I(Ca) inactivation, an effect not seen with extracellular Ang II. Moreover, in control animals, no change in the rate of I(Ca) inactivation was seen with intracellular Ang II. Thapsigargin (1 micromol/L), a potent inhibitor of sarcoplasmic reticulum (SR) ATPase, which depletes the SR, decreased the rate of I(Ca) inactivation elicited by intracellular Ang II, although the cytoplasmic calcium concentration was highly buffered with 10 mmol/L EGTA. These findings might indicate that intracellular Ang II releases calcium from the SR and inactivates I(Ca). The effect of intracellular Ang II on peak I(Ca) was not altered by extracellular losartan (10(-7) mmol/L), supporting the notion that the peptide acted intracellularly. Other studies showed that intracellular Ang I administration (10(-8) mmol/L) enhanced the peak I(Ca) density and the rate of I(Ca) inactivation, an effect that was reduced by intracellular enalaprilat (10(-8) mmol/L). Moreover, intracellular enalaprilat by itself reduced the peak I(Ca) density. These observations might indicate that endogenous Ang II is contributing to I(Ca) modulation in the failing heart.

Cell coupling and impulse propagation in the failing heart

Cell coupling and impulse propagation were investigated in the ventricle of cardiomyopathic hamsters at an advanced stage of heart failure. An appreciable decline in junctional conductance was found, a phenomenon in part related to activation of the plasma and cardiac renin-angiotensin systems. Decreased expression of connexin43 or an alteration of junctional proteins also might be implicated in the decreased cell coupling. Morphologic abnormalities such as fibrosis, necrosis, and rupture of cell contacts contribute to the decline of conduction velocity or to the blockade of impulse propagation in some areas of the ventricle, creating the conditions for anisotropic conduction and cardiac arrhythmias. The decrease in membrane potential found in myopathic cells is related in part to depression of Na-KATPase activity, and the lack of action of beta-adrenergic agonists on junctional conductance is explained by down-regulation of beta receptors and an abnormality of adenyl cyclase.

Effect of tedisamil on cell communication, impulse propagation, and excitability of the failing heart

In the present work, the effect of tedisamil on gap junctional conductance (gj) and conduction velocity was investigated in the failing heart of cardiomyopathic hamsters (TO-2 strain). It was found that tedisamil (10(-7) M) increased gj by 53.8+/-1% (n = 23) in cell pairs isolated from 2 months old cardiomyopathic hamsters. The effect of tedisamil was suppressed by intracellular dialysis of an inhibitor of protein kinase A and also by adenosine indicating that the drug increases gj through the activation of adenylcyclase. Tedisamil also increased the conduction velocity and cardiac refractoriness of ventricular muscle from young cardiomyopathic hamsters. At an advanced stage of the disease, however, when the beta-adrenoceptor, adenylcyclase signaling system is impaired, tedisamil was unable to increase gj. The present results indicate that the antiarrhythmic action of tedisamil is in part related to an increase in junctional conductance and conduction velocity.

Atrial natriuretic factor reduces cell coupling in the failing heart, an effect mediated by cyclic GMP

The influence of the atrial natriuretic factor (ANF) on heart-cell communication was investigated in cell pairs isolated from the ventricle of cardiomyopathic hamsters (BIO TO-2; 11 months old), and the results were compared with controls (F1B) of same age. The results indicated that ANF (10(-8) M) added to the bath caused a decline in junctional conductance (gj) of 48 +/- 2% (n = 15) within 90 s. The effect of ANF was suppressed by HS-142-1, a specific antagonist of guanylyl cyclase ANF receptor. Moreover, the decline in gj elicited by ANF was related to the synthesis of cyclic guanosine monophosphate (cGMP). Indeed, dibutyryl-cGMP (10(-4) M) decreased gj by 80 +/- 3.5% (n = 15) within 90 s, and zaprinast, a selective inhibitor of cGMP phosphodiesterase, enhanced the effect of ANF on gj. The possible relationship between ischemia, ANF release, and impairment of cell coupling is discussed.

Opioid peptide gene expression in the primary hereditary cardiomyopathy of the Syrian hamster. I. Regulation of prodynorphin gene expression by nuclear protein kinase C

Prodynorphin gene expression was investigated in adult ventricular myocytes isolated from normal (F1B) or cardiomyopathic (BIO 14.6) hamsters. Prodynorphin mRNA levels were higher in cardiomyopathic than in control myocytes and were stimulated by treatment of control cells with the protein kinase C (PKC) activator 1, 2-dioctanoyl-sn-glycerol. Both chelerythrine and calphostin C, two PKC inhibitors, abolished the stimulatory effect of the diglyceride and significantly reduced prodynorphin gene expression in cardiomyopathic myocytes. Nuclear run-off experiments indicated that the prodynorphin gene was regulated at the transcriptional level and that treatment of nuclei isolated from control cells with 1, 2-dioctanoyl-sn-glycerol increased prodynorphin gene transcription, whereas chelerythrine or calphostin C abolished this transcriptional effect. Direct exposure of nuclei isolated from cardiomyopathic myocytes to these inhibitors markedly down-regulated the rate of gene transcription. The expression of PKC-alpha, -delta, and -epsilon, as well as PKC activity, were increased in nuclei of cardiomyopathic myocytes compared with nuclei from control cells. The levels of both intracellular and secreted dynorphin B, a biologically active product of the gene, were higher in cardiomyopathic than in control cells and were stimulated or inhibited by cell treatment with 1,2-dioctanoyl-sn-glycerol or PKC inhibitors, respectively.

Opioid peptide gene expression in the primary hereditary cardiomyopathy of the Syrian hamster. II. Role of intracellular calcium loading

We have previously shown that prodynorphin gene expression was markedly increased in adult myocytes of BIO 14.6 cardiomyopathic hamsters and that nuclear protein kinase C (PKC) may be involved in the induction of this opioid gene. Here we report that the cytosolic Ca2+ concentration was significantly increased in resting and in KCl-depolarized cardiomyopathic myocytes compared with normal cells. In normal and in cardiomyopathic cells, KCl significantly increased prodynorphin mRNA levels and prodynorphin gene transcription. These effects were abolished by the Ca2+ channel blocker verapamil. In control myocytes, the KCl-induced increase in prodynorphin mRNA expression was in part attenuated by chelerythrine or calphostin C, two selective PKC inhibitors. In these cells, KCl induced the translocation of PKC-alpha into the nucleus, increasing nuclear PKC activity. In resting cardiomyopathic myocytes, the increase in prodynorphin mRNA levels and gene transcription were significantly attenuated by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxy-methylester being completely abolished when the chelating agent was administered in the presence of PKC inhibitors. KCl and the PKC activator 1,2-dioctanoyl-sn-glycerol additively stimulated prodynorphin gene expression both in normal and in cardiomyopathic cells. Therefore, we conclude that PKC activation and intracellular Ca2+ overload may represent the two major signaling mechanisms involved in the induction of the prodynorphin gene in cardiomyopathic cells.

Renin-angiotensin system and cell communication in the failing heart

The influence of heart failure on the process of cell communication was investigated in cell pairs isolated from the ventricle of cardiomyopathic hamsters (11 months old) and the results compared with age-matched normal hamsters. The gap junctional conductance (gj) was measured with two voltage-clamp amplifiers. The results showed two major populations of cell pairs with respect to gj values: one with very low values (0.8 to 2.5 nS) and the other with higher values (7 to 35 nS). In normal hamsters, the most frequent gj values were in the range of 40 to 100 nS. Angiotensin II (Ang 11, 1 microg/mL) caused cell uncoupling in myopathic myocytes with low gj but reduced gj by 53 +/- 6.6 percent (+/- SE) in cell pairs with higher gj values (7 to 35 nS). The effect of Ang II on gj of myopathic cell pairs was suppressed by losartan (10(-7) mol/L). In cardiomyopathic cell pairs with low gj (0.8 to 2.5 nS), enalapril (1 microg/mL) caused an appreciable increase in gj (219 +/- 20.3 percent), whereas in cell pairs with higher gj (7 to 35 nS), the gj increment was smaller (80 +/- 10.8 percent) but still larger than that seen in controls (33 +/- 5.4 percent). Intracellular dialysis of Ang I (10(-8) mol/L) abolished cell communication in myopathic cell pairs with low gj (0.8 to 2.5 nS) and reduced gj by 66 +/- 1.7 percent in the other pairs (7 to 35 nS). The effect of Ang I on gj was greatly reduced by enalaprilat (10(-9) mol/L) added to the cytosol. Dialysis of Ang II (10(-8) mol/L) into the myopathic cell reduced gj by 48 +/- 4.2 percent, an effect abolished by losartan (10(-8) mol/L). The results indicate that the decline in gj seen in the ventricle of cardiomyopathic hamsters is in part due to activation of the cardiac renin-angiotensin system.

Effect of U-50,488H on the contractile response of cardiomyopathic hamster ventricular myocytes

We examined the effects of a selective kappa opioid receptor agonist (U-50,488H) on the contractile properties of single ventricular myocytes from 127 day old control (F1B) and cardiomyopathic (BIO 14.6) hamsters. Myocytes in bicarbonate buffered solution with 1.5 mM [Ca2+] were electrically stimulated with field electrodes in the bath. Length changes were monitored via myocyte edge tracking. Twitch amplitude and the velocity of cell shortening were less in the cardiomyopathic hamster myocytes than in age-matched hamsters (P less than or equal to 0.05). There was a concentration-dependent effect of U-50,488H (0.1-20 microM) to decrease twitch amplitude and shortening velocity in both control and cardiomyopathic myocytes (P less than or equal to 0.001). In cells loaded with the Ca2+ indicator indo-1 the negative inotropic action of U-50,488H was associated with a decreased indo-1 fluorescence transient amplitude. There was no difference in the negative inotropic effect of U-50,488H on control and cardiomyopathic cells. Thus, the CM hamster does not demonstrate a different contractile response to U-50,488H.

Diminished beta-adrenergic receptor responsiveness and cardiac dilation in hearts of myopathic Syrian hamsters (BIO 53.58) are associated with a functional abnormality of the G stimulatory protein

Previous studies have demonstrated a diminution in the bioactivity of the guanine nucleotide-binding regulatory protein that stimulates adenylyl cyclase (Gs) in hearts of the hypertrophic BIO 14.6 Syrian hamster. In this study, we measured functional activity and immunodetectable levels of Gs in a mutant strain of hamsters (BIO 53.58) that develop a dilated cardiomyopathy. Pathological studies demonstrated that 100-day-old BIO 53.58 hamsters had substantial ventricular dilation when compared with age-matched F1B controls. Additionally, these 100-day-old hamsters demonstrated diminished contractile response to beta-adrenergic receptor stimulation. The pathological and hemodynamic changes were associated with defective coupling of Gs to adenylyl cyclase as adenylyl cyclase activation was distinctly decreased in the presence of isoproterenol, fluoride ion, guanine nucleotides, and forskolin. Additionally, the ability of the alpha-subunit of Gs to reconstitute isoproterenol-stimulated adenylyl cyclase activity in S49 cyc- membranes was reduced approximately 65%. By contrast, cyc- complementation assays did not reveal a difference between the functional activity of Gs in hearts from 30-day-old BIO 53.58 hamsters and F1B controls. Furthermore, beta-adrenergic receptor stimulation of adenylyl cyclase in the membranes of the young BIO 53.58 hamsters was not significantly different from controls. The substantial alterations in Gs bioactivity in hearts of the 100-day-old BIO 53.58 hamsters was not associated with alterations in the immunodetectable levels of either alpha Gs or alpha Gi on Western Blots. These results suggest that G protein changes are associated with ventricular dilation in BIO 53.58 hamsters and that G protein levels are not always reflective of G protein bioactivity.

Verapamil ameliorates clinical, pathologic and biochemical manifestations of experimental chagasic cardiomyopathy in mice

The influence of long-term verapamil administration on the consequences of Trypanosoma cruzi infection in mice was studied with regard to animal mortality, morbidity, myocardial pathologic features and myocardial beta-adrenergic adenylate cyclase activity. Verapamil administration dramatically decreased the mortality rate from 60% to 6% during the 70 day period of infection. Three clinical stages of infection were evident. In the acute stage (17 days after infection with maximal parasitemia), verapamil treatment not only decreased the incidence of myocardial disease (fibrosis and inflammation), but also protected myocardial beta-adrenergic adenylate cyclase activity. In addition, there was no increase in total body weight, which was regarded as an index of right-sided heart failure. In the subacute stage (30 to 60 days after infection), administration of verapamil continued to decrease myocardial disease and preserve beta-adrenergic adenylate cyclase activity. In addition, verapamil ameliorated the morbidity and mortality associated with this stage of infection. The chronic stage of infection was characterized by a decrease in myocardial disease and in beta-adrenergic adenylate cyclase activity. Thus, independent of the state of infection, long-term verapamil treatment enhanced beta-adrenergic adenylate cyclase activity. In addition, verapamil ameliorated the morbidity associated with infection. Although the relation among these various effects of verapamil in the setting of T. cruzi infection remains to be determined, collectively the results suggested that verapamil administration attenuated the consequences of T. cruzi infection.

Alterations in calcium antagonist receptors and sodium-calcium exchange in cardiomyopathic hamster tissues

The Syrian cardiomyopathic (CM) hamster (BIO 14.6) develops a progressive cardiomyopathy characterized by cellular necrosis, hypertrophy, and, eventually, cardiac dilatation and congestive heart failure. Several lines of evidence implicate cellular calcium overload as an important etiologic factor. We previously reported an increased number of receptors for calcium antagonist drugs, which block voltage-dependent calcium channels, in heart, skeletal muscle, and brain tissue of these hamsters in the early necrotic stage of the disease. To better characterize the pathophysiological significance of this abnormality we evaluated calcium antagonist receptor binding and Na+-Ca2+ exchange in CM and control hamsters at different stages of disease as documented by quantitative histopathologic assessment. In CM hamsters as young as 10 days, an age previously thought to be before the onset of disease, we identified cardiac myocyte hypertrophy, a twofold increase in calcium antagonist receptor binding in heart and brain, and a 50% increase in skeletal muscle. Overt histological lesions were present in skeletal muscle at 25 days and in heart between 28-30 days. The size of cardiac lesions increased over time and changed from necrotic foci with cellular infiltration to fibrotic or calcified lesions by 360 days. Myocardial cellular hypertrophy persisted through the late stages of the disease (360 days), but increased calcium antagonist binding was present in heart only to 6 months of age, in skeletal muscle to 90 days, and in brain to 30 days. Na+-Ca2+ exchange in heart was normal until 15 days and then increased by 400% at 30 days suggesting that this augmentation might be a secondary response to the earlier increase in calcium antagonist receptors. At 360 days cardiac Na+-Ca2+ exchange was decreased by 50%, likely reflecting progressive cardiac damage. The increase in calcium antagonist receptors in CM animals as young as 10 days supports the hypothesis that abnormalities in voltage-dependent calcium channels play a role in the pathophysiology of CM hamsters.