Altered cardiac mechanism and sarcoplasmic reticulum function in pressure overload-induced cardiac hypertrophy in rats

RyR2 and Calcium Release in Heart Failure

Heart Failure (HF) is defined as the inability of the heart to efficiently pump out enough blood to maintain the body's needs, first at exercise and then also at rest. Alterations in Ca2+ handling contributes to the diminished contraction and relaxation of the failing heart. While most Ca2+ handling protein expression and/or function has been shown to be altered in many models of experimental HF, in this review, we focus in the sarcoplasmic reticulum (SR) Ca2+ release channel, the type 2 ryanodine receptor (RyR2). Various modifications of this channel inducing alterations in its function have been reported. The first was the fact that RyR2 is less responsive to activation by Ca2+ entry through the L-Type calcium channel, which is the functional result of an ultrastructural remodeling of the ventricular cardiomyocyte, with fewer and disorganized transverse (T) tubules. HF is associated with an elevated sympathetic tone and in an oxidant environment. In this line, enhanced RyR2 phosphorylation and oxidation have been shown in human and experimental HF. After several controversies, it is now generally accepted that phosphorylation of RyR2 at the Calmodulin Kinase II site (S2814) is involved in both the depressed contractile function and the enhanced arrhythmic susceptibility of the failing heart. Diminished expression of the FK506 binding protein, FKBP12.6, may also contribute. While these alterations have been mostly studied in the left ventricle of HF with reduced ejection fraction, recent studies are looking at HF with preserved ejection fraction. Moreover, alterations in the RyR2 in HF may also contribute to supraventricular defects associated with HF such as sinus node dysfunction and atrial fibrillation.

CBL aggravates Ang II-induced cardiac hypertrophy via the VHL/HIF-1α pathway

CBL (Casitas B cell lymphoma), an important ubiquitin protein ligase, is involved in protein folding, protein maturation, and proteasome-dependent protein catabolism in different cells. However, its role in cardiac hypertrophy is still unclear. In this study, we found that expression of CBL is increased in an Ang II-induced mouse cardiac hypertrophy animal model and in Ang II-treated H9C2 cells. Interference with CBL expression attenuates the degree of myocardial hypertrophy as well as the expression of hypertrophy-related genes in H9C2 cells. Further research found that CBL aggravates myocardial hypertrophy by activating HIF-1α, which is an aggravating factor for hypertrophy. The effect of CBL on promoting myocardial hypertrophy was reversed by interference with HIF-1α. Mechanistically, we found that CBL directly interacted with and degraded VHL by increasing its ubiquitination level, which is a widely accepted regulatory factor of HIF-1α. Finally, our results showed that CBL was partially dependent on degradation of VHL and that activation of HIF-1α promoted myocardial hypertrophy. Collectively, these findings suggest that strategies based on activation of the CBL/HIF-1α axis might be promising for the treatment of hypertrophic cardiomyopathy.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Effects of feeding and digestion on myocardial contractility and expression of calcium-handling proteins in Burmese pythons (Python molurus)

Pythons are important models of studies on postprandial metabolism because their physiological responses are exacerbated when digesting large prey. Prior studies of these animals have shown hypertrophy of the cardiac tissue 2 to 3 days after feeding, coinciding with the peak of the specific dynamic action (SDA), but the consequences of this remodeling in myocardial contractility have not been studied, which is the purpose of this work. Specimens of Python molurus were divided into two groups: a Digesting group (2 days after feeding, at the peak of SDA), and a Fasting group (28 days after feeding). When compared to the Fasting group, the Digesting group showed higher relative ventricular mass and calcium-handling protein expression such as sarcoplasmic reticulum Ca²⁺-ATPase (SERCA), phospholamban (PLB), and the Na⁺/Ca²⁺ exchanger (NCX). Digesting pythons also exhibited significant increases in the cardiac contraction force (Fc), rates of force development and relaxation, and cardiac pumping capacity. Therefore, the higher SERCA, PLB and NCX expression levels increased cytosolic Ca²⁺ transient amplitude, improving myofilament force. These changes are crucial to maintain cardiac output and a relatively high and continuous blood flow required by metabolic expenditure that occurs in postprandial animals.

Sequential alterations in Akt, GSK3β, and calcineurin signalling in the mouse left ventricle after thoracic aortic constriction

Left ventricular hypertrophy (LVH) is an adaptive response to chronic biomechanical stress that generally progresses to maladaptive hypertrophy and heart failure (HF). We studied the activation of protein kinase B (Akt/PKB), glycogen synthase kinase 3 beta (GSK3β), and calcineurin (Cn) at 3, 7, 15, 30, and 60 days following transverse aortic constriction (TAC) in 4-week-old mice. Following TAC, GSK3β inactivation at day 3 was associated with Akt activation, whereas at days 15 and 30, it appeared to be controlled by other kinases. Moderate nonsignificant Cn activation occurred at the early stages, and peak activation at day 30, concomitant with GSK3β inactivation and overt LVH and HF. At the latest stage (day 60), despite further progression of LVH and HF, Cn activation appeared attenuated. Early stages of LVH were associated with Ca2+-handling protein upregulation, whereas major Cn activation, associated with GSK3β inactivation, appeared to engage maladaptive hypertrophy and progression to HF associated with Ca2+-handling protein downregulation.

Down-regulation of the human ether-a-go-go-related gene in rat cardiac hypertrophy

INTRODUCTION

Cardiac hypertrophy is a risk factor for QT prolongation and cardiac sudden death. In this study, the authors examined the expressional regulation on the rat human ether-a-go-go-related gene (HERG), which encodes a structural subunit of the rapid component of the delayed rectifier potassium current (I(Kr)), during myocardial hypertrophy using rat as a model system.

METHODS

Cardiac hypertrophy was established in Sprague-Dawley rats by coarctation of the abdominal aorta [left ventricular hypertrophy (LVH) group]. Sham-operated rats were defined as control group (Ctrl group). Hemodynamic, morphologic and histologic parameters were recorded 6 weeks after operation. In addition, the expression of HERG was also determined using a combination of real-time polymerase chain reaction, Western blot and immunohistochemical analyses.

RESULTS

Compared with the sham-operated Ctrl group, abdominal aortic coarctation induced LVH in the LVH group, as evidenced by significantly increased ratios of heart weight/left ventricular weight to body weight and enlarged left ventricular myocytes in the histologic sections. The hemodynamic profile revealed significant increases in heart rate and left ventricular end-diastolic pressure, as well as a decrease in the maximal rate of left ventricular pressure fall in the LVH rats, when compared with the Ctrl rats. Electrocardiograms showed prolonged QT and corrected QT intervals. On the molecular level, a significant reduction of HERG, messengerRNA and protein was observed in LVH group, which was inversely correlated with prolonged corrected QT (r = -0.842, P = 0.000).

CONCLUSION

The expressional down-regulation of HERG gene may constitute a novel mechanism for QT prolongation during cardiac hypertrophy.

Enhanced calcium mobilization in rat ventricular myocytes during the onset of pressure overload-induced hypertrophy

Early cardiovascular changes evoked by pressure overload (PO) may reveal adaptive strategies that allow immediate survival to the increased hemodynamic load. In this study, systolic and diastolic Ca2+ cycling was analyzed in left ventricular rat myocytes before ( day 2, PO-2d group) and after ( day 7, PO-7d group) development of hypertrophy subsequent to aortic constriction, as well as in myocytes from time-matched sham-operated rats (sham group). Ca2+ transient amplitude was significantly augmented in the PO-2d group. In the PO-7d group, intracellular Ca2+ concentration ([Ca2+]i) was reduced during diastole, and mechanical twitch relaxation (but not [Ca2+]i decline) was slowed. In PO groups, fractional sarcoplasmic reticulum (SR) Ca2+ release at a twitch, SR Ca2+ content, SR Ca2+ loss during diastole, and SR-dependent integrated Ca2+ flux during twitch relaxation were significantly greater than in sham-operated groups, whereas the relaxation-associated Ca2+ flux carried by the Na+/Ca2+ exchanger was not significantly changed. In the PO-7d group, mRNA levels of cardiac isoforms of SR Ca2+-ATPase (SERCA2a), phospholamban, calsequestrin, ryanodine receptor, and NCX were not significantly altered, but the SERCA2a-to-phospholamban ratio was increased 2.5-fold. Moreover, greater sensitivity to the inotropic effects of the β-adrenoceptor agonist isoproterenol was observed in the PO-7d group. The results indicate enhanced Ca2+ cycling between SR and cytosol early after PO imposition, even before hypertrophy development. Increase in SR Ca2+ uptake may contribute to enhancement of excitation-contraction coupling (augmented SR Ca2+ content and release) and protection against arrhythmogenesis due to buildup of [Ca2+]i during diastole.

Subcellular remodeling as a viable target for the treatment of congestive heart failure

It is now well known that congestive heart failure (CHF) is invariably associated with cardiac hypertrophy, and changes in the shape and size of cardiomyocytes (cardiac remodeling) are considered to explain cardiac dysfunction in CHF. However, the mechanisms responsible for the transition of cardiac hypertrophy to heart failure are poorly understood. Several lines of evidence both from various experimental models of CHF and from patients with different types of CHF have indicated that the functions of different subcellular organelles such as extracellular matrix, sarcolemma, sarcoplasmic reticulum, myofibrils, mitochondria, and nucleus are defective. Subcellular abnormalities for protein contents, gene expression, and enzyme activities in the failing heart become evident as a consequence of prolonged hormonal imbalance, metabolic derangements, and cation maldistribution. In particular, the occurrence of oxidative stress, development of intracellular Ca2+ overload, activation of proteases and phospholipases, and alterations in cardiac gene expression result in changes in the biochemical composition, molecular structure, and function of different subcellular organelles (subcellular remodeling). Not only does subcellular remodeling appear to be intimately involved in the transition of cardiac hypertrophy to heart failure, the mismatching of the function of different subcellular organelles leads to the development of cardiac dysfunction. Although blockade of the renin-angiotensin system, sympathetic nervous system, and various other hormonal actions have been reported to produce beneficial effects on cardiac remodeling and heart dysfunction in CHF, the actions of various cardiac drugs on subcellular remodeling have not been examined extensively. Some recent studies have indicated that both the angiotensin-converting enzyme inhibitors and angiotensin receptor antagonists attenuate changes in sarcolemma, sarcoplasmic reticulum, and myofibril enzyme activities, protein contents, and gene expression, and partly improve cardiac function in the failing hearts. It is suggested that subcellular remodeling is an excellent target for the development of improved drug therapy for CHF. Furthermore, extensive studies should investigate the effects of different agents individually or in combination on reverse subcellular remodeling, cardiac remodeling, and cardiac dysfunction in various experimental models of CHF.

High Ambient Pressure Produces Hypertrophy and Up-Regulates Cardiac Sarcoplasmic Reticulum Ca2+ Regulatory Proteins in Cultured Rat Cardiomyocytes

Previously, we demonstrated in vivo that the nature of the alterations in sarcoplasmic reticulum (SR) function and SR Ca2+ regulatory proteins depends both on the type of mechanical overload imposed and on the duration of the heart disorder. The purpose of the present study was to determine in vitro whether an extrinsic mechanical overload (in the form of high ambient pressure) would cause an up-regulation of ryanodine receptor (RyR) and Ca2+-ATPase, as we previously reported mildly pressure-overloaded, hypertrophied rat hearts. Primary cultures of neonatal rat cardiomyocytes were prepared and high ambient pressure was produced using an incubator and pressure-overloading apparatus. Cells were exposed to one of two conditions for 72 h: atmospheric pressure conditions (APC) or high pressure conditions (HPC; HPC=APC+200 mmHg). The expression levels of RyR and Ca2+-ATPase were quantified and functional characteristics were monitored. The cell area was significantly greater under HPC. After 6 h exposure, the physiological properties of cardiomyocytes were impaired, but they returned to the baseline level within 24 h. After 24 h exposure, the expression level of RyR was significantly higher under HPC, and for Ca2+-ATPase, the expression level was significantly higher under HPC after 6 h exposure. HPC caused hypertrophy and up-regulated the expression of Ca2+ regulatory proteins and their genes. We suggest that this in vitro pressure-overloading model may prove useful, as is a stretch-overloading model, for investigation of the intracellular Ca2+ regulatory pathways responsible for the development of cardiac hypertrophy.

TRANSCRIPTIONAL REGULATION OF CARDIAC SARCOPLASMIC RETICULUM CALCIUM-ATPase GENE DURING THE PROGRESSION OF SEPSIS

Changes in sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) gene expression in the rat heart during different phases of sepsis were studied. Sepsis was induced by cecal ligation and puncture (CLP). Septic rats were divided into two groups: the early hyperdynamic (9 h after CLP, early sepsis) and the late hypodynamic (18 h after CLP; late sepsis) groups. Western blot analyses reveal that SERCA2a protein level remained unaltered during early sepsis but was decreased by 59% during late sepsis. Northern blot analyses show that the steady-state level of SERCA2a mRNA stayed unchanged during the early phase but was decreased by 43% during the late phase of sepsis. Nuclear runoff assays show that the transcription rate of SERCA2a gene transcript remained unaffected during early sepsis but was decreased by 34% during late sepsis. The actinomycin D pulse-chase studies indicate that the half-life of SERCA2a mRNA was unaffected during the early and the late phases of sepsis. These findings demonstrate that during the early phase of sepsis, the protein level, the mRNA abundance, and the transcription rate of SERCA2a remained unaltered, whereas during the late phase of sepsis, the rate of transcription of SERCA2a was decreased, and the decreased transcription rate was associated with decreases in SERCA2a mRNA abundance and SERCA2a protein level in the rat heart. Based on these data, it is concluded that SERCA2a gene expression decreased during the late phase of sepsis in the rat heart and that the decreased expression was regulated at the transcriptional level.

Possible role of acylphosphatase, Bcl-2 and Fas/Fas-L system in the early changes of cardiac remodeling induced by volume overload

To identify early adaptive processes of cardiac remodeling (CR) in response to volume overload, we investigated the molecular events that may link intracellular Ca(2+) homeostasis alterations and cardiomyocyte apoptosis. In swine heart subjected to aorto-cava shunt for 6, 12, 24, 48 and 96 h sarcoplasmic reticulum (SR) Ca(2+) pump activity was reduced until 48 h (-30%), but a recovery of control values was found at 96 h. The decrease in SR Ca(2+)-ATPase (SERCA2a) expression at 48 h, was more marked (-60%) and not relieved by a subsequent recovery, while phospholamban (PLB) concentration and phosphorylation were unchanged at all the considered times. Conversely, acylphosphatase activity and expression significantly increased from 48 to 96 h (+40%). Bcl-2 expression increased significantly from 6 to 24 h, but at 48 h, returned to control values. At 48 h, microscopic observations showed that overloaded myocardium underwent substantial damage and apoptotic cell death in concomitance with an enhanced Fas/Fas-L expression. At 96 h, apoptosis appeared attenuated, while Fas/Fas-L expression was still higher than control values and cardiomyocyte hypertrophy became to develop. These data suggest that in our experimental model, acylphosphatase could be involved in the recovery of SERCA2a activity, while cardiomyocyte apoptosis might be triggered by a decline in Bcl-2 expression and a concomitant activation of Fas.

Assessment of sarcoplasmic reticulum Ca2+-uptake during the development of left ventricular hypertrophy

Cardiac sarcoplasmic reticulum (SR) sequesters Ca(2+) and plays a crucial role in the regulation of intracellular Ca(2+). Its functional properties are central to the excitation-contraction cycle of cardiac muscle. In this study, we hypothesized that alterations in SR function occur during the development of left ventricular (LV) hypertrophy. LV hypertrophy was produced in Lewis rats by the one-kidney, one-clip (1K1C) procedure. LV tissues were obtained from 1K1C rats 1 week (mild, N=7), 4 weeks (moderate, N=7), and 8 weeks (severe, N=7) post-surgery and from the corresponding age-matched, sham-operated controls (N=7 at each stage). In all of these rats, the ratio of LV weight (g) to body weight (kg) was determined and considered as an index for LV hypertrophy. In addition, the ratio of lung weight (g) to body weight (kg) was determined and considered as an index for pulmonary congestion and heart failure. In each LV specimen, SR Ca(2+)-uptake and tissue Ca(2+)-ATPase (CAA) level were determined. In 1K1C rats, LV hypertrophy increased by 21, 40, and 90% at 1, 4, and 8 weeks post-surgery, respectively, compared to the age-matched, sham-operated rats, whereas pulmonary congestion did not occur at 1 and 4 weeks but increased significantly by about 21% at 8 weeks. Further, both SR Ca(2+)-uptake and immunodetectable CAA level did not change at 1 week, increased (54%) to the same extent at 4 weeks, and decreased (42%) by approximately the same extent at 8 weeks in 1K1C rats compared to the age-matched, sham-operated rats. In summary, as LV hypertrophy evolved, Ca(2+)-uptake and CAA expression did not change in the early, increased in the moderate, and then declined in the later stages of hypertrophy development. The increase in Ca(2+)-uptake and CAA expression suggests, at the cellular level, a compensatory response to LV hypertrophy, while the decline at later stages indicates the transition to heart failure.

Mechanisms underlying delayed afterdepolarizations in hypertrophied left ventricular myocytes of rats

Cardiac hypertrophy was induced in rats by daily injection of isoproterenol (5 mg/kg ip) for 7 days. Membrane voltage and currents were recorded using the whole cell patch-clamp technique in left ventricular myocytes from control and hypertrophied hearts. Ryanodine-sensitive delayed afterdepolarizations (DADs) and transient inward current (I(ti)) appeared in hypertrophied cells more often and were of larger amplitude than in control cells. DADs and I(ti) are carried principally by Na/Ca exchange with smaller contributions from a nonselective cation channel and from a Cl- channel. The latter is expressed only in hypertrophied myocytes. In hypertrophy, the density of caffeine-induced Na/Ca exchange current (I(Na/Ca)) was increased by 26%, sarcoplasmic reticulum (SR) Ca2+ content as assessed from the integral of I(Na/Ca) was increased by 30%, the density of Na-pump current (I(pump)) was reduced by 40%, and the intracellular Na+ content, measured by Na+-selective microelectrodes was increased by 55%. The results indicate that DADs and I(ti) are generated by spontaneous Ca2+ release from an overloaded SR caused by a downregulated Na pump and an upregulated Na/Ca exchange. These findings may explain the propensity for arrhythmias seen in this model of hypertrophy.

Effects of cardioplegic arrest and reperfusion on rabbit cardiac ryanodine receptors

Calcium overload is considered to be a primary contributor to ischemia-reperfusion injury. Cardiac sarcoplasmic reticulum (SR), the main regulator of intracellular Ca2+ concentration under normal conditions, is a target for ischemic myocardial injury. The ryanodine receptor (RyR) is the SR Ca2+ release channel. Previous reports have shown that a reduction in RyR activity during global myocardial ischemia correlates with concomitant myocardial dysfunction. Crystalloid cardioplegia, a technique for myocardial protection during heart operations, reduces Ca2+ accumulation during global ischemia. Hence, the effects of cardioplegia on RyR in isolated rabbit hearts was investigated. The study also compared [3H] ryanodine binding before ischemia (control group), after 30 min of ischemia (either global ischemia (GI group) or cardioplegic arrest (CA group)), and after 20 min of reperfusion. The GI group, but not the CA group, showed a significant reduction in the maximum number of binding sites (Bmax) for RyR compared with the control group (Control vs GI group: after ischemia, 1.33+/-0.27 vs 0.83+/-0.12 pmol/mg protein, p<0.05; after reperfusion, 1.33+/-0.27 vs 0.80+/-0.08 pmol/mg protein; p<0.05). CA group: after ischemia, 1.22+/-0.20 pmol/mg protein; after reperfusion, 1.15+/-0.28 pmol/mg protein). The affinity (Kd) values for [3H] ryanodine binding were not different among the 3 groups at any point. The preservation of RyR numbers during cardioplegia correlated with the concomitant preservation of cardiac functions. The results indicate that number of functional RyR was much better preserved during cardioplegia than during global ischemia. It is postulated that cardioplegia-induced protection of cardiac RyR may result in the protection of SR function during ischemia-reperfusion.

Reduced ryanodine receptor to dihydropyridine receptor ratio may underlie slowed contraction in a rabbit model of left ventricular cardiac hypertrophy

Cardiac hypertrophy is associated with contractile dysfunction, a feature of which is a slowing of the time to reach peak contraction. We have examined the main mechanisms involved in the initiation of contraction and investigated if their functions are changed during cardiac hypertrophy. Cardiac hypertrophy was induced by constriction of the ascending aorta in the rabbit. After 6 weeks left ventricular myocytes were isolated or left ventricular and septal mixed membrane preparations were produced for electrophysiological and radioligand binding studies, respectively. Aortic constriction resulted in a 24% and 23% increase in heart weight to body weight ratio and cell capacitance, respectively. Action potential duration and time-to-reach 50% and 90% peak contraction (TTP(50)and TTP(90), respectively) were significantly prolonged in myocytes from hypertrophied hearts. The prolongation of TTP(50)and TTP(90)could not be explained by altered peak calcium current density or SR calcium content which were unchanged in hypertrophy. Radioligand binding studies performed on tissue preparations from the same hearts, revealed a 34% reduction in ryanodine receptor (RYR) density with no change in dihydropyridine receptor (DHPR) density. This resulted in a reduction in the ratio of RYR to DHPR from 4.4:1 to 3.3:1 in hypertrophy. Ryanodine receptor Ca(2+)-sensitivity was unchanged between sham operated and hypertrophied groups. A reduction in the ratio of RYRs to DHPRs may result in a degree of "functional uncoupling" causing defective release of Ca(2+)from the SR. These findings may underlie the slowed TTP of myocyte contraction in hypertrophy.

Effect of milrinone on left ventricular relaxation and Ca(2+) uptake function of cardiac sarcoplasmic reticulum

Milrinone, a phosphodiesterase 3 (PDE3) inhibitor, is known to enhance left ventricular (LV) contractility by an inhibition of the breakdown of cAMP through the mechanism inhibiting PDE3. However, it is unclear whether milrinone also exerts positive lusitropy, like dobutamine. Here, we assessed the effects of milrinone on in vivo LV relaxation, as well as the Ca(2+)-ATPase activity and the Ca(2+) uptake function of the cardiac sarcoplasmic reticulum (SR), compared with the effect of dobutamine on those functions. After dobutamine (3 microg x kg(-1) x min(-1)) was administered, the peak value of the first derivative of LV pressure (+dP/dt) increased by 46%, whereas the time constant (tau) of LV pressure decay decreased by 6.9%, respectively. After milrinone (10 microg/kg) was administered, the peak +dP/dt increased to a similar extent as dobutamine (46%), whereas tau decreased much more than dobutamine (19.9%; P < 0.05). In LV crude homogenate, the thapsigargin-sensitive, Ca(2+)-ATPase activity-cAMP relationships was significantly less increased by milrinone compared with dobutamine (P < 0.05), indicating the higher sensitivity of the SR Ca(2+)-ATPase activity on cAMP by milrinone than by dobutamine. In the SR vesicles purified from LV muscles, the addition of cAMP increased the SR Ca(2+) uptake in a dose-dependent fashion, and the PDE3 inhibitors (milrinone and cGMP) significantly augmented this response (P < 0.05). Hence, milrinone substantially improved LV relaxation in association with an acceleration of the SR Ca(2+)-ATPase activity and the SR Ca(2+) uptake. This acceleration might be due to an inhibition of the membrane-bound PDE3 in the SR, leading to a local elevation of cAMP.

Early changes induced in the left ventricle by pressure overload. An experimental study on swine heart

The purpose of this study was to evaluate the early changes in sarcoplasmic reticulum (SR) function and the parallel morphological and hemodynamic modifications occurring in the heart following pressure overload. As regards SR function, we also explored the levels of acylphosphatase, an enzyme which might have a regulatory effect on the SR Ca(2+) pump by hydrolyzing the phosphorylated intermediate of this transport system. Pigs subjected to pressure overload by aortic stenosis for 6, 12, 24, 48, 72, and 96 h were compared to sham-operated controls. At each of the considered times both SR Ca(2+)-ATPase activity and Ca(2+) uptake, as well as acylphosphatase activity, were significantly enhanced in the pressure overloaded compared to the control hearts, with a maximal increase at 6 h; moreover, a positive and significant correlation was found between these parameters. The modifications in the activities of Ca(2+)-ATPase and acylphosphatase reflected an increased expression of these proteins, while phospholamban did not show significant changes in its concentration nor in its phosphorylation status. As for hemodynamic parameters, rapid changes in the left ventricular function were observed and especially the early hours following the aortic stenosis appeared to be crucial for the adjustment of heart function. The most relevant morphological finding was a focal disarrangement of the myofibrillar pattern which was very evident at 6 h, and progressively attenuated at later times. Taken together our data suggest that an early adaptation to the increased hemodynamic working overload is a consistent activation of the contractile apparatus which reflects, at least in part, an enhanced SR function. Besides the changes in Ca(2+) pump protein expression, increased acylphosphatase levels might also contribute to this effect.

Alterations in cardiac sarcoplasmic reticulum Ca2+ regulatory proteins in the atrial tissue of patients with chronic atrial fibrillation

OBJECTIVES

Our purpose was to determine whether atrial fibrillation (AF) patients have alterations in sarcoplasmic reticulum (SR) Ca2+ regulatory proteins in the atrial myocardium.

BACKGROUND

Clinically, AF is the most frequently encountered arrhythmia. Recent studies indicate that an inability to maintain intracellular Ca2+ homeostasis with a consequent increase in membrane-triggered activity could be the primary initiating factor in some circumstances, and that cytosolic Ca2+ abnormalities are an important mediator of sustained AF.

METHODS

We measured the maximum number of [3H]ryanodine binding sites (Bmax) and the expression levels of ryanodine receptor (RyR) mRNA and calcium-adenosine triphosphatase (Ca2+-ATPase) mRNA in atrial myocardial tissue from 13 patients with AF due to mitral valvular disease (MVD) and 9 patients with normal sinus rhythm (NSR).

RESULTS

In AF patients, 1) Bmax was significantly lower in each atrium (0.21+/-0.03 pmol/mg [right], 0.16+/-0.04 pmol/mg [left]) than in the right atrium (0.26+/-0.08 pmol/mg) of NSR patients; 2) Bmax was significantly lower in the left atrium than in the right atrium; 3) Bmax in the left atrium was significantly lower at higher levels of pulmonary capillary wedge pressure; 4) the expression level of RyR mRNA was significantly lower in both the left (1.24 x 10(-2)+/-1.28 x 10(-2)) and right (1.70 x 10(-2)+/-1.78 x 10(-2)) atrium than in the right atrium of NSR patients (6.11 x 10(-2)+/-2.79 x 10(-2)); and 5) the expression level of Ca2+-ATPase mRNA was significantly lower in both the left (5.67 x 10(-2)+/-4.01 x 10(-2)) and right (7.71 x 10(-2)+/-3.56 x 10(-2)) atrium than in the right atrium (12.60 x 10(-2)+/-3.92 x 10(-2)) of NSR patients.

CONCLUSIONS

These results provide the first direct evidence of abnormalities in the Ca2+ regulatory proteins of the atrial myocardium in chronic AF patients. Conceivably, such abnormalities may be involved in the initiation and/or perpetuation of AF.

Reduced subendocardial ryanodine receptors and consequent effects on cardiac function in conscious dogs with left ventricular hypertrophy

The goal of this study was to examine the transmural distribution of ryanodine receptors in left ventricular (LV) hypertrophy (LVH) and its in vivo consequences. Dogs were chronically instrumented with an LV pressure gauge, ultrasonic crystals for measurement of LV internal diameter and wall thickness, and a left circumflex coronary blood flow velocity transducer. Severe LVH was induced by chronic banding of the aorta (12+/-1 months), which resulted in a 78% increase in LV/body weight. When ryanodine was infused directly into the circumflex coronary artery, it did not affect LV global function or systemic hemodynamics; however, it reduced LV wall thickening and delayed relaxation in the posterior wall in control dogs but was relatively ineffective in dogs with LVH. In LV sarcolemmal preparations, [3H]ryanodine ligand binding revealed a subendocardial/subepicardial gradient in normal dogs. In LVH there was a 45% decrease in ryanodine receptor binding and a loss in the natural subendocardial/subepicardial gradient, which roughly correlated inversely with the extent of LVH and directly with regional wall motion. Both mRNA and Western analyses revealed similar findings, with a reduction of the transmural mRNA levels and a loss in the natural gradient between subendocardial and subepicardial layers in LVH. Thus, ryanodine receptor message and binding in LVH is reduced preferentially in the subendocardium with consequent attenuation of the action of ryanodine in vivo. The selectively altered ryanodine regulation subendocardially in LVH could reconcile some of the controversy in this field and may play a role in mediating decompensation from stable LVH.

Myocardial beta-adrenergic reactivity in pressure overload-induced cardiac hypertrophy in the rat

The purpose of this study was to evaluate the changes in myocardial beta-adrenergic reactivity in animals undergoing a 4 week cardiac pressure-overload. Abdominal aortic constriction (AAC) or sham operation (sham) were performed in male Wistar rats, and 4 weeks later, isoprenaline dose-effects (chronotropic, inotropic and lusitropic properties) were studied after pithing. Noradrenaline (NA) and adrenaline (A) concentrations and NA turn-over index (DHPG /NE ratio) were evaluated in heart ventricles, while beta-adrenoceptor characteristics in ventricle homogenates and slices with [125I]iodocyanopindolol and the beta (1)/beta (2)-adrenoceptor ratio were estimated. Four weeks of cardiac pressure overload resulted in a 70% increase in ventricle weight/body weight ratio (from 2.5 +/- 0.1 to 4.2 +/- 0.3 mg/g in sham and AAC rats, respectively) and a 24% increase in protein contents (from 11.3 +/- 0.7 to 14.0 +/- 1.1 mg/100 mg ventricle in sham and AAC rats respectively). The ventricle NA content was similar in AAC and sham, while the ventricle A content and NA turn-over index were significantly increased in AAC rats (35 and 80% vs sham, respectively). Dose response of isoprenaline was significantly shifted to the right for all studied effects in AAC rats. However, maximal response (in relative values) was similar in AAC and sham rats only for heart rate but not for parameters depending on left ventricle contractile response. The beta-adrenoceptor density was significantly decreased in AAC by 30% without apparent affinity change and due to decreases in beta (1)-sites in septum and to beta (1)- and beta (2)-adrenoceptors in left ventricle endocardium. Decreases in isoprenaline-induced cardiac responses in AAC rats are associated with beta (1)-adrenoceptor density reduction and modification of beta (1)- and beta (2)-adrenoceptor ratio. These modifications are not the only reason for such dose response changes, at least for contractile response.

Growth hormone preserves cardiac sarcoplasmic reticulum Ca2+ release channels (ryanodine receptors) and enhances cardiac function in cardiomyopathic hamsters

OBJECTIVE

Growth hormone (GH) improves cardiac function in experimental models of heart failure and human dilated cardiomyopathy. However, the mechanism by which GH increases myocardial contractility is not entirely clear. Our aim was to examine the effects of GH on cardiac function and cardiac sarcoplasmic reticulum Ca2+ release channels (ryanodine receptors, RyR) in the hearts of UM-X7.1 cardiomyopathic hamsters during the development of heart failure.

METHODS

Experimental and healthy control hamsters were examined at the age of 20 weeks. Recombinant human GH (2 mg/kg/day, s.c.) or vehicle was then administered for 3 weeks. We examined (i) the in vivo left ventricular (LV) size and LV systolic function using transthoracic echocardiography, (ii) the density (Bmax) and affinity (Kd) of high-affinity [3H] ryanodine binding sites in crude homogenates from normal and cardiomyopathic hamster hearts.

RESULTS

Vehicle-treated UM-X7.1 hamsters exhibited significant increases in left ventricular end-diastolic diameter and end-systolic diameter (LVESd), and a significant decrease in LV fractional shortening (FS). GH-treatment attenuated the increase in LVESd and reduced the LV chamber size, and also significantly increased LVFS. Vehicle-treated UM-X7.1 hamsters exhibited a significantly lower Bmax than control hamsters (0.34 +/- 0.04 vs 0.44 +/- 0.06 pmol/mg, p < 0.05), and the treatment with GH in UM-X7.1 hamsters significantly attenuated the reduction of Bmax (0.42 +/- 0.03 pmol/mg vs vehicle-treated group (0.34 +/- 0.04 pmol/mg), p < 0.05). Kd did not differ significantly between the experimental groups. In normal control hamsters, GH treatment with this dose did not significantly enhance LV systolic function or the density of RyRs. There was no significant difference in terms of the connective-tissue volume-fraction, myocyte size and capillary density between the GH- and vehicle-treated groups of UM-X7.1 hamsters.

CONCLUSIONS

GH treatment may improve cardiac function by preserving the density of RyRs and enhancing cellular function in cardiomyopathic hamster hearts.

Polylysine-induced rapid Ca2+ release from cardiac sarcoplasmic reticulum

The rapid kinetics of polylysine-induced Ca2+ release from cardiac sarcoplasmic reticulum (SR) was assessed in combination with its effect on ryanodine binding. SR vesicles were isolated from canine cardiac SR. The time course of SR Ca2+ release was continuously monitored by a stopped-flow apparatus, and [3H]ryanodine binding was done by using the filtration method. The initial rate of polylysine-induced Ca2+ release from cardiac SR revealed different concentration dependence from those observed in skeletal SR. The initial rate peaked at 0.11 microM, followed by a decrease at higher concentrations in skeletal SR, whereas it increased to 3.7 microM in cardiac SR. The [3H]ryanodine binding was also stimulated by polylysine with an identical parallelism with Ca2+ release in terms of polylysine concentration dependence. Thus we demonstrated that the cardiac SR Ca2+ release channel is sensitive to activation by polylysine and that there is a difference in the concentration dependence of polylysine-induced activation of cardiac and skeletal SR Ca2+ release channels.

Effects of dantrolene sodium on progression of left ventricular hypertrophy induced by pressure overload in rats

We studied the long-term effects of dantrolene sodium (D), a specific sarcoplasmic reticulum (SR) Ca2+-release inhibitor, on the progression of left ventricular pressure-overloaded hypertrophy in rats. We treated abdominal aorta-constricted rats with one of two doses of D for 4 weeks. The extent of hypertrophy was expressed as the ratio of left ventricle to body weight. Hemodynamic parameters were measured by using a microtip catheter manometer. Although a low dose of D (500 mg/L in drinking water) decreased blood pressure to normal levels, the progression of cardiac hypertrophy was not inhibited. In contrast, a high dose of D (5 mg/kg, i.p.) also reduced blood pressure and inhibited the progression of cardiac hypertrophy. Dantrolene sodium had no effect on cardiac function in sham-operated rats. Thus control of Ca2+ release from the SR might be crucial in regulating the progression of cardiac hypertrophy, the final mediator possibly being intracellular Ca2+ concentration.

Alterations in cardiac SR Ca(2+)-release channels during development of heart failure in cardiomyopathic hamsters

The cardiomyopathic Syrian hamster develops a progressive cardiomyopathy characterized by cellular necrosis, hypertrophy, cardiac dilatation, and congestive heart failure. This study aimed to identify alterations in cardiac mechanical function and in the cellular content of sarcoplasmic reticulum (SR) Ca(2+)-release channels (ryanodine receptors, RyR) in the heart of the UM-X7.1 cardiomyopathic hamster during the development of heart failure. Experimental and healthy control hamsters were examined at 8, 18, and 28 wk of age. The UM-X7.1 hamsters had developed left ventricular (LV) hypertrophy at 8 wk and a marked LV dilatation at 18-28 wk. During the latter stage, the UM-X7.1 hamster hearts showed global hypokinesis. Equilibrium binding assays of high-affinity sites for [3H]ryanodine were performed in ventricular homogenate preparations. There was no significant difference between the two groups in the maximum number of [3H]ryanodine binding sites (Bmax) at either 8 or 18 wk of age, although the cardiac pump function was impaired in UM-X7.1 hamsters at 18 wk of age. By 28 wk, Bmax was significantly lower in the UM-X7.1 hamsters. Quantitative immunoblot assay revealed that the content of RyR protein in cardiomyopathic hearts, which was increased at the early stage, declined to below normal as heart failure advanced. These results suggest that the number of RyR in the UM-X7.1 cardiomyopathic hamsters was preserved at both the hypertrophic and early stages of heart failure with a possibly compensatory increase in the level of protein expression, although the cardiac function already showed a tendency to be impaired.