Superinhibition of sarcoplasmic reticulum function by phospholamban induces cardiac contractile failure

To determine whether selective impairment of cardiac sarcoplasmic reticulum (SR) Ca(2+) transport may drive the progressive functional deterioration leading to heart failure, transgenic mice, overexpressing a phospholamban Val(49) --> Gly mutant (2-fold), which is a superinhibitor of SR Ca(2+)-ATPase affinity for Ca(2+), were generated, and their cardiac phenotype was examined longitudinally. At 3 months of age, the increased EC(50) level of SR Ca(2+) uptake for Ca(2+) (0.67 +/- 0.09 microm) resulted in significantly higher depression of cardiomyocyte rates of shortening (57%), relengthening (31%), and prolongation of the Ca(2+) signal decay time (165%) than overexpression (2-fold) of wild type phospholamban (68%, 64%, and 125%, respectively), compared with controls (100%). Echocardiography also revealed significantly depressed function and impaired beta-adrenergic responses in mutant hearts. The depressed contractile parameters were associated with left ventricular remodeling, recapitulation of fetal gene expression, and hypertrophy, which progressed to dilated cardiomyopathy with interstitial tissue fibrosis and death by 6 months in males. Females also had ventricular hypertrophy at 3 months but exhibited normal systolic function up to 12 months of age. These results suggest a causal relationship between defective SR Ca(2+) cycling and cardiac remodeling leading to heart failure, with a gender-dependent influence on the time course of these alterations.     

Effect of dietary polyunsaturated fatty acids on contractile function of hearts isolated from sedentary and trained rats

Moderate physical training induced a decrease in arterial blood pressure in fish oil-fed rats as compared to sunflower seed oil-fed rats. The purpose of this study was to determine if these changes were due to modifications of the left ventricular function of the heart. Forty rats were fed a semi-purified diet containing either 10% sunflower seed oil or 10% fish oil (EPAX 3000TG, Pronova). Each dietary group was assigned to two sub-groups, one being constituted by sedentary animals and the other by trained animals. Training was achieved by daily running for 60 minutes at moderate intensity for three weeks. At the end of the training period, the animals were sacrificed and their hearts were immediately perfused according to the working mode. The phospholipid fatty acid composition and parameters of the left ventricular function were determined. Feeding fish oil markedly reduced the proportion of n-6 polyunsaturated fatty acids (PUFA, 18:2 n-6, 20:4 n-6, 22:4 n-6 and 22:5 n-6) in cardiac phospholipids. The n-6 PUFA were replaced by n-3 PUFA (mainly docosahexaenoic acid). In sedentary animals, the fluid dynamic (aortic and coronary flow, cardiac output) was not modified by the diet. The heart rate was reduced (-10%) in n-3 PUFA-rich hearts. Physical training did not markedly alter the polyunsaturated fatty acid profile of cardiac phospholipids. Conversely, it reduced the heart rate, aortic flow and cardiac output (-11, -21 and -14%, respectively) at a similar extent in the two dietary groups. In a second set of experiments, the training period was repeated in animals fed a commercially available diet (A103, UAR) which simultaneously provided n-6 and n-3 fatty acids. In these dietary conditions, neither the aortic flow nor the heart rate was decreased by physical exercise. These results suggest that both n-6 and n-3 PUFA in the diet are necessary to ensure a good cardiac adaptation to moderate physical training. Furthermore, the fish oil-induced decrease in arterial blood pressure in trained animals was not related to changes in cardiac contractility, but to a decrease in vascular resistances. Moderate physical training + dietary n-3 PUFA might be used to prevent hypertension and cardiovascular diseases.     

Alterations in cardiac contractile performance and sarcoplasmic reticulum function in sucrose-fed rats is associated with insulin resistance

Diabetes mellitus (DM) causes the development of a specific cardiomyopathy that results from the metabolic derangements present in DM and manifests as cardiac contractile dysfunction. Although myocardial dysfunction in Type 1 DM has been associated with defects in the function and regulation of the sarcoplasmic reticulum (SR), very little is known about SR function in Type 2 DM. Accordingly, this study examined whether abnormalities in cardiac contractile performance and SR function occur in the prestage of Type 2 DM (i.e., during insulin resistance). Sucrose feeding was used to induce whole body insulin resistance, whereas cardiac contractile performance was assessed by echocardiography and SR function was measured by SR calcium (Ca²⁺) uptake. Sucrose-fed rats exhibited hyperinsulinemia, hyperglycemia, and hyperlipidemia relative to control rats. Serial echocardiographic assessments in the sucrose-fed rats revealed early abnormalities in diastolic function followed by late systolic dysfunction and concurrent alterations in myocardial structure. The hearts of the 10-wk sucrose-fed rats showed depressed SR function demonstrated by a significant reduction in SR Ca²⁺ uptake. The decline in SR Ca²⁺ uptake was associated with a significant decrease in the cAMP-dependent protein kinase and Ca²⁺/calmodulin-dependent protein kinase II-mediated phosphorylation of phospholamban. The results show that abnormalities in cardiac contractile performance and SR function occur at an insulin-resistant stage before the manifestation of overt Type 2 DM. cardiomyopathy; diabetes mellitus; echocardiography     

Effect of hypertension on hearts of rats trained by swimming

To determine whether a prior chronic swimming program would alter the heart's response to chronic hypertension, female rats were made to swim for 10 wk, and then the left renal artery was stenosed. Heart perfusions were performed 10 wk later. The five groups studied were: control (C), normotensive swimmers (Sw), sedentary hypertensives (H), swimming rats made hypertensive and then allowed to be sedentary (Sw-H-Sd); and swimming animals made hypertensive and continued in a swimming program (Sw-H-Sw). Total heart and left ventricular weights were increased in increasing degrees in the sequence Sw, H, Sw-H-Sd, and Sw-H-Sw. Right ventricular weight was only increased in Sw and Sw-H-Sw. Swimming before the onset of hypertension enhanced total cardiac output and stroke work. Ejection fractions and mean velocity of circumferential fiber shortening (Vcf) were increased in Sw-H-Sd or Sw-H-Sw vs. controls. Myocardial O2 extraction was increased and coronary flow and myocardial O2 consumption were diminished in all hypertensive groups. However, lactate production was similar in all groups. Myosin adenosinetriphosphatase activity was increased in Sw but decreased in the three H groups. The percent of V1 myosin isozyme was greater and the percent of V3 less in Sw than in C; V1 was diminished and V3 increased in H and Sw-H-Sd; isozymes were normal in Sw-H-Sw.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiac contractile proteins and autoimmune myocarditis

Concerning cardiac contractile proteins, antigenicity and myocarditogenicity were discussed. In normal states, these proteins are immunologically tolerant, and can not provoke any heart-specific disease. Why the proteins can provoke such lethal autoimmune myocarditis has not been completely elucidated. Shortly after cardiac infection or myocardial ischemia, these proteins may work as the antigen for the autoimmune myocardites. First of all, the role of cardiac myosin has been strongly emphasized. But, the antigen determinants: epitope proteins remain unclear. Either cross-activity to the streptococcal M protein and/or the -helical coiled-coil protein may be an important factor to determine antigenicity. In this autoimmune myocarditis, the roles of T-lymphocyte and cardiac dendritic cell are noticeable. Through further study on the relation between antigen epitope and the infectious agents in the heart; on cardio-cytotoxity of the T-lymphocyte and on the precise contribution of cardiac dendritic cells, this autoimmune myocarditis will be more clarified.     

Localization of sarcoplasmic reticulum proteins in rat skeletal muscle by immunofluorescence

Ca++-Mg++-dependent ATPase and calsequestrin, the major intrinsic and extrinsic proteins, respectively, of the sarcoplasmic reticulum, were localized in cryostat sections of adult rat skeletal muscle by immunofluorescent staining and phase-contrast microscopy. Relatively high concentrations of both the ATPase and calsequestrin were found in fast-twitch myofibers while a very low concentration of the ATPase and a moderate concentration of calsequestrin were found in slow-twitch myofibers. These findings are consistent with previous biochemical studies of the isolated sarcoplasmic reticulum of slow-twitch and fast-twitch mammalian muscles. The distribution of the ATPase in muscle fibers is distinctly different from that of calsequestrin. While calsequestrin is present only near the interface between the I- and A-band regions of the sarcomere, the ATPase is found throughout the I-band region as well as in the center of the A-band region. In comparing these results with in situ ultrastructural studies of the distribution of sarcoplasmic reticulum in fast-twitch muscle, it appears that the ATPase is rather uniformly distributed throughout the sarcoplasmic reticulum while calsequestrin is almost exclusively confined to those regions of the membrane system which correspond to terminal cisternae. Fluorescent staining with these antisera was not observed in vascular smooth muscle cells present in the cryostat sections of the mammalian skeletal muscle used in this study.     

Dexamethasone treatment improves sarcoplasmic reticulum function and contractile performance in aged myocardium

Corticosteroids are thought to be involved in the maintenance of normal myocardial function by mechanisms incompletely understood. This study investigated the potential therapeutic benefit of the synthetic glucocorticoid, dexamethasone, in reversing age-associated deterioration in cardiac contractile performance and Ca2+ sequestration function of the sarcoplasmic reticulum. Dexamethasone was administered to senescent (26-28-month old), male Fischer 344 rats at a rate of 4 microg/h for 5 days via subcutaneously implanted osmotic mini pumps. Control rats received vehicle solution in similar manner. Contractile performance was assessed in Langendorff-perfused, electrically paced hearts from control and dexamethasone-treated rats. The results obtained showed that dexamethasone-treatment of aged rats resulted in significant improvement in myocardial contractile performance as evidenced by (i) increase (approximately 30-60%) in developed peak tension at a wide range of beating frequencies (2-6 Hz), (ii) unaltered time to peak tension, and (iii) decrease (approximately 8-15%) in time to half-relaxation. Also, SR isolated from dexamethasone-treated rats displayed approximately 2-fold higher rates of ATP-energized Ca2+ uptake compared to SR from control rats. The deficits in contractile performance of the senescent heart (prolonged contraction duration and diminished contractile force) are reversible through a glucocorticoid-mediated improvement in SR Ca2+ pump function.     

Impaired sarcoplasmic reticulum function leads to contractile dysfunction and cardiac hypertrophy

Sarcoplasmic reticulum (SR)-mediated Ca(2+) sequestration and release are important determinants of cardiac contractility. In end-stage heart failure SR dysfunction has been proposed to contribute to the impaired cardiac performance. In this study we tested the hypothesis that a targeted interference with SR function can be a primary cause of contractile impairment that in turn might alter cardiac gene expression and induce cardiac hypertrophy. To study this we developed a novel animal model in which ryanodine, a substance that alters SR Ca(2+) release, was added to the drinking water of mice. After 1 wk of treatment, in vivo hemodynamic measurements showed a 28% reduction in the maximum speed of contraction (+dP/dt(max)) and a 24% reduction in the maximum speed of relaxation (-dP/dt(max)). The slowing of cardiac relaxation was confirmed in isolated papillary muscles. The late phase of relaxation expressed as the time from 50% to 90% relaxation was prolonged by 22%. After 4 wk of ryanodine administration the animals had developed a significant cardiac hypertrophy that was most prominent in both atria (right atrium +115%, left atrium +100%, right ventricle +23%, and left ventricle +13%). This was accompanied by molecular changes including a threefold increase in atrial natriuretic factor mRNA and a sixfold increase in beta-myosin heavy chain mRNA. Sarcoplasmic endoplasmic reticulum Ca(2+) mRNA was reduced by 18%. These data suggest that selective impairment of SR function in vivo can induce changes in cardiac gene expression and promote cardiac growth.     

Cardiac sarcoplasmic reticulum isolation from slaughterhouse beef heart

A method by which slaughterhouse beef heart may be used to obtain a cardiac sarcoplasmic reticulum with high sarcoplasmic reticulum activities and low non-sarcoplasmic reticulum contamination is described. Moreover conditions which influence the yield and activity of this preparation are extensively explored. The characteristics of this sarcoplasmic reticulum preparation are compared with the preparations obtained by the other available methods.     

Functional aspects of skeletal muscle contractile apparatus and sarcoplasmic reticulum after fatigue

This study examined the effects of fatigue on the functionalaspects of the contractile apparatus and sarcoplasmic reticulum (SR).Frog semitendinosus muscles were stimulated to fatigue, and skinnedfibers or a homogenate fraction was prepared from both fatigued andrested contralateral muscles. In fatigued fibers, maximalCa²⁺-activated force of thecontractile apparatus was unaltered, whereas maximal actomyosin-ATPaseactivity was depressed by 20%. TheCa²⁺ sensitivity of force wasincreased, whereas that of actomyosin-ATPase was not altered. Also, therate constant for tension redevelopment was decreased at submaximalCa²⁺ concentration. These latterfindings suggest that fatigue slows the dissociation offorce-generating myosin cross bridges.Ca²⁺ uptake andCa²⁺-ATPase activity of the SRwere depressed by 46 and 21%, respectively, in the fatigued muscles.Fatigue also reduced the rates of SR Ca²⁺ release evoked byAgNO₃ and4-chloro-m-cresol by 38 and 45%, respectively. During fatigue, the contractile apparatus and SR undergointrinsic functional alterations. These changes likely result inaltered force production and energy consumption by the intact muscle.

     

Structural membrane proteins and loosely associated proteins of the sarcoplasmic reticulum

The protein composition of sarcoplasmic-reticulum vesicles, either unpurified or after fractionation on sucrose gradients, and with or without previous osmotic shock and sonication, was investigated by electrophoresis in acid polyacrylamide gels. The pattern of release of loosely bound proteins is discussed with respect to their localization in the interior of the vesicles.     

Microarray profiling of skeletal muscle sarcoplasmic reticulum proteins

Microarrays were developed to profile the level of proteins associated with calcium regulation in sarcoplasmic reticulum (SR) isolated from porcine Longissimus muscle. The microarrays consisted of SR preparations printed onto to glass slides and probed with monoclonal antibodies to 7 target proteins. Proteins investigated included: ryanodine receptor, (RyR), dihydropyridine receptor, (DHPR), triadin (TRI), calsequestrin (CSQ), 90 kDa junctional protein (JSR90), and fast-twitch and slow-twitch SR calcium ATPases (SERCA1 and SERCA2). Signal from a fluorescently-labeled detection antibody was measured and quantitated using a slide reader. The microarray developed was also employed to profile Longissimus muscle SR proteins from halothane genotyped animals. Significant (P<0.05) reductions in levels of several proteins were found including: RyR, CSQ, TRI, DHPR and SERCA2 in SR samples from halothane positive animals. The results illustrate the potential of microarrays as a tool for profiling SR proteins and aiding investigations of calcium regulation.     

Cardiac force-interval relationship,adrenaline and sarcoplasmic reticulum in rainbow trout

The force-interval relationship was examined at 20 and 10 °C in electrically paced atrial and ventricular tissue of rainbow trout,Oncorhynchus mykiss, regarding dependence on the sarcoplasmic reticulum and influence of adrenaline. In both tissues, adrenaline (10^-6 mol·l^-1) doubled control force developed at 0.5 Hz. In atrial but not in ventricular tissue it also shortened the diastolic interval needed for recovery of a given fraction of the control force. In atrial tissue and in ventricular tissue at 20 °C, the fraction of force recovered in the presence of adrenaline was diminished by 10 mol·l^-1 of ryanodine, a specific inhibitor of the sarcoplasmic reticulum. In atrial tissue not exposed to adrenaline and in ventricular tissue at 10 °C irrespective of adrenaline, ryanodine did not affect recovery. In atrial but not in ventricular tissue it also diminished control force. In conclusion, the cardiac sarcoplasmic reticulum of trout seems to support force development during adrenaline dependent increases in heart rate, and in atrial tissue also the force at steady state.Abbreviations E-C coupling excitation-contraction coupling - P-R potential - SR sarcoplasmic reticulum - SE standard error of the mean     

Cardiac sarcoplasmic reticulum contains a low-affinity site for phenylalkylamines

The distribution of the bovine cardiac binding sites for the organic calcium-channel blockers was studied. Crude microsomal membranes were separated into three fractions, which contained mainly membranes derived from sarcolemma, 'junctional' sarcoplasmic reticulum containing transversal tubuli, and free sarcoplasmic reticulum. The high-affinity binding site for the dihydropyridines, determined in the presence of nitrobenzylthioinosine, was enriched 12-fold and 17-fold in sarcolemma and junctional sarcoplasmic reticulum. The binding sites for the phenylalkylamines, determined with [3H]verapamil or [3H](-)desmethoxyverapamil, were enriched 1.5-3.4-fold in sarcolemma and junctional sarcoplasmic reticulum but 6-10-fold in free sarcoplasmic reticulum. The phenylalkylamine-binding site, present in free sarcoplasmic reticulum, was partially destroyed by chymotrypsin or phospholipase A2 and C treatment. Specific binding was proportional to the concentration of the added membrane protein. The binding of (-)desmethoxyverapamil was half-maximally inhibited by 6.5 mM calcium chloride and was optimal in the presence of 5 mM EGTA. In three out of five preparations (-)desmethoxyverapamil bound to a single site with an apparent Kd value of 191 +/- 42.8 nM and a density of 34.5 +/- 7.7 pmol/mg protein. In two out of five preparations an additional high-affinity site (Kd approximately 0.67 nM) was detected. The low-affinity site bound other phenylalkylamines, but stereospecific binding of phenylalkylamines was not observed. Binding of phenylalkylamines to the low-affinity site was inhibited by some but not all calmodulin 'antagonists'. Furthermore dihydropyridines did not affect the binding of (--)desmethoxyverapamil suggesting that the low-affinity site differs considerably from the high-affinity sarcolemmal site. These results suggest that free sarcoplasmic reticulum contains a binding site for phenylalkylamines at a relative high density, which is not related to the high-affinity site present in the voltage-dependent calcium channel.     

Assembly of the sarcoplasmic reticulum proteins in differentiating rat skeletal muscle cell cultures: localization by immunofluorescence of sarcoplasmic reticulum proteins in differentiating rat skeletal muscle cell cultures

Immunofluorescent staining techniques were used to study the distribution of the Ca(2) + Mg(2+)-dependent ATPase and calsequestrin in primary cultures of differentiating rat skeletal muscle cells, grown for different periods of time under various culture conditions. In mononucleated myoblasts calsequestrin was detected after 45 h in culture whereas the ATPase was not detected until 60 h. After cell fusion began, both proteins could be identified in all multinucleated cells. Myoblasts grown for longer than 60 h in low Ca(2+) medium contained calsequestrin and the ATPase, even though they were unable to fuse. These studies at the cellular level confirm biochemical findings on the biosynthesis of calsequestrin and the ATPase. Immunofluorescent staining of myoblasts showed that calsequestrin first appears in a well-defined region of the cell near one end of the nucleus. At later times, the staining occupied progressively larger regions adjacent to the nucleus and took on a fibrous appearance. This suggests that calsequestrin first accumulates in the Golgi region and then gradually spreads throughout the cell. In contrast, the ATPase appeared to be concentrated in many small patches or foci throughout the cytoplasm and was never confined to one particular region, although some parts of the cell often stained more intensely than others. In multinucleated cells, alternating dark and fluorescent strands parallel to the longitudinal axis of the cells were evident.