The role of phospholamban in the regulation of calcium transport by cardiac sarcoplasmic reticulum

Targeting calcium-mediated inter-organellar crosstalk in cardiac diseases

INTRODUCTION

Abnormal calcium signaling between organelles such as the sarcoplasmic reticulum (SR), mitochondria and lysosomes is a key feature of heart diseases. Calcium serves as a secondary messenger mediating inter-organellar crosstalk, essential for maintaining the cardiomyocyte function.

AREAS COVERED

This article examines the available literature related to calcium channels and transporters involved in inter-organellar calcium signaling. The SR calcium-release channels ryanodine receptor type-2 (RyR2) and inositol 1,4,5-trisphosphate receptor (IP₃R), and calcium-transporter SR/ER-ATPase 2a (SERCA2a) are illuminated. The roles of mitochondrial voltage-dependent anion channels (VDAC), the mitochondria Ca²⁺ uniporter complex (MCUC), and the lysosomal H⁺/Ca²⁺ exchanger, two pore channels (TPC), and transient receptor potential mucolipin (TRPML) are discussed. Furthermore, recent studies showing calcium-mediated crosstalk between the SR, mitochondria, and lysosomes as well as how this crosstalk is dysregulated in cardiac diseases are placed under the spotlight.

EXPERT OPINION

Enhanced SR calcium release via RyR2 and reduced SR reuptake via SERCA2a, increased VDAC and MCUC-mediated calcium uptake into mitochondria, and enhanced lysosomal calcium-release via lysosomal TPC and TRPML may all contribute to aberrant calcium homeostasis causing heart disease. While mechanisms of this crosstalk need to be studied further, interventions targeting these calcium channels or combinations thereof might represent a promising therapeutic strategy.

Implications of SGLT Inhibition on Redox Signalling in Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained (atrial) arrhythmia, a considerable global health burden and often associated with heart failure. Perturbations of redox signalling in cardiomyocytes provide a cellular substrate for the manifestation and maintenance of atrial arrhythmias. Several clinical trials have shown that treatment with sodium-glucose linked transporter inhibitors (SGLTi) improves mortality and hospitalisation in heart failure patients independent of the presence of diabetes. Post hoc analysis of the DECLARE-TIMI 58 trial showed a 19% reduction in AF in patients with diabetes mellitus (hazard ratio, 0.81 (95% confidence interval: 0.68-0.95), n = 17.160) upon treatment with SGLTi, regardless of pre-existing AF or heart failure and independent from blood pressure or renal function. Accordingly, ongoing experimental work suggests that SGLTi not only positively impact heart failure but also counteract cellular ROS production in cardiomyocytes, thereby potentially altering atrial remodelling and reducing AF burden. In this article, we review recent studies investigating the effect of SGLTi on cellular processes closely interlinked with redox balance and their potential effects on the onset and progression of AF. Despite promising insight into SGLTi effect on Ca²⁺ cycling, Na⁺ balance, inflammatory and fibrotic signalling, mitochondrial function and energy balance and their potential effect on AF, the data are not yet conclusive and the importance of individual pathways for human AF remains to be established. Lastly, an overview of clinical studies investigating SGLTi in the context of AF is provided.

Progress in gene therapy for heart failure

Recent advances in our understanding of the pathophysiology of myocardial dysfunction in the setting of congestive heart failure have created a new opportunity in developing nonpharmacological approaches to treatment. Gene therapy has emerged as a powerful tool in targeting the molecular mechanisms of disease by preventing the ventricular remodeling and improving bioenergetics in heart failure. Refinements in vector technology, including the creation of recombinant adeno-associated viruses, have allowed for safe and efficient gene transfer. These advancements have been coupled with evolving delivery methods that include vascular, pericardial, and direct myocardial approaches. One of the most promising targets, SERCA2a, is currently being used in clinical trials. The recent success of the Calcium Upregulation by Percutaneous Administration of Gene Therapy in Cardiac Disease phase 2 trials using adeno-associated virus 1-SERCA2a in improving outcomes highlights the importance of gene therapy as a future tool in treating congestive heart failure.

The role of CaMKII regulation of phospholamban activity in heart disease

Phospholamban (PLN) is a phosphoprotein in cardiac sarcoplasmic reticulum (SR) that is a reversible regulator of the Ca²⁺-ATPase (SERCA2a) activity and cardiac contractility. Dephosphorylated PLN inhibits SERCA2a and PLN phosphorylation, at either Ser¹⁶ by PKA or Thr¹⁷ by Ca²⁺-calmodulin-dependent protein kinase (CaMKII), reverses this inhibition. Through this mechanism, PLN is a key modulator of SR Ca²⁺ uptake, Ca²⁺ load, contractility, and relaxation. PLN phosphorylation is also the main determinant of β1-adrenergic responses in the heart. Although phosphorylation of Thr¹⁷ by CaMKII contributes to this effect, its role is subordinate to the PKA-dependent increase in cytosolic Ca²⁺, necessary to activate CaMKII. Furthermore, the effects of PLN and its phosphorylation on cardiac function are subject to additional regulation by its interacting partners, the anti-apoptotic HAX-1 protein and Gm or the anchoring unit of protein phosphatase 1. Regulation of PLN activity by this multimeric complex becomes even more important in pathological conditions, characterized by aberrant Ca²⁺-cycling. In this scenario, CaMKII-dependent PLN phosphorylation has been associated with protective effects in both acidosis and ischemia/reperfusion. However, the beneficial effects of increasing SR Ca²⁺ uptake through PLN phosphorylation may be lost or even become deleterious, when these occur in association with alterations in SR Ca²⁺ leak. Moreover, a major characteristic in human and experimental heart failure (HF) is depressed SR Ca²⁺ uptake, associated with decreased SERCA2a levels and dephosphorylation of PLN, leading to decreased SR Ca²⁺ load and impaired contractility. Thus, the strategy of altering SERCA2a and/or PLN levels or activity to restore perturbed SR Ca²⁺ uptake is a potential therapeutic tool for HF treatment. We will review here the role of CaMKII-dependent phosphorylation of PLN at Thr¹⁷ on cardiac function under physiological and pathological conditions.

Cardiomyocyte-specific p65 NF-κB deletion protects the injured heart by preservation of calcium handling

NF-κB is a well-known transcription factor that is intimately involved with inflammation and immunity. We have previously shown that NF-κB promotes inflammatory events and mediates adverse cardiac remodeling following ischemia reperfusion (I/R). Conversely, others have pointed to the beneficial influence of NF-κB in I/R injury related to its anti-apoptotic effects. Understanding the seemingly disparate influence of manipulating NF-κB is hindered, in part, by current approaches that only indirectly interfere with the function of its most transcriptionally active unit, p65 NF-κB. Mice were generated with cardiomyocyte-specific deletion of p65 NF-κB. Phenotypically, these mice and their hearts appeared normal. Basal and stimulated p65 expression were significantly reduced in whole hearts and completely ablated in isolated cardiomyocytes. When compared with wild-type mice, transgenic animals were protected from both global I/R by Langendorff as well as regional I/R by coronary ligation and release. The protected, transgenic hearts had less cytokine activity and decreased apoptosis. Furthermore, p65 ablation was associated with enhanced calcium reuptake by the sarcoplasmic reticulum. This influence on calcium handling was related to increased expression of phosphorylated phospholamban in conditional p65 null mice. In conclusion, cardiomyocyte-specific deletion of the most active, canonical NF-κB subunit affords cardioprotection to both global and regional I/R injury. The beneficial effects of NF-κB inhibition are related, in part, to modulation of intracellular calcium homeostasis.

Modulation of cardiac contractility by the phospholamban/SERCA2a regulatome

Heart disease remains the leading cause of death and disability in the Western world. Current therapies aim at treating the symptoms rather than the subcellular mechanisms, underlying the etiology and pathological remodeling in heart failure. A universal characteristic, contributing to the decreased contractile performance in human and experimental failing hearts, is impaired calcium sequestration into the sarcoplasmic reticulum (SR). SR calcium uptake is mediated by a Ca(2+)-ATPase (SERCA2), whose activity is reversibly regulated by phospholamban (PLN). Dephosphorylated PLN is an inhibitor of SERCA and phosphorylation of PLN relieves this inhibition. However, the initial simple view of a PLN/SERCA regulatory complex has been modified by our recent identification of SUMO, S100 and the histidine-rich Ca-binding protein as regulators of SERCA activity. In addition, PLN activity is regulated by 2 phosphoproteins, the inhibitor-1 of protein phosphatase 1 and the small heat shock protein 20, which affect the overall SERCA-mediated Ca-transport. This review will highlight the regulatory mechanisms of cardiac contractility by the multimeric SERCA/PLN-ensemble and the potential for new therapeutic avenues targeting this complex by using small molecules and gene transfer methods.

Elevated levels of protein phosphatase 1 and phosphatase 2A may contribute to cardiac dysfunction in diabetes

Although protein phosphorylation and dephosphorylation are known to regulate the activities of different enzymes, sufficient information on the role of dephosphorylation in cardiac function is not available. Since protein phosphatases mediate dephosphorylation, it is possible that cardiac dysfunction induced by diabetes may be due to alterations in the activities of these enzymes. We therefore determined cardiac protein phosphatase activity as well as protein contents of phosphatase 1 and phosphatase 2A in diabetic animals. For this purpose, rats were made diabetic by administering a single intravenous injection of streptozotocin (65 mg/kg body weight) and hearts were examined after 1, 2, 3, 4 and 8 weeks. Some of the 4-week diabetic animals received subcutaneous injections of insulin (3 U/day) for a further period of 4 weeks. Cardiac dysfunction was evident after 2 weeks of inducing diabetes and deteriorated further with time. A significant increase in protein phosphatase activity appeared after 1 week and persisted until 8 weeks. Increased protein phosphatase activity in the diabetic heart was associated with a corresponding increase in the protein contents of both phosphatase 1 and phosphatase 2A. Insulin treatment partly prevented the changes observed in diabetic animals. The results suggest that increased protein phosphatase activities and subsequent enhanced protein dephosphorylation may play a role in diabetes-induced cardiac dysfunction.

Phase I and phase II of short-term mechanical restitution in perfused rat left ventricles

We examined the contributions of the Ca(2+) channels of the sarcolemma and of the sarcoplasmic reticulum to electromechanical restitution. Extrasystoles (F(1)) were interpolated 40-600 ms following a steady-state beat (F(0)) in perfused rat ventricles paced at 2 or 3 Hz. Plots of F(1)/F(0) versus the extrasystolic interval consisted of phase I, which occurred before relaxation of the steady-state beat, and phase II, which occurred later. Phase I exhibited a period of enhanced left ventricular pressure development that coincided with action potential prolongation. Phase I was eliminated by -BAY K 8644 (100 nM) and FPL 64176 (150 nM), augmented by 3 microM thapsigargin plus 200 nM ryanodine and unaffected by KN-93 and KB-R7943. Phase II was accelerated by the Ca(2+) channel agonists and by isoproterenol but was eliminated by thapsigargin plus ryanodine. The results suggest that phase I of electromechanical restitution is caused by a transient L-type Ca(2+) current facilitation, whereas phase II represents the recovery of the ability of the sarcoplasmic reticulum to release Ca(2+).

Angiotensin type 1 receptor antagonism with irbesartan inhibits ventricular hypertrophy and improves diastolic function in the remodeling post-myocardial infarction ventricle

To evaluate the role of angiotensin II (AII) on diastolic function during post-myocardial infarction (MI) ventricular remodeling, coronary ligation or sham operation was performed in male Sprague-Dawley rats. Experimental animals were maintained on either irbesartan, a selective AT1-receptor antagonist, or no treatment. Measurement of cardiac hypertrophy, diastolic function, and sarcoendoplasmic reticulum adenosine triphosphatase (ATPase; SERCA) and phospholamban (PLB) gene expression was assessed at 6 weeks after MI. Myocardial infarction caused a significant increase in myocardial mass and left ventricular (LV) filling pressure, whereas LV systolic pressure and +dP/dt were reduced. The time constant of isovolumic relaxation (tau) was markedly prolonged after MI. Post-MI hypertrophy was associated with substantial increases in the messenger RNA (mRNA) expression of atrial natriuretic peptide (ANP), but no significant changes in SERCA or PLB levels. Although irbesartan treatment did not significantly alter post-MI LV systolic or filling pressures, it nevertheless effectively decreased ventricular hypertrophy, improved tau, and normalized ANP expression. These results demonstrate that AT1-receptor antagonism has important effects on myocardial hypertrophy and ANP gene expression, which are independent of ventricular loading conditions. In addition, the improvement in diastolic function was not related to changes in SERCA and PLB gene expression, suggesting that enhanced myocardial relaxation was related to the blockade of AII effects on myocyte function or through a reduction of ventricular hypertrophy itself or both.

Phospholamban: protein structure, mechanism of action, and role in cardiac function

A comprehensive discussion is presented of advances in understanding the structure and function of phospholamban (PLB), the principal regulator of the Ca2+-ATPase of cardiac sarcoplasmic reticulum. Extensive historical studies are reviewed to provide perspective on recent developments. Phospholamban gene structure, expression, and regulation are presented in addition to in vitro and in vivo studies of PLB protein structure and activity. Applications of breakthrough experimental technologies in identifying PLB structure-function relationships and in defining its interaction with the Ca2+-ATPase are also highlighted. The current leading viewpoint of PLB's mechanism of action emerges from a critical examination of alternative hypotheses and the most recent experimental evidence. The potential physiological relevance of PLB function in human heart failure is also covered. The interest in PLB across diverse biochemical disciplines portends its continued intense scrutiny and its potential exploitation as a therapeutic target.

Phosphorylation and regulation of the Ca(2+)-pumping ATPase in cardiac sarcoplasmic reticulum by calcium/calmodulin-dependent protein kinase

In cardiac muscle, a membrane-associated Ca2+/calmodulin-dependent protein kinase (CaM kinase) phosphorylates the Ca(2+)-pumping ATPase in addition to its previously characterized substrates, phospholamban and Ca(2+)-release channel (ryanodine receptor). The phosphorylated amino acid in the Ca(2+)-ATPase has been identified as serine. Posphorylation of the Ca(2+)-ATPase is rapid and is reversible by a membrane-associated protein phosphatase, Ca(2+)-ATPase purified from cardiac SR underwent phosphorylation by exogenous CaM kinase, and the phosphorylated enzyme displayed twofold greater catalytic activity without alteration in its Ca(2+)-sensitivity. The phosphorylation of the Ca(2+)-ATPase was found to be isoform-specific in that the cardiac and slow-twitch skeletal muscle isoform (SERCA 2), but not the fast-twitch skeletal muscle isoform (SERCA 1), underwent phosphorylation by CaM kinase. Studies using SERCA 1 and SERCA 2 isoforms and their mutants expressed in a heterelogous cell system have resulted in i) confirmation of the isoform specificity of Ca(2+)-ATPase phosphorylation by CaM kinase, ii) identification of Ser38 as the site in SERCA 2 phosphorylated by CaM kinase, and iii) demonstration of phosphorylation-induced increase in Vmax of Ca2+ transport by the SERCA 2 enzyme. These observations suggest that in cardiac and slow-twitch skeletal muscle direct phosphorylation of the SR Ca(2+)-ATPase by the membrane-bound CaM kinase may serve to stimulate Ca2+ sequestration and therefore, the speed of muscle relaxation.

Role of an aprotinin-sensitive protease in the activation of Ca(2+)-ATPase by superoxide radical (O2-.) in microsomes of pulmonary vascular smooth muscle

We have investigated the role of an aprotinin-sensitive protease in regulating Ca(2+)-ATPase activity and Ca2+ uptake (ATP-dependent and Na(+)-dependent) in microsomes of bovine pulmonary vascular smooth muscle during treatment with the O2(-.)-generating system hypoxanthine plus xanthine oxidase. Treatment of the smooth muscle microsomes with the O2(-.)-generating system produced a protease in a gelatin-containing zymogram with an apparent molecular mass of 16 kDa. This 16 kDa proteolytic protein was found to be inhibited by superoxide dismutase (SOD) and aprotinin but not by PMSF. Using polyclonal antiserum to aprotinin, we found that it is an ambient antiprotease of the smooth muscle microsomes. Treatment of the microsomes with the O2(-.)-generating system stimulated protease activity tested with a synthetic substrate N-benzoyl-DL-arginine p-nitroanilide and also enhanced Ca(2+)-ATPase activity. It also stimulated ATP-dependent Ca2+ uptake. In contrast, Na(+)-dependent Ca2+ uptake was found to be inhibited by the O2(-.)-generating system. Pretreatment of the microsomes with SOD and aprotinin preserved the increase in protease activity, Ca(2+)-ATPase activity and ATP-dependent Ca2+ uptake. In addition, O2(-.)-caused inhibition of the Na(+)-dependent Ca2+ uptake which was reversed by SOD and aprotinin. Pretreatment with PMSF did not cause any discernible alteration in the protease activity, Ca(2+)-ATPase activity. ATP-dependent Ca2+ uptake and Na(+)-dependent Ca2+ uptake in the microsomes caused by the O2(-.)-generating system. These results suggest that an aprotinin-sensitive protease plays a pivotal role in regulating Ca(2+)-ATPase and Ca(2+)-uptake activities in microsomes of pulmonary vascular smooth muscle under oxidant O2(-.)-triggered conditions.

Cardiac-specific overexpression of phospholamban alters calcium kinetics and resultant cardiomyocyte mechanics in transgenic mice

Phospholamban is the regulator of the cardiac sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity and an important modulator of basal contractility in the heart. To determine whether all the SR Ca(2+)-ATPase enzymes are subject to regulation by phospholamban in vivo, transgenic mice were generated which overexpressed phospholamban in the heart, driven by the cardiac-specific alpha-myosin heavy chain promoter. Quantitative immunoblotting revealed a twofold increase in the phospholamban protein levels in transgenic hearts compared to wild type littermate hearts. The transgenic mice showed no phenotypic alterations and no changes in heart/body weight, heart/lung weight, and cardiomyocyte size. Isolated unloaded cardiac myocytes from transgenic mice exhibited diminished shortening fraction (63%) and decreased rates of shortening (64%) and relengthening (55%) compared to wild type (100%) cardiomyocytes. The decreases in contractile parameters of transgenic cardiomyocytes reflected decreases in the amplitude (83%) of the Ca2+ signal and prolongation (131%) in the time for decay of the Ca2+ signal, which was associated with a decrease in the apparent affinity of the SR Ca(2+)-ATPase for Ca2+ (56%), compared to wild type (100%) cardiomyocytes. In vivo analysis of left ventricular systolic function using M mode and pulsed-wave Doppler echocardiography revealed decreases in fractional shortening (79%) and the normalized mean velocity of circumferential shortening (67%) in transgenic mice compared to wild type (100%) mice. The differences in contractile parameters and Ca2+ kinetics in transgenic cardiomyocytes and the depressed left ventricular systolic function in transgenic mice were abolished upon isoproterenol stimulation. These findings indicate that a fraction of the Ca(2+)-ATPases in native SR is not under regulation by phospholamban. Expression of additional phospholamban molecules results in: (a) inhibition of SR Ca2+ transport; (b) decreases in systolic Ca2+ levels and contractile parameters in ventricular myocytes; and (c) depression of basal left ventricular systolic function in vivo.

Differential phospholamban gene expression in murine cardiac compartments. Molecular and physiological analyses

Phospholamban, the regulator of the Ca2+ pump in cardiac sarcoplasmic reticulum, is differentially expressed between murine atrial and ventricular muscles. Quantitative analyses of RNA isolated from atrial flaps and ventricular apices indicated that the phospholamban gene transcript copy number is 2.5-fold higher in the ventricle compared with the atrium of the FVB/N mouse and 6-fold higher in the ventricle compared with the atrium of the B6D2/F1 mouse strain. These findings were corroborated by in situ hybridization studies of cardiopulmonary sections from both murine strains, and phospholamban transcripts were also observed in pulmonary myocardia of both strains. Analyses of phospholamban transcript levels relative to alpha-myosin heavy chain (alpha-MHC) revealed a 3-fold higher phospholamban abundance in the ventricle compared with the atrium of the FVB/N murine strain. However, the relative mRNA level of Ca(2+)-ATPase (ratio of sarcoplasmic reticulum Ca(2+)-ATPase [SERCA2] to alpha-MHC) in the ventricle was 80% of that in the atrium. Consequently, the relative ratio of phospholamban to SERCA2 mRNA was 4.2-fold lower in the atrium than in the ventricle. The lower transcript ratio of phospholamban to SERCA2 in the atrium was associated with significantly shortened times to half-relaxation (17.40 +/- 0.71 milliseconds for atrium versus 30.58 +/- 2.04 milliseconds for ventricle), assessed in isolated superfused cardiac tissue preparations recorded at maximum length tension. Contraction times, measured as times to peak tension, were also significantly shortened in atrial muscle (27.36 +/- 0.82 milliseconds) compared with ventricular muscle (44.60 +/- 2.55 milliseconds), assessed in the same tissue preparations. These findings suggest that phospholamban gene expression is differentially regulated in murine atrial and ventricular muscles and that this differential expression may be associated with differences in the contractile parameters of these cardiac compartments.

The failing human heart is unable to use the Frank-Starling mechanism

There is evidence that the failing human left ventricle in vivo subjected to additional preload is unable to use the Frank-Starling mechanism. The present study compared the force-tension relation in human nonfailing and terminally failing (heart transplants required because of dilated cardiomyopathy) myocardium. Isometric force of contraction of electrically driven left ventricular papillary muscle strips was studied under various preload conditions (2 to 20 mN). To investigate the influence of inotropic stimulation, the force-tension relation was studied in the presence of the cardiac glycoside ouabain. In skinned-fiber preparations of the left ventricle, developed tension was measured after stretching the preparations to 150% of the resting length. To evaluate the length-dependent activation of cardiac myofibrils by Ca2+ in failing and nonfailing myocardium, the tension-Ca2+ relations were also measured. After an increase of preload, the force of contraction gradually increased in nonfailing myocardium but was unchanged in failing myocardium. There were no differences in resting tension, muscle length, or cross-sectional area of the muscles between both groups. Pretreatment with ouabain (0.02 mumol/L) restored the force-tension relation in failing myocardium and preserved the force-tension relation in nonfailing tissue. In skinned-fiber preparations of the same hearts, developed tension increased significantly after stretching only in preparations from nonfailing but not from failing myocardium. The Ca2+ sensitivity of skinned fibers was significantly higher in failing myocardium (EC50, 1.0; 95% confidence limit, 0.88 to 1.21 mumol/L) compared with nonfailing myocardium (EC50, 1.7; 95% confidence limit, 1.55 to 1.86 mumol/L). After increasing the fiber length by stretching, a significant increase in the sensitivity of the myofibrils to Ca2+ was observed in nonfailing but not in failing myocardium. These experiments provide evidence for an impaired force-tension relation in failing human myocardium. On the subcellular level, this phenomenon might be explained by a failure of the myofibrils to increase the Ca2+ sensitivity after an increase of the sarcomere length.

Purification, amino-terminal sequence and functional properties of a 64 kDa cytosolic protein from heart muscle capable of modulating calcium transport across the sarcoplasmic reticulum in vitro

In previous studies we have described the inhibitory action of a cytosolic protein fraction from heart muscle on ATP-dependent Ca2+ uptake by the sarcoplasmic reticulum (SR); further this inhibition was shown to be blocked by an inhibitor antagonist, also derived from the cytosol (Narayanan et al., Biochim. Biophys. Acta. 735: 53-66, 1983; Can. J. Physiol. Pharmacol. 67: 999-1006, 1989). Here we report the complete purification of the antagonist protein (AP) and characterization of its functional properties. AP was purified to homogeneity from rabbit heart cytosol using two procedures, one utilizing sequential DE52-cellulose and hydroxylapatite chromatography, and the other utilizing anion exchange chromatography on Mono Q HR 5/5 column in a Pharmacia FPLC system. The purified AP has an apparent molecular weight of 64 kDa; it is made up of about 43% hydrophobic and 57% hydrophilic residues with the following amino-terminal sequence: E-A-H-K-S-E-I-A-H-R-F-N-D-V-G-E-E-H-F-I-G-L-V-L-I-T-F-S-Q-Y-L-Q-K-X-P-Y- E-E-H-A . This partial amino acid sequence data indicate strong sequence homology to serum albumin (sequence homology: 85% to rat serum albumin and 74% to sheep and bovine serum albumin). The purified AP caused concentration-dependent-blockade of the inhibition of Ca2+ uptake by SR observed in the presence of the cytosolic Ca2+ uptake inhibitor protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Processes that remove calcium from the cytoplasm during excitation-contraction coupling in intact rat heart cells

1. The processes that remove Ca2+ rapidly from the cytoplasm were studied in isolated rat ventricular myocytes subjected to whole-cell voltage clamp and internal perfusion with the Ca2+ indicator, indo-1. Na(+)-Ca2+ exchange was eliminated in most experiments by removing Na+ both internally and externally. 2. When the Ca(2+)-pumping ATPase of the sarcoplasmic reticulum (SR) was inhibited with cyclopiazonic acid and ryanodine interfered with the release of Ca2+ from the SR, [Ca2+]i transients rose slowly and declined extremely slowly. We concluded that transport of Ca2+ by mitochondria and the surface membrane Ca(2+)-pumping ATPase would be negligible over the time course of a single [Ca2+]i transient. 3. The influence of cytoplasmic Ca2+ ligands was characterized by internal perfusion with high concentrations of diffusible Ca2+ ligands (indo-1) or by superfusion with the membrane-permeant Ca2+ ligand, BAPTA AM. As the concentration of indo-1 in the cell increased from < 0.1 mM to at least 0.5 mM, the time constant of the decline of [Ca2+]i increased from about 0.15 s to nearly 3 s. 4. Calcium bound to endogenous Ca2+ ligands during depolarizing clamp pulses was characterized quantitatively as the difference between the total Ca2+ entering the cell via L-type Ca2+ channels and [Ca2+]i, in experiments in which SR function had been abolished. As total calcium increased during the entry of Ca2+, total calcium was found to agree reasonably well with that predicted by assuming that Ca2+ could bind to endogenous intracellular Ca2+ ligands and to indo-1. 5. The results indicate that, in the absence of Na+, the major factors determining the removal of cytoplasmic free Ca2+ are the Ca(2+)-pumping ATPase of the SR and the binding of Ca2+ to endogenous and exogenous Ca2+ ligands. 6. Several hypothetical 'Ca2+ removal functions' were fitted to the declining phase of [Ca2+]i transients. The best fit was one in which the flux of Ca2+ through the SR Ca(2+)-pumping ATPase was described by a Michaelis-Menten-type equation. The decline of the [Ca2+]i transient was thus described by a linear, first-order differential equation having terms giving the rate of Ca2+ transport by the SR Ca(2+)-pumping ATPase (Vmax and KM), the rates of complexation of Ca2+ with the various Ca2+ ligands (L), and a leak of Ca2+ into the cytoplasm from the SR (FSR,leak).(ABSTRACT TRUNCATED AT 400 WORDS)

Inotropic and lusitropic dysfunction in myocardium from patients with dilated cardiomyopathy

Isometric force of contraction (DT), peak rate of tension increase (+T), peak rate of tension decrease (-T), time to peak tension (TPT), and time to half-relaxation (T 1/2 T) were measured in electrically driven human papillary muscle strips (New York Heart Association [NYHA] class IV heart transplants, dilated cardiomyopathy; nonfailing (NF) donor hearts, brain dead) (1 Hz, 37 degrees C) under basal conditions (1.8 mmol/L Ca2+) and after stimulation with isoprenaline, ouabain, and Ca2+. There was no difference in the isometric contraction (+T, -T, TPT, and T 1/2 T) between NYHA IV hearts and NF hearts under basal conditions. Inotropic stimulation above 300% of basal DT increased -T significantly more in NF hearts (p less than 0.05) compared with NYHA IV hearts. The effectiveness of ouabain and Ca2+ to increase DT was not significantly changed in NYHA IV hearts compared with NF hearts. The isoprenaline-mediated increase in DT was reduced (p less than 0.05) in NYHA IV hearts to a similar extent (70%) as beta-adrenoceptors were downregulated. When the rate of stimulation was increased to 3 Hz (force-frequency relationship), force of contraction increased only in NF preparations, whereas it decreased in NYHA IV myocardium (p less than 0.05). It was concluded that the contractile apparatus in terminally failing human myocardium is sufficient to maximally increase DT. During inotropic stimulation, abnormalities in diastolic rather than systolic contraction become evident. This may indicate abnormal intracellular Ca2+ handling.

Ontogeny of cytosolic proteins capable of modulating sarcoplasmic reticulum calcium transport in heart muscle

In a previous study we described the inhibitory action of a cytosolic protein fraction from heart muscle on ATP-dependent Ca2+ uptake by sarcoplasmic reticulum (SR); further, this inhibition was shown to be blocked by an inhibitor antagonist, also derived from the cytosol (Narayanan et al. Biochim Biophys Acta 735: 53-66, 1983). The present study investigated the ontogenetic expression of the activities of Ca2+ transport inhibitor and inhibitor antagonist in heart cytosol during fetal and postnatal development of the rat. The SR Ca2+ transport inhibitor activity was undetectable in the cytosol of fetal (15- or 20-days gestation) rat heart but was manifested in the cytosol as early as one day after birth and increased progressively thereafter to reach almost adult levels within the first two weeks of postnatal development. The activity of the SR Ca2+ transport inhibitor antagonist was barely detectable in the near-term (20 days gestation) fetus but increased substantially during early postnatal development, in parallel with the rise in activity of the inhibitor. The ontogenetic appearance and increase in the activities of the Ca2+ transport inhibitor and its antagonist correlated well with the concurrent appearance and increase in the amounts of two polypeptides of apparent molecular weights 43 kDa and 64 kDa, which we have tentatively identified as the inhibitor and inhibitor antagonist, respectively. The co-ordinated expression of both the inhibitor and inhibitor antagonist activities in the cytosol during the early postnatal period parallels the morphogenesis and functional maturation of SR in cardiac muscle suggesting likely involvement of these cytosolic proteins in the physiological regulation of SR function.