Intracellular calcium related to force development in twitch contraction of mammalian myocardium

Deficiency of mitochondrial calcium uniporter abrogates iron overload-induced cardiac dysfunction by reducing ferroptosis

Iron overload associated cardiac dysfunction remains a significant clinical challenge whose underlying mechanism(s) have yet to be defined. We aim to evaluate the involvement of the mitochondrial Ca2+ uniporter (MCU) in cardiac dysfunction and determine its role in the occurrence of ferroptosis. Iron overload was established in control (MCUfl/fl) and conditional MCU knockout (MCUfl/fl-MCM) mice. LV function was reduced by chronic iron loading in MCUfl/fl mice, but not in MCUfl/fl-MCM mice. The level of mitochondrial iron and reactive oxygen species were increased and mitochondrial membrane potential and spare respiratory capacity (SRC) were reduced in MCUfl/fl cardiomyocytes, but not in MCUfl/fl-MCM cardiomyocytes. After iron loading, lipid oxidation levels were increased in MCUfl/fl, but not in MCUfl/fl-MCM hearts. Ferrostatin-1, a selective ferroptosis inhibitor, reduced lipid peroxidation and maintained LV function in vivo after chronic iron treatment in MCUfl/fl hearts. Isolated cardiomyocytes from MCUfl/fl mice demonstrated ferroptosis after acute iron treatment. Moreover, Ca2+ transient amplitude and cell contractility were both significantly reduced in isolated cardiomyocytes from chronically Fe treated MCUfl/fl hearts. However, ferroptosis was not induced in cardiomyocytes from MCUfl/fl-MCM hearts nor was there a reduction in Ca2+ transient amplitude or cardiomyocyte contractility. We conclude that mitochondrial iron uptake is dependent on MCU, which plays an essential role in causing mitochondrial dysfunction and ferroptosis under iron overload conditions in the heart. Cardiac-specific deficiency of MCU prevents the development of ferroptosis and iron overload-induced cardiac dysfunction.

Cardiac sarcomere mechanics in health and disease

The sarcomere is the fundamental structural and functional unit of striated muscle and is directly responsible for most of its mechanical properties. The sarcomere generates active or contractile forces and determines the passive or elastic properties of striated muscle. In the heart, mutations in sarcomeric proteins are responsible for the majority of genetically inherited cardiomyopathies. Here, we review the major determinants of cardiac sarcomere mechanics including the key structural components that contribute to active and passive tension. We dissect the molecular and structural basis of active force generation, including sarcomere composition, structure, activation, and relaxation. We then explore the giant sarcomere-resident protein titin, the major contributor to cardiac passive tension. We discuss sarcomere dynamics exemplified by the regulation of titin-based stiffness and the titin life cycle. Finally, we provide an overview of therapeutic strategies that target the sarcomere to improve cardiac contraction and filling.

Calcium and Heart Failure: How Did We Get Here and Where Are We Going?

The occurrence and prevalence of heart failure remain high in the United States as well as globally. One person dies every 30 s from heart disease. Recognizing the importance of heart failure, clinicians and scientists have sought better therapeutic strategies and even cures for end-stage heart failure. This exploration has resulted in many failed clinical trials testing novel classes of pharmaceutical drugs and even gene therapy. As a result, along the way, there have been paradigm shifts toward and away from differing therapeutic approaches. The continued prevalence of death from heart failure, however, clearly demonstrates that the heart is not simply a pump and instead forces us to consider the complexity of simplicity in the pathophysiology of heart failure and reinforces the need to discover new therapeutic approaches.

Emergence of Mechano-Sensitive Contraction Autoregulation in Cardiomyocytes

The heart has two intrinsic mechanisms to enhance contractile strength that compensate for increased mechanical load to help maintain cardiac output. When vascular resistance increases the ventricular chamber initially expands causing an immediate length-dependent increase of contraction force via the Frank-Starling mechanism. Additionally, the stress-dependent Anrep effect slowly increases contraction force that results in the recovery of the chamber volume towards its initial state. The Anrep effect poses a paradox: how can the cardiomyocyte maintain higher contractility even after the cell length has recovered its initial length? Here we propose a surface mechanosensor model that enables the cardiomyocyte to sense different mechanical stresses at the same mechanical strain. The cell-surface mechanosensor is coupled to a mechano-chemo-transduction feedback mechanism involving three elements: surface mechanosensor strain, intracellular Ca2+ transient, and cell strain. We show that in this simple yet general system, contractility autoregulation naturally emerges, enabling the cardiomyocyte to maintain contraction amplitude despite changes in a range of afterloads. These nontrivial model predictions have been experimentally confirmed. Hence, this model provides a new conceptual framework for understanding the contractility autoregulation in cardiomyocytes, which contributes to the heart's intrinsic adaptivity to mechanical load changes in health and diseases.

Force and Calcium Transients Analysis in Human Engineered Heart Tissues Reveals Positive Force-Frequency Relation at Physiological Frequency

Force measurements in ex vivo and engineered heart tissues are well established. Analysis of calcium transients (CaT) is complementary to force, and the combined analysis is meaningful to the study of cardiomyocyte biology and disease. This article describes a model of human induced pluripotent stem cell cardiomyocyte-derived engineered heart tissues (hiPSC-CM EHTs) transduced with the calcium sensor GCaMP6f followed by sequential analysis of force and CaT. Average peak analysis demonstrated the temporal sequence of the CaT preceding the contraction twitch. The pharmacological relevance of the test system was demonstrated with inotropic indicator compounds. Force-frequency relationship was analyzed in the presence of ivabradine (300 nM), which reduced spontaneous frequency and unmasked a positive correlation of force and CaT at physiological human heart beating frequency with stimulation frequency between 0.75 and 2.5 Hz (force +96%; CaT +102%). This work demonstrates the usefulness of combined force/CaT analysis and demonstrates a positive force-frequency relationship in hiPSC-CM EHTs.

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Analysis of calcium transients and force in engineered heart tissues

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Accurate replications of drug effects on calcium transients and force analysis

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Positive force- and calcium transients-frequency relationship

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Reverse correlation between omecamtiv mecarbil's inotropic effect and frequency

AAV9.I-1c delivered via direct coronary infusion in a porcine model of heart failure improves contractility and mitigates adverse remodeling

BACKGROUND

Heart failure is characterized by impaired function and disturbed Ca2+ homeostasis. Transgenic increases in inhibitor-1 activity have been shown to improve Ca2 cycling and preserve cardiac performance in the failing heart. The aim of this study was to evaluate the effect of activating the inhibitor (I-1c) of protein phosphatase 1 (I-1) through gene transfer on cardiac function in a porcine model of heart failure induced by myocardial infarction.

METHODS AND RESULTS

Myocardial infarction was created by a percutaneous, permanent left anterior descending artery occlusion in Yorkshire Landrace swine (n=16). One month after myocardial infarction, pigs underwent intracoronary delivery of either recombinant adeno-associated virus type 9 carrying I-1c (n=8) or saline (n=6) as control. One month after myocardial infarction was created, animals exhibited severe heart failure demonstrated by decreased ejection fraction (46.4±7.0% versus sham 69.7±8.5%) and impaired (dP/dt)max and (dP/dt)min. Intracoronary injection of AAV9.I-1c prevented further deterioration of cardiac function and led to a decrease in scar size.

CONCLUSIONS

In this preclinical model of heart failure, overexpression of I-1c by intracoronary in vivo gene transfer preserved cardiac function and reduced the scar size.

Myofilament length dependent activation

The Frank-Starling law of the heart describes the interrelationship between end-diastolic volume and cardiac ejection volume, a regulatory system that operates on a beat-to-beat basis. The main cellular mechanism that underlies this phenomenon is an increase in the responsiveness of cardiac myofilaments to activating Ca(2+) ions at a longer sarcomere length, commonly referred to as myofilament length-dependent activation. This review focuses on what molecular mechanisms may underlie myofilament length dependency. Specifically, the roles of inter-filament spacing, thick and thin filament based regulation, as well as sarcomeric regulatory proteins are discussed. Although the "Frank-Starling law of the heart" constitutes a fundamental cardiac property that has been appreciated for well over a century, it is still not known in muscle how the contractile apparatus transduces the information concerning sarcomere length to modulate ventricular pressure development.

Intracellular devastation in heart failure

End-stage heart failure is characterized by a number of abnormalities at the cellular level, which include changes in excitation-contraction coupling, alterations in contractile proteins and activation/deactivation of signaling pathways. Even though many of these changes are adaptive to the high workload and stress in heart failure, a significant number of these alterations are deeply deleterious to the cardiac cell. In this article, we will review the changes in calcium cycling that occur in myopathic hearts and how they can be effectively targeted. We will also focus on protein misfolding in the setting of cardiac dysfunction.

Design of a phase 1/2 trial of intracoronary administration of AAV1/SERCA2a in patients with heart failure

BACKGROUND

Heart failure (HF) remains a major cause of morbidity and mortality in North America. With an aging population and an unmet clinical need by current pharmacologic and device-related therapeutic strategies, novel treatment options for HF are being explored. One such promising strategy is gene therapy to target underlying molecular anomalies in the dysfunctional cardiomyocyte. Prior animal and human studies have documented decreased expression of SERCA2a, a major cardiac calcium cycling protein, as a major defect found in HF.

METHODS AND RESULTS

We hypothesize that increasing the activity of SERCA2a in patients with moderate to severe HF will improve their cardiac function, disease status, and quality of life. Gene transfer of SERCA2a will be performed via an adeno-associated viral (AAV) vector, derived from a nonpathogenic virus with long-term transgene expression as well as a clinically established favorable safety profile.

CONCLUSIONS

We describe the design of a phase 1 clinical trial of antegrade epicardial coronary artery infusion (AECAI) administration of AAVI/SERCA2a (MYDICAR) to subjects with HF divided into 2 stages: in Stage 1, subjects will be assigned open-label MYDICAR in one of up to 4 sequential dose escalation cohorts; in Stage 2, subjects will be randomized in parallel to 2 or 3 doses of MYDICAR or placebo in a double-blinded manner.

Improvement of cardiac sarcoplasmic reticulum calcium cycling in dogs with heart failure following long-term therapy with the Acorn Cardiac Support Device

Abnormal Ca(2+)-homeostasis is a hall-marked characteristic of the failing heart. In the normal myocardium, the sarcoplasmic reticulum (SR) is a principal organelle that controls intracellular Ca(2+) concentration during the cardiac cycle. The SR consists of longitudinal and terminal cisternea regions. The former contains the Ca(2+)-ATPase pump or SERCA-2a whose function is to transport cytosolic Ca(2+) into the lumen of the SR during diastole and whose activity is regulated by reversible phosphorylation of the endogenously SR-bound phospholamban (PLB). The SR's terminal cisternea region contains ryanodine-sensitive Ca(2+)-release channels (RR), the activity of which is regulated by direct and indirect reversible phosphorylation. These channels release the SR-stored Ca(2+) during contraction. We have shown that in left ventricular (LV) myocardium from dogs with coronary microembolization-induced heart failure, ability of the SR to sequester and release Ca(2+) during the cardiac cycles is impaired. This abnormality was associated with reduced expression (protein and mRNA) levels of Ca(2+)-ATPase, PLB, and reduced PLB phosphorylation. Long-term therapy with the Acorn Cardiac Support Device (CSD) is associated with restoration of the ability of the SR to sequester Ca(2+). This improvement in SR function following chronic CSD therapy was due primarily to increased affinity of the SERCA-2a for calcium. The later was associated with (1) increased phosphorylation of PLB at serine 16 and threonine 17, (2) unchanged protein expression of PLB and (3) unchanged protein expression of SERCA-2a in LV myocardium of CSD-treated dogs compared to controls. This review summarizes our current understanding of the role of the CSD in modulating SR calcium cycling in an experimental canine model of chronic heart failure produced by multiple sequential intracoronary microembolizations.

Cardiac myofilaments: mechanics and regulation

The mechanical properties of the cardiac myofilament are an important determinant of pump function of the heart. This report is focused on the regulation of myofilament function in cardiac muscle. Calcium ions form the trigger that induces activation of the thin filament which, in turn, allows for cross-bridge formation, ATP hydrolysis, and force development. The structure and protein-protein interactions of the cardiac sarcomere that are responsible for these processes will be reviewed. The molecular mechanism that underlies myofilament activation is incompletely understood. Recent experimental approaches have been employed to unravel the mechanism and regulation of myofilament mechanics and energetics by activator calcium and sarcomere length, as well as contractile protein phosphorylation mediated by protein kinase A. Central to these studies is the question whether such factors impact on muscle function simply by altering thin filament activation state, or whether modulation of cross-bridge cycling also plays a part in the responses of muscle to these stimuli.

Myofibrillar responsiveness to cAMP, PKA, and caffeine in an animal model of heart failure

We investigated whether an alteration of myofilament calcium responsiveness and contractile activation may in part contribute to heart failure. A control group of Broad Breasted White turkey poults was given regular feed without additive, whereas the experimental group was given the control ration with 700 ppm of furazolidone at 1 week of age for 3 weeks (DCM). At 4 weeks of age, left ventricular trabeculae carneae were isolated from hearts and calcium-force relationships studied. No differences in calcium-activation between fibers from control or failing hearts were noted under standard experimental conditions. Also failing hearts demonstrated no significant shift in the population of troponin T isoforms but we did observe a significant 4-fold decrease in TnT content in failing hearts compared to non-failing hearts. Addition of caffeine, however, resulted in a greater leftward shift on the calcium axis in fibers from failing hearts. At pCa 6, caffeine increased force by 26+/-2.1% in control fibers and 44.5+/-8.7% in myopathic fibers. Cyclic AMP resulted in a greater rightward shift on the calcium axis in failing myocardium. In control muscles, the frequency of minimum stiffness (f(min)) was higher than in muscles from failing hearts. cAMP and caffeine both shifted f(min) to higher frequencies in control fibers whereas in fibers from failing hearts both caused a greater shift. These results lead us to conclude that heart failure exerts differential effects on cAMP and caffeine responsiveness. Our data suggest that changes at the level of the thin myofilaments may alter myofilament calcium responsiveness and contribute to the contractile dysfunction seen in heart failure.

Effects of temperature and calcium availability on ventricular myocardium from rainbow trout

We studied the mechanical and electrophysiological properties of ventricular myocardium from rainbow trout (Oncorhynchus mykiss) in vitro at 4, 10, and 18 degrees C from fish acclimated at 10 degrees C. Temperature alone did not significantly alter the contractile force of the myocardium, but the time to peak tension and time to 80% relaxation were prolonged at 4 degrees C and shortened at 18 degrees C. The duration of the action potential was also prolonged at 4 degrees C and progressively shortened at higher temperatures. An alteration of the stimulation frequency did not affect contraction amplitude at any temperature. Calcium influx via L-type calcium channels was increased by raising extracellular calcium concentration (¿Ca(2+)(o)) or including Bay K 8644 (Bay K) and isoproterenol in the bathing medium. These treatments significantly enhanced the contractile force at all temperatures. Calcium channel blockers had a reverse-negative inotropic effect. Unexpectedly, the duration of the action potential at 10 degrees C was shortened as ¿Ca(2+)(o) increased. However, Bay K prolonged the plateau phase at 4 degrees C. Caffeine, which promotes the release of sarcoplasmic reticulum (SR) calcium, increased contractile force eightfold at all three temperatures, but the SR blocker ryanodine was only inhibitory at 4 degrees C. Our results suggest that contractile force in ventricular myocardium from Oncorhynchus mykiss is primarily regulated by sarcolemmal calcium influx and that ventricular contractility is maintained during exposure to a wide range of temperatures.

Intracellular calcium and the relationship to contractility in an avian model of heart failure

Global contractile heart failure was induced in turkey poults by furazolidone feeding (700 ppm). Abnormal calcium regulation appears to be a key factor in the pathophysiology of heart failure, but the cellular mechanisms contributing to changes in calcium fluxes have not been clearly defined. Isolated ventricular myocytes from non-failing and failing hearts were therefore used to determine whether the whole heart and ventricular muscle contractile dysfunctions were realized at the single cell level. Whole cell current- and voltage-clamp techniques were used to evaluate action potential configurations and L-type calcium currents, respectively. Intracellular calcium transients were evaluated in isolated myocytes with fura-2 and in isolated left ventricular muscles using aequorin. Action potential durations were prolonged in failing myocytes, which correspond to slowed cytosolic calcium clearing. Calcium current-voltage relationships were normal in failing myocytes; preliminary evidence suggests that depressed transient outward potassium currents contribute to prolonged action potential durations. The number of calcium channels (as measured by radioligand binding) were also similar in non-failing and failing hearts. Isolated ventricular muscles from failing hearts had enhanced inotropic responses, in a dose-dependent fashion, to a calcium channel agonist (Bay K 8644). These data suggest that changes in intracellular calcium mobilization kinetics and longer calcium-myofilament interaction may be able to compensate for contractile failure. We conclude that the relationship between calcium current density and sarcoplasmic reticulum calcium release is a dynamic process that may be altered in the setting of heart failure at higher contraction rates.

Modulation of Ca2+ transient decay by tension and Ca2+ removal in hyperthyroid myocardium

We investigated the contribution of sarcoplasmic reticulum (SR) and Na+/Ca2+ exchanger in the tension-dependent change in the decay of the Ca2+ transients (CaT) in euthyroid (Eu) and hyperthyroid (Hy) myocardium. Hy was induced by thyroxine treatment to enhance the rate of SR Ca2+ uptake. With the use of the aequorin method, CaT and tension in twitch contraction were simultaneously measured under various conditions (changing muscle length and Ca2+ concentration in solution). In both groups, the decay time of CaT (DT) showed a significant dependence on the developed tension, but the tension dependence of DT in Hy was significantly less than in Eu. In the presence of caffeine (3 mM), the tension dependence of DT in Hy became apparent as in Eu. Inhibition of Na+/Ca2+ exchanger by replacing Na+ with Li+ did not affect the dependence in Hy. The normalized extra Ca2+, which is the Ca2+ concentration change in response to a quick length change, in Hy was similar to that in Eu. pCa-tension relations of skinned trabeculae measured at different lengths (1.9 and 2.3 micrometer) were nearly identical in both groups. These results indicate that the tension-dependent change in the affinity of troponin C for Ca2+ works in both Eu and Hy myocardium and that the tension-dependent change in DT is influenced by the Ca2+ uptake rate of SR.

Insulin-like growth factor-1 but not growth hormone augments mammalian myocardial contractility by sensitizing the myofilament to Ca2+ through a wortmannin-sensitive pathway: studies in rat and ferret isolated muscles

A growing body of evidence has been accumulated recently suggesting that growth hormone (GH) and insulin-like growth factor-1 (IGF-1) affect cardiac function, but their mechanism(s) of action is unclear. In the present study, GH and IGF-1 were administered to isolated isovolumic aequorin-loaded rat whole hearts and ferret papillary muscles. Although GH had no effect on the indices of cardiac function, IGF-1 increased isovolumic developed pressure by 24% above baseline. The aequorin transients were abbreviated and demonstrated decreased amplitude. The positive inotropic effects of IGF-1 were not associated with increased intracellular Ca2+ availability to the contractile machinery but to a significant increase of myofilament Ca2+ sensitivity. Accordingly, the Ca2+-force relationship obtained under steady-state conditions in tetanized muscle was shifted significantly to the left (EC50, 0.44+/-0.02 versus 0.52+/-0.03 micromol/L with and without IGF-1 in the perfusate, respectively; P<0.05); maximal Ca2+-activated tetanic pressure was increased significantly by 12% (211+/-3 versus 235+/-2 mm Hg in controls and IGF-1-treated hearts, respectively; P<0.01). The positive inotropic actions of IGF-1 were not associated with changes in either pHi or high-energy phosphate content, as assessed by 31P nuclear magnetic resonance spectroscopy, and were blocked by the phosphatidylinositol 3-kinase inhibitor wortmannin. Concomitant administration of IGF binding protein-3 blocked IGF-1-positive inotropic action in ferret papillary muscles. In conclusion, IGF-1 is an endogenous peptide that through a wortmannin-sensitive pathway displays distinct positive inotropic properties by sensitizing the myofilaments to Ca2+ without increasing myocyte [Ca2+]i.

Effect of inotropic interventions on contraction and Ca2+ transients in the human heart

The present study investigated the influences of inotropic intervention on the intracellular Ca2+ transient (intracellular Ca2+ concentration ([Ca2+]i)) and contractile twitch. Isometric twitch and [Ca2+]i (fura 2 ratio method) were measured simultaneously (1 Hz, 37 degrees C) after stimulation with Ca2+ (0.9-3.2 mM), the cardiac glycoside ouabain (Oua; 0.1 microM), the beta1- and beta2-adrenoceptor-agonist isoprenaline (Iso; 1-10 nM), and the Ca2+ sensitizer EMD-57033 (30 microM) by using isolated human nonfailing right auricular trabeculae (n = 19). Inotropic interventions increased force of contraction and peak rate of tension rise (+T) significantly. Only Iso stimulated peak rate of tension decay (-T) higher than +T (P < 0.05), thereby reducing time of contraction (Ttwitch). EMD-57033 increased +T more effectively than -T and prolonged Ttwitch (P < 0.05). Ca2+, Oua, and Iso, but not EMD-57033, increased systolic Ca2+. Diastolic Ca2+ increased after stimulation with Oua or Ca2+, but not in the presence of EMD-57033. Iso shortened the Ca2+ transient and did not influence diastolic Ca2+. In conclusion, positive inotropic agents differently affect force and [Ca2+]i depending on their mode of action. Inotropic interventions influence diastolic Ca2+ and thus may be less advantageous in a situation with altered intracellular Ca2+ homeostasis (e.g., heart failure due to dilated cardiomyopathy).

Long-term angiotensin-converting enzyme inhibition with fosinopril improves depressed responsiveness to Ca2+ in myocytes from aortic-banded rats

BACKGROUND

We have previously shown that long-term ACE inhibition with fosinopril prolongs survival and improves ventricular function despite persistent severe left ventricular pressure overload in ascending aortic-banded rats with left ventricular hypertrophy during the transition from compensation to failure.

METHODS AND RESULTS

To study the cellular mechanism of the effects of long-term ACE inhibition on the modification of the transition to failure in pressure-overload hypertrophy, we measured simultaneous intracellular Ca2+ transients and myocyte shortening in isolated left ventricular myocytes from fosinopril-treated aortic-banded rats (n = 9), untreated aortic-banded rats (n = 9), and normal age-matched control rats (n = 10). Fosinopril therapy was begun 6 weeks after banding and was continued until week 21 after banding, when the animals were killed. Collagenase-dissociated myocytes loaded with indo 1-AM were paced at 3 Hz at 36 degrees C and superfused at [Ca2+]o of 0.6, 1.2, and 3.0 mmol/L. In myocytes from untreated aortic-banded rats, peak systolic [Ca2+]i was higher than in control myocytes, and the relationship between myocyte shortening and [Ca2+]i was depressed relative to control myocytes, implicating impaired responsiveness to Ca2+. Long-term fosinopril treatment improved both myocyte shortening and the relationship of shortening to [Ca2+]i (P < .05 versus myocytes from untreated aortic-banded rats). Maximal Ca(2+)-activated force was depressed in chemically skinned left ventricular fibers from untreated aortic-banded hypertrophied rats relative to age-matched controls but not in the fosinopril-treated aortic-banded rats.

CONCLUSIONS

Long-term ACE inhibition improves responsiveness to Ca2+ in the presence of normalization of maximal Ca(2+)-activated force in aortic-banded rats subjected to persistent pressure overload. This may contribute to the favorable effects whereby ACE inhibition modifies the transition from compensated hypertrophy to failure.

Exogenously administered growth hormone and insulin-like growth factor-I alter intracellular Ca2+ handling and enhance cardiac performance. In vitro evaluation in the isolated isovolumic buffer-perfused rat heart

It has been proposed that chronic treatment with growth hormone (GH) or insulin-like growth factor-I (IGF-I) in the rat may enhance cardiac function in vivo. To confirm these findings and elucidate the mechanisms by which cardiac function is modulated, we studied isolated buffer-perfused rat hearts after 4 weeks of treatment with high doses of GH and IGF-I alone or in combination. Mechanical parameters were measured at 50% of the intracardiac balloon volume at which maximal developed pressure (DevP) occurred. EC50 of the force-Ca2+ relationship and maximal Ca(2+)-activated systolic wall stress (max sigma s) were assessed by increasing Ca2+ in the perfusate in a stepwise fashion and plotting systolic wall stress (sigma s) versus intracellular peak systolic Ca2+, measured by the aequorin bioluminescence method. We found a marked increase of systolic pressure (Ps), DevP, and (+dP/dt)/DevP in the treated groups compared with the control group. The combination group showed a blunted effect. sigma s was increased in all treated groups for a perfusate Ca2+ concentration of > 1.5 mmol/L. The enhanced systolic performance can be explained by an increase of the overall Ca2+ responsiveness due to an increased maximal response to Ca2+ even though the EC50 of the Ca(2+)-dose response was also slightly increased. Ps was further enhanced by an increase of the relative wall thickness induced by the treatment. Diastolic pressure, diastolic Ca2+, and the amplitude and time course of the Ca2+ transient were not influenced by any treatment protocol. All treatments caused increases of body and heart weight. These data support the hypothesis that both IGF-I and GH directly affect cardiac performance by altering cardiac geometry as well as by enhancing max sigma s.

Doxorubicin alters Ca(2+) transients but fails to change Ca(2+) sensitivity of contractile proteins

Doxorubicin produced a transient increase and a subsequent decrease in the amplitude of twitch contraction in myocytes isolated from guinea-pig heart and loaded with fura-2. These changes were associated with an increase and a subsequent decrease, respectively, in the amplitude of Ca(2+) transients (peak minus diastolic Ca(2+) concentrations). Doxorubicin increased the diastolic Ca(2+) concentration with a concomitant shortening of the diastolic myocyte length. The time to peak Ca(2+) transients and the time to peak twitch contraction increased in parallel. Doxorubicin failed to affect the Ca(2+) concentration-contraction curve in skinned fibers obtained from atrial muscle. We conclude that biphasic inotropic effects of doxorubicin result from biphasic changes in Ca(2+) transients, and that doxorubicin fails to alter Ca(2+) sensitivity of contractile proteins. These findings are consistent with the hypothesis that doxorubicin enhances Ca(2+) release and impairs Ca(2+) uptake by the sarcoplasmic reticulum.

Effect of haemodynamic pressure overload of the adult ferret right ventricle on inotropic responsiveness to external calcium and rest periods

The inotropic effects of external calcium concentration ([Ca2+]o] and rest periods have been compared in papillary muscles isolated from control (n = 4) and pressure-overloaded right (n = 5) ventricles of adult ferrets. Hypertrophy was induced by pulmonary artery clipping for 30-45 days. Under control conditions (3 mM [Ca2+]o, 0.1 Hz), the isometric twitch force of hypertrophied muscles was decreased by 75%, time to peak was increased by 30% and time to half-relaxation was increased by 50% compared with non-hypertrophied preparations. The sensitivity of contraction to [Ca2+]o was decreased in hypertrophied muscles compared with control ([Ca2+] required for half-maximal contraction: 4.1 mM vs 1.7 mM) and the maximal contraction reached at high [Ca2+]o was smaller in pressure-overloaded muscles compared with control (8.3 +/- 2.0 mN mm-2 vs 19.0 +/- 2.1 mN mm-2 respectively). In both groups, rest periods longer than the steady-state interval were initially accompanied by a potentiation of the first post-rest contraction compared with steady-state. Peak potentiation occurred after a rest of 120 s in hypertrophied muscles and after a rest of 60 s in control. The maximal relative potentiation, i.e. compared with the steady-state twitch, was higher in hypertrophied muscles (+75%) than in control (+20%). After peak potentiation, the amplitude of the first post-rest contraction progressively decreased with increasing periods of rest, although at a slower rate in hypertrophy compared with control. The time constants of post-rest decay were 1203 +/- 99 s and 528 +/- 24 s respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Responsiveness of the myofilaments to Ca2+ in human heart failure: implications for Ca2+ and force regulation

Myofilament calcium sensitivity and maximal calcium-activated force are fundamental properties of the contractile proteins in the heart. We examined these properties in normal human right-ventricular trabeculae carneae obtained from hearts of brain-dead patients with no known cardiac disease, and from patients with end-stage heart failure undergoing cardiac transplantation. There were no differences in calcium-activation of the control and myopathic muscles from chemically-skinned trabeculae or from intact tetanized preparations. We then tested the effect of DPI 201-106 (4-[3-(4-diphenylmethyl-1-piperazinyl)-2-hydroxypropoxy]-1H-indole - carbonitrile), a new inotropic agent, in both preparations. In myopathic muscles, 1 microM DPI sensitized the myofilaments to Ca2+, as evidenced by a significant shift of the [Ca2+]-force relationship towards lower [Ca2+], in both skinned and intact preparations. On the other hand, the same concentration of DPI did not affect the calcium-activation in control muscles in both preparations. We also found that the twitch [Ca2+]-force relationship, which has been used as an indication of myofilament sensitivity, was dissociated from the steady-state [Ca2+]-force relationship, and was shifted along the [Ca2+] axis by modulation in the time-course of the twitch and [Ca2+]i, and not by the sensitivity of the myofilaments to Ca2+. Protein kinase C stimulation differentially altered the responsiveness of the myofilaments to Ca2+ in normal and myopathic muscle fibers. We propose that even though calcium activation and maximal calcium-activated force are unaltered in myopathic hearts there are changes in thin filament regulation in myopathic hearts that result in altered responses to agents that directly act on the thin filaments, and that the potential for force development is similar in normal and myopathic human hearts.

Contractile deactivation and uncoupling of crossbridges. Effects of 2,3-butanedione monoxime on mammalian myocardium

We investigated the effects of 1 and 3 mM 2,3-butanedione monoxime (BDM, diacetyl monoxime) on excitation and contraction of cardiac muscle in several types of preparations at various levels of organization. We selected a concentration of BDM that was not expected to affect sarcolemmal calcium flux and action potential duration in cardiac tissue. Two indicators were used to record intracellular calcium. Aequorin, a bioluminescent calcium indicator, was used in studies with ferret papillary muscle preparations, and fura-2, a fluorescent calcium indicator, was used in studies with guinea pig cardiac myocytes. In both cases, addition of BDM resulted in a reduction of peak intracellular calcium released from the sarcoplasmic reticulum and a reduction of peak twitch force. The duration of the action potential of isolated myocytes was slightly abbreviated in the presence of BDM. In studies on the calcium current in the myocytes, addition of BDM was associated with reduced calcium current at any potential. Peak calcium current was reduced by 7.9 +/- 1% in the presence of BDM. In tetanized ferret papillary muscles, BDM reduced maximal calcium-activated force by 30 +/- 5% and increased the calcium ion concentration required for half-maximal force by 0.1 +/- 0.01 microM. The Hill coefficient was reduced from 5.00 +/- 0.11 to 3.40 +/- 0.20. Maximal shortening velocity of ferret papillary muscles was increased in the presence of BDM from 1.55 +/- 0.24 to 2.04 +/- 0.33 mm/sec. Ca2+ binding to troponin C in skinned fiber preparations from guinea pig, bovine, and canine hearts was unaffected by addition of up to 10 mM BDM. Our results indicate that BDM affects both calcium availability and responsiveness of the myofilaments to Ca2+. Uncoupling of contractile activation from excitation may also result from altered crossbridge kinetics.