Regulation Of Cardiac Contractility Research Articles

The effects of nickel chloride on papillary muscle contractility under normothermic and hypothermic conditions: Comparison of active and hibernating ground squirrels (Urocitellus undulatus) with Wistar rats.

Extracellular Ca2+ plays a pivotal role in the regulation of cardiac contractility under normal and extreme conditions. Here, by using nickel chloride (NiCl2), a non-specific blocker of extracellular Ca2+ influx, we studied the input of extracellular Ca2+ on the regulation of papillary muscle (PM) contractility under normal and hypothermic conditions in ground squirrels (GS), and rats. By measuring isometric force of contraction, we studied how NiCl2 affects force-frequency relationship and the rest effect in PM of these species at 30 °C and 10 °C. We found that at 30 °C 1.5 mM NiCl2 significantly reduced force of contraction across entire frequency range in active GS and rats, whereas in hibernating GS force of contraction was reduced at low and high frequency range. Additionally, NiCl2 evoked spontaneous contractility in rats but not GS PM. The rest effect was significantly reduced by NiCl2 for active GS and rats but not hibernating GS. At 10 °C, NiCl2 fully reduced contractility in active GS and, to a lesser extent, in rats, whereas in hibernating GS it was significant only at 0.3 Hz. The rest effect was significantly reduced by NiCl2 in both active and hibernating GS, whereas it was unmasked in rats that had high contractility under hypothermic conditions in control. Our results show a significant contribution of extracellular Ca2+ to myocardial contractility in GS not only in active but also in hibernating states, especially under hypothermic conditions, whereas limitation of extracellular Ca2+ influx in rats under hypothermia can play protective role for myocardial contractility.

Abstract P3128: Synapse-associated Protein 97 Is Necessary For Enhancing Cardiac Contractility Via Beta1 Adrenoceptor-neuronal Nitric Oxide Synthase Pathway And Guanosine3',5'-cyclic Monophosphate Signaling

Objective: Synapse-associated protein 97 (SAP97) is a scaffold protein widely expressed in the brain, heart, and kidney. SAP97 has been suggested function in the regulation of cardiac contractility. SAP97 directly binds to cardiac beta1 adrenceptor. Our recent studies have shown that increased cAMP and cGMP levels are involved in enhancing excitation-contraction (E-C) coupling after sympathetic nervous system activation. We aim to study the role of SAP97 in beta1 adrenoceptor-induced increases in cyclic nucleotides and cardiac E-C coupling. Results and Methods: Cardiac function in mice with cardiomyocyte-specific deletion of the SAP97 (SAP97-cKO) was comparable to that in flox/flox control littermates. Ligand-induced cGMP signal in freshly isolated adult ventricular myocytes (AVMs) using FRET biosensor Gi500. The cGMP responsiveness caused by β1AR agonist as dobutamine was reduced in SAP97-cKO versus f/f cells. No significant difference between the groups using nonselective beta-adrenergic stimulation isoproterenol (ISO). In comparison, the dobutamine-induced cAMP levels were not reduced in SAP97-cKO when compared to f/f controls. The dobutamine-induced ejection fraction in hearts and sarcomere shortening in AVMs were significantly attenuated in SAP97-cKO hearts and AVMs when compared to f/f controls, respectively. On the other hand, no difference was observed between f/f and cKO AVMs and hearts after stimulation by ISO. To further examine the LTCC activity, whole-cell currents (ICa) were recorded from freshly isolated AVMs. The results showed that the LTCC current was attenuated in SAP97-cKO AVMs than cKO controls when exposed to dobutamine. These results show SAP97 is necessary for increasing intracellular cGMP level, LTCC current, and cardiac E-C coupling and contractility after β1AR stimulation. Conclusion: SAP97 plays a crucial role in β 1 AR signaling, which is necessary for triggering E-C coupling through cAMP and cGMP pathways.

Phosphodiesterases and Compartmentation of cAMP and cGMP Signaling in Regulation of Cardiac Contractility in Normal and Failing Hearts.

Cardiac contractility is regulated by several neural, hormonal, paracrine, and autocrine factors. Amongst these, signaling through β-adrenergic and serotonin receptors generates the second messenger cyclic AMP (cAMP), whereas activation of natriuretic peptide receptors and soluble guanylyl cyclases generates cyclic GMP (cGMP). Both cyclic nucleotides regulate cardiac contractility through several mechanisms. Phosphodiesterases (PDEs) are enzymes that degrade cAMP and cGMP and therefore determine the dynamics of their downstream effects. In addition, the intracellular localization of the different PDEs may contribute to regulation of compartmented signaling of cAMP and cGMP. In this review, we will focus on the role of PDEs in regulating contractility and evaluate changes in heart failure.

Phosphorylation and Fragmentation of the Cardiac Troponin T: Mechanisms, Role in Pathophysiology and Laboratory Diagnosis

Cardiac troponin T (cTnT), a protein essential for calcium-regulated, myofibrillar ATPase activity, is extremely sensitive to the action of a significant number of intra- and extracellular enzymes, the action of which causes post-translational modifications (PTMs) of amino acid structure and functioning cTnT. PTMs of cTnT may play important roles in the regulation of cardiac contractility. The vast majority of cTnT modifications involve the phosphorylation by a variety of Ser/Thr kinases, including PKC. At the same time, the activity of cTnT phosphorylation can change under physiological conditions and in some CVDs, including HF, AMI, and arrhythmias. Along with cTnT phosphorylation, cTnT fragmentation occurs, the activity of which can also change. This article discusses the mechanisms of cTnT phosphorylation and fragmentation, discusses the important role of these processes in the pathophysiology and laboratory diagnosis of some CVD, and notes promising directions for further research.

Investigating the role of PKA type I in the heart

Norepinephrine and epinephrine provide the most powerful stimulation of cardiac function through activation of β-adrenergic receptors and the cAMP-dependent protein kinase (PKA). The PKA holoenzyme consists of a dimer of regulatory (R) subunits that binds and inhibits two catalytic subunits (C). Two types of PKA are expressed in the heart, type I and type II, differing by their R subunit, RIα and RIIα. Whereas PKA type II plays an important role in the regulation of cardiac contractility, the role of the PKA type I is little understood. To investigate the role of PKA type I in the adult heart. The cardiac phenotype of two mouse models was investigated. The first is a cardiac-specific and conditional knock-out of RIα (RIα-KO) which leads to a constitutive activation of PKA type I in the heart. The second is a knock-in mutation in the cAMP-binding site of RIα leading to a higher activation threshold for PKA type I (ADO-MUT). Echocardiography shows that young RIα-KO mice have an increased ejection fraction (EF) associated with greater phosphorylation of the key players of excitation-contraction coupling but no change in heart rate (HR). Besides, with age, these mice develop cardiac hypertrophy, dilatation and die from heart failure (HF). In contrast, young ADO-MUT mice have a slightly decreased EF and HR, and do not develop HF with age. In RIα-KO mice, gene and RIα protein deletion were not efficient in the sino-atrial node (SAN). Thus, a third model was developed in which RIα can be specifically and conditionally invalidated in the SAN and conduction system by the use of a HCN4 promoter to investigate the role of PKA type I in cardiac automatism. Thus far, our results indicate a previously unsuspected role of PKA type I in cardiac contraction and possibly also in automatism.

Zbtb20 deficiency causes cardiac contractile dysfunction in mice.

The zinc-finger protein ZBTB20 regulates development and metabolism in multiple systems, and is essential for postnatal survival in mice. However, its potential role in the cardiovascular system remains undefined. Here, we demonstrate that ZBTB20 is critically involved in the regulation of cardiac contractility and blood pressure in mice. At the age of 16days, the relatively healthy Zbtb20-null mice exhibited hypotension without obvious change of heart rate or other evidence for heart failure. Moreover, Zbtb20 deletion led to a marked reduction in heart size, left ventricular wall thickness, and cell size of cardiomyocytes, which was largely proportional to the decreased body growth. Notably, echocardiographic and hemodynamic analyses showed that cardiac contractility was greatly impaired in the absence of ZBTB20. Mechanistically, ZBTB20 deficiency decreased cardiac ATP contents, and compromised the enzyme activity of mitochondrial complex I in heart as well as L-type calcium current density in cardiomyocytes. Furthermore, the developmental activation of some mitochondrial function-related genes was significantly attenuated in Zbtb20-null myocardium, which included Hspb8, Ckmt2, Cox7a1, Tfrc, and Ogdhl. Put together, these results suggest that ZBTB20 plays a crucial role in the regulation of heart development, energy metabolism, and contractility.

Abstract MP135: Cardiac Sarcoplasmic Reticulum Calcium Cycling is Regulated by Kinase Independent Function of Pi3kgamma

Genetic deletion of Phosphoinositide 3-kinase (PI3Kγ) in mice (PI3Kγ -/- ) results in increased cAMP levels and enhanced ventricular contractility. We investigated whether the lack of PI3Kγ plays a role in cardiac contractility by altering intracellular calcium recycling. Isolated cardiomyocytes from PI3Kγ -/- mice showed significantly reduced calcium reuptake by sarcoendoplasmic reticulum (SR) following caffeine induced calcium release indicating that PI3Kγ locally regulates the function of SR. The intracellular calcium remained at elevated levels in the cardiomyocytes of PI3Kγ -/- for a prolonged period after caffeine treatment. This could be due to changes in phosphorylation of SERCA2, Ryanodine receptor (RyR 2 ) or phospholamban (PLN). In fact, when we looked at phosphorylation of PLN in cardiac lysates, a major regulator of cardiac contractility and relaxation, PI3Kγ -/- mice showed significantly reduced PLN phosphorylation compared to littermate controls. Previous studies from our laboratory suggested that absence of PI3Kγ leads to increase in protein phosphatase (PP) activity which could be possible reason for rapid dephosphorylation of PLN, resulting in inhibition of SERCA2 pump. We observed increased SR associated PP activity and PLN associated PP activity in PI3Kγ -/- mice. We also observed increased association of PP-1 and PP2A with PLN in the absence of PI3Kγ. The altered calcium handling in the cardiomyocytes of PI3Kγ -/- mice could be restored to the level of WT controls by okadaic acid mediated inhibition of PP, suggesting that PI3Kγ plays a role in regulating PP activity associated with SR. To test whether PI3Kγ activity is required for PLN dephosphorylation and SR calcium cycling, we used mice with cardiac specific overexpression of kinase dead PI3Kγ (PI3Kγ inact ) in global PI3Kγ -/- mice (PI3Kγ inact /PI3Kγ -/- ). PI3Kγ inact /PI3Kγ -/- mice showed restored PLN phosphorylation, improved caffeine induced calcium reuptake, decreased SR and PLN associated PP activity. These studies show a novel regulation of PP and SR calcium regulation by kinase independent function of PI3Kγ. The underlying mechanism of PP regulation by PI3Kγ will be presented.

Loss of miR-132/212 Has No Long-Term Beneficial Effect on Cardiac Function After Permanent Coronary Occlusion in Mice.

Background: Myocardial infarction (MI) is caused by occlusion of the coronary artery and induces ischemia in the myocardium and eventually a massive loss in cardiomyocytes. Studies have shown many factors or treatments that can affect the healing and remodeling of the heart upon infarction, leading to better cardiac performance and clinical outcome. Previously, miR-132/212 has been shown to play an important role in arteriogenesis in a mouse model of hindlimb ischemia and in the regulation of cardiac contractility in hypertrophic cardiomyopathy in mice. In this study, we explored the role of miR-132/212 during ischemia in a murine MI model.Methods and Results: miR-132/212 knockout mice show enhanced cardiac contractile function at baseline compared to wild-type controls, as assessed by echocardiography. One day after induction of MI by permanent occlusion, miR-132/212 knockout mice display similar levels of cardiac damage as wild-type controls, as demonstrated by infarction size quantification and LDH release, although a trend toward more cardiomyocyte cell death was observed in the knockout mice as shown by TUNEL staining. Four weeks after MI, miR-132/212 knockout mice show no differences in terms of cardiac function, expression of cardiac stress markers, and fibrotic remodeling, although vascularization was reduced. In line with these in vivo observation, overexpression of miR-132 or miR-212 in neonatal rat cardiomyocyte suppress hypoxia induced cardiomyocyte cell death.Conclusion: Although we previously observed a role in collateral formation and myocardial contractility, the absence of miR-132/212 did not affect the overall myocardial performance upon a permanent occlusion of the coronary artery. This suggests an interplay of different roles of this miR-132/212 before and during MI, including an inhibitory effect on cell death and angiogenesis, and a positive effect on cardiac contractility and autophagic response. Thus, spatial or tissue specific manipulation of this microRNA family may be essential to fully understand the roles and to develop interventions to reduce infarct size.

1178Unsuspected role of the cardiac PKA type I in excitation-contraction coupling and in heart failure development

Abstract The cAMP-dependent protein kinase (PKA) consists of two regulatory (R) and two catalytic (C) subunits and comprises two subtypes, PKAI and PKAII, defined by the nature of their regulatory subunits, RIα and RIIα respectively. Whereas PKAII is thought to play a key role in β-adrenergic (β-AR) regulation of cardiac contractility, the function of PKAI is unclear. To address this question, we generated mice with cardiomyocyte-specific and conditional invalidation of the RIα subunit of PKA. Tamoxifen injection in 8 weeks-old mice resulted in a >70% decrease in RIα protein without modification of other PKA subunits, which was associated with ∼2-fold increased basal PKA activity in RIα-KO mice (p<0.05, N=6/group). This translated into enhanced cardiac contraction and relaxation, as observed in vivo by increased fractional shortening and E-wave velocity (p<0.05, N=10/group) and ex vivo by increased LV pressure and maximal rate of contraction and relaxation (p<0.05, N=9/group). L-type Ca2+ current density was increased in ventricular myocytes from RIα-KO, and β-AR stimulation was decreased by ∼50% (p<0.05, n=38 cells for WT, and, n=40 for RIα-KO). Consistently, Ca2+ transients amplitude and relaxation kinetics were increased, along with increased occurrence of Ca2+ sparks and waves (p<0.05, n=44 cells for WT, and, n=50 for RIα KO). Phosphorylation of Ca2+ channels (CaV1.2), PLB, RyR2 and cMyBP-C at PKA sites was increased >2-fold (p<0.05, N=6/group) in RIα KO without modification of total protein expression. With age, these mice developed a congestive heart failure (HF) phenotype with massive hypertrophy and fibrosis which eventually led to death in 50% of RIα-KO mice at 50 weeks (versus 0% in WT, p<0.01). These results reveal a previously unsuspected role of PKA type I in cardiac excitation-contraction coupling and HF.

Abstract 109: Site-specific Acetylation of Cardiac Troponin I Regulates Myofilament Relaxation and Calcium Sensitivity

Objective: Cardiac troponin I (cTnI) is an essential regulator of cardiac contractility and relaxation. Mutations within key regions of this regulator lead to cardiomyopathies. Further, post-translational modification of cTnI through phosphorylation impacts myofibril relaxation and calcium sensitivity. Recent studies have also demonstrated that myofibril proteins are acetylated leading to faster relaxation. These studies highlight the potential significance of myofilament acetylation; however, it is not known if site-specific acetylation of cTnI can lead to mechanical changes in the myofibril. The objective of this study was to determine if acetylation at a single site of cTnI (lysine-132; K132) is sufficient to alter myofibril protein mechanics. Methods: Adult rat ventricular cardiomyocytes infected with adenoviral constructs expressing either cTnI K132 replaced with glutamine (K132Q; to mimic acetylation) or K132 replaced with arginine (K132R; to prevent acetylation) were subjected to cell contractility and isolated myofibril mechanic measurements. Additionally, skinned myofibrils were exchanged with troponin containing wildtype (WT) or K132Q cTnI and mechanics assessed. Finally, dynamics of reconstituted thin filaments containing WT or K132Q cTnI were assessed by in vitro motility assay. Results: Cardiomyocytes expressing cTnI K132Q relaxed faster and had decreased calcium sensitivity compared to WT cTnI at the whole cell and myofibril level. Relaxation or calcium sensitivity did not differ between cardiomyocytes infected with WT cTnI and cTnI K132R. Myofibrils with cTnI K132Q exchanged ex vivo demonstrate faster relaxation and decreased calcium sensitivity as well as decreased motility. Conclusions: Our results indicate for the first time that acetylation at a specific cTnI lysine can alter myofibril relaxation and calcium sensitivity. This work underscores the importance of understanding how acetylation affects specific sarcomeric proteins in the context of cardiac disease, and suggests that modulation of myofilament lysine acetylation may represent a novel therapeutic target to alter cardiac relaxation.

The cAMP-dependent protein kinase type I regulates cardiac excitation-contraction coupling

The cAMP-dependent protein kinase (PKA) consists of two regulatory (R) and two catalytic (C) subunits and comprises two subtypes, PKAI and PKAII, defined by the nature of their regulatory subunits, RIα and RIIα respectively. Whereas PKAII is thought to play a key role in β-adrenergic (β-AR) regulation of cardiac contractility, the function of PKAI is unclear. To address this question, we generated mice with cardiomyocyte-specific and conditional invalidation of the RIα subunit of PKA. Tamoxifen injection in 8 weeks-old mice resulted in a > 70% decrease in RIα protein without modification of other PKA subunits, which was associated with ∼2-fold increased basal PKA activity in RIα-KO mice ( P < 0.05, N = 6/group). This translated into enhanced cardiac contraction and relaxation, as observed in vivo by increased fractional shortening and E-wave velocity ( P < 0.05, N = 10/group) and ex vivo by increased LV pressure and maximal rate of contraction and relaxation ( P < 0.05, N = 9/group). L-type Ca 2+ current density was increased in ventricular myocytes from RIα-KO, and β-AR stimulation was decreased by ∼50% ( P < 0.05, N = 38 cells for WT, and, N = 40 for RIα-KO). Consistently, Ca 2+ transients amplitude and relaxation kinetics were increased, along with increased occurrence of Ca 2+ sparks and waves ( P < 0.05, N = 44 cells for WT, and, N = 50 for RIα KO). Phosphorylation of Ca 2+ channels (CaV1.2), PLB, RyR2 and cMyBP-C at PKA sites was increased > 2-fold ( P < 0.05, N = 6/group) in RIα KO without modification of total protein expression. With age, these mice developed a congestive heart failure (HF) phenotype with massive hypertrophy and fibrosis which eventually led to death in 50% of RIα-KO mice at 50 weeks (versus 0% in WT, P < 0.01). These results reveal a previously unsuspected role of PKA type I in cardiac excitation-contraction coupling and HF.

Structural and functional impact of troponin C-mediated Ca2+ sensitization on myofilament lattice spacing and cross-bridge mechanics in mouse cardiac muscle

Acto-myosin cross-bridge kinetics are important for beat-to-beat regulation of cardiac contractility; however, physiological and pathophysiological mechanisms for regulation of contractile kinetics are incompletely understood. Here we explored whether thin filament-mediated Ca2+ sensitization influences cross-bridge kinetics in permeabilized, osmotically compressed cardiac muscle preparations. We used a murine model of hypertrophic cardiomyopathy (HCM) harboring a cardiac troponin C (cTnC) Ca2+-sensitizing mutation, Ala8Val in the regulatory N-domain. We also treated wild-type murine muscle with bepridil, a cTnC-targeting Ca2+ sensitizer. Our findings suggest that both methods of increasing myofilament Ca2+ sensitivity increase cross-bridge cycling rate measured by the rate of tension redevelopment (kTR); force per cross-bridge was also enhanced as measured by sinusoidal stiffness and I1,1/I1,0 ratio from X-ray diffraction. Computational modeling suggests that Ca2+ sensitization through this cTnC mutation or bepridil accelerates kTR primarily by promoting faster cross-bridge detachment. To elucidate if myofilament structural rearrangements are associated with changes in kTR, we used small angle X-ray diffraction to simultaneously measure myofilament lattice spacing and isometric force during steady-state Ca2+ activations. Within in vivo lattice dimensions, lattice spacing and steady-state isometric force increased significantly at submaximal activation. We conclude that the cTnC N-domain controls force by modulating both the number and rate of cycling cross-bridges, and that the both methods of Ca2+ sensitization may act through stabilization of cTnC's D-helix. Furthermore, we propose that the transient expansion of the myofilament lattice during Ca2+ activation may be an additional factor that could increase the rate of cross-bridge cycling in cardiac muscle. These findings may have implications for the pathophysiology of HCM.

Abstract 495: The Loss Of Pka Activity And Activation In Cardiomyocytes Does Not Induce Cardiac Abnormalities

Protein Kinase A (PKA) is the major mediator of β-adrenergic (βAR) regulation of cardiac contractility but other mediators (e.g., EPAC) have also been suggested. Reduced PKA basal activity and activation are linked to cardiac diseases. However, the roles of PKA in cardiac physiology and pathology have not been clearly defined because it has been difficult to knockout all expressed PKA catalytic subunit genes in cardiomyocytes. We ablated PKA activity and activation in cardiomyocytes by expressing a PKA inhibitor peptide (PKAi-GFP) in cardiomyocytes in transgenic (TG) mice. We made novel discoveries that: (1) Basal PKA-mediated phosphorylation was reduced for proteins in myofilaments and longitudinal SR but not for proteins in subsarcolemmal and dyadic spaces in PKAi TG hearts, suggesting basal PKA activity is different in individual cellular compartments; (2) PKAi increased the expression of CaMKII, the L-type Ca 2+ channel β subunits and current, but decreased CaMKII phosphorylation; (3) These changes resulted in significantly enhanced myofilament Ca 2+ sensitivity with prolonged contraction and relaxation, decreased myocyte Ca 2+ transient decay rate but enhanced myocyte contractility. (4) PKA inhibition fully abolishes the increases of heart rate, and cardiac and myocyte contractility by a β-adrenergic agonist (isoproterenol, ISO), showing a surprising dominant role of PKA in heart rate regulation and a minimal role of PKA-independent βAR signaling in regulating cardiac function; (5) The increased phosphorylation of proteins mediated by PKA and CaMKII after ISO was prevented by PKAi because increased PKA and CaMKII activation by ISO was eliminated by PKAi in vivo and in vitro . (6) PKAi TG mice maintained partial exercise capability by enhancing basal cardiac function, vascular constriction and ventricular filling (i.e., preload) during βAR stimulation. These features resemble some of the characteristics of diseased hearts. However, PKAi TG mice did not show any cardiac functional or structural abnormalities during the 1-year study period. In conclusion, PKA is the essential mediator of acute βAR regulation of cardiac function and exercise capacity, but the loss of PKA basal activity and activation by βAR agonists do not cause cardiac disease.

Abstract 366: The Role of Mitochondrial Calcium Uniporter Complex in Cardiac Contractility and Function

Cardiac contractility is regulated by the intracellular Ca 2+ concentration fluxes which are maintained by multiple channels and transporters. Mitochondrial Calcium Uniporter (MCU) is the highly selective Ca 2+ ion channel responsible for the electrophoretic Ca 2+ uptake into the mitochondrial matrix. Earlier studies on the cardiac-specific acute MCU knockout and transgenic dominant-negative MCU mice have demonstrated that mitochondrial Ca 2+ ( m Ca 2+ ) signaling is necessary for cardiac ‘‘fight-or-flight’’ contractile response, however, the role of m Ca 2+ in the beat-to-beat regulation of cardiac contractility is highly debated. The MCU channel function is regulated by varied molecular regulatory components as well as ROS mediated modification at the Cys-96 residue of the core MCU protein. Our earlier studies have demonstrated that loss of MCU Regulator 1 (MCUR1) in cardiomyocytes results in the impaired m Ca 2+ uptake. To dissect the precise role of MCU in regulating cytosolic Ca 2+ transients associated with excitation-contraction (E-C) coupling and cardiac functions, we have employed the loss-of-function, cardiac-specific MCUR1 knockout mouse and a gain-of-function MCU C96A knockin mouse generated by CRISPR/Cas9 approach. Using these novel mouse models, both by in vivo analyses of cardiac physiology and in vitro experiments including single-cell cardiac contractility, calcium transients, and electrophysiology measurements, our study demonstrates that m Ca 2+ buffering actively participates in E-C coupling. MCUR1/MCU regulated m Ca 2+ buffering in cardiomyocytes, although insignificant under basal condition, becomes critical in stress-induced conditions and actively participates in regulating the c Ca 2+ transients during adrenergic stimulation. Our data indicate that MCUR1 ablation offers protection against pressure-overload cardiac hypertrophy and MCU C96A KI mice exhibit significantly decreased LV ejection fraction at baseline. In summary, our results provide critical insights into the mechanisms by which the MCU channel contributes in regulating the contractile function of the cardiomyocytes and the role of m Ca 2+ in the development and progression of heart failure.

The cAMP-dependent protein kinase type I regulates cardiac excitation-contraction coupling

The cAMP-dependent protein kinase (PKA) consists of two regulatory (R) and two catalytic (C) subunits and comprises two subtypes, PKAI and PKAII, defined by the nature of their regulatory subunits, RIα and RIIα respectively. Whereas PKAII is thought to play a key role in β-adrenergic (β-AR) regulation of cardiac contractility, the function of PKAI is unclear. To address this question, we generated mice with cardiomyocyte-specific and conditional invalidation of the RIα subunit of PKA. Tamoxifen injection in 8 weeks-old mice resulted in a > 70% decrease in RIα protein without modification of other PKA subunits, which was associated with ∼2-fold increased basal PKA activity in RIα-KO mice (P 2-fold (P

Functional significance of C-terminal mobile domain of cardiac troponin I

Ca2+-regulation of cardiac contractility is mediated through the troponin complex, which comprises three subunits: cTnC, cTnI, and cTnT. As intracellular [Ca2+] increases, cTnI reduces its binding interactions with actin to primarily interact with cTnC, thereby enabling contraction. A portion of this regulatory switching involves the mobile domain of cTnI (cTnI-MD), the role of which in muscle contractility is still elusive. To study the functional significance of cTnI-MD, we engineered two cTnI constructs in which the MD was truncated to various extents: cTnI(1–167) and cTnI(1–193). These truncations were exchanged for endogenous cTnI in skinned rat papillary muscle fibers, and their influence on Ca2+-activated contraction and cross-bridge cycling kinetics was assessed at short (1.9 μm) and long (2.2 μm) sarcomere lengths (SLs). Our results show that the cTnI(1–167) truncation diminished the SL-induced increase in Ca2+-sensitivity of contraction, but not the SL-dependent increase in maximal tension, suggesting an uncoupling between the thin and thick filament contributions to length dependent activation. Compared to cTnI(WT), both truncations displayed greater Ca2+-sensitivity and faster cross-bridge attachment rates at both SLs. Furthermore, cTnI(1–167) slowed MgADP release rate and enhanced cross-bridge binding. Our findings imply that cTnI-MD truncations affect the blocked-to closed-state transition(s) and destabilize the closed-state position of tropomyosin.

Abstract 464: Two Different Microdomains of β 1 -adrenoreceptor Signaling Revealed by Live Cell Imaging

Background: 3’,5’-cyclic adenosine monophosphate (cAMP) is an ubiquitous second messenger and a crucial regulator of cardiac function and disease. In cardiomyocytes, it is produced predominantly after activation of β 1 -adrenergic receptors (β 1 -ARs) by catecholamines and acts intracellularly in discrete functionally relevant microdomains formed, for example, around calcium-handling proteins. Previously, we reported that β 1 -ARs are distributed across various cardiomyocyte membrane areas, including transverse (T)-tubules and cell crests. However, it is unknown whether these two β 1 -AR pools contribute differentially to the regulation of cardiac contractility and gene expression. Methods and Results: To directly visualize receptor-microdomain communication in cardiomyocytes, we established a combination of scanning ion conductance microscopy (SICM) with transgenically expressed targeted Förster resonance energy transfer (FRET)-based biosensors. Using this approach, we measured local cAMP responses in distinct microdomains of mouse ventricular cardiomyocytes (such as plasma membrane, cytosol and nucleus) after localized stimulation of β 1 -AR on different membrane structures of healthy and diseased cardiomyocytes. Using a plasma membrane targeted cAMP biosensor, we found that β 1 -AR stimulation at the crest induced stronger cAMP signals compared to β 1 -AR stimulated in the T-tubuli where cAMP was highly confined by PDE3. This difference was abolished in a pressure overload hypertrophy model due to submembrane redistribution of PDEs. Interestingly, crest β 1 -AR signals could propagate deeper inside the cell, inducing higher nuclear cAMP responses than recorded from receptors stimulated in the T-tubules. Conclusions: in the present study, we have demonstrated that β 1 -ARs located in T-tubuli and cell crests form two differentially regulated cAMP microdomains, each having its typical PDE repertoire and generating distinct second messenger signals. More detailed understanding of these two microdomains at different subsarcolemmal locations may contribute to new therapeutic strategies including more specific β-blockers.

Protein kinase C mechanisms that contribute to cardiac remodelling.

Protein phosphorylation is a highly-regulated and reversible process that is precisely controlled by the actions of protein kinases and protein phosphatases. Factors that tip the balance of protein phosphorylation lead to changes in a wide range of cellular responses, including cell proliferation, differentiation and survival. The protein kinase C (PKC) family of serine/threonine kinases sits at nodal points in many signal transduction pathways; PKC enzymes have been the focus of considerable attention since they contribute to both normal physiological responses as well as maladaptive pathological responses that drive a wide range of clinical disorders. This review provides a background on the mechanisms that regulate individual PKC isoenzymes followed by a discussion of recent insights into their role in the pathogenesis of diseases such as cancer. We then provide an overview on the role of individual PKC isoenzymes in the regulation of cardiac contractility and pathophysiological growth responses, with a focus on the PKC-dependent mechanisms that regulate pump function and/or contribute to the pathogenesis of heart failure.

Does Protein Sulfuration Take Place in the Heart?

Endogenous hydrogen sulfide (H2S) has been suggested to play a physiological role through protein sulfuration (also referred as sulfhydration) in the regulation of cardiac contractility. The sulfuration is a process by which H2S combines to free cysteine residue on proteins to create persulfide (protein‐S‐SH), changing protein conformation, such as in calcium channels, and functions. Since any new molecule of free H2S produced will immediately combine with the iron present on myoglobin or will be oxidized, our objective was to determine whether increasing endogenous H2S production can actually increase the pool of H2S combined with cysteine residues in the heart.To achieve this goal, we infused exogenous H2S up to a level that will alter cardiac contractility in order to create conditions wherein soluble H2S concentrations increase to levels higher than those reached by endogenous sulfide in the heart. The presence of sulfurated protein in the heart was identified by measuring gaseous H2S released after reducing disulfide bonds. TCEP was chosen as a potent reducing agent instead of dithiothreitol (DTT), previously used for this purpose in the literature, as DTT possesses two thiol groups (−SH) a significant source of H2S. Hearts were removed from control animals and animals exposed to 0.8 mg/min NaHS until cardiac depression‐induced hypotension was produced. The hearts were immediately homogenized in glycine‐NaOH buffer (pH 9), to prevent any evaporation of gaseous H2S. The homogenate was transferred into the 50‐ml chamber filled with sodium phosphate buffer (final pH ~6). At this pH, HS− will easily convert into gaseous H2S, in keeping with H2S partial pressure, and measured in the chamber headspace. The headspace was continuously flowed with N2, and H2S was directly measured by a voltammetry H2S analyzer before and after TCEP (5 mM).We found that the level of H2S produced by TCEP was similar and very low (below 0.5 nmol/g of protein) in both the control and intoxicated hearts, despite levels of free H2S in the blood above 10 microM in the latter. In contrast, the pool of H2S in albumin solution (10% in saline) incubated with H2S for 10 min, increased as a function of H2S concentrations using 0, 2, and 20 microM NaHS and reaching 10.6 ± 1.1, 11.8 ± 0.7 and 13.3 ± 0.4 nmol/g respectively. The change in H2S remained however extremely modest.Toxic levels of soluble H2S were unable to increase the pool of H2S liberated from the heart in a strong reducing environment. This suggests that free H2S in the heart is immediately trapped by other compounds such metallo‐proteins, e.g. myoglobin, present in the heart or is oxidized in the mitochondria before reacting with proteins, in contrast to a solution of albumin unable to produce these effects. Even in the latter, the pool of H2S produced by an increase in free H2S appears to be extremely small and to require toxic levels of sulfide to be affected.

Reaktívoxigén-származékok szerepe a szívműködés szabályozásában

Increased production of reactive oxygen species has been implicated in the pathogenesis of congestive heart failure. However, emerging evidence suggests a role for reactive oxygen species in regulating various physiological cellular processes in the myocardium. The authors summarize the current understanding of involvement of reactive oxygen species in the regulation of cardiac contractility under physiological conditions.