Phosphorylation of cardiac troponin inhibitory subunit (troponin I) and tropomyosin-binding subunit (troponin T) by cardiac phospholipid-sensitive Ca2+-dependent protein kinase

Considering impact of age and sex on cardiac cytoskeletal components

The cardiac cytoskeletal components are integral to cardiomyocyte function and are responsible for contraction, sustaining cell structure, and providing scaffolding to direct signaling. Cytoskeletal components have been implicated in cardiac pathology; however, less attention has been paid to age-related modifications of cardiac cytoskeletal components and how these contribute to dysfunction with increased age. Moreover, significant sex differences in cardiac aging have been identified, but we still lack a complete understanding to the mechanisms behind these differences. This review summarizes what is known about how key cardiomyocyte cytoskeletal components are modified because of age, as well as reported sex-specific differences. Thorough consideration of both age and sex as integral players in cytoskeletal function may reveal potential avenues for more personalized therapeutics.

TNNT1, TNNT2, and TNNT3: Isoform genes, regulation, and structure-function relationships

Troponin T (TnT) is a central player in the calcium regulation of actin thin filament function and is essential for the contraction of striated muscles. Three homologous genes have evolved in vertebrates to encode three muscle type-specific TnT isoforms: TNNT1 for slow skeletal muscle TnT, TNNT2 for cardiac muscle TnT, and TNNT3 for fast skeletal muscle TnT. Alternative splicing and posttranslational modifications confer additional structural and functional variations of TnT during development and muscle adaptation to various physiological and pathological conditions. This review focuses on the TnT isoform genes and their molecular evolution, alternative splicing, developmental regulation, structure-function relationships of TnT proteins, posttranslational modifications, and myopathic mutations and abnormal splicing. The goal is to provide a concise summary of the current knowledge and some perspectives for future research and translational applications.

Sizing up models of heart failure: Proteomics from flies to humans

Cardiovascular disease is the leading cause of death in the western world. Heart failure is a heterogeneous and complex syndrome, arising from various etiologies, which result in cellular phenotypes that vary from patient to patient. The ability to utilize genetic manipulation and biochemical experimentation in animal models has made them indispensable in the study of this chronic condition. Similarly, proteomics has been helpful for elucidating complicated cellular and molecular phenotypes and has the potential to identify circulating biomarkers and drug targets for therapeutic intervention. In this review, the use of human samples and animal model systems (pig, dog, rat, mouse, zebrafish, and fruit fly) in cardiac research is discussed. Additionally, the protein sequence homology between these species and the extent of conservation at the level of the phospho-proteome in major kinase signaling cascades involved in heart failure are investigated.

The role of protein kinase C-mediated phosphorylation of sarcomeric proteins in the heart-detrimental or beneficial?

Protein kinase C (PKC) is a family of serine/threonine protein kinases, and alterations have been found in PKC isoform expression and localization in the failing heart. These alterations in PKC activation levels influence the PKC-mediated phosphorylation status of cellular target proteins involved in Ca²⁺-handling and sarcomeric contraction. The differences observed in the effects due to PKC-mediated phosphorylation may underlie part of the contractile dysfunction observed in the failing heart. It is therefore important to establish the beneficial and detrimental effects of this kinase in the healthy and failing heart. The function of PKC has been studied intensively; however, the complexity of the regulation of this kinase makes the interpretation of the different effects difficult. The main focus of this review is the (patho)physiological impact of phosphorylation of sarcomeric proteins, myosin light chain-2, troponin I and T, desmin, myosin binding protein-C, and titin by PKC.

Phosphorylation, but not alternative splicing or proteolytic degradation, is conserved in human and mouse cardiac troponin T

Cardiac troponin T (cTnT), the tropomyosin binding subunit of the troponin complex, plays a pivotal regulatory role in the Ca(2+)-mediated interaction between actin thin filament and myosin thick filament. The post-translational modifications (PTMs) and alternative splicing of cTnT may represent important regulatory mechanisms of cardiac contractility. However, a complete characterization of PTMs and alternatively spliced isoforms in cTnT present in vivo is lacking. Top-down protein mass spectrometry (MS) analyzes whole proteins, thus providing a global view of all types of modifications, including PTMs and sequence variants, simultaneously in one spectrum without a priori knowledge. In this study, we applied an integrated immunoaffinity chromatography and top-down MS approach to comprehensively characterize PTMs and alternatively spliced isoforms of cTnT purified from healthy human and wild-type mouse heart tissue. High-resolution Fourier transform MS revealed that human cTnT (hcTnT) and mouse cTnT (mcTnT) have similar phosphorylation patterns, whereas higher molecular heterogeneity was observed for mcTnT than hcTnT. Further MS/MS fragmentation of monophosphorylated hcTnT and mcTnT by electron capture dissociation and collisionally activated dissociation unambiguously identified Ser1 as the conserved in vivo phosphorylation site. In contrast, we identified a single spliced isoform for hcTnT but three alternatively spliced isoforms for mcTnT. Moreover, we observed distinct proteolytic degradation products for hcTnT and mcTnT. This study also demonstrates the advantage of top-down MS/MS with complementary fragmentation techniques for the identification of modification sites in the highly acidic N-terminal region of cTnT.

Troponin T isoforms and posttranscriptional modifications: Evolution, regulation and function

Troponin-mediated Ca²(+)-regulation governs the actin-activated myosin motor function which powers striated (skeletal and cardiac) muscle contraction. This review focuses on the structure-function relationship of troponin T, one of the three protein subunits of the troponin complex. Molecular evolution, gene regulation, alternative RNA splicing, and posttranslational modifications of troponin T isoforms in skeletal and cardiac muscles are summarized with emphases on recent research progresses. The physiological and pathophysiological significances of the structural diversity and regulation of troponin T are discussed for impacts on striated muscle function and adaptation in health and diseases.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Calcium signaling phenomena in heart diseases: a perspective

Ca(2+) is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca(2+))(i) level in situ. The Ca(2+) signal inducing contraction in cardiac muscle originates from two sources. Ca(2+) enters the cell through voltage dependent Ca(2+) channels. This Ca(2+) binds to and activates Ca(2+) release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca(2+) induced Ca(2+) release (CICR) process. Entry of Ca(2+) with each contraction requires an equal amount of Ca(2+) extrusion within a single heartbeat to maintain Ca(2+) homeostasis and to ensure relaxation. Cardiac Ca(2+) extrusion mechanisms are mainly contributed by Na(+)/Ca(2+) exchanger and ATP dependent Ca(2+) pump (Ca(2+)-ATPase). These transport systems are important determinants of (Ca(2+))(i) level and cardiac contractility. Altered intracellular Ca(2+) handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the beta-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca(2+) release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca(2+) regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca(2+) handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Myofibrillar remodeling in cardiac hypertrophy, heart failure and cardiomyopathies

BACKGROUND

A wide variety of pathological conditions have been shown to result in cardiac remodelling and myocardial dysfunction. However, the mechanisms of transition from adaptive to maladaptive alterations, as well as those for changes in cardiac performance leading to heart failure, are poorly understood.

OBSERVATIONS

Extensive studies have revealed a broad spectrum of progressive changes in subcellular structures and function, as well as in signal transduction and metabolism in the heart, among different cardiovascular disorders. The present review is focused on identifying the alterations in molecular and biochemical structure of myofibrils (myofibrillar remodelling) in hypertrophied and failing myocardium in different types of heart diseases. Numerous changes at the level of gene expression for both contractile and regulatory proteins have already been reported in failing hearts and heart diseases; these changes are potential precursors for heart failure such as cardiac hypertrophy and cardiomyopathies. Myofibrillar remodelling, as a consequence of proteolysis, oxidation, and phosphorylation of some functional groups in both contractile and regulatory proteins in hearts failing due to different etiologies, has also been described.

CONCLUSIONS

Although myofibrillar remodelling appears to be associated with cardiac dysfunction, alterations in both contractile and regulatory proteins are dependent on the type and stage of heart disease.

Dephosphorylation specificities of protein phosphatase for cardiac troponin I, troponin T, and sites within troponin T

Protein dephosphorylation by protein phosphatase 1 (PP1), acting in concert with protein kinase C (PKC) and protein kinase A (PKA), is a pivotal regulatory mechanism of protein phosphorylation. Isolated rat cardiac myofibrils phosphorylated by PKC/PKA and dephosphorylated by PP1 were used in determining dephosphorylation specificities, Ca²⁺-stimulated Mg²⁺ATPase activities, and Ca²⁺ sensitivities. In reconstituted troponin (Tn) complex, PP1 displayed distinct substrate specificity in dephosphorylation of TnT preferentially to TnI, in vitro. In situ phosphorylation of cardiomyocytes with calyculin A, a protein phosphatase inhibitor, resulted in an increase in the phosphorylation stiochiometry of TnT (0.3 to 0.5 (67%)), TnI (2.6 to 3.6 (38%)), and MLC2 (0.4 to 1.7 (325%)). These results further confirmed that though MLC2 is the preferred target substrate for protein phosphatase in the thick filament, the Tn complex (TnI and TnT) from thin filament and C-protein in the thick filament are also protein phosphatase substrates. Our in vitro dephosphorylation experiments revealed that while PP1 differentially dephosphorylated within TnT at multiple sites, TnI was uniformly dephosphorylated. Phosphopeptide maps from the in vitro experiments show that TnT phosphopeptides at spots 4A and 4B are much more resistant to PP1 dephosphorylation than other TnT phosphopeptides. Mg²⁺ATPase assays of myofibrils phosphorylated by PKC/PKA and dephosphorylated by PP1 delineated that while PKC and PKA phosphorylation decreased the Ca²⁺-stimulated Mg²⁺ATPase activities, dephosphorylation antagonistically restored it. PKC and PKA phosphorylation decreased Ca²⁺ sensitivity to 3.6 µM and 5.0 µM respectively. However, dephosphorylation restored the Mg²⁺ATPase activity of PKC (99%) and PKA (95%), along with the Ca²⁺ sensitivities (3.3 µM and 3.0 µM, respectively).

Angiotensin II-induced negative inotropy in rat ventricular myocytes: role of reactive oxygen species and p38 MAPK

The octapeptide angiotensin II (ANG II) can modulate cardiac contractility and is increased in heart failure, where contractile function is impaired. In rat cardiac myocytes, 1 μM of ANG II produces a negative inotropic effect (NIE) (24.6 ± 5% reduction). However, the subcellular signaling involved in this effect remains elusive. We examined the mechanisms and signaling events involved in the reduction in contractile function induced by the peptide in indo-1-loaded rat cardiomyocytes. The results showed that the NIE of ANG II was not associated with a parallel decrease in the intracellular Ca2+transient, indicating that a decrease in myofilament responsiveness to Ca2+underlies the reduction in contractility. We assessed the role of PKC, tyrosine kinases, reactive oxygen species (ROS), and mitogen-activated protein kinases (MAPKs) in the NIE of the peptide. Pretreatment of cells with the NAD(P)H oxidase inhibitor diphenyleneiodonium chloride or with the superoxide scavenger 4,5-dihydroxy-1,3-benzene-disulfonic acid did not affect the ANG II-induced NIE. Moreover, ANG II-induced ROS production, after 20 min of incubation with the peptide, could not be detected with the use of either the fluorophore 5-(6)-chloromethyl-2′, 7′-dichlorodihydrofluorecein diacetate or lucigenin-enhanced chemiluminescence. In contrast, the ANG II-induced NIE was abrogated by the inhibitors of PKC (calphostin C), tyrosine kinase (genistein), and p38 MAPK (SB-202190). Furthermore, the NIE was significantly exacerbated (60 ± 10% reduction) by p38 MAPK overexpression. These results exclude the participation of ROS in the NIE of the peptide and point to PKC and tyrosine kinase as upstream mediators. Furthermore, they reveal p38 MAPK as the putative effector of the reduction in myofilament responsiveness to Ca2+and the decrease in contractility induced by the peptide.

The Special Structure and Function of Troponin I in Regulation of Cardiac Contraction and Relaxation

In this chapter I review evidence for a pivotal role of the sarcomeric thin filament protein, troponin I, in cardiac muscle activation and its modulation by covalent modifications, sarcomere length, and intracellular pH. This evidence demonstrates that the cardiac variant of troponin I (cTnI), which is the only isoform expressed in the adult myocardium, has unique structure and function that are specialized for extrinsic and intrinsic control of cardiac contraction and relaxation.

Effects of kappa-opioid receptor activation on myocardium

Kappa-opioid receptor stimulation of the heart transiently increases twitch amplitude and decreases Ca2+-dependent actomyosin Mg2+-ATPase activity through an undetermined mechanism. One purpose of the present study was to determine if the increase in twitch amplitude is due to changes in myofilament Ca2+ sensitivity. We also wanted to determine if kappa-opioid receptor activation alters maximum actin-myosin ATPase activity and Ca2+ sensitivity of tension in a way consistent with protein kinase A or protein kinase C (PKC) action. Rat hearts were treated with U50,488H (a kappa-opioid receptor agonist), phenylephrine plus propranolol (alpha-adrenergic receptor stimulation), isoproterenol (a beta-adrenergic receptor agonist), or phorbol 12-myristate 13-acetate (PMA, receptor independent activator of PKC) or were untreated (control), and myofibrils were isolated. U50,488H, phenylephrine plus propranolol, and PMA all decreased maximum Ca2+-dependent actomyosin Mg2+-ATPase activity, whereas isoproterenol treatment increased maximum Ca2+-dependent actomyosin Mg2+- ATPase activity. Untreated myofibrils exposed to exogenous PKC-epsilon, but not PKC-delta, decreased maximum actomyosin Mg2+-ATPase activity. Langendorff-perfused hearts treated with U50,488H, phenylephrine plus propranolol, or isoproterenol had significantly higher ventricular ATP levels compared with control hearts. PKC inhibitors abolished the effects of U50,488H on Ca2+-dependent actomyosin Mg2+-ATPase activity and myocardial ATP levels. U50,488H and PMA treatment of isolated ventricular myocytes increased Ca2+ sensitivity of isometric tension compared with control myocytes at pH 7.0. The U50,488H-dependent increase in Ca2+ sensitivity of tension was retained at pH 6.6. Together, these findings are consistent with the hypotheses that 1) the positive inotropy associated with kappa-opioid receptor activation may be due in part to a PKC-mediated increase in myofilament Ca2+-sensitivity of tension and 2) the kappa-opioid receptor-PKC pathway is a modulator of myocardial energy status through reduction of actomyosin ATP consumption.

Role for PKC in the adenosine-induced decrease in shortening velocity of rat ventricular myocytes

We previously demonstrated that both adenosine receptor activation and direct activation of protein kinase C (PKC) decrease unloaded shortening velocity (V(max)) of rat ventricular myocytes. The goal of this study was to further investigate a possible link among adenosine receptors, phosphoinositide-PKC signaling, and V(max) in rat ventricular myocytes. We determined that the adenosine receptor agonist R-phenylisopropyladenosine (R-PIA, 100 microM) and the alpha-adrenergic receptor agonist phenylephrine (Phe, 10 microM) increased turnover of inositol phosphates. PKC translocation from the cytosol to the sarcolemma was used as an indicator of PKC activation. Western blot analysis demonstrated an increased PKC-epsilon translocation after exposure to R-PIA, Phe, and the PKC activators dioctanoylglycerol (50 microM) and phorbol myristate acetate (1 microM). PKC-alpha, PKC-delta, and PKC-zeta did not translocate to the membrane after R-PIA exposure. Finally, PKC inhibitors blocked R-PIA-induced decreases in V(max) as well as Ca(2+)-dependent actomyosin ATPase in rat ventricular myocytes. These results support the conclusions that adenosine receptors activate phosphoinositide-PKC signaling and that adenosine receptor-induced PKC activation mediates a decrease in V(max) in ventricular myocytes.

Role of protein kinase C in alpha(1)-adrenergic regulation of a(Na)(i) in guinea pig ventricular myocytes

We investigated the role of protein kinase C (PKC) in alpha(1)-adrenergic regulation of intracellular Na(+) activity (a(Na)(i)) in single guinea pig ventricular myocytes. a(Na)(i) and membrane potentials were measured with the Na(+)-sensitive indicator sodium-binding benzofuran isophthalate and conventional microelectrodes, respectively, at room temperature (24-26 degrees C) while myocytes were stimulated at a rate of 0.25-0.3 Hz. The PKC activator 4beta-phorbol 12-myristate 13-acetate (PMA) decreased a(Na)(i) in a concentration-dependent manner. PMA (100 nM) produced a maximal decrease in a(Na)(i) of 1.5 mM from 6.5 +/- 0.4 to 5.0 +/- 0.4 mM (means +/- SE, n = 12, P < 0.01). The PMA concentration required for a half-maximal decrease in a(Na)(i) was 0.46 +/- 0.13 nM (n = 3, P < 0.01). An inactive phorbol, 4alpha-phorbol 12-myristate 13-acetate, did not decrease a(Na)(i). The decrease caused by PMA could be blocked by the PKC inhibitors staurosporine and bisindolylmaleimide I (GF-109203X). Stimulation of the alpha(1)-adrenoceptor with 50 microM phenylephrine decreased a(Na)(i) from 6.1 +/- 0.3 to 4.6 +/- 0.3 mM (n = 11, P < 0.01). The decrease in a(Na)(i) produced by phenylephrine was blocked by pretreatment with staurosporine, GF-109203X, or PMA. The decrease in a(Na)(i) produced by PMA was not prevented by pretreatment with tetrodotoxin but was blocked by pretreatment with strophanthidin or high extracellular K(+) concentration. The results suggest that alpha(1)-adrenergic receptor activation results in a decrease in a(Na)(i) via PKC-induced stimulation of the Na(+)-K(+) pump in cardiac myocytes.

Myofilament calcium regulation in human myocardium

BACKGROUND

We investigated whether decreased myofilament calcium contractile activation may, in part, contribute to heart failure.

METHODS AND RESULTS

Calcium concentration required for 50% activation and Hill coefficient for fibers from nonfailing and failing human hearts at pH 7.1 were not different. Maximum calcium-activated force (F(max)) was also not different. However, at pH 6.8 and 6.9, differences were seen in myofilament calcium activation between nonfailing and failing hearts. At lower pH, failing myocardium was shifted left on the calcium axis compared with nonfailing myocardium, which suggested an increase in myofilament calcium responsiveness. Increased inorganic phosphate concentration decreased maximal force development by 56% in nonfailing and 36% in failing myocardium and shifted the calcium-force relationship by 2.01+/-0.22 versus 0.86+/-0.13 micromol/L, respectively (P<0.05). Addition of cAMP resulted in a 0. 56 micromol/L shift toward higher intracellular calcium concentrations in nonfailing myocardium and a 1.04 micromol/L shift in failing myocardium. Protein kinase A in the presence of cAMP resulted in a further rightward shift in nonfailing human myocardium but did not further shift the calcium-force relationship in fibers from failing hearts. cGMP also resulted in a greater decrease in myofilament calcium sensitivity in fibers from failing hearts.

CONCLUSIONS

We propose that changes at the level of the thin myofilaments result in differential responses to changes in the intracellular milieu in nonfailing versus failing myocardium.

Exogenous calcium preconditions myocardium from patients taking oral sulfonylurea agents

We have previously reported that atrial trabeculae from patients taking oral sulfonylurea hypoglycemic agents cannot be preconditioned by transient ischemia, which may, in part, explain the increased cardiovascular mortality historically associated with the use of these agents (J. C. Cleveland et al., 1997, Circulation 96, 29-32). Recently, we reported that clinically accessible and acceptable exogenous Ca(2+) pretreatment protects human atrial trabeculae from subsequent ischemia (B. S. Cain et al., 1998, Ann. Thoracic Surg. 65, 1065-1070). It remains unknown whether this preconditioning strategy could confer protection to trabeculae from patients taking oral sulfonylurea drugs. We therefore hypothesized that exogenous Ca(2+) confers ischemic protection to trabeculae from patients taking oral sulfonylureas. Human atrial trabeculae were suspended in organ baths and field stimulated at 1 Hz, and force development was recorded. Following 90 min equilibration, trabeculae from patients taking oral sulfonylurea agents (n = 6 patients) were subjected to ischemia/reperfusion (I/R; 45/120 min) with or without Ca(2+) (1 mM increase x 5 min) 10 min prior to I/R. I/R decreased postischemic human myocardial contractility in trabeculae from patients on oral hypoglycemics to 15.3 +/- 2.0% baseline developed force (%BDF). Ca(2+) pretreatment increased postischemic human myocardial developed force to 35.3 +/- 2.9 %BDF in these patients (P < 0.05 vs I/R, ANOVA and Bonferroni/Dunn). We conclude that atrial muscle from patients taking oral hypoglycemic agents can be preconditioned with exogenous Ca(2+). This therapy may offer a clinically relevant means to precondition the myocardium of diabetics taking oral hypoglycemic agents prior to clinical interventions such as coronary angioplasty or cardiac bypass.

Troponin I: inhibitor or facilitator

TN-I occurs as a homologous group of proteins which form part of the regulatory system of vertebrate and invertebrate striated muscle. These proteins are present in vertebrate muscle as isoforms, Mr 21000-24000, that are specific for the muscle type and under individual genetic control. TN-I occupies a central position in the chain of events starting with the binding of calcium to troponin C and ending with activation of the Ca2+ stimulated MgATPase of the actomyosin filament in muscle. The ability of TN-I to inhibit the MgATPase of actomyosin in a manner that is accentuated by tropomyosin is fundamental to its role but the molecular mechanism involved is not yet completely understood. For the actomyosinATPase to be regulated the interaction of TN-I with actin, TN-C and TN-T must undergo changes as the calcium concentration in the muscle cell rises, which result in the loss of its inhibitory activity. A variety of techniques have enabled the sites of interaction to be defined in terms of regions of the polypeptide chain that must be intact to preserve the biological properties of TN-I. There is also evidence for conformational changes that occur when the complex with TN-C binds calcium. Nevertheless a detailed high resolution structure of the troponin complex and its relation to actin/tropomyosin is not yet available. TN-I induces changes in those proteins with which it interacts, that are essential for their function. In the special case of cardiac TN-I its effect on the calcium binding properties of TN-C is modulated by phosphorylation. It has yet to be determined whether TN-I acts directly as an inhibitor or indirectly by interacting with associated proteins to facilitate their role in the regulatory system.

Troponin T: genetics, properties and function

Troponin T (TnT) is present in striated muscle of vertebrates and invertebrates as a group of homologous proteins with molecular weights usually in the 31-36 kDa range. It occupies a unique role in the regulatory protein system in that it interacts with TnC and TnI of the troponin complex and the proteins of the myofibrillar thin filament, tropomyosin and actin. In the myofibril the molecule is about 18 nm long and for much its length interacts with tropomyosin. The ability of TnT to form a complex with tropomyosin is responsible for locating the troponin complex with a periodicity of 38.5 nm along the thin filament of the myofibril. In addition to it structural role, TnT has the important function of transforming the TnI-TnC complex into a system, the inhibitory activity of which, on the tropomyosin-actomyosin MgATPase of the myofibril, becomes sensitive to calcium ions. Different genes control the expression of TnT in fast skeletal, slow skeletal and cardiac muscles. In all muscles, and particularly in fast skeletal, alternative splicing of mRNA produces a series of isoforms in a developmentally regulated manner. In consequence TnT exists in many more isoforms than any of the other thin filament proteins, the TnT superfamily. Despite the general homology of TnT isoforms, this alternative splicing leads to variable regions close to the N- and C-termini. As the isoforms have slightly different effects on the calcium sensitivity of the actomyosin MgATPase, modulation of the contractile response to calcium can occur during development and in different muscle types. TnT has recently aroused clinical interest in its potential for detecting myocardial damage and the association of mutations in the cardiac isoform with hypertrophic cardiomyopathy.

Cardiac protein phosphorylation: functional and pathophysiological correlates

Protein phosphorylation acts a pivotal mechanism in regulating the contractile state of the heart by modulating particular levels of autonomic control on cardiac force/length relationships. Early studies of changes in cardiac protein phosphorylation focused on key components of the excitation-coupling process, namely phospholamban of the sarcoplasmic reticulum and myofibrillar troponin I. In more recent years the emphasis has shifted towards the identification of other phosphoproteins, and more importantly, the delineation of the mechanistic and signaling pathways regulating the various known phosphoproteins. In addition to cAMP- and Ca(2+)-calmodulin-dependent kinase processes, these have included regulation by protein kinase C and the ever-emerging family of growth factor-related kinases such as the tyrosine-, mitogen- and stress-activated protein kinases. Similarly, the role of protein dephosphorylation by protein phosphatases has been recognized as integral in modulating normal cardiac cellular function. Recent studies involving a variety of cardiovascular pathologies have demonstrated that changes in the phosphorylation states of key cardiac regulatory proteins may underlie cardiac dysfunction in disease states. The emphasis of this comprehensive review will be on discussing the role of cardiac phosphoproteins in regulating myocardial function and pathophysiology based not only on in vitro data, but more importantly, from ex vivo experiments with corroborative physiological and biochemical evidence.

Benefits of combination therapy in hypertensive patients with associated coronary artery disease: a subgroup with specific demands

Although prevention of coronary artery disease (CAD) is one of the main goals of antihypertensive therapy, when first seen hypertensive patients often have associated CAD. These patients need a therapy that can exert an acute anti-ischemic action, such as ad hoc relief of angina pectoris, and can also reduce the incidence of myocardial infarction (MI) or reinfarction. Reduction in blood pressure (BP) alone does not appear to be adequate because in hypertensive patients CAD is a complex and multifactorial process involving not only hemodynamic, neurohormonal, and metabolic factors but also hypertension-induced myocardial and vascular structural changes, which appear independently to contribute to risk for CAD. In theory, antihypertensive combination therapy, by summing the different effects of various drugs, appears to have a greater capacity for comprehensive management of hypertensive patients with CAD. Simultaneous administration of angiotensin-converting enzyme (ACE) inhibitors and calcium-channel blockers appears to be particularly effective. In several clinical trials with long-term follow-up, ACE inhibitor therapy has been associated with a substantial reduction in the risk for major ischemic events. The antiproliferative action of ACE inhibitors on myocardium and the vascular wall, their hemodynamic effects, antiatherogenic actions, neurohormonal attenuation, and certain genetic issues may account for the ability of this class of drugs to reduce the risk for CAD-related events. Although ACE inhibitors can be expected to increase coronary blood flow when the renin-angiotensin system is activated and to reduce BP, ventricular filling pressure, and sympathetic drive, thus far an acute anti-ischemic action of these drugs has not been demonstrated. Unlike ACE inhibitors, which usually have class-specific effects, there are important differences in the clinical effects of various calcium antagonists. The first generation of dihydropyridine calcium-entry blockers has failed to demonstrate efficacy in secondary prevention of coronary artery events. However, verapamil reduces mortality in patients with normal left ventricular function. The antihypertensive efficacy of verapamil, its antiatherogenic action, and its ability to reverse left ventricular hypertrophy, to improve diastolic function, and to interfere with endothelium-derived contracting factors may also account for the improved survival of patients with CAD treated with this drug. Moreover, verapamil is also effective in the treatment of all types of angina because it reduces myocardial oxygen consumption as a result of its hypotensive effect and its ability to reduce heart rate, and it may also improve oxygen delivery to the myocardium because of its action on coronary vasodilatation. It is also important to consider that ACE inhibitors and calcium antagonists often induce the same beneficial effects through different mechanisms, thus allowing a synergistic action when the two classes of drugs are administered together.

Calcium preconditioning in human myocardium

BACKGROUND

Ischemic stress and other protein kinase C (PKC)-linked receptor stimuli can induce rapid cardiac protection against ischemia-reperfusion injury. We and others have demonstrated that exogenous calcium (Ca2+) pretreatment confers PKC-mediated cardiac functional and infarct protection in animal models, but it remains unknown whether Ca2+ preconditioning confers similar postischemic functional protection in human myocardium, and, if so, whether the mechanism is mediated by PKC. We postulated that Ca2+ preconditioning confers ischemic tolerance to human myocardium by a PKC-dependent mechanism.

METHODS

Human atrial trabeculae were suspended in organ baths and paced at 1 Hz, and force development was recorded. After 90 minutes of equilibration, all trabeculae were subjected to ischemia (45 minutes) and reperfusion (120 minutes). Exogenous CaCl2 (3.0 mmol/L for 5 minutes) or vehicle (saline solution) was administered before simulated ischemia, with or without concurrent PKC inhibition (bisindolylmaleimide I, 150 nmol/L).

RESULTS

Ischemia-reperfusion resulted in decreased postischemic developed force, Ca2+ preconditioning protected human myocardium against ischemia-reperfusion injury (p < 0.05 versus control ischemia-reperfusion), and concurrent PKC inhibition abolished the salutary effect of Ca2+ preconditioning in human myocardium (p < 0.05 versus Ca2+ preconditioning).

CONCLUSIONS

Preconditioning with Ca2+ represents a potent means of accessing PKC-mediated protection of the human myocardium against ischemia-reperfusion injury.

Possible mechanism of the negative inotropic effect of alpha1-adrenoceptor agonists in rat isolated left atria after exposure to free radicals

1. This study was designed to investigate the mechanism(s) of the negative inotropic effects of alpha1-adrenoceptor agonists observed in rat isolated left atria after exposure to free radicals. 2. Ouabain and calphostin C were used in contraction experiments to block the sodium pump and protein kinase C. Methoxamine-induced phospholipase C and Na+/K+ ATPase activities were measured. 3. Methoxamine (300 microM) increased contractile force by 1.6 +/- 0.2 mN in control atria but decreased contractile force in electrolysis-treated atria by 2.0 +/- 0.1 mN (P < 0.05), as determined 10 min after methoxamine addition. In contrast, the positive inotropic effects of endothelin-1 (30 nM) and isoprenaline (10 microM) were reduced from 2.6 +/- 0.3 to 1.3 +/- 0.1 mN and from 2.6 +/- 0.3 to 1.7 +/- 0.2 mN, respectively, by electrolysis treatment (P < 0.05), but not converted into a negative inotropic action. 4. In an inositol phosphate assay we observed that the stimulation of phospholipase C by methoxamine was attenuated by electrolysis when the (electrolyzed) medium from the organ bath was used, but the phospholipase C responses were restored by the use of fresh medium. However, fresh medium did not counteract the negative inotropic effect of methoxamine. Accordingly, the negative inotropic effect of methoxamine is not directly related to the impaired phospholipase C responses seen in atria subjected to electrolysis. 5. Ouabain (10 microM) and the protein kinase C inhibitor calphostin C (50 nM), completely prevented the negative inotropic effect of 300 microM methoxamine in electrolysis-treated atria. 6. Measurement of the Na+/K+ ATPase activity, revealed that in control atria, alpha1-adrenoceptor stimulation with 300 microM methoxamine, decreased the Na+/K+ ATPase activity by 14.4 +/- 7.7%. In contrast, methoxamine increased the Na+/K+ ATPase activity by 48.8 +/- 8.9% (P < 0.05) in electrolysis-treated atria. Interestingly, this increase in Na+/K+ ATPase activity was completely counteracted by calphostin C (1.4 +/- 0.1% over basal). 7. These results indicate that the negative inotropic effects of alpha1-adrenoceptor agonists, observed in rat isolated left atria exposed to free radicals, are likely to be caused by protein kinase C-mediated phosphorylation and subsequent activation of the Na+/K+ ATPase.

The coatomer protein beta'-COP, a selective binding protein (RACK) for protein kinase Cepsilon

Distinct subcellular localization of activated protein kinase C (PKC) isozymes is mediated by their binding to isozyme-specific RACKs (receptors for activated C-kinase). Our laboratory has previously isolated one such protein, RACK1, and demonstrated that this protein displays specificity for PKCbeta. We have recently shown that at least part of the PKCepsilon RACK-binding site on PKCepsilon lies within the unique V1 region of this isozyme (Johnson, J. A., Gray, M. O., Chen, C.-H., and Mochly-Rosen, D. (1996) J. Biol. Chem. 271, 24962-24966). Here, we have used the PKCepsilon V1 region to clone a PKCepsilon-selective RACK, which was identified as the COPI coatomer protein, beta'-COP. Similar to RACK1, beta'-COP contains seven repeats of the WD40 motif and fulfills the criteria previously established for RACKs. Activated PKCepsilon colocalizes with beta'-COP in cardiac myocytes and binds to Golgi membranes in a beta'-COP-dependent manner. A role for PKC in control of secretion has been previously suggested, but this is the first report of direct protein/protein interaction of PKCepsilon with a protein involved in vesicular trafficking.

alpha 1-adrenoceptor-mediated negative inotropy of adrenaline in rat myocardium

1. The effect of alpha- and beta-adrenoceptor stimulation on isotonic contraction was investigated on right ventricular papillary muscles of the rat, stimulated at a rate of 0.5 Hz. 2. Adrenaline (0.5 microM) induced a slight but significant negative inotropic effect: shortening decreased from 0.137 +/- 0.058 to 0.122 +/- 0.059 muscle lengths (mean +/- S.D.; -11%, P < 0.0001) and maximum shortening velocity from 2.9 +/- 1.2 to 2.7 +/- 1.3 muscle lengths s-1 (-7%, P < 0.025). 3. The negative inotropic effect of adrenaline was enhanced after blocking the beta-adrenoceptors with 50 microM atenolol. On the other hand, exposure to adrenaline after blocking the alpha-adrenoceptors with 50 microM phentolamine resulted in an increase in shortening as well as in maximum shortening velocity. 4. Stimulation of the beta-adrenoceptors with 0.5 microM isoprenaline caused marked positive inotropic effects, whereas stimulation of the alpha 1-adrenoceptors with 0.5 microM phenylephrine regularly resulted in a long-lasting decrease in shortening and maximum shortening velocity. 5. 1,2-Dioctanoyl-sn-glycerol (1,2-DOG) and adrenaline induced an activation of protein kinase C (PKC) with translocation of this enzyme from the cytosol to the sarcolemma. 6. Activation of PKC with 10 microM 1,2-DOG and 0.5 microM adrenaline was accompanied by a decrease in shortening and maximum shortening velocity. Inhibition of PKC with 0.1 microM staurosporine abolished the negative inotropic effect of adrenaline. 7. From these results we conclude that a low dose of adrenaline stimulates not only beta-but also alpha-adrenoceptors and that the observed negative inotropic effect of adrenaline is mediated by alpha 1-adrenoceptors, linked to the diacylglycerol-PKC signal transduction pathway.

Effects of endothelin-1 on [Ca2+]i-shortening trajectory and Ca2+ sensitivity in rabbit single ventricular cardiomyocytes loaded with indo-1/AM: comparison with the effects of phenylephrine and angiotensin II

In most mammalian species, activation of myocardial endothelin as well as alpha1-adrenergic and angiotensin receptors leads to an increase in contractile function and myocardial cell hypertrophy, in association with acceleration of PI hydrolysis and with resultant production of IP3 and diacylglycerol. Therefore, these receptors may share a common intracellular signal transduction process in cardiac regulation. Although the pathophysiological relevance of endothelin- and angiotensin-mediated signal transduction has been postulated to play a key role in the progress of congestive heart failure, the details of the regulation are still controversial. We carried out experiments to further study the regulation induced by activation of these receptors. In spite of a wide range of species-dependent variation among mammals in the induction of the cardiotonic effect via these receptors, there is an excellent correlation between the extent of acceleration of PI hydrolysis and the positive inotropic effect (associated with a negative lusitropic effect) of the respective receptor agonists under most experimental conditions in rabbit ventricular myocardium. In isolated rabbit ventricular cardiomyocytes loaded with indo-1/AM, activation of these receptors elicited a very similar changes in the relationship between [Ca2+]i and cell shortening: the [Ca2+]i-shortening trajectory was shifted mainly upwards and the relationship of peak shortening vs peak [Ca2+]i was shifted to the left, an indication that the PIE of these agonists is consistently associated with an increase in [Ca2+]i and in the sensitivity of myofilaments to Ca2+ ions under the same experimental condition. Pieces of evidence in biochemical and pharmacological analyses imply that the products of PI hydrolysis, namely diacylglycerol and subsequent activation of protein kinase C, might play a crucial role in the regulation of cardiac function that is induced upon activation of endothelin, angiotensin and alpha-adrenergic receptors in the rabbit ventricular myocardium.

Phosphorylation specificities of protein kinase C isozymes for bovine cardiac troponin I and troponin T and sites within these proteins and regulation of myofilament properties

Protein kinase C (PKC) isozymes alpha, delta, epsilon, and zeta, shown to be expressed in adult rat cardiomyocytes, displayed distinct substrate specificities in phosphorylating troponin I and troponin T subunits in the bovine cardiac troponin complex. Thus, because they have different substrate affinities, PKC-alpha, -delta, and -epsilon phosphorylated troponin I more than troponin T, but PKC-zeta conversely phosphorylated the latter more than the former. Furthermore, PKC isozymes exhibited discrete specificities in phosphorylating distinct sites in these proteins as free subunits or in the troponin complex. Unlike other isozymes, PKC-delta was uniquely able to phosphorylate Ser-23/Ser-24 in troponin I, the bona fide phosphorylation sites for protein kinase A (PKA); and consequently, like PKA, it reduced Ca2+ sensitivity of Ca2+-stimulated MgATPase of reconstituted actomyosin S-1. In addition, PKC-delta, like PKC-alpha, readily phosphorylated Ser-43/Ser-45 (sites common for all PKC isozymes) and reduced maximal activity of MgATPase. In this respect, PKC-delta functioned as a hybrid of PKC-alpha and PKA. In contrast to PKC-alpha, -delta, and -epsilon, PKC-zeta exclusively phosphorylated two previously unknown sites in troponin T. Phosphorylation of troponin T by PKC-alpha resulted in decreases in both Ca2+ sensitivity and maximal activity, whereas phosphorylation by PKC-zeta resulted in a slight increase of the Ca2+ sensitivity without affecting the maximal activity of MgATPase. Most of the in vitro phosphorylation sites in troponin I and troponin T were confirmed in situ in adult rat cardiomyocytes. The present study has demonstrated for the first time distinct specificities of PKC isozymes for phosphorylation of two physiological substrates in the myocardium, with functional consequences.

Signalling by protein kinase C isoforms in the heart

Understanding transmembrane signalling process is one of the major challenge of the decade. In most tissues, since Fisher and Krebs's discovery in the 1950's, protein phosphorylation has been widely recognized as a key event of this cellular function. Indeed, binding of hormones or neurotransmitters to specific membrane receptors leads to the generation of cytosoluble second messengers which in turn activate a specific protein kinase. Numerous protein kinases have been so far identified and roughly classified into two groups, namely serine/threonine and tyrosine kinases on the basis of the target acid although some more recently discovered kinases like MEK (or MAP kinase kinase) phosphorylate both serine and tyrosine residues. Protein kinase C is a serine/threonine kinase that was first described by Takai et al. [1] as a Ca- and phospholipid-dependent protein kinase. Later on, Kuo et al. [2] found that PKC was expressed in most tissues including the heart. The field of investigation became more complicated when it was found that the kinase is not a single molecular entity and that several isoforms exist. At present, 12 PKC isoforms and other PKC-related kinases [3] were identified in mammalian tissues. These are classified into three groups. (1) the Ca-activated alpha-, beta-, and gamma-PKCs which display a Ca-binding site (C2); (2) the Ca-insensitive delta-, epsilon-, theta-, eta-, and mu-PKCs. The kinases that belong to both of these groups display two cysteine-rich domains (C1) which bind phorbol esters (for recent review on PKC structure, see [4]). (3) The third group was named atypical PKCs and include zeta, lambda, and tau-PKCs that lack both the C2 and one cysteine-rich domain. Consequently, these isoforms are Ca-insensitive and cannot be activated by phorbol esters [5]. In the heart, evidence that multiple PKC isoforms exist was first provided by Kosaka et at. [6] who identified by chromatography at least two PKC-related isoenzymes. Numerous studies were thus devoted to the biochemical characterization of these isoenzymes (see [7] for review on cardiac PKCs) as well as to the identification of their substrates. This overview aims at updating the present knowledge on the expression, activation and functions of PKC isoforms in cardiac cells.

Cardiac alpha(1)-adrenoceptors that regulate contractile function: subtypes and subcellular signal transduction mechanisms

Activation of alpha(1)-adrenoceptors as well as endothelin (ET) and angiotensin II (Ang II) receptors in cardiac muscle is coupled to acceleration of the hydrolysis of phosphoinositide (PI), with resultant production of inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol. There is an excellent correlation between the extent of acceleration of the PI hydrolysis and the positive inotropic effect (PIE) under most experimental conditions after the administration of a-adrenoceptor agonists, ET and Ang II in the rabbit ventricular muscle. The PIE of the alpha-adrenoceptor agonists, ET and Ang II is associated with a negative lusitropic effect and an increase in the sensitivity of myofilaments to Ca(2)+ ions. The PIE can be selectively inhibited by inhibitors of protein kinase C (PKC) such as staurosporine, NA 0345 and H-7, with little effect on the PI hydrolysis and the PIE of isoproterenol and Bay k 8644. Surprisingly, an activator of PKC, phorbol 12,13-dibutyrate (PDBu), selectively and more completely inhibited the PIE and acceleration of PI hydrolysis induced by the alpha-adrenoceptor agonists as well as by ET and Ang II in the rabbit. These receptor agonists consistently cause intracellular alkalinization by activation of Na+-H+ exchange, while the effects on membrane ion channel activities are divergent. For example, alpha-adrenoceptor agonists cause monophasic prolongation of the action potential, the time course of which coincides well with that of the PIE, while ET and Ang II produce a biphasic change in action potential duration, i.e., the long-lasting prolongation preceded by a transient abbreviation. Alpha-adrenoceptor agonists scarcely affect I(ca), whereas ET elicits a biphasic alteration of the current. In addition, the potassium current, I(K1), is markedly suppressed by alpha-adrenoceptor agonists, but this effect is not revealed with Ang II under the same experimental condition. These results indicate that the effects of alpha(1)-adrenoceptor stimulation are partially shared by those of FT and Ang II receptor activation in the heart. Approximately 60% of the total population of alpha(1)-adrenoceptors in the rabbit ventricle are composed of alpha(1A) subtype, which is susceptible to chlorethylclonidine (CEC) and is predominantly responsible for the alpha(1)-mediated PIE and PI hydrolysis. The remaining fraction that belongs to alpha(1A) subtype is further subclassified into the WB 4101-sensitive (partly coupled to PI hydrolysis) and the niguldipine-sensitive (PI hydrolysis-unrelated) subtypes.

Cardiac troponin I mutants. Phosphorylation by protein kinases C and A and regulation of Ca(2+)-stimulated MgATPase of reconstituted actomyosin S-1

The significance of site-specific phosphorylation of cardiac troponin I (TnI) by protein kinase C and protein kinase A in the regulation of Ca(2+)-stimulated MgATPase of reconstituted actomyosin S-1 was investigated. The TnI mutants used were T144A, S43A/S45A, and S43A/S45A/T144A (in which the identified protein kinase C phosphorylation sites, Thr-144 and Ser-43/ Ser-45, were, respectively, substituted by Ala) and S23A/S24A and N32 (in which the protein kinase A phosphorylation sites Ser-23/Ser-24 were either substituted by Ala or deleted). The mutations caused subtle changes in the kinetics of phosphorylation by protein kinase C, and all mutants were maximally phosphorylated to various extents (1.3-2.7 mol of phosphate/mol of protein). Protein kinase C could cross-phosphorylate protein kinase A sites but the reverse essentially could not occur. Compared to wild-type TnI and T144A, un-phosphorylated S43A/S45A, S43A/S45A/T144, S23A/ S24A, and N32 caused a decreased Ca2+ sensitivity of Ca(2+)-stimulated MgATPase of reconstituted actomyosin. S-1. Phosphorylation by protein kinase C of wild-type and all mutants except S43A/S45A and S43A/S45A/T144A caused marked reductions in both the maximal activity of Ca(2+)-stimulated MgATPase and apparent affinity of myosin S-1 for reconstitued (regulated) actin. It was further noted that protein kinase C acted in an additive manner with protein kinase A by phosphorylating Ser-23/Ser-24 to bring about a decreased Ca2+ sensitivity of the myofilament. It is suggested that Ser-43/Ser-45 and Ser-23/Ser-24 in cardiac TnI are important for normal Ca2+ sensitivity of the myofilament, and that phosphorylation of Ser-43/Ser-45 and Ser-23/Ser-24 is primarily involved in the protein kinase C regulation of the activity and Ca2+ sensitivity, respectively, of actomyosin S-1 MgATPase.

Regulation of phospholamban and troponin-I phosphorylation in the intact rat cardiomyocytes by adrenergic and cholinergic stimuli: roles of cyclic nucleotides, calcium, protein kinases and phosphatases and depolarization

Protein phosphorylation was investigated in [32P]-labeled cardiomyocytes isolated from adult rat heart ventricles. The beta-adrenergic stimulation (by isoproterenol, ISO) increased the phosphorylation of inhibitory subunit of troponin (TN-I), C-protein and phospholamban (PLN). Such stimulation was largely mediated by increased adenylyl cyclase (AC) activity, increased myoplasmic cyclic AMP and increased cyclic AMP dependent protein kinase (A-kinase)-catalyzed phosphorylation of these proteins in view of the following observations: (a) dibutyryl-and bromo-derivatives of cyclic AMP mimicked the stimulatory effect of ISO on protein phosphorylation while (b) Rp-cyclic AMP was found to attenuate ISO-dependent stimulation. Unexpectedly, 8-bromo cyclic GMP was found to markedly increase TN-I and PLN phosphorylation. Both beta 1- and beta 2-adrenoceptors were present and ISO binding to either receptor was found to stimulate myocyte AC. However, the stimulation of the beta 2-AR only marginally increased while the stimulation of beta 1-AR markedly increased PLN phosphorylation. Other stimuli that increase tissue cyclic AMP levels also increased PLN and TN-I phosphorylation and these included isobutylmethylxanthine (non-specific phosphodiesterase inhibitor), milrinone (inhibits cardiotonic inhibitable phosphodiesterase, sometimes called type III or IV) and forskolin (which directly stimulates adenylyl cyclase). Cholinergic agonists acting on cardiomyocyte M2-muscarinic receptors that are coupled to AC via pertussis toxin(PT)-sensitive G proteins inhibited AC and attenuated ISO-dependent increases in PLN and TN-I phosphorylation. The in vivo PT treatment, which ADP-ribosylated Gi-like protein(s) in the myocytes, markedly attenuated muscarinic inhibitory effect on PLN and TN-I phosphorylation on one hand and, increased the beta-adrenergic stimulation, on the other. Controlled exposure of isolated myocytes to N-ethyl maleimide, also led to the findings similar to those seen following the PT treatment. Exposure of myocytes to phorbol, 12-myristate, 13-acetate (PMA) increased the protein phosphorylation, augmenting the stimulation by ISO, and such augmentation was antagonized by propranolol suggesting modulation of the beta-adrenoceptor coupled AC pathway by PMA. Okadaic acid (OA) exposure of myocytes also increased protein phosphorylation with the results supporting the roles for type 1 and 2A protein phosphatases in the dephosphorylation of PLN and TN-I. Interestingly OA treatment attenuated the muscarinic inhibitory effect which was restored by subsequent brief exposure of myocytes to PMA. While the stimulation of alpha adrenoceptors exerted little effect on the phosphorylation of PLN and TN-I, inactivation of alpha adrenoceptors by chloroethylclonidine (CEC), augmented beta-adrenergically stimulated phosphorylation. KCl-dependent depolarization of myocytes was observed to potentiate ISO-dependent increase in phosphorylation (incubation period 15 sec to 1 min) as well as to accelerate the time-dependent decline in this phosphorylation seen upon longer incubation. Verapamil decreased ISO-stimulated protein phosphorylation in the depolarized myocytes. Depolarization was found to have little effect on the muscarinic inhibitory action on phosphorylation. Prior treatment of myocytes with PMA, was found to augment ISO-stimulated protein phosphorylation in the depolarized myocytes. Such augmented increases were completely blocked by propranolol. Forskolin also stimulated PLN and TN-I phosphorylation. Prior exposure of myocytes to forskolin followed by incubation in the depolarized and polarized media showed that PLN was dephosphorylated more rapidly in the depolarized myocytes. The results support the view that both cyclic AMP and calcium signals cooperatively increase the rates of phosphorylation of TN-I and PLN in the depolarized cardiomyocytes during beta-adrenergic stimulation. (ABSTRACT TRUNCATED)

Specific degradation of troponin T and I by mu-calpain and its modulation by substrate phosphorylation

The degradation of troponin (Tn) subunits by calpain was studied by incubating either isolated cardiac Tns or myocardial cryosections with two different calpain isoenzymes isolated from rat skeletal muscle. Western-blot analysis with monoclonal antibodies against TnI and TnT showed that mu-calpain was at least ten times more active than m-calpain in degrading TnI and TnT both in vitro and in situ. TnC was completely resistant to both proteinase forms. Phosphorylation by cyclic AMP-dependent protein kinase (PKA) isolated from rat skeletal muscle reduced the sensitivity of TnI to degradation. This effect in combination with an increased efficiency of the endogenous inhibitor [Salamino, De Tullio, Michetti, Mengotti, Melloni and Pontremoli (1994) Biochem. Biophys. Res. Commun. 199, 1326-1332] probably reduces the proteolytic activity of calpain in cells on PKA stimulation. Conversely, phosphorylation by protein kinase C (PKC) resulted in a twofold increase in the degradation of TnI. Degradation by m-calpain was not modified by Tn phosphorylation. The different sensitivity to mu-calpain might be related to changes in TnI oligomeric structure. Indeed, on PKC phosphorylation, the apparent molecular mass of TnI calculated from the distribution coefficient of Tn complex in Sephadex G-100 matrix was reduced from 90 to 30 kDa suggesting dissociation of the Tn complex.

Endothelin-1 does not phosphorylate phospholamban and troponin I in intact beating rat hearts

To determine a role of phosphorylation of specific cardiac regulatory proteins in the positive inotropic effect of endothelin-1, we examined phosphorylation of sarcoplasmic reticulum and myofibrillar proteins in perfused beating rat hearts treated with endothelin-1. In parallel experiments, the effects of isoprenaline and phorbol-12,13-dibutyrate (PDB) on protein phosphorylation were also tested. In 32Pi-labeled hearts, perfusion with isoprenaline (100 nM) caused 4.4- and 10.4-fold increases in the degree of phosphorylation of phospholamban in sarcoplasmic reticulum and of troponin I in myofibrils, respectively. In contrast, neither endothelin-1 (100 nM) nor PDB (1 microM) significantly changed the phosphorylation state of these proteins. These findings provide evidence that phosphorylation of major cardiac regulatory proteins is not responsible for the positive inotropic action of endothelin-1.

The effects of various drugs on the myocardial inotropic response

1. The signal transduction process mediated by cyclic AMP that leads to the characteristic positive inotropic effect (PIE) in association with a positive lusitropic effect (acceleration of rate of twitch relaxation) has been well established. Relationships between accumulation of cyclic AMP, changes in intracellular Ca2+ transients and the PIE differ, however, depending on the mechanism of particular drugs that affect different steps in the metabolism of cyclic AMP. Selective partial agonists of beta 1-adrenoceptors and inhibitors of phosphodiesterase (PDE) III cause the accumulation of less cyclic AMP for a given PIE than does isoproterenol. In addition, in aequorin-microinjected canine ventricular muscle, selective inhibitors of PDE III, OPC 18790 and Org 9731, produced smaller decreases in the responsiveness of myofilaments to Ca2+ ions than isoproterenol, while a partial agonist of beta 1-adrenoceptors, denopamine, elicits a decrease in Ca2+ responsiveness of the same extent as does isoproterenol. 2. Activation of myocardial alpha 1-adrenoceptors, as well as stimulation of receptors for endothelin and angiotensin II, which accelerates hydrolysis of phosphoinositide (PI) to result in production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are associated with very similar inotropic regulation: (1) the dependence on the species of animals of induction of the PIE; (2) an excellent correlation between the extent of acceleration of hydrolysis of PI and the PIE; (3) isometric contraction curves associated with a negative lusitropic effect; (4) the PIE associated with increases in myofibrillar responsiveness to Ca2+ ions; and (5) the selective inhibition of the PIE by an activator of protein kinase C (PKC), phorbol 12,13-dibutyrate (PDBu), with little effect on the PIE of isoproterenol and Bay k 8644. 3. A novel class of cardiotonic agents, namely, Ca2+ sensitizers such as EMD 53998 and Org 30029, act on the Ca(2+)-binding site of troponin C, increasing the affinity of these sites for Ca2+ ions, or at the actin-myosin interface to facilitate the cycling of cross-bridges. These agents produce a PIE with little change or decrease in Ca2+ transients and may bring about a significant breakthrough in the development of drugs for reversal of myocardial failure in the treatment of congestive heart failure.

Endothelin-1 does not alter Ca2+ responsiveness in saponin-skinned ferret papillary muscles

Endothelin is a potent vasoconstrictor and a positive inotropic agent in myocardium. Endothelin has been reported to increase myocardial contractility with little or no increase in intracellular Ca2+, thus apparently enhancing myofilament responsiveness to Ca2+. We investigated the effects of endothelin on tension development and Ca2+ responsiveness in both intact and saponin-skinned ferret right ventricular papillary muscles. Isolated ferret papillary muscles were stimulated for 2 h in the presence or absence of endothelin (100 nM). The muscles were then chemically skinned with saponin and exposed to relaxing and contracting solutions containing varying amounts of Ca2+, and the developed force of contraction was measured. The [Ca2+] required for half-maximal activation (pCa50) was determined by fitting force versus Ca2+ data to the Hill equation. In isometrically contracting muscles, endothelin (100 nM) caused a mean percent increase in developed tension of 34.7% +/- 11.3% (mean +/- S.E.). In muscles that were exposed to endothelin for 2 h and then skinned, neither the pCa50 nor the maximal Ca(2+)-activated force (Fmax) were significantly different from control skinned papillary muscles. After skinning, when endothelin (100 nM) was added to the Ca2+ buffers, both pCa50 and Fmax were significantly decreased. When papillary muscles were pretreated with phorbol 12-myristate 13-acetate (PMA) and then skinned, there was a significant increase in the pCa50. These results indicate that endothelin acts directly on the myofilaments to impair force development by directly decreasing the Ca2+ responsiveness of myofilaments.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C phosphorylation of cardiac troponin T decreases Ca(2+)-dependent actomyosin MgATPase activity and troponin T binding to tropomyosin-F-actin complex

Effects of phosphorylation of bovine cardiac troponin T (TnT) by protein kinase C on the Ca(2+)-stimulated MgATPase activity of reconstituted actomyosin complex and the binding of TnT to tropomyosin(Tm)-F-actin were investigated. The Ca(2+)-stimulated MgATPase of actomyosin containing phosphorylated TnT (1.8 mol of P/mol), compared with that containing unphosphorylated TnT, was decreased by up to 48%. Phosphorylation of TnT also decreased (up to 48%) its maximum binding to Tm-F-actin, which was accompanied by a decrease (up to 3.5-fold) in its apparent binding affinity. The findings indicate that the effects of phosphorylated TnT in decreasing actomyosin MgATPase might be secondary to its decreased interactions with the other components of the thin filament, representing a new mechanism underlying the negative inotropic responses of various cardiac preparations to protein kinase C-activating phorbol esters.

Ca2+ and pH determine the interaction of chromaffin cell scinderin with phosphatidylserine and phosphatidylinositol 4,5,-biphosphate and its cellular distribution during nicotinic-receptor stimulation and protein kinase C activation

Nicotinic stimulation and high K(+)-depolarization of chromaffin cells cause disassembly of cortical filamentous actin networks and redistribution of scinderin, a Ca(2+)-dependent actin filament-severing protein. These events which are Ca(2+)-dependent precede exocytosis. Activation of scinderin by Ca2+ may cause disassembly of actin filaments leaving cortical areas of low cytoplasmic viscosity which are the sites of exocytosis (Vitale, M. L., A. Rodríguez Del Castillo, L. Tchakarov, and J.-M. Trifaró. 1991. J. Cell. Biol. 113:1057-1067). It has been suggested that protein kinase C (PKC) regulates secretion. Therefore, the possibility that PKC activation might modulate scinderin redistribution was investigated. Here we report that PMA, a PKC activator, caused scinderin redistribution, although with a slower onset than that induced by nicotine. PMA effects were independent of either extra or intracellular Ca2+ as indicated by measurements of Ca2+ transients, and they were likely to be mediated through direct activation of PKC because inhibitors of the enzyme completely blocked the response to PMA. Scinderin was not phosphorylated by the kinase and further experiments using the Na+/H+ antiport inhibitors and intracellular pH determinations, demonstrated that PKC-mediated scinderin redistribution was a consequence of an increase in intracellular pH. Moreover, it was shown that scinderin binds to phosphatidylserine and phosphatidylinositol 4,5-biphosphate liposomes in a Ca(2+)-dependent manner, an effect which was modulated by the pH. The results suggest that under resting conditions, cortical scinderin is bound to plasma membrane phospholipids. The results also show that during nicotinic receptor stimulation both a rise in intracellular Ca2+ and pH are observed. The rise in intracellular pH might be the result of the translocation and activation of PKC produced by Ca2+ entry. This also would explain why scinderin redistribution induced by nicotine is partially (26-40%) inhibited by inhibitors of either PKC or the Na+/H+ antiport. In view of these findings, a model which can explain how scinderin redistribution and activity may be regulated by pH and Ca2+ in resting and stimulated conditions is proposed.

Protein kinase C activation by phorbol esters induces chromaffin cell cortical filamentous actin disassembly and increases the initial rate of exocytosis in response to nicotinic receptor stimulation

Nicotinic stimulation and high K+ depolarization of bovine chromaffin cells cause disassembly of cortical filamentous actin networks. Previous work from our laboratory has demonstrated that disassembly of actin filaments is Ca(2+)-dependent, precedes exocytosis and occurs in cortical areas of low cytoplasmic viscosity which are the sites of exocytosis. It has also been suggested that protein kinase C is involved in catecholamine secretion from chromaffin cells. Therefore, the possibility that protein kinase C activation might be implicated in cortical filamentous actin disassembly was investigated. Here we report that phorbol myristate acetate, a protein kinase C activator, causes cortical filamentous actin disassembly. Short-term phorbol ester treatment does not alter the morphology of chromaffin cells; however, 1 h after phorbol ester exposure an increase in cell flattening and membrane ruffling is observed. Phorbol ester-induced cortical filamentous actin disassembly is inhibited by protein kinase C activity inhibitors, is independent of extracellular Ca2+ and has a slower time course than that induced by either nicotinic receptor stimulation or K(+)-depolarization. Phorbol ester effects are likely to be mediated by activation of protein kinase C and not by any changes in intracellular Ca2+ levels, as indicated by measurements of Ca2+ transients. Pretreatment of chromaffin cells with phorbol myristate acetate increases the initial rate of nicotine-evoked catecholamine release. Nicotine-induced cortical actin filament disassembly and catecholamine secretion are partially (29-40%) inhibited by pretreatment of cells with either calphostin C, staurosporine or sphingosine. The results suggest that protein kinase C may be involved in the reorganization of the cortical actin filament network priming the cells for release by removing a barrier to secretory granule mobility. However, its role in exocytosis is modulatory but not essential.

Different effects of alpha- and beta-adrenergic stimulation on cytosolic pH and myofilament responsiveness to Ca2+ in cardiac myocytes

alpha-Adrenergic stimulation (alpha-AS) and beta-adrenergic stimulation (beta-AS) of the myocardium are associated respectively with an increase and a decrease in myofilament responsiveness to Ca2+. We hypothesized that changes in cytosolic pH (pH(i)) may modulate these opposite actions of alpha-AS and beta-AS. The effects of alpha-AS (50 microM phenylephrine and 1 microM nadolol) and beta-AS (0.05 microM isoproterenol) on contraction and either cytosolic Ca2+ (Cai) or pH(i) were assessed in adult rat ventricular myocytes bathed in bicarbonate buffer (pH 7.36 +/- 0.05). In cells loaded with the ester derivative (AM form) of indo-1, the 410/490-nm ratio of emitted fluorescence indexed Cai. Myofilament responsiveness to Ca2+ was assessed by the relaxation phase of the length-indo-1 fluorescence relation during a twitch. alpha-AS and beta-AS shifted this relation in opposite directions, indicating that alpha-AS increased and beta-AS decreased myofilament responsiveness to Ca2+. In addition, the positive inotropic action of alpha-AS was associated with an increased Cai transient amplitude in 50% of the myocytes (n = 12), whereas beta-AS always increased Cai (n = 5). In cells loaded with the fluorescent pH(i) probe SNARF-1 AM, the emitted 590/640-nm fluorescence is a measure of pH(i). The effect of alpha-AS on the extent of cell shortening during the twitch (ES) was expressed as the percentage of resting cell length. Both ES and pH(i) were assessed in myocytes bathed in 1.5 mM [Ca2+] and stimulated at 0.5 Hz (control ES, 7.4 +/- 1.5%; control pH(i), 7.11 +/- 0.05; n = 10). alpha-AS enhanced both ES (delta ES, 1.8 +/- 0.6%; p less than 0.05) and pH(i) (delta pH(i), 0.06 +/- 0.01; p less than 0.005), and there was a significant correlation between delta ES and delta pH(i) (r = 0.76, p less than 0.05). A similar effect of alpha-AS on pH(i) was observed in the absence of electrical stimulation (n = 8). The alpha-AS-induced enhancement of ES and pH(i) was abolished by 10 microM ethylisopropylamiloride, a Na(+)-H+ exchange inhibitor (n = 7). In additional experiments, myocytes were preincubated either with 0.2 microM 4 beta-phorbol 12-myristate 13-acetate (n = 8) or with 5 nM staurosporine (n = 8), which have been shown to downregulate and inhibit Ca(2+)-activated phospholipid-dependent protein kinase C, respectively. In either group, alpha-AS had no effect on pH(i) and decreased ES to approximately 60% of control.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane interactions of mastoparan analogues related to their differential effects on protein kinase C, Na, K-ATPase and HL60 cells

Membrane interactions of tetradecapeptide toxin mastoparan (MP) and analogues (MP-3, MP-X and polistes MP), as indicated by inhibition of various enzymatic and cellular activities, were investigated. MP-3 was found to be the least active in inhibiting protein kinase C (PKC; activated by phosphatidylserine vesicles, synaptosomal membranes or phorbol ester), synaptosomal membrane Na,K-ATPase and proliferation and viability of leukemia HL60 cells. MP-3, however, was as active as others in inhibiting PKC activated by arachidonate monomers and phorbol ester binding. The unique properties of MP-3, the [des-Ile1-Asn2]-analogue of MP, might be related to its low functional amphiphilicity compared to others and useful in further delineating biological activities associated with or regulated by membranes.

12-O-tetradecanoylphorbol-13-acetate induces selective desquamation of superficial cells in rat urinary bladder epithelium

12-O-tetradecanoylphorbol-13-acetate (TPA) is known to affect the proliferation and/or differentiation of several types of cells. We injected TPA directly into the lumen of rat bladder to determine, using scanning and transmission electron microscopy, its effects on the bladder epithelium in vivo. At 1 h after TPA injection (1 microgram/ml), the superficial cells of the epithelium had changed their morphology, and large spherical vacuoles occupied their cytoplasm. In some areas, the underlying intermediate cells were exposed by the desquamation of the superficial cells. During the next few hours, TPA was excreted from the bladder lumen by voluntary micturition, but the desquamation of the superficial cells proceeded further. All the superficial cells were lost from the luminal surface by 24 h after TPA injection. The changes noted were specific for the superficial cells and were not observed in the intermediate or basal cells. After 24 h, part of the epithelium had a three-layer structure, indicating that regeneration was taking place. These results demonstrate that TPA selectively affects and desquamates superficial cells in a short period of time. This experimental system may be useful for studying in vivo cell proliferation and/or differentiation.

Cytoskeleton and molecular mechanisms in neurotransmitter release by neurosecretory cells

The process of exocytosis is a fascinating interplay between secretory vesicles and cellular components. Secretory vesicles are true organelles which not only store and protect neurotransmitters from inactivation but also provide the cell with efficient carriers of material for export. Different types of secretory vesicles are described and their membrane components compared. Associations of several cytoplasmic proteins and cytoskeletal components with secretory vesicles and the importance of such associations in the mechanism of secretion are discussed. A description of possible sites of action for Ca2+ as well as possible roles for calmodulin, G-proteins and protein kinase C in secretion are also presented. Important aspects of the cytoskeleton of neurosecretory cells are discussed. The cytoskeleton undergoes dynamic changes as a result of cell stimulation. These changes (i.e. actin filament disassembly) which are a prelude to exocytosis, play a central role in secretion. Moreover, advanced electrophysiological techniques which allow the study of secretory vesicle-plasma membrane fusion in real-time resolution and at the level of the single secretory vesicle, have also provided a better understanding of the secretory process.

Effect of alpha-adrenergic stimulation on activation of protein kinase C and phosphorylation of proteins in intact rabbit hearts

The intracellular events and specifically the role of protein kinase C-mediated protein phosphorylation, after alpha-adrenergic receptor stimulation of the heart, are not well understood. We examined the phosphorylation of sarcolemmal, sarcoplasmic reticular, myofibrillar, and cytosolic proteins in perfused beating rabbit hearts on activation of protein kinase C by phenylephrine. Perfusion of rabbit hearts with phenylephrine was associated with a positive inotropic response, which was dose and time dependent. Maximal stimulation (1.54-fold increase in +dP/dt) was obtained with 10 microM phenylephrine at 4 minutes. Examination of the activity levels of protein kinase C in these hearts revealed a redistribution of this activity from the cytosolic to the membranous fraction, suggesting the activation of this enzyme in vivo. Prazosin, an alpha 1-adrenergic antagonist, prevented the increase in the inotropy and the redistribution of protein kinase C activity mediated by phenylephrine. Examination of the degree of phosphorylation of membranous, myofibrillar, and cytosolic proteins revealed that activation of protein kinase C in vivo was associated with increased phosphorylation of a 15-kd sarcolemmal protein and a 28-kd cytosolic protein. There were no increases in the degree of phosphorylation of phospholamban in the sarcoplasmic reticulum and of troponin I, troponin T, and C protein in the myofibrils, although these proteins were found to be substrates for protein kinase C in vitro. These findings provide evidence that protein kinase C is activated in response to alpha-adrenergic stimulation and that activation is associated with increased phosphorylation of a 15-kd sarcolemmal protein and a 28-kd cytosolic protein in the myocardium.