Arachidonic acid-dependent phosphorylation of troponin I and myosin light chain 2 in cardiac myocytes

Multi-Omics Approach Profiling Metabolic Remodeling in Early Systolic Dysfunction and in Overt Systolic Heart Failure

Metabolic remodeling plays an important role in the pathophysiology of heart failure (HF). We sought to characterize metabolic remodeling and implicated signaling pathways in two rat models of early systolic dysfunction (MOD), and overt systolic HF (SHF). Tandem mass tag-labeled shotgun proteomics, phospho-(p)-proteomics, and non-targeted metabolomics analyses were performed in left ventricular myocardium tissue from Sham, MOD, and SHF using liquid chromatography–mass spectrometry, n = 3 biological samples per group. Mitochondrial proteins were predominantly down-regulated in MOD (125) and SHF (328) vs. Sham. Of these, 82% (103/125) and 66% (218/328) were involved in metabolism and respiration. Oxidative phosphorylation, mitochondrial fatty acid β-oxidation, Krebs cycle, branched-chain amino acids, and amino acid (glutamine and tryptophan) degradation were highly enriched metabolic pathways that decreased in SHF > MOD. Glycogen and glucose degradation increased predominantly in MOD, whereas glycolysis and pyruvate metabolism decreased predominantly in SHF. PKA signaling at the endoplasmic reticulum–mt interface was attenuated in MOD, whereas overall PKA and AMPK cellular signaling were attenuated in SHF vs. Sham. In conclusion, metabolic remodeling plays an important role in myocardial remodeling. PKA and AMPK signaling crosstalk governs metabolic remodeling in progression to SHF.

K ATP channel agonists preserve connexin43 protein in infarcted rats by a protein kinase C-dependent pathway

Downward remodelling of gap junctional proteins between myocytes may trigger ventricular arrhythmia after myocardial infarction. We have demonstrated that ATP-sensitive potassium (K_ATP) channel agonists attenuated post-infarction arrhythmias. However, the involved mechanisms remain unclear. The purpose of this study was to determine whether K_ATP channel agonists can attenuate arrhythmias through preserving protein kinase C (PKC)-–dependent connexin43 level after myocardial infarction. Male Wistar rats after ligating coronary artery were randomized to either vehicle, nicorandil, pinacidil, glibenclamide or a combination of nicorandil and glibenclamide or pinacidil and glibenclamide for 4 weeks. To elucidate the role of PKC in the modulation of connexin43 level, carbachol and myristoylated PKC V1–2 peptide were also assessed. Myocardial connexin43 level was significantly decreased in vehicle-treated infarcted rats compared with sham. Attenuated connexin43 level was blunted after administering K_ATP channel agonists, assessed by immunofluorescent analysis, Western blotting, and real-time quantitative reverse transcription-PCR of connexin43. Arrhythmic scores during programmed stimulation in the K_ATP channel agonists-treated rats were significantly lower than those treated with vehicle. The beneficial effects of K_ATP channel agonists were blocked by either glibenclamide or 5-hydroxydecanoate. Addition of the PKC activator, phorbol 12-myristate 13-acetate and the specific PKC agonist, carbachol, blocked the effects of nicorandil on connexin43 phosphorylation and dye permeability. The specific PKC antagonist, myristoylated PKC V1–2 peptide, did not have additional beneficial effects on connexin43 phosphorylation compared with rats treated with nicorandil alone. Chronic use of K_ATP channel agonists after infarction, resulting in enhanced connexin43 level through a PKC-dependent pathway, may attenuate the arrhythmogenic response to programmed electrical stimulation.

The significance of regulatory light chain phosphorylation in cardiac physiology

It has been over 35 years since the first identification of phosphorylation of myosin light chains in skeletal and cardiac muscle. Yet only in the past few years has the role of these phosphorylations in cardiac dynamics been more fully understood. Advances in this understanding have come about with further evidence on the control mechanisms regulating the level of phosphorylation by kinases and phosphatases. Moreover, studies clarifiying the role of light chain phosphorylation in short and long term control of cardiac contractility and as a factor in cardiac remodeling have improved our knowledge. Especially important in these advances has been the use of gain and loss of function approaches, which have not only testedthe role of kinases and phosphatases, but also the effects of loss of RLC phosphorylation sites. Major conclusions from these studies indicate that (i) two negatively-charged post-translational modifications occupy the ventricular RLC N-terminus, with mouse RLC being doubly phosphorylated (Ser 14/15), and human RLC being singly phosphorylated (Ser 15) and singly deamidated(Asn14/16 to Asp); (ii)a distinct cardiac myosin light kinase (cMLCK) and a unique myosin phosphatase targeting peptide (MYPT2) control phosphoryl group transfer;and (iii) ablation of RLC phosphorylationdecreases ventricular power, lengthens the duration of ventricular ejection, and may also modify other sarcomeric proteins (e.g., troponin I) as substrates for kinases and/or phosphatases. A long term effect of low levels of RLC phosphorylation in mouse models also involves remodeling of the heart with hypertrophy, depressed contractility, and sarcomeric disarray. Data demonstrating altered levels of RLC phosphorylation in comparisons of samples from normal and stressed human hearts indicate the significance of these findings in translational medicine.

Effect of pravastatin on ventricular arrhythmias in infarcted rats: role of connexin43

Epidemiologic studies showed that men treated with statins appear to have a lower incidence of sudden death than men without statins. However, the specific factor for this remained disappointingly elusive. We assessed whether pravastatin enhanced connexin43 expression after myocardial infarction through attenuation of endothelin-1. Twenty-four hours after ligation of the anterior descending artery, male Wistar rats were randomized to vehicle, pravastatin, mevalonate, bosentan, or a combination of pravastatin and mevalonate or pravastatin and bosentan for 4 wk. Myocardial endothelin-1 levels were significantly elevated in vehicle-treated rats at the border zone compared with sham-operated rats. Myocardial connexin43 expression at the border zone was significantly decreased in vehicle-treated infarcted rats compared with sham-operated rats. Attenuated connexin43 expression was blunted after administration of pravastatin, as assessed by immunofluorescence analysis, Western blotting, and real-time quantitative RT-PCR of connexin43. Bosentan enhanced connexin43 amount in infarcted rats and did not have additional beneficial effects on pravastatin-treated rats. Arrhythmic scores during programmed stimulation in vehicle-treated rats were significantly higher than scores in those treated with pravastatin. In contrast, the beneficial effects of pravastatin-induced connexin43 were abolished by the addition of mevalonate and a protein kinase C inducer. In addition, the amount of connexin43 showed significant increase after addition of bisindolylmaleimide, implicating that protein kinase C is a relevant target in endothelin-1-mediated connexin43 expression. Thus chronic use of pravastatin after infarction, resulting in enhanced connexin43 amount by attenuation of mevalonate-dependent endothelin-1 through a protein kinase C-dependent pathway, may attenuate the arrhythmogenic response to programmed electrical stimulation.

Evidence for regulation of mitochondrial function by the L-type Ca2+ channel in ventricular myocytes

The L-type Ca(2+) channel is responsible for initiating contraction in the heart. Mitochondria are responsible for meeting the cellular energy demands and calcium is required for the activity of metabolic intermediates. We examined whether activation of the L-type Ca(2+) channel alone is sufficient to alter mitochondrial function. The channel was activated directly with the dihydropyridine agonist BayK(-) or voltage-clamp of the plasma membrane and indirectly by depolarization of the membrane with high KCl. Activation of the channel increased superoxide production (assessed as changes in dihydroethidium fluorescence), NADH production and metabolic activity (assessed as formation of formazan from tetrazolium) in a calcium-dependent manner. Activation of the channel also increased mitochondrial membrane potential assessed as changes in JC-1 fluorescence. The response was reversible upon inactivation of the channel during voltage-clamp of the plasma membrane and did not appear to require calcium. We examined whether the response may be mediated through movement of cytoskeletal proteins. Depolymerization of actin or exposing cells to a peptide directed against the alpha-interacting domain of the alpha(1C)-subunit of the channel (thereby preventing movement of the beta-subunit) attenuated the increase in mitochondrial membrane potential. We conclude that activation of the L-type Ca(2+) channel can regulate mitochondrial function and the response appears to be modulated by movement through the cytoskeleton.

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.

Negative inotropic effects of endothelin-1 in mouse cardiomyocytes: evidence of a role for Na+-Ca2+ exchange

Endothelin-1 (ET-1) is a peptide hormone produced within the myocardium which may modulate myocardial contractility in a paracrine-autocrine fashion. In the majority of species, ET-1 has a direct positive inotropic effect on the myocardium that involves both increased myofilament Ca(2+) sensitivity and increased Ca(2+) transients. Ca(2+) entry through reverse-mode Na(+)-Ca(2+) exchange, involving both indirect effects via elevation of intracellular [Na(+)] and direct activation of the Na(+)-Ca(2+) exchanger, have been suggested to contribute to the increase in Ca(2+) transients. Conversely, mouse cardiomyocytes show an exclusively negative inotropic response to ET-1. Here, Nishimaru and colleagues present novel evidence that the negative inotropic effect of ET-1 in mouse cardiomyocytes involves both a reduction in myofilament Ca(2+) sensitivity and increased Ca(2+) extrusion, via Na(+)-Ca(2+) exchange. Data obtained using the selective Na(+)-Ca(2+) exchange blocker, SEA0400, suggest that a re-assessment of the role of the exchanger in Ca(2+)-handling by mouse cardiomyocytes may be necessary.

The MLCK-mediated alpha1-adrenergic inotropic effect in atrial myocardium is negatively modulated by PKCepsilon signaling

The present study examined the role of myosin light chain kinase (MLCK), PKC isozymes, and inositol 1,4,5-trisphosphate (IP(3)) receptor in the positive inotropic effect of alpha(1)-adrenergic stimulation in atrial myocardium. We measured inotropic effects of phenylephrine (0.3-300 microM) in isolated left atrial preparations (1 Hz, 37 degrees C, 1.8 mM Ca(2+), 0.3 microM nadolol) from male 8-week FVB mice (n=200). Phenylephrine concentration-dependently increased force of contraction from 1.5+/-0.1 to 2.8+/-0.1 mN (mean+/-s.e.m., n=42), which was associated with increased MLC-2a phosphorylation at serine 21 and 22 by 67% and translocation of PKCepsilon but not PKCalpha to membrane (+30%) and myofilament (+50%) fractions.MLCK inhibition using ML-7 or wortmannin right-shifted the concentration-response curve of phenylephrine, reducing its inotropic effect at 10 microM by 73% and 81%, respectively. The compound KIE1-1 (500 nM), an intracellularly acting PKCepsilon translocation inhibitor peptide, prevented PKCepsilon translocation and augmented the maximal inotropic effect of phenylephrine by 40%. In contrast, inhibition of Ca(2+)-dependent PKC translocation (KIC1-1, 500 nM) had no effect. Chelerythrine, a PKC inhibitor, decreased basal force without changing the inotropic effect of phenylephrine. The IP(3) receptor blocker 2-APB (2 and 20 microM) concentration-dependently decreased basal force, but did not affect the concentration-response curve of phenylephrine. These results indicate that activation of MLCK is required for the positive inotropic effect of alpha(1)-adrenergic stimulation, that the Ca(2+)-independent PKCepsilon negatively modulates this effect, and that PKCalpha and IP(3) receptor activation is not involved.

Differential contribution of troponin I phosphorylation sites to the endothelin-modulated contractile response

Cardiac troponin I is a phosphorylation target for endothelin-activated protein kinase C. Earlier work in cardiac myocytes expressing nonphosphorylatable slow skeletal troponin I provided evidence that protein kinase C-mediated cardiac troponin I phosphorylation accelerates relaxation. However, replacement with the slow skeletal isoform also alters the myofilament pH response and the Ca2+ transient, which could influence endothelin-mediated relaxation. Here, differences in the Ca2+ transient could not explain the divergent relaxation response to endothelin in myocytes expressing cardiac versus slow skeletal troponin I nor could activation of Na+/H+ exchange. Three separate clusters within cardiac troponin I are phosphorylated by protein kinase C, and we set out to determine the contribution of the Thr144 and Ser23/Ser24 clusters to the endothelin-mediated contractile response. Myocyte replacement with a cardiac troponin I containing a Thr144 substituted with the Pro residue found in slow skeletal troponin I resulted in prolonged relaxation in response to acute endothelin compared with control myocytes. Ser23/Ser24 also is a target for protein kinase C phosphorylation of purified cardiac troponin I, and although this cluster was not acutely phosphorylated in intact myocytes, significant phosphorylation developed within 1 h after adding endothelin. Replacement of Ser23/Ser24 with Ala indicated that this cluster contributes significantly to relaxation during more prolonged endothelin stimulation. Overall, results with these mutants provide evidence that Thr144 plays an important role in the acute acceleration of relaxation, whereas Ser23/Ser24 contributes to relaxation during more prolonged activation of protein kinase C by endothelin.

p38 MAP kinase-mediated negative inotropic effect of HIV gp120 on cardiac myocytes

Myocardial dysfunction leading to dilated cardiomyopathy has been documented with surprisingly high frequency in human immunodeficiency virus (HIV)-infected individuals. p38 MAP kinase has been implicated as a mediator of myocardial dysfunction. We previously reported p38 MAP kinase activation by the HIV coat protein gp120 in neonatal rat cardiac myocytes. We now report the direct inotropic effects of HIV gp120 on adult rat ventricular myocytes (ARVM). ARVM were continuously superfused with gp120, and percent fractional shortening (FS) was determined by automated border detection and simultaneous intracellular ionized free Ca2+concentration ([Ca2+]i) measured by fura 2-AM fluorescence: gp120 alone increased FS and increased [Ca2+]iwithin 5 min and then depressed FS without a decrease in [Ca2+]iby 20–60 min, which persisted for at least 2 h. Exposure of ARVM to gp120 also resulted in the phosphorylation of the upstream regulator of p38 MAP kinase MKK3/6, p38 MAP kinase itself, and its downstream effector, ATF-2, over a similar time course. ERK (p44/42) and JNK stress signaling pathways were not similarly activated. The effects of the p38 MAP kinase inhibitor were concentration dependent. SB-203580 (10 μM) blocked both p38 MAP kinase phosphorylation and the delayed negative inotropic effect of gp120. SB-203580 (5 μM) selectively blocked phosphorylation of ATF-2 without blocking the phosphorylation of MKK3/6 or p38 MAP kinase itself. SB-203580 (5 μM) administered before, with, or after gp120 blocked the negative inotropic effect of gp120 in ARVM. p38 MAP kinase activation may be a common stress-response mechanism contributing to myocardial dysfunction in HIV and other nonischemic as well as ischemic cardiomyopathies.

Protein kinase C and A sites on troponin I regulate myofilament Ca2+ sensitivity and ATPase activity in the mouse myocardium

Cardiac troponin I (cTnI) is a phosphoprotein subunit of the troponin-tropomyosin complex that is thought to inhibit cardiac muscle contraction during diastole. To investigate the contributions of cTnI phosphorylation to cardiac regulation, transgenic mice were created with the phosphorylation sites of cTnI mutated to alanine. Activation of protein kinase C (PKC) by perfusion of hearts with phorbol-12-myristate-13-acetate (PMA) or endothelin-1 (ET-1) inhibited the maximum ATPase rate by up to 25 % and increased the Ca2+ sensitivity of ATPase activity and of isometric tension by up to 0.15 pCa units. PKC activation no longer altered cTnI phosphorylation, depressed ATPase rates or enhanced myofilament Ca2+ sensitivity in transgenic mice expressing cTnI that could not be phosphorylated on serines43/45 and threonine144 (PKC sites). Modest changes in myosin regulatory light chain phosphorylation occurred in all mouse lines, but increases in myofilament Ca2+ sensitivity required the presence of phosphorylatable cTnI. For comparison, the beta-adrenergic agonist isoproterenol caused a 38 % increase in maximum ATPase rate and a 0.12 pCa unit decrease in myofilament Ca2+ sensitivity. These beta-adrenergic effects were absent in transgenic mice expressing cTnI that could not be phosphorylated on serines23/24 (protein kinase A, PKA, sites). Overall, the results indicate that PKC and PKA exert opposing effects on actomyosin function by phosphorylating cTnI on distinct sites. A primary role of PKC phosphorylation of cTnI may be to reduce the requirements of the contractile apparatus for both Ca2+ and ATP, thereby promoting efficient ATP utilisation during contraction.

Role of troponin I phosphorylation in protein kinase C-mediated enhanced contractile performance of rat myocytes

Our goal was to define the role of phosphorylated cardiac troponin-I in the adult myocyte contractile performance response to activated protein kinase C. In agreement with earlier work, endothelin enhanced both adult rat myocyte contractile performance and cardiac troponin-I phosphorylation. Protein kinase C participated in both responses. The role of cardiac troponin-I phosphorylation in the contractile function response to protein kinase C was further investigated using gene transfer into myocytes of troponin-I isoforms/mutants lacking one or more phosphorylation sites previously identified in purified cardiac troponin-I. Sarcomeric replacement with slow skeletal troponin-I-abrogated protein kinase C-mediated troponin-I phosphorylation. In functional studies, endothelin slowed relaxation in myocytes expressing slow skeletal troponin-I, while the relaxation rate increased in myocytes expressing cardiac troponin-I. Based on these results, acceleration of myocyte relaxation during protein kinase C activation largely depended on cardiac troponin-I phosphorylation. Experiments with troponin-I isoform chimeras provided evidence that phosphorylation sites in the amino portion of cardiac troponin I-mediated the protein kinase C acceleration of relaxation. The cardiac troponin-I Thr-144 phosphorylation site identified in earlier biochemical studies was not significantly phosphorylated during the acute contractile response. Thus, amino-terminal protein kinase C-dependent phosphorylation sites in cardiac troponin-I are likely responsible for the accelerated relaxation observed in adult myocytes.

An epsilonPKC-selective inhibitor attenuates back phosphorylation of a low molecular weight protein in cardiac myocytes

We have studied epsilon PKC-mediated phosphorylation events in neonatal cardiac myocytes using back phosphorylation. 3 nM 4-beta 12-myristate-13-acetate (PMA)-intact cell treatment preferentially activates epsilon PKC in these cells (Circ. Res. 76 (1995) 654) and caused decreased 32P incorporation (back phosphorylation) into an approximately 18-kDa protein. This response required physiological levels of free Mg(2+) and short (3-5 min) incubation periods in back phosphorylation assays. Introduction of a selective epsilon PKC translocation inhibitor (epsilon V1) into these cells attenuated the 3 nM PMA-induced back phosphorylation response while translocation inhibitors to the classical PKC or deltaPKC isozymes were without effect. Pretreatment of our cells with endothelin-1 (ET1) had similar effects to 3 nM PMA albeit the magnitude of the ET1 back phosphorylation response was about one-half that of 3 nM PMA. Our results suggest that epsilon PKC phosphorylates an approximately 18-kDa protein found in the particulate cell fraction of neonatal cardiac myocytes. Epsilon PKC modulates diverse cardiac responses including contraction, ion channel functions, hypertrophy, and ischemic preconditioning. Characterization of epsilon PKC-selective phosphotransferase events may reveal novel regulatory mechanisms for this enzyme in neonatal cardiac myocytes.

Quantitation and distribution of beta-tubulin in human cardiac myocytes

Increasing evidence suggests that derangements of cytoskeletal proteins contribute to alterations in intracellular signaling, myocyte function, and the coupling of myocytes to the extracellular matrix during cardiac hypertrophy and failure. Data from animal studies have shown an increased density of beta-tubulin protein in the right or left ventricle subjected to pressure overload, and have demonstrated that interfering with excess polymerization of beta-tubulin improves contractility. We tested the hypothesis that beta-tubulin is increased in human left ventricular hypertrophy and end-stage heart failure. Confocal microscopy of fluorescently labeled beta-tubulin protein revealed an increased density of the beta-tubulin network in cardiomyocytes from both hypertrophied and failing human hearts as compared to cells from nonfailing hearts. Western blot analysis on total heart homogenate showed no change in beta-tubulin when data were normalized to either actin or calsequestrin, although there was a significant increase in failing human hearts when data were normalized only for a constant amount of protein per heart. The mRNA for beta-tubulin was not changed in hypertrophied hearts, but was significantly decreased in failing human hearts. Thus, similar to animal models, we have shown that the density of the microtubular network within the cardiomyocyte is increased in end-stage failing human hearts. We have also shown for the first time that beta-tubulin density is increased in cells from hypertrophied human hearts. Although the functional implications of this finding in the human heart remain to be explored, data from animal studies suggest that increased beta-tubulin protein contributes to cardiac dysfunction.

Troponin I serines 43/45 and regulation of cardiac myofilament function

We studied Ca(2+) dependence of tension and actomyosin ATPase rate in detergent extracted fiber bundles isolated from transgenic mice (TG), in which cardiac troponin I (cTnI) serines 43 and 45 were mutated to alanines (cTnI S43A/S45A). Basal phosphorylation levels of cTnI were lower in TG than in wild-type (WT) mice, but phosphorylation of cardiac troponin T was increased. Compared with WT, TG fiber bundles showed a 13% decrease in maximum tension and a 20% increase in maximum MgATPase activity, yielding an increase in tension cost. Protein kinase C (PKC) activation with endothelin (ET) or phenylephrine plus propranolol (PP) before detergent extraction induced a decrease in maximum tension and MgATPase activity in WT fibers, whereas ET or PP increased maximum tension and stiffness in TG fibers. TG MgATPase activity was unchanged by ET but increased by PP. Measurement of protein phosphorylation revealed differential effects of agonists between WT and TG myofilaments and within the TG myofilaments. Our results demonstrate the importance of PKC-mediated phosphorylation of cTnI S43/S45 in the control of myofilament activation and cross-bridge cycling rate.

Transforming growth factor beta-1 attenuates endothelin-1-induced functions in neonatal cardiac myocytes

In the present study we characterized a "crosstalk" mechanism between transforming growth factor beta-1 (TGF beta-1) and endothelin-1 (ET1) signaling pathways in neonatal cardiac myocytes. A 5 minute pretreatment with 1 ng/ml concentrations of TGF beta-1 attenuated ET1-induced negative chronotropic effects and translocation of the alpha, delta and varepsilonPKC isozymes to the particulate cell fraction. We found no effect of TGF beta-1 on responses induced by the P(2) purinergic agonist ATP or phorbol ester. Treatment of cardiac myocytes with acidic fibroblast growth factor (aFGF) did not alter ET1- or ATP-mediated effects on contraction rate or translocation of PKC isozymes to the particulate fraction. Our studies suggest that TGF beta-1 may act as a negative modulator of ET1- but not ATP- or phorbol ester-induced PKC isozyme signaling events in neonatal cardiac myocytes. A better understanding of the complex ET1 and TGF beta-1 signaling mechanisms in neonatal heart cells should enhance our knowledge regarding the interplay between these pathways.

Effects of endothelin-1 on K(+) currents from rat ventricular myocytes

It has been suggested that the positive inotropic effect of the vasoactive peptide hormone, endothelin-1 (ET-1), involves inhibition of cardiac K(+) currents. In order to identify the K(+) currents modulated by ET-1, the outward K(+) currents of isolated rat ventricular myocytes were investigated using whole-cell patch-clamp recording techniques. Outward currents were elicited by depolarisation to +40 mV for 200 ms from the holding potential of -60 mV. Currents activated rapidly, reaching a peak (I(pk)) of 1310 +/- 115 pA and subsequently inactivating to an outward current level of 1063 +/- 122 pA at the end of the voltage-pulse (I(late)) (n = 11). ET-1 (20 nM) reduced I(pk) by 247.6 +/- 60.7 pA (n = 11, P < 0.01) and reduced I(late) by 323.2 +/- 43.9 pA (P < 0.001). The effects of ET-1 were abolished in the presence of the nonselective ET receptor antagonist, PD 142893 (10 microM, n = 5). Outward currents were considerably reduced and the effects of ET-1 were not observed when K(+) was replaced with Cs(+) in the experimental solutions; this indicates that ET-1 modulated K(+)-selective currents. A double-pulse protocol was used to investigate the inactivation of the currents. The voltage-dependent inactivation of the currents from potentials positive to -80 mV was fitted by a Boltzmann equation revealing the existence of an inactivating transient outward component (I(to)) and a noninactivating steady-state component (I(ss)). ET-1 markedly inhibited I(ss) by 43.0 +/- 3.8% (P < 0.001, n = 7) and shifted the voltage-dependent inactivation of I(to) by +3.3 +/- 1.2 mV (P < 0.05). Although ET-1 had little effect on the onset of inactivation of the currents elicited from a conditioning potential of -70 mV, the time-independent noninactivating component of the currents was markedly inhibited. In conclusion, the predominant effect of ET-1 was to inhibit a noninactivating steady-state background K(+) current (I(ss)). These results are consistent with the hypothesis that I(ss) inhibition contributes to the inotropic effects of ET-1.

The ischemic heart--experimental models

Results obtained by experimental studies of the ischemic heart have been of tremendous importance for the understanding of physiology, biochemistry and lately also the molecular genetics of the heart. Experimental models in use for the study of the ischemic heart involve studies on the integrated organism, experiments with isolated hearts or multicellular preparation, and also studies of cells isolated from the heart. Regional ischemia in the anaesthetized animal has been a standard model. Knowledge about infarct size limitation as well as heart function in acute and chronic ischemia has been obtained based on experiments in a wide variety of species. The isolated perfused heart has been subjected to extensive use. As a result, the understanding of intracellular processes is constantly developing. Cell models and transgenic-mice models represent promising additions. Each model and each species has certain advantages and disadvantages. Variability in susceptibility towards ischemia and reperfusion is also present. The consequences of ischemia can be described as contractile dysfunction and stunning, arrhythmia and infarction each representing different endpoints of injury. The experimental model is also heavily dependent on the endpoint that is chosen for the study. Results obtained in one experimental model can, therefore, not be generalized into universal conclusions about the ischemic heart. With respect to the human and the disease caused by myocardial ischemia, fragments of knowledge put together from different types of experimental models create the background for successful design of potential treatment.

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.

Troponin I phosphorylation and myofilament calcium sensitivity during decompensated cardiac hypertrophy

We have measured myocyte cell shortening, troponin-I (Tn-I) phosphorylation, Ca2+ dependence of actomyosin adenosinetriphosphatase (ATPase) activity, adenosine 3',5'-cyclic monophosphate (cAMP) levels, and myofibrillar isoform expression in the spontaneously hypertensive rat (SHR) during decompensated cardiac hypertrophy (76 wk old) and in age-matched Wistar-Kyoto rat (WKY) controls. The decreased inotropic response to beta-adrenergic stimulation previously observed in myocytes from 26-wk-old SHR was further reduced at 76 wk of age. In response to beta-adrenergic stimulation, Tn-I phosphorylation was greater in the 76-wk-old SHR than in the WKY, although cAMP-dependent protein kinase A (PKA)-dependent Tn-I phosphorylation in the SHR did not increase with progression from compensated (26 wk) to decompensated (76 wk) hypertrophy. We also observed a dissociation between the increased PKA-dependent Tn-I phosphorylation and decreased cAMP levels in the 76-wk-old SHR versus WKY during beta-adrenergic stimulation. Baseline Tn-I phosphorylation was significantly reduced in 76-wk-old SHR versus WKY and was associated with decreased basal cAMP levels and increased Ca2+ sensitivity of actomyosin ATPase activity. The change in myofilament Ca2+ sensitivity during beta-adrenergic stimulation in the 76-wk-old SHR (0.65 pCa units) was over twofold greater than in the 76-wk-old WKY (0.30 pCa units). We also determined whether embryonic troponin T isoforms were reexpressed in decompensated hypertrophy and observed significant reexpression of the embryonic cardiac troponin T isoforms in the 76-wk-old SHR. The significant decrease in Ca2+ sensitivity with beta-adrenergic stimulation in 76-wk-old SHR may contribute to the severely impaired inotropic response during decompensated hypertrophy in the SHR.

Mechanism of action of endothelin in rat cardiac muscle: cross-bridge kinetics and myosin light chain phosphorylation

1. The molecular mechanism of inotropic action of endothelin was investigated in rat ventricular muscle by studying its effects on characteristics of isometric twitch, barium-induced steady contracture and the level of incorporation of 32Pi into myosin light chain 2. 2. Exposure of rat papillary muscle to endothelin caused an increase in isometric twitch force but did not alter the twitch-time parameters. 3. Endothelin did not significantly change the maximum contracture tension but did cause an increase in contracture tension at submaximal levels of activation, without changes in the tension-to-stiffness ratio and kinetics of attached cross-bridges. Kinetics of attached cross-bridges were deduced during steady contracture from complex-stiffness values, and in particular from the frequency at which muscle stiffness assumes a minimum value, fmin. Endothelin did not alter fmin. 4. Endothelin caused an increase in the level of incorporation of 32Pi into myosin light chain 2 without a concurrent change in the level of incorporation of 32Pi into troponin I. 5. We conclude that the inotropic action of endothelin is not due to an increase in the kinetics of attached cross-bridges, nor due to a change in the force per unit cross-bridge, but may result from an increased divalent cation sensitivity caused by elevated myosin light chain 2 phosphorylation, resembling post-tetanic potentiation in fast skeletal muscle fibres.

Arachidonic acid drives mini-glucagon action in cardiac cells

Recent studies have shown that glucagon is processed by cardiac cells into its COOH-terminal (19-29) fragment, mini-glucagon, and that this metabolite is an essential component of the contractile positive inotropic effect of glucagon (Sauvadet, A., Rohn, T., Pecker, F. and Pavoine, C. (1996) Circ. Res. 78, 102-109). We now show that mini-glucagon triggers arachidonic acid (AA) release from [3H]AA-loaded embryonic chick ventricular myocytes via the activation of a phospholipase A2 sensitive to submicromolar Ca2+ concentrations. The phospholipase A2 inhibitor, AACOCF3, prevented mini-glucagon-induced [45Ca2+] accumulation into the sarcoplasmic reticulum, but inhibitors of lipoxygenase, cyclooxygenase, or epoxygenase pathways were ineffective. AA applied exogenously, at 0. 3 microM, reproduced the effects of mini-glucagon on Ca2+ homeostasis and contraction. Thus AA: (i) caused [45Ca2+] accumulation into a sarcoplasmic reticulum compartment sensitive to caffeine; 2) potentiated caffeine-induced Ca2+ mobilization from cells loaded with Fura-2; 3) acted synergistically with glucagon or cAMP to increase both the amplitude of Ca2+ transients and contraction of electrically stimulated cells. AA action was dose-dependent and specific since it was mimicked by its non-hydrolyzable analog 5,8,11,14-eicosatetraynoic acid but not reproduced by other lipids such as, arachidic acid, linolenic acid, cis-5,8,11,14,17-eicosapentaenoic acid, cis-4,7,10,13,16, 19-docosahexaenoic acid, or arachidonyl-CoA, even in the micromolar range. We conclude that AA drives mini-glucagon action in the heart and that the positive inotropic effect of glucagon on heart contraction relies on both second messengers, cAMP and AA.

Myofibrillar calcium sensitivity of isometric tension is increased in human dilated cardiomyopathies: role of altered beta-adrenergically mediated protein phosphorylation

To examine the role of alterations in myofibrillar function in human dilated cardiomyopathies, we determined isometric tension-calcium relations in permeabilized myocytesized myofibrillar preparations (n = 16) obtained from left ventricular biopsies from nine patients with dilated cardiomyopathy (DCM) during cardiac transplantation or left ventricular assist device implantation. Similar preparations (n = 10) were obtained from six normal hearts used for cardiac transplantation. Passive and maximal Ca2+-activated tensions were similar for the two groups. However, the calcium sensitivity of isometric tension was increased in DCM compared to nonfailing preparations ([Ca2+]50=2.46+/-0.49 microM vs 3.24+/-0.51 microM, P < 0.001). In vitro treatment with the catalytic subunit of protein kinase A (PKA) decreased calcium sensitivity of tension to a greater degree in failing than in normal preparations. Further, isometric tension-calcium relations in failing and normal myofibrillar preparations were similar after PKA treatment. These findings suggest that the increased calcium sensitivity of isometric tension in DCM may be due at least in part to a reduction of the beta-adrenergically mediated (PKA-dependent) phosphorylation of myofibrillar regulatory proteins such as troponin I and/or C-protein.