Differential effects of pH on calcium activation of myofilaments of adult and perinatal dog hearts. Evidence for developmental differences in thin filament regulation

Myofilament calcium sensitization delays decompensated hypertrophy differently between the sexes following myocardial infarction

Contractile dysfunction is common to many forms of cardiovascular disease. Approaches directed at enhancing cardiac contractility at the level of the myofilaments during heart failure (HF) may provide a means to improve overall cardiovascular function. We are interested in gender-based differences in cardiac function and the effect of sarcomere activation agents that increase contractility. Thus, we studied the effect of gender and time on integrated arterial-ventricular function (A-V relationship) following myocardial infarction (MI). In addition, transgenic mice that overexpress the slow skeletal troponin I isoform were used to determine the impact of increased myofilament Ca(2+) sensitivity following MI. Based on pressure-volume (P-V) loop measurements, we used derived parameters of cardiovascular function to reveal the effects of sex, time, and increased myofilament Ca(2+) sensitivity among groups of post-MI mice. Analysis of the A-V relationship revealed that the initial increase was similar between the sexes, but the vascular unloading of the heart served to delay the decompensated stage in females. Conversely, the vascular response at 6 and 10 wk post-MI in males contributed to the continuous decline in cardiovascular function. Increasing the myofilament Ca(2+) sensitivity appeared to provide sufficient contractile support to improve contractile function in both male and female transgenic mice. However, the improved contractile function was more beneficial in males as the concurrent vascular response contributed to a delayed decompensated stage in female transgenic mice post-MI. This study represents a quantitative approach to integrating the vascular-ventricular relationship to provide meaningful and diagnostic value following MI. Consequently, the data provide a basis for understanding how the A-V relationship is coupled between males and females and the enhanced ability of the cardiovascular system to tolerate pathophysiological stresses associated with HF in females.

Heart failure-associated changes in RNA splicing of sarcomere genes

BACKGROUND

Alternative mRNA splicing is an important mechanism for regulation of gene expression. Altered mRNA splicing occurs in association with several types of cancer, and a small number of disease-associated changes in splicing have been reported in heart disease. However, genome-wide approaches have not been used to study splicing changes in heart disease. We hypothesized that mRNA splicing is different in diseased hearts compared with control hearts.

METHODS AND RESULTS

We used the Affymetrix Exon array to globally evaluate mRNA splicing in left ventricular myocardial RNA from controls (n=15) and patients with ischemic cardiomyopathy (n=15). We observed a broad and significant decrease in mRNA splicing efficiency in heart failure, which affected some introns to a greater extent than others. The profile of mRNA splicing separately clustered ischemic cardiomyopathy and control samples, suggesting distinct changes in mRNA splicing between groups. Reverse transcription-polymerase chain reaction validated 9 previously unreported alternative splicing events. Furthermore, we demonstrated that splicing of 4 key sarcomere genes, cardiac troponin T (TNNT2), cardiac troponin I (TNNI3), myosin heavy chain 7 (MYH7), and filamin C, gamma (FLNC), was significantly altered in ischemic cardiomyopathy and in dilated cardiomyopathy and aortic stenosis. In aortic stenosis samples, these differences preceded the onset of heart failure. Remarkably, the ratio of minor to major splice variants of TNNT2, MYH7, and FLNC classified independent test samples as control or disease with >98% accuracy.

CONCLUSIONS

Our data indicate that mRNA splicing is broadly altered in human heart disease and that patterns of aberrant RNA splicing accurately assign samples to control or disease classes.

Differential effects of a green tea-derived polyphenol (-)-epigallocatechin-3-gallate on the acidosis-induced decrease in the Ca(2+) sensitivity of cardiac and skeletal muscle

(-)-Epigallocatechin-3-gallate (EGCg), a green tea-derived polyphenol, has received much attention as a protective agent against cardiovascular diseases. In this study, we determined its effects on the acidosis-induced change in the Ca(2+) sensitivity of myofilaments in myofibrils prepared from porcine ventricular myocardium and chicken pectoral muscle. EGCg (0.1 mM) significantly inhibited the decrease caused by lowering the pH from 7.0 to 6.0 in the Ca(2+) sensitivity of myofibrillar ATPase activity in cardiac muscle, but not in skeletal muscle. Studies on recombinant mouse cardiac troponin C (cTnC) and chicken fast skeletal troponin C (sTnC) using circular dichroism and intrinsic and extrinsic fluorescence spectroscopy showed that EGCg bound to cTnC with a dissociation constant of approximately 3-4 muM, but did not bind to sTnC. By presumably binding to the cTnC C-lobe, EGCg decreased Ca(2+) binding to cTnC and overcame the depressant effect of protons on the Ca(2+) sensitivity of the cardiac contractile response. To demonstrate isoform-specific effects of the action of EGCg, the pH sensitivity of the Ca(2+) response was examined in cardiac myofibrils in which endogenous cTnC was replaced with exogenous sTnC or cTnC and in skeletal myofibrils in which the endogenous sTn complex was replaced with whole cardiac Tn complex (cTn). The results suggest that the binding of EGCg to the cardiac isoform-specific TnC or Tn complex alters the effect of pH on myofilament Ca(2+) sensitivity in striated muscle.

Troponin and cardiomyopathy

The troponin complex was discovered over thirty years ago and since then much insight has been gained into how this complex senses fluctuating levels of Ca(2+) and transmits this signal to the myofilament. Advances in genetics methods have allowed identification of mutations that lead to the phenotypically distinct cardiomyopathies: hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM) and dilated cardiomyopathy (DCM). This review serves to highlight key in vivo studies of mutation effects that have followed many years of functional studies and discusses how these mutations alter energetics and promote the characteristic remodeling associated with cardiomyopathic diseases. Studies have been performed that examine alterations in signaling and genomic methods have been employed to isolate upregulated proteins, however these processes are complex as there are multiple roads to hypertrophy or dilation associated with genetic cardiomyopathies. This review suggests future directions to explore in the troponin field that would heighten our understanding of the complex regulation of cardiac muscle contraction.

Single amino acid substitutions define isoform-specific effects of troponin I on myofilament Ca2+ and pH sensitivity

Troponin I isoforms play a key role in determining myofilament Ca2+ sensitivity in cardiac muscle. The goal here was to identify domain clusters and residues that confer troponin I isoform-specific myofilament Ca2+ and pH sensitivities of contraction. Key domains/residues that contribute to troponin I isoform-specific Ca2+ and pH sensitivity were studied using gene transfer of a slow skeletal troponin I (ssTnI) template, with targeted cardiac troponin I (cTnI) residue substitutions. Substitutions in ssTnI with cognate cTnI residues R125Q, H132A, and V134E, studied both independently and together (ssTnIQAE), resulted in efficient stoichiometric replacement of endogenous myofilament cTnI in adult cardiac myocytes. In permeabilized myocytes, the pCa50 of tension ([Ca2+] required for half maximal force), and the acidosis-induced rightward shift of pCa50 were converted to the cTnI phenotype in myocytes expressing ssTnIQAE or ssTnIH132A, and there was no functionally additive effect of ssTnIQAE versus ssTnIH132A. Interestingly, only the acidosis-induced shift in Ca2+ sensitivity was comparable to cTnI in myocytes expressing ssTnIV134E, while ssTnIR125Q fully retained the ssTnI phenotype. An additional ssTnIN141H substitution, which lies within the same structural region of TnI as V134, produced a shift in myofilament Ca2+ sensitivity comparable to cTnI at physiological pH, while the acidic pH response was similar to the effect of wild-type ssTnI. Analysis of sarcomere shortening in intact adult cardiac myocytes was consistent with the force measurements. Targeted substitutions in the carboxyl portion of TnI produced residue-specific influences on myofilament Ca2+ and pH sensitivity of force and give new molecular insights into the TnI isoform dependence of myofilament function.

Heart failure, ischemia/reperfusion injury and cardiac troponin

Over the forty years since its discovery, there has been a profound transition in thinking with regard to the role of troponin in the control of cardiac function. This transition involved a change in perception oftroponin as a passive molecular switch responding to membrane controlled fluctuations in cytoplasmic Ca2+ to a perception of troponin as a critical element in signaling cascades that actively engage in control of cardiac function. Evidence demonstrating functionally significant developmental and mutant isoform switches and post-translational modifications of cardiac troponin complex proteins, troponin I (cTnI) and troponin T (cTnT) provided convincing evidence for a more complicated role of troponin in control of cardiac function and dynamics. The physiological role of these modifications of troponin is reviewed in this monograph and has also been reviewed elsewhere (Solaro and Rarick, 1998; Gordon et al., 2000; Solaro et al., 2002a; Kobayashi and Solaro, 2005). Our focus here is on studies related to modifications in troponin that appear important in the processes leading from compensated hypertrophy to heart failure. These studies reveal the potentially significant role of post-translational modifications of troponin in these processes. Another focus is on troponin as a target for inotropic agents. Pharmacological manipulation of troponin by small molecules remains an important avenue of approach for the treatment of acute and chronic heart failure (Kass and Solaro, 2006).

Beta-myosin heavy chain myocytes are more resistant to changes in power output induced by ischemic conditions

During ischemia intracellular concentrations of P(i) and H+ increase. Also, changes in myosin heavy chain (MHC) isoform toward beta-MHC have been reported after ischemia and infarction associated with coronary artery disease. The purpose of this study was to investigate the effects of myoplasmic changes of P(i) and H+ on the loaded shortening velocity and power output of cardiac myocytes expressing either alpha- or beta-MHC. Skinned cardiac myocyte preparations were obtained from adult male Sprague-Dawley rats (control or treated with 5-n-propyl-2-thiouracil to induce beta-MHC) and mounted between a force transducer and servomotor system. Myocyte preparations were subjected to a series of isotonic force clamps to determine shortening velocity and power output during Ca2+ activations in each of the following solutions: 1) pCa 4.5 and pH 7.0; 2) pCa 4.5, pH 7.0, and 5 mM P(i); 3) pCa 4.5 and pH 6.6; and 4) pCa 4.5, pH 6.6, and 5 mM P(i). Added P(i) and lowered pH each caused isometric force to decline to the same extent in alpha-MHC and beta-MHC myocytes; however, beta-MHC myocytes were more resistant to changes in absolute power output. For example, peak absolute power output fell 53% in alpha-MHC myocytes, whereas power fell only 38% in beta-MHC myocytes in response to elevated P(i) and lowered pH (i.e., solution 4). The reduced effect on power output was the result of a greater increase in loaded shortening velocity induced by P(i) in beta-MHC myocytes and an increase in loaded shortening velocity at pH 6.6 that occurred only in beta-MHC myocytes. We conclude that the functional response to elevated P(i) and lowered pH during ischemia is MHC isoform-dependent with beta-MHC myocytes being more resistant to declines in power output.

Characterization of Troponin T Dilated Cardiomyopathy Mutations in the Fetal Troponin Isoform

The major goal of this study was to elucidate how troponin T (TnT) dilated cardiomyopathy (DCM) mutations in fetal TnT and fetal troponin affect the functional properties of the fetal heart that lead to infantile cardiomyopathy. The DCM mutations R141W and DeltaK210 were created in the TnT1 isoform, the primary isoform of cardiac TnT in the embryonic heart. In addition to a different TnT isoform, a different troponin I (TnI) isoform, slow skeletal TnI (ssTnI), is the dominant isoform in the embryonic heart. In skinned fiber studies, TnT1-wild-type (WT)-treated fibers reconstituted with cardiac TnI.troponin C (TnC) or ssTnI.TnC significantly increased Ca(2+) sensitivity of force development when compared with TnT3-WT-treated fibers at both pH 7.0 and pH 6.5. Porcine cardiac fibers treated with TnT1 that contained the DCM mutations (R141W and DeltaK210), when reconstituted with either cardiac TnI.TnC or ssTnI.TnC, significantly decreased Ca(2+) sensitivity of force development compared with TnT1-WT at both pH values. The R141W mutation, which showed no significant change in the Ca(2+) sensitivity of force development in the TnT3 isoform, caused a significant decrease in the TnT1 isoform. The DeltaK210 mutation caused a greater decrease in Ca(2+) sensitivity and maximal isometric force development compared with the R141W mutation in both the fetal and adult TnT isoforms. When complexed with cardiac TnI.TnC or ssTnI.TnC, both TnT1 DCM mutations strongly decreased maximal actomyosin ATPase activity as compared with TnT1-WT. Our results suggest that a decrease in maximal actomyosin ATPase activity in conjunction with decreased Ca(2+) sensitivity of force development may cause a severe DCM phenotype in infants with the mutations.

Troponin T isoforms alter the tolerance of transgenic mouse cardiac muscle to acidosis

Troponin T (TnT) is an essential protein in the Ca2+ regulatory system of striated of muscle. Three fiber type-specific TnT genes have evolved in higher vertebrates to encode cardiac, slow and fast skeletal muscle TnT isoforms. To understand the functional significance of TnT isoforms, we studied the effects of acidosis on the contractility of transgenic mouse cardiac muscle that expresses fast skeletal muscle TnT. Contractility analysis of intact cardiac muscle strips showed that while no differences were detected at physiological pH, the transgenic cardiac muscle had significantly greater decreases in +dF/dtmax at acidic pH than that of the wild-type control. Contractility of skinned cardiac muscles demonstrated that the presence of fast TnT resulted in significantly larger decreases in force and Ca2+ sensitivity at acidic pH than that of the wild-type control. The effect of TnT isoforms on the tolerance of muscle to acidosis may explain the higher tolerance of embryonic versus adult cardiac muscles. The results are consistent with the hypothesis that charge differences in TnT isoforms contribute to the contractility of muscle. The data further support a hypothesis that slow TnT is similar to the cardiac, but not fast, and TnT may contribute to the higher tolerance of slow muscles to stress conditions. Therefore, TnT isoform diversity may contribute to the compatibility of muscle thin filaments to cellular environments in different fiber types, during development and functional adaptation.

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.

Cardiac troponin T isoforms demonstrate similar effects on mechanical performance in a regulated contractile system

Alteration of troponin T (TnT) isoform expression has been reported in human and animal models of myocardial failure. The two adult beef cardiac TnT isoforms (TnT(3) and TnT(4)) were isolated for comparative functional analysis. Thin filaments were reconstituted containing pure populations of the isoforms. The in vitro motility assay was used to directly compare the effect of the two TnT isoforms on force and unloaded shortening as a function of free calcium. We found no significant differences between the two isoforms in terms of calcium sensitivity, cooperativity, or maximal activation (velocity and force) as assessed in a fully calcium-regulated system. Activation by myosin strong binding was similar for thin filaments containing either of the two TnT isoforms. Whereas maximally activated velocity and cooperativity was depressed at pH 6.5, no difference between thin filaments containing the two isoforms was detected. From the small magnitude of the TnT isoform shifts detected in myocardial failure and the lack of significant mechanical effect detected in the motility assay, variable TnT isoform expression is unlikely to be any functional significance in heart failure.

Expression of slow skeletal troponin I in adult transgenic mouse heart muscle reduces the force decline observed during acidic conditions

1. Acidosis in cardiac muscle is associated with a decrease in developed force. We hypothesized that slow skeletal troponin I (ssTnI), which is expressed in neonatal hearts, is responsible for the observed decreased response to acidic conditions. To test this hypothesis directly, we used adult transgenic (TG) mice that express ssTnI in the heart. Cardiac TnI (cTnI) was completely replaced by ssTnI either with a FLAG epitope introduced into the N-terminus (TG-ssTnI) or without the epitope (TG-ssTnI) in these mice. TG mice that express cTnI were also generated as a control TG line (TG-cTnI). Non-transgenic (NTG) littermates were used as controls. 2. We measured the force-calcium relationship in all four groups at pH 7.0 and pH 6.5 in detergent-extracted fibre bundles prepared from left ventricular papillary muscles. The force-calcium relationship was identical in fibre bundles from NTG and TG-cTnI mouse hearts, therefore NTG mice served as controls for TG-ssTnIand TG-ssTnI mice. Compared to NTG controls, the force generated by fibre bundles from TG mice expressing ssTnI was more sensitive to Ca(2+). The shift in EC(50) (the concentration of Ca(2+) at which half-maximal force is generated) caused by acidic pH was significantly smaller in fibre bundles isolated from TG hearts compared to those from NTG hearts. However, there was no difference in the force-calcium relationship between hearts from the TG-ssTnIand TG-ssTnI groups. 3. We also isolated papillary muscles from the right ventricle of NTG and TG mouse hearts expressing ssTnI and measured isometric force at extracellular pH 7.33 and pH 6.75. At acidic pH, after an initial decline, twitch force recovered to 60 +/- 3 % (n = 7) in NTG papillary muscles, 98 +/- 2 % (n = 5) in muscles from TG-ssTnIand 96 +/- 3 % (n = 7) in muscles from TG-ssTnI hearts. Our results indicate that TnI isoform composition plays a crucial role in the determination of myocardial force sensitivity to acidosis.

Localization of regions of troponin I important in deactivation of cardiac myofilaments by acidic pH

Ca2+-activation of cardiac muscle myofilaments is more sensitive to depression by acidic pH than is the case with skeletal myofilaments. We tested the hypothesis that this difference is related to specific regions of the TnI (troponin I) isoforms in these muscles. We exchanged native Tn complex in detergent-extracted fiber bundles from mouse ventricles with Tn containing various combinations of fast (fsTnI) or slow skeletal (ssTnI) complexed with either cardiac TnC (cTnC) or fsTnC, and with cTnC complexed with the following chimeras: (1) fsTnI N-terminal region (fN) plus cTnI inhibitory peptide (cIp) and cTnI C-terminal region (cC); and (2) cTnI N-terminal region (cN)-cIp-fsTnI C-terminal region (fC). We determined the change in half maximal Ca2+(DeltaEC50) for tension activation at pH 7.0 and pH 6.5. Similar DeltaEC50 values were obtained for unextracted controls (5.53+/-0.30 microm), for preparations containing cTnI-cTnC (5.74+/-0.40 microm), and preparations exchanged with cTnI-fsTnC (5.63+/-0.40 microm). However, replacement of cTnI with fsTnI significantly decreased DeltaEC50 to 3.95+/-0.17 microm. Replacement of cTnI with ssTnI also significantly depressed DeltaEC50 to 2.07+/-0.15 microm. Results of studies using the chimeras demonstrated that the C-terminal domains of cTnI and fsTnI are responsible for these differences. This conclusion also fits with data from experiments in which we measured Ca2+-binding to the regulatory site of cTnC in binary complexes containing cTnC with cTnI, fsTnI, or the chimeras. Our results localize a region of TnI important in effects of acidosis on cardiac myofilaments and extend our earlier data indicating that C-terminal regions of cTnI outside the Ip are critical for activation by Ca2+.

A pH-sensitive interaction of troponin I with troponin C coupled with strongly binding cross-bridges in cardiac myofilament activation

Slow skeletal muscle troponin I (ssTnI) expressed predominantly in perinatal heart confers a marked resistance to acidic pH on Ca(2+) regulation of cardiac muscle contraction. To explore the molecular mechanism underlying this phenomenon, we investigated the roles of TnI isoforms (ssTnI and cardiac TnI (cTnI)) in the thin filament activation by strongly binding cross-bridges, by exchanging troponin subunits in cardiac permeabilized muscle fibers. Fetal cardiac muscle showed a marked resistance to acidic pH in activation of the thin filament by strongly binding cross-bridges compared to adult muscle. Exchanging ssTnI into adult fibers altered the pH sensitivity from adult to fetal type, indicating that ssTnI also confers a marked resistance to acidic pH on the cross-bridge-induced thin filament activation. However, the adult fibers containing ssTnI or cTnI but lacking TnC showed no pH sensitivity. These findings provide the first evidence for the coupling between strongly binding cross-bridges and a pH-sensitive interaction of TnI with TnC in cardiac muscle contraction, as a molecular basis of the mechanism conferring the differential pH sensitivity on Ca(2+) regulation.

Role of troponin I isoform switching in determining the pH sensitivity of Ca(2+) regulation in developing rabbit cardiac muscle

Skinned muscle fibers prepared from fetal rabbit heart (28 days of gestation) showed a marked resistance to acidic pH in the Ca(2+) regulation of force generation, compared to the fibers prepared from adult heart. SDS-PAGE and immunoblot analysis showed that the slow skeletal troponin I was predominantly expressed in the fetal cardiac muscle, while the cardiac isoform was predominantly expressed in the adult cardiac muscle. Direct exchange of purified slow skeletal and cardiac troponin I isoforms into these skinned muscle fibers revealed that cardiac troponin I made the Ca(2+) regulation of contraction sensitive to acidic pH just as in the adult fibers, whereas slow skeletal troponin I made the Ca(2+) regulation of contraction resistant to acidic pH just as in the fetal fibers. These results demonstrate that the troponin I isoform switching accounts fully for the change in the pH dependence of Ca(2+) regulation of contraction in developmental cardiac muscle.

Comparison of cross-bridge cycling kinetics in neonatal vs. adult rat ventricular muscle

The developmental shift in contractile protein isoform expression in the rodent heart likely affects actin-myosin cross-bridge interactions. We compared the Ca2+ sensitivity for force generation and cross-bridge cycling kinetics in neonatal (postnatal days 0-3) and adult (day 84) rats. The force-pCa relationship was determined in Triton-X skinned muscle bundles activated at pCa 9.0 to 4.0. In strips maximally activated at pCa 4.0, the following parameters of cross-bridge cycling were measured: (1) rate of force redevelopment following rapid shortening and restretching (ktr); and (2) isometric stiffness at maximal activation and in rigor. The fraction of attached cross-bridges (alpha fs) and apparent rate constants for cross-bridge attachment (fapp) and detachment (gapp) were derived assuming a two-state model for cross-bridge cycling. Compared to the adult, the force-pCa curve for neonatal cardiac muscle was significantly shifted to the left. Neonatal cardiac muscle also displayed significantly smaller alpha fs, slower ktr and fapp; however, gapp was not significantly different between age groups. These data indicate that weaker force production in neonatal cardiac muscle involves, at least in part, less efficient cross-bridge cycling kinetics.

Tropomyosin modulates pH dependence of isometric tension

We investigated the effect of pH on isometric tension in actin filament-reconstituted and thin filament-reconstituted bovine cardiac muscle fibers in the pH range of 6.0-7.4. Thin filament was reconstituted from purified G-actin with either bovine cardiac tropomyosin (Tm) or rabbit skeletal Tm in conjunction with cardiac or skeletal troponin (Tn). Results showed that isometric tension decreased linearly with a decrease in pH. The slope of the pH-tension relation, DeltaF/DeltapH (Deltarelative tension/Deltaunit pH), was 0.28 and 0.44 in control cardiac fibers and skeletal fibers, respectively. In actin filament-reconstituted fibers without regulatory proteins, DeltaF/DeltapH was 0.62, namely larger than that in cardiac or skeletal fibers. When reconstituted with cardiac Tm-Tn complex (nTm), DeltaF/DeltapH recovered to 0.32, close to the value obtained in control cardiac fibers. When reconstituted with skeletal nTm, DeltaF/DeltapH recovered to 0.48, close to the value for control skeletal fibers. To determine whether Tm or Tn is responsible for the inhibitory effects of nTm on the tension decrease caused by reduced pH, thin filament was reconstituted with cardiac Tm and skeletal Tn, or with skeletal Tm and cardiac Tn. When cardiac Tm was used, pH dependence of isometric tension coincided with that of control cardiac fibers. When skeletal Tm was used, the pH dependence coincided with that of control skeletal fibers. Furthermore, closely similar results were obtained in fibers reconstituted with actin and either cardiac or skeletal Tm without Tn. These results demonstrate that Tm but not Tn modulates the pH dependence of active tension.

Regulation of the rat cardiac troponin I gene by the transcription factor GATA-4

Troponin I is a thin-filament contractile protein expressed in striated muscle. There are three known troponin I genes which are expressed in a muscle-fibre-type-specific manner in mature animals. Although the slow skeletal troponin I isoform is expressed in fetal and neonatal heart, the cardiac isoform is restricted in its expression to the myocardium at all developmental stages. To study the regulation of this cardiac-specific and developmentally regulated gene in vitro, the rat cardiac troponin I gene was cloned. Transient transfection assays were performed with troponin I-luciferase fusion plasmids to characterize the regulatory regions of the gene. Proximal regions of the upstream sequence were sufficient to support high levels of expression of the reporter gene in cardiocytes and relatively low levels in other cell types. The highest luciferase activity in the cardiocytes was noted with a plasmid that included the region spanning -896 to +45 of the troponin I genomic sequence. Co-transfection of GATA-4, a recently identified cardiac transcription factor, with troponin I-luciferase constructs permitted high levels of luciferase expression in non-cardiac cells. Electrophoretic mobility-shift assays demonstrated specific binding of GATA-4 to oligonucleotides representative of multiple sites of the troponin I sequence. Mutation of a proximal GATA-4 DNA-binding site decreased transcriptional activation in transfected cardiocytes. These results indicate that the proximal cardiac troponin I sequence is sufficient to support high levels of cardiac-specific gene expression and that the GATA-4 transcription factor regulates troponin I-luciferase expression in vitro.

The heart and development

The perspective from which the developing heart is viewed can lead to differing conclusions about the effects of development on cardiac function. The hearts of the embryo, fetus and adult, viewed from a global perspective, sustain the circulation through the same basic mechanisms of developing pressure and ejecting blood. The failure of the embryonic heart to perform these tasks results in growth failure, edema, and embryonic death, just as in the infant and adult such failure results in premature death. Furthermore, from the viewpoint of gross anatomy, following embryonic morphogenesis, the developing and adult hearts appear in general to be structurally similar, differing only in size and mass. However, a closer view shows, in the molecular and structural makeup of the myocardium, richly complex changes that can modulate the basic physiological properties of the cardiac myocyte. This article focuses on how these changes and the effects of birth and development alter ventricular function.

Troponin I isoform expression is developmentally regulated in differentiating embryonic stem cell-derived cardiac myocytes

We studied troponin I (TnI) isoform expression in the mouse embryonic stem (ES) cell model of cardiogenesis as an essential first step to understanding the relationship between TnI isoform transitions and myofibrillar function. Cultures of differentiating ES cells were grown on coverslips to permit microscopic inspection of foci of spontaneously contracting cardiac myocytes developing in culture. TnI expression was followed over time to test whether the cardiac myocytes undergo the developmental pattern of expression characteristic of vertebrate cardiogenesis, in which slow skeletal TnI (ssTnI) is expressed initially, followed by induction of cardiac (cTnI) isoform expression. Cardiac TnI expression was examined using the cardiac-specific, monoclonal TI-1 antibody (Ab) while all striated muscle ThI isoforms were detected using the monoclonal TI-4 Ab. Cardiac-specific TnI expression was detected in only 8% (8/96) of foci contracting less than 5 days while TI-4 positive staining was present in 95% (71/73) of foci. These results indicate that other striated muscle TnI isoforms were being expressed in most of the TI-4 positive staining foci. The proportion of contracting foci expressing the cardiac isoform increased steadily over time, such that 100% of foci contracting more than 20 days (13/13) stained positive with the TI-1 Ab. Dual labeling experiments with both TI-1 and TI-4 anti-TnI Abs in the same culture confirmed that within each foci, the area expressing cTnI increased with the days of spontaneous contraction. Western blot analysis of micro-dissected ES cell derived cardiac myocytes confirmed that TI-4 immunostaining at early developmental time points represented ssTnI, and not the fast skeletal TnI isoform. We conclude that ES cell-derived cardiac myocytes display the developmental induction of cardiac TnI expression characteristic of vertebrate cardiac development. Thus, this model should be useful for studying the regulation and functional significance of TnI isoform expression during in vitro cardiogenesis.

Recombinant troponin I substitution and calcium responsiveness in skinned cardiac muscle

Using treatment with vanadate solutions, we extracted native cardiac troponin I and troponin C (cTnI and cTnC) from skinned fibers of porcine right ventricles. These proteins were replaced by exogenously supplied TnI and TnC isoforms, thereby restoring Ca2+-dependent regulation. Force then depended on the negative logarithm of Ca2+ concentration (pCa) in a sigmoidal manner, the pCa for 50% force development, pCa50, being about 5.5. For reconstitution we used fast-twitch rabbit skeletal muscle TnI and TnC (sTnI and sTnC), bovine cTnI and cTnC or recombinant sTnIs that were altered by site-directed mutagenesis. Incubation with TnI inhibited isometric tension in TnI-extracted fibers in the absence of Ca2+, but restoration of Ca2+ dependence required incubation with both TnI and TnC. Relaxation at low Ca2+ levels and the steepness of the force/pCa relation depended on the concentration of exogenously supplied TnI in the reconstitution solution (range 20-150 "mu"M), while Ca2+ sensitivity, i.e. the pCa50, was dependent on the isoform, and also on the concentration of TnC in the reconstitution solution. At pH 6.7, skinned fibers reconstituted with optimal concentrations of sTnC and sTnI (120 "mu"M and 150 "mu"M, respectively) were more sensitive to Ca2+ than those reconstituted with cTnC and cTnI (difference in pCa50 approx. 0.2 units). Rabbit sTnI was cloned and expressed in Escherichia coli using a high yield expression plasmid. We introduced point mutations into the TnI inhibitory region comprising the sequence of the minimal common TnC/actin binding site (-G104-K-F-K-R-P-P-L-R-R-V-R115-). The four mutants produced by substitution of T for P110, G for P110, G for L111, and G for K105 were chosen, based on previous work with synthetic peptides showing that single amino acid substitution in this region diminished the capacity of these peptides to inhibit acto-S1 ATPase or contraction of skinned fibers. Therefore, all amino acid residues of the inhibitory region are thought to contribute to biological activity of TnI. However, each of the recombinant TnIs could substitute for endogenous TnI. In combination with exogenous TnC, Ca2+ dependence could be restored when gly110sTnI, thr110sTnI or gly111sTnI was used for reconstitution. The mutant gly105sTnI, on the other hand, reduced the ability of skinned fibers to relax at low Ca2+ concentrations and it caused an increase in Ca2+ sensitivity.

Effects of troponin C isoforms on pH sensitivity of contraction in mammalian fast and slow skeletal muscle fibres

1. The effects of troponin C (TnC) isoforms on the acidic pH-induced rightward shift in the tension-pCa (-log[Ca2+]) relationship were examined in slow soleus and fast psoas skeletal muscle fibers. Endogenous TnC was partially extracted from skinned single fibres and the extracted fibres were subsequently reconstituted with purified TnC. The pCa producing one-half maximal tension (pCa50) was determined at pH 7.00 and 6.20 in each fibre and then the pH-induced shift in pCa50 (delta pCa50) was calculated. 2. In control fast fibres which express fast skeletal TnC (sTnC), the delta pCa50 was 0.64 +/- 0.02 pCa units (n = 10), and this increased significantly to 0.78 +/- 0.04 pCa units (n = 8) following extraction and reconstitution with cardiac TnC (cTnC). In each fibre, the reconstituted delta pCa50 was subtracted from the control delta pCa50 which yielded a significant shift of -0.13 +/- 0.05 pCa units (n = 8; P < 0.05). Thus, the pH sensitivity of contraction was increased in the cTnC-reconstituted psoas fibres. 3. In extracted psoas fibres that were reconstituted with fast sTnC the pH sensitivity of contraction was unchanged, indicating that the above effects were related to the TnC isoform and not a non-specific effect of the extraction procedure. 4. In a second series of experiments cTnC was specifically extracted from slow soleus fibres which were subsequently reconstituted with purified fast sTnC. Skeletal TnC reconstituted soleus fibres demonstrated a significant decrease in pH sensitivity. In each fibre, the reconstituted delta pCa50 (mean, 0.58 +/- 0.02 pCa units) was subtracted from the control delta pCa50 (mean, 0.63 +/- 0.02 pCa units) which yielded a significant shift of 0.05 +/- 0.01 pCa units (n = 4; P < 0.05). The pH sensitivity was not altered in cTnC-reconstituted soleus fibres (-0.01 +/- 0.01 pCa units, n = 4). 5. These findings indicate that TnC isoforms alter the pH sensitivities of contraction in slow and fast skeletal muscle fibres. However, the magnitude of the change in pH sensitivity is muscle lineage dependent, indicating that differential expression of other myofilament protein isoforms, together with TnC, is necessary to confer full pH sensitivity of contraction in striated muscles.

Reconstitution of skinned cardiac fibres with human recombinant cardiac troponin-I mutants and troponin-C

Troponin C (TnC) could be extracted from skinned porcine cardiac muscle fibres and their Ca2+ sensitivity restored by reconstitution with recombinant human cardiac TnC. After extraction of troponin I (TnI) and TnC using the vanadate treatment method of Strauss et al. [Strauss, J. D., Zeugner, C., Van Eyk, J.E., Bletz, C., Troschka, M. and Rüegg, J.C. (1992) FEBS Lett. 310, 229-234], skinned porcine cardiac muscle fibres were reconstituted with wild-type recombinant human cardiac TnC and either wild-type cardiac TnI or several mutant isoforms of human TnI. Reconstitution with wild-type proteins restored the Ca2+ sensitivity of the tissue and phosphorylation of the TnI with the catalytic subunit of protein kinase A reduced the Ca2+ sensitivity (i.e.-log[Ca2+] for 50% of maximal force) as has been shown by others. However, reconstitution with the TnI mutant Ser-23Asp/Ser-24Asp mimicking the phosphorylated form of cardiac TnI, led to a reduced Ca2+ sensitivity compared with reconstitution with wild-type TnI, whereas the mutant Ser-23Ala/Ser-24Ala behaved as the dephosphorylated form of TnI. These data confirm the importance of negative charge in this region of the TnI molecule in altering the Ca2+ responsiveness in this system.

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.

Troponin I isoforms and differential effects of acidic pH on soleus and cardiac myofilaments

Differences in pH sensitivity of tension generation between developing and adult cardiac myofilaments, which contain the same isoform of troponin C (TnC), have been proposed to be due to troponin I (TnI) isoform switching from the slow skeletal (ss) to cardiac (c) TnI isoforms (21). We investigated the effects of acidic pH on Ca(2+)-activation of force in chemically skinned preparations of adult rat trabeculae and single soleus fibers that also share the same TnC isoform. Compared with the soleus fibers, trabeculae demonstrated a greater suppression of tension and a rightward shift in pCa50 (-log half-maximally activating molar Ca2+ concentration) when pH was decreased from 7.0 to 6.2. The pH-induced shift in pCa50 in soleus fibers did not change with sarcomere length. Troponin subunit interactions were also investigated, using cardiac troponin C (cTnCIA) labeled with a fluorescent probe, 2-(4'-iodoacetamidoanilino)-naphthalene-6-sulfonic acid. Under acidic conditions, cTnCIA demonstrated a decrease in Ca(2+)-affinity. This decrease was amplified both in the binary complex cTnCIA-cTnI and in the complex cTnCIA-cTnI-cTnT-tropomyosin to the same extent. In contrast, substitution of ssTnI for cTnI in these complexes produced the same decrease in Ca2+ affinity in response to acidic pH as cTnCIA alone. These results support our hypothesis that differential effects of pH on tension generation and Ca2+ sensitivity between soleus fibers and trabeculae are due to the presence of different isoforms of TnI.

Mechanisms of myocardial depression after bolus injection of sodium bicarbonate

PURPOSE

The classic model for the effects of NaHCO3 on myocardial function predicts transient myocardial depression after an intravenous bolus of sodium bicarbonate in association with myocardial acidosis.

METHODS

Five anesthetized, paralyzed, and ventilated dogs underwent midline sternotomy. Myocardial global function was assessed by cardiac output, left ventricular (LV) dp/dt, LV end-systolic, and LV end-diastolic pressures. Regional myocardial function assessed by measuring the LV regional end-systolic, LV end-diastolic lengths, and LAD coronary blood flow. Coronary sinus, intramyocardial and arterial pH were measured as was free serum Ca++. Animals were made acidemia by infusion of 0.3 N HCl and then given a bolus of sodium bicarbonate. This produced transient depression followed by recovery of myocardial function.

RESULTS

During the depression phase there was no significant decrease in interstitial pH or an increase in A-VCO2 difference as predicted by the current model. However, there was a significant decrease in the serum free Ca++ that coincided with myocardial depression.

CONCLUSION

We could not confirm the predictions of the classic model and hypothesize that myocardial depression may be caused by decreased availability of free Ca++ of decreased Ca++ flux rather than intracellular acidosis.

Sprint-training effects on some contractile properties of single skinned human muscle fibres

The effects of sprint training on the contractile properties of human muscle fibres obtained by needle biopsy were investigated. Individual fibres were mechanically skinned and activated by Ca(2+)- and Sr(2+)-buffered solutions at pH 7.1, and allocated to distinct populations on the basis of their contractile characteristics. The majority of fibres sampled pre-training could be separated into the three major fibre groups: Populations I (24/70, 34%), II (25/70, 36%) and III (18/70, 26%), which exhibited characteristics similar to those of histochemically classified type I, IIA and IIB fibres, respectively. The remainder (3/70, 4%) represented another fibre group, with intermediate characteristics. The muscle fibres were also activated by Ca2+ at a reduced pH of 6.6, to mimic the intracellular acidification that occurs during intense exercise. Lowering pH increased the threshold for contraction by Ca2+, reduced Ca2+ sensitivity, and increased the steepness of the force-pCa relationship, in all fibres sampled from the three major fibre groups. Maximum force was not significantly reduced in any fibre population. In the post-training sample, the three major fibre types were present in different proportions: Populations I (10/52, 19%), II (20/52, 38.5%) and III (11/52, 21%). Three other fibre groups sampled in low numbers exhibited contractile characteristics intermediate between Population I and Population II. Following sprint training all of the three main fibre populations exhibited higher thresholds for contraction by, and lower sensitivities to, Sr2+ but not Ca2+, compared with the fibres sampled pre-training. Maximum force was significantly lower in Population II fibres after sprint training. At pH 6.6, post-trained Population III fibres exhibited even lower Ca2+ sensitivity, with concomitant increases in the threshold for contraction and force-pCa curve steepness.

Troponin replacement in permeabilized cardiac muscle. Reversible extraction of troponin I by incubation with vanadate

Calcium-dependent regulation of tension and ATPase activity in permeabilized porcine ventricular muscle was lost after incubation with 10 mM vanadate. After transfer from vanadate to a vanadate-free, low-Ca2+ solution (pCa greater than 8), the permeabilized muscle produced 84.8% +/- 20.1% (+/- S.D., n = 98) of the isometric force elicited by high Ca2+ (pCa approximately 4.5) prior to incubation with vanadate. Transfer back to a high Ca2+ solution elicited no additional force (83.2% +/- 18.7% of control force). SDS-PAGE and immunoblot analysis of fibers and solutions demonstrated substantial extraction (greater than 90%) of Troponin I (TnI). Calcium dependence was restored after incubation with solutions containing either whole cardiac troponin or a combination of TnI and troponin C subunits. This reversible extraction of troponin directly demonstrates the role of TnI in the regulation of striated muscle contractility and permits specific substitution of the native TnI with exogenously supplied protein.

Troponin-tropomyosin abnormalities in hamster cardiomyopathy

Cardiac myofibrils from cardiomyopathic hamsters exhibit elevated Mg2+ ATPase activity and a parallel upward shift of the calcium ATPase dose response curve. To explore the mechanism, myofibrils from control and cardiomyopathic hamster hearts were incubated with isolated troponin-tropomyosin complex (Tn.Tm) from cardiomyopathic and control hamster or from dog hearts. Tn.Tm from control hamster or dog hearts restored normal Mg2+ ATPase activities to myofibrils from myopathic hearts. However, the maximum ATPase response to calcium stimulation was less in cardiomyopathic myofibrils compared to controls, even when control Tn.Tm was included. Electrophoretic patterns of Tn.Tm from myopathic and control hearts were similar. Electrophoresis of the hamster myofibrils mixed with dog cardiac Tn.Tm and then washed demonstrated binding of this complex to myopathic myofibrils. To further confirm that the incubation experiments resulted in binding, 125I troponin-tropomyosin was cross-hybridized with myofibrils, extensively washed, and then analyzed enzymatically and autoradiographically. Autoradiograms demonstrated similar percent binding of 125I Tn.Tm to all myofibrillar preparations and enzymatic effects like those found using cold Tn.Tm. These studies suggest that Tn.Tm from cardiomyopathic hearts inhibits Mg2+ myofibrillar ATPase activity to a lesser degree than Tn.Tm from control hearts. Decreased stimulation by calcium in myopathic preparations may be due to abnormalities in troponin-tropomyosin and/or to the decreased myosin ATPase activity observed previously.

Effects of changes of pH on the contractile function of cardiac muscle

It has been known for over 100 years that acidosis decreases the contractility of cardiac muscle. However, the mechanisms underlying this decrease are complicated because acidosis affects every step in the excitation-contraction coupling pathway, including both the delivery of Ca2+ to the myofilaments and the response of the myofilaments to Ca2+. Acidosis has diverse effects on Ca2+ delivery. Actions that may diminish Ca2+ delivery include 1) inhibition of the Ca2+ current, 2) reduction of Ca2+ release from the sarcoplasmic reticulum, and 3) shortening of the action potential, when such shortening occurs. Conversely, Ca2+ delivery may be increased by the prolongation of the action potential that is sometimes observed and by the rise of diastolic Ca2+ that occurs during acidosis. This rise, which will increase the uptake and subsequent release of Ca2+ by the sarcoplasmic reticulum, may be due to 1) stimulation of Na+ entry via Na(+)-Ca2+ exchange; 2) direct inhibition of Na(+)-Ca2+ exchange; 3) mitochondrial release of Ca2+; and 4) displacement of Ca2+ from cytoplasmic buffer sites by H+. Acidosis inhibits myofibrillar responsiveness to Ca2+ by decreasing the sensitivity of the contractile proteins to Ca2+, probably by decreasing the binding of Ca2+ to troponin C, and by decreasing maximum force, possibly by a direct action on the cross bridges. Thus the final amount of force developed by heart muscle during acidosis is the complex sum of these changes.

[Mechanisms of action of halogenated anesthetics on isolated cardiac muscle]

The mechanisms responsible for the direct negative inotropic effects of the three currently used volatile anesthetics (halothane, enflurane and isoflurane) are reviewed. These agents interfere at each step of excitation-contraction coupling, i.e. sarcolemmal membrane, sarcoplasmic reticulum and contractile proteins. At the myofilament level, they decrease both calcium sensitivity and maximal developed force of cardiac skinned fibers of various species, a preparation in which all functional membranes are destroyed and thus allowing to study the direct effects of volatile anesthetics on myocardial contractile proteins. The effects of the three volatile anesthetics are similar at equipotent concentrations. The site of action seems to involve the regulatory proteins of the thin myofilament, especially troponin-tropomyosin complex. At the sarcolemmal level, all three anesthetics decrease Ca++ entry through the voltage-dependent calcium channels, an effect that seems slightly more important for both halothane and enflurane than for isoflurane. However, these two sites of action (contractile proteins and sarcolemmal membrane) are not sufficient to explain their overall negative inotropic effect. The third site of action involves the sarcoplasmic reticulum. Halothane and enflurane produce an initial liberation of Ca++ from internal stores, while isoflurane does not. All three agents decrease the net uptake of Ca++ and increase the permeability of sarcoplasmic reticulum to Ca++, similar to the effect of caffeine. However, the resulting effect, i.e. a reduction of sarcoplasmic reticulum Ca++ content occurs at clinical concentrations of halothane or enflurane, while much higher concentrations of isoflurane are required to produce a similar reduction. This differential effect on the sarcoplasmic reticulum function (which is quantitative but not qualitative) seems to be mainly responsible for the lesser negative inotropic effect of isoflurane as observed in intact cardiac muscles of various species including humans. The knowledge of the mechanisms of action of volatile anesthetics is important for understanding the potential consequences associated with their use in patients receiving cardiac drugs, especially calcium blockers and phosphodiesterase inhibitors.

Developmental changes in the expression of rabbit left ventricular troponin T

We examined cardiac troponin T (TnT) isoform expression in rabbit left ventricular myocardium at three different stages of postnatal development. Using sodium dodecyl sulfate gel electrophoresis (PAGE), we resolved five isoforms: TnT1, TnT2, TnT3, TnT4, and TnT5. TnT1 had the slowest electrophoretic mobility and TnT5 the fastest. The predominant isoforms were TnT2, TnT3, and TnT4. The relative amounts of TnT2, TnT3, TnT4, and TnT5 were examined in myocardium from three age-groups: 3 days (Group 1), 21-22 days (Group 2), and 99-109 days (Group 3). The amount of TnT2 relative to the total amount of TnT (determined by the ratio of the areas under the densitometric curves) decreased significantly (p less than 0.01) with age from 42 +/- 4% in Group 1 to 25 +/- 3% in Group 3. In contrast, the relative amount of TnT4 increased with age from 23 +/- 2% in Group 1 to 33 +/- 4% in Group 3 (p less than 0.01). The relative amounts of the other two isoforms change biphasically with development: TnT3 decreased from Group 1 to Group 2 and increased from Group 2 to Group 3. TnT5, a minor isoform, increased from Group 1 to Group 2 and decreased from Group 2 to Group 3. These developmental changes in troponin T expression may account for some of the maturational changes observed in the physiological and biochemical properties of the myocardium.

Effects of acidosis on ventricular muscle from adult and neonatal rats

We compared the response of ventricular muscle from adult and neonatal rats to hypercapnic acidosis. In adult muscle, acidosis caused an initial rapid fall of developed tension to 30 +/- 5% of control (mean +/- SEM, n = 6). However, tension recovered slowly to a steady state that was 56 +/- 6% of control. In neonatal muscle, acidosis caused a significantly smaller initial fall in tension to 43 +/- 3% (n = 8, p less than 0.05), but the tension then showed a subsequent slower fall to a steady state that was 29 +/- 4% of control, significantly less than in the adult (p less than 0.01). We have attempted to identify the mechanisms underlying these differences in response. In detergent-skinned myofibrils, reducing the pH from 7.0 to 6.5 caused a reduction in the pCa50 of 0.61 units in the adult muscle, but only 0.27 units in the neonatal ventricular muscle. Myofibrillar Ca2+ sensitivity in neonatal ventricular muscle is thus less susceptible to the effects of acidic pH than that of adult muscle. Since intracellular pH decreases rapidly on application of increased external CO2, these results are consistent with the finding that, initially, developed tension in neonatal muscles is less sensitive to the effects of acidosis. Sodium dodecylsulfate gel electrophoresis of myofibrillar preparations from adult and neonatal rats demonstrated differences in thin filament proteins, including troponin I, which may underlie the observed differences in Ca2+ sensitivity. In adult rat ventricular muscles, the slow recovery of tension during acidosis is associated with an increase in the amplitude of the Ca2+ transients to 263 +/- 34% of control (n = 4).(ABSTRACT TRUNCATED AT 250 WORDS)