News Physiol Sci 16: 278-281, 2001;
1548-9213/01 $5.00
News in Physiological Sciences, Vol. 16, No. 6, 278-281,
December 2001
© 2001 Int. Union Physiol. Sci./Am. Physiol. Soc.
Troponin I Isoforms and Chimeras: Tuning the Molecular Switch of Cardiac Contraction
Margaret V. Westfall1,2 and
Joseph M. Metzger1
1 Departments of Physiology and
2 Surgery, University of Michigan, Ann Arbor, Michigan 48109-0686
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Abstract
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Troponin I, a key myofilament protein, plays a critical role in regulating force generation in striated muscle by acting as a Ca2+-dependent molecular switch. Domains contributing to the functional properties of troponin I have recently been defined in intact myofilaments of adult cardiac myocytes. This has been attained using gene transfer of chimeras derived from two troponin I isoforms expressed during cardiac development.
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Introduction
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The primary function of the heart is to pump blood, and this function depends directly on the capacity of cardiac muscle to generate force. Parallel arrays of thick and thin myofilaments organized into sarcomeres within cardiac myocytes are directly responsible for force generation. Specifically, myosin cross-bridges on thick filaments attach to actin on thin filaments to produce force. This actomyosin interaction is regulated by Ca2+ binding to troponin, a heterotrimeric complex on thin filaments that is comprised of the Ca2+-binding subunit troponin C (TnC), inhibitory subunit troponin I (TnI), and tropomyosin-binding subunit troponin T (TnT). A fundamental goal of research on the troponin regulatory complex is to understand the unique functions of each subunit within intact myofilaments of muscle cells.
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Changes in TnI function are linked to alterations in cardiac performance
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The influence of TnI on cardiac function has received increasing attention due to correlations between modification of cardiac TnI (cTnI) and changes in cardiac function. Normally, TnI behaves as a molecular switch within the myofilament by toggling between actin and TnC in a Ca2+-dependent manner (14). In cardiac muscle, events such as the developmental transition from slow skeletal to cardiac TnI isoform expression (12), as well as ß-adrenergic-activated protein kinase A (PKA)-mediated phosphorylation of cTnI, modulate the tension response (Fig. 1
; Refs. 11 and 14). Several clinically important pathophysiological conditions known to compromise heart function have also been linked to TnI (Fig. 1). For example, release of cTnI from the heart is now used clinically as a key indicator of myocardial ischemia (3). Ischemia and other cardiovascular conditions are also associated with changes in TnI phosphorylation state (20), and changes in cellular ions that can accompany myocardial ischemia, such as acidic pH (10), are associated with TnI dysfunction. Moreover, work in transgenic animals has shown that removal of 17 amino acids at the carboxy terminus of cTnI is sufficient to produce myocardial stunning. Myocardial stunning is a clinical event resulting from reduced perfusion (9) in which TnI degradation occurs and causes decreased myocardial function. In addition, mutations within cTnI are linked to the human disease state hypertrophic cardiomyopathy (8). Together, these points underscore the importance of investigating the function of TnI within the myofilaments of cardiac myocytes.
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FIGURE 1. Physiological and pathophysiological conditions in which troponin I (TnI) is linked to alterations in cardiac function.
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TnI influences myofilament Ca2+ sensitivity of tension
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Insights into the function of TnI have been gained by studying cardiac development. In comparative studies of fetal/neonatal and adult myocardium, developmental transitions in TnI isoform expression correlated with significant differences in Ca2+-activated force development (13). Results from these studies indicated that expression of the slow skeletal isoform of TnI (ssTnI) in fetal/neonatal myocardium correlated with heightened Ca2+ sensitivity compared with adult hearts, which exclusively express the cTnI isoform. This developmental relationship is complicated by near-simultaneous isoform transitions in numerous sarcomeric proteins, making it challenging to assign a specific physiological function to an isoform transition for a single contractile protein (12).
Another approach to examining TnI function has been to use purified protein or fragments reconstituted into actomyosin preparations (6, 11, 14). Important strides toward understanding the fundamental functions of this protein have been achieved using this approach. Biochemical studies provided the first evidence supporting the idea that TnI acted as a molecular switch. At low Ca2+ levels, such as in diastole, TnI bound tightly to actin and inhibited actomyosin ATPase activity, whereas increasing Ca2+ levels, as during systole, resulted in high-affinity binding of TnI for TnC and reduced affinity for actin (6, 11, 14). Experiments with purified full-length protein and fragments of TnI have revealed important information about TnI structure and function by showing that TnI interacts with TnC in an antiparallel arrangement (6) and contains multiple actin- and TnC-binding regions (15). Despite these advances in our understanding of TnI, contractile protein stoichiometry within these preparations cannot be duplicated in biochemical studies, and actomyosin ATPase activity measured in biochemical studies may not reflect the Ca2+-activated myofilament tension response under some ionic conditions (5). Thus it is critical to investigate TnI function within the intact myofilaments of cardiac myocytes to determine how changes in TnI function influence force output.
To study TnI function within the intact myofilament requires specific manipulation of TnI within sarcomeres, followed by measurement of force produced by the myocyte. Gene transfer into myocytes represents a new tool for the precise genetic dissection of individual contractile proteins in the adult cardiac sarcomere. Rod-shaped adult myocytes are used for these studies because they display the required parallel arrays of myofilaments necessary for the measurement of force. Recombinant adenoviral vectors facilitate rapid and efficient expression and myofilament incorporation of individual contractile proteins within sarcomeres of adult myocytes (18). Gene sequences up to ~7 kb can be engineered into first-generation, replication-deficient adenovirus vectors, whereas longer sequences up to ~30 kb can be included with gutted viral vectors (reviewed in Ref. 4). After gene delivery, myocytes are cultured under serum-free conditions to prevent dedifferentiation of the contractile apparatus (Fig. 2; Refs. 16–18). This approach results in nearly complete replacement of endogenous cTnI with newly expressed TnI in the myofilaments of adult cardiac myocytes without detected changes in the stoichiometry and/or isoform expression of the contractile proteins. As a result, the influence of TnI isoforms and novel TnI proteins on Ca2+-activated myofilament tension can be followed in individual adult cardiac myocytes as the nascent TnI gene products are expressed and incorporated into the myofilaments of living myocytes.
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FIGURE 2. Flow diagram of steps for viral-mediated gene transfer. A: the intact heart from adult rats is perfused, and ventricular myocytes are isolated and plated on laminin-coated glass coverslips in 5% serum-containing media for 2 h. B: a shuttle vector is prepared containing the TnI gene of interest. This vector contains a human cytomegalovirus promoter (CMV) and SV40 polyadenylation signal (polyA). Recombinant adenovirus is prepared by homologous recombination of the shuttle vector with full-length adenovirus containing an insert that extends this adenovirus beyond the range for packaging. Recombinant virus particles can be packaged but lack the E1 region and therefore replicate only in cell lines expressing this region in trans. C: adenovirus is added to myocyte cultures in serum-free media and incubated for 1 h before adding 2 ml of additional serum-free media. Myocytes are then cultured with 95% O2-5% CO2 at 37°C. D: myocyte structure and function are determined after gene transfer. Western blot analysis is carried out to monitor TnI expression as well as the stoichiometry and isoform expression of other contractile proteins. Myocytes also are immunolabeled with antibodies recognizing specific TnI proteins to assess myofilament incorporation and sarcomere structure. In functional studies, Ca2+-activated tension is measured in membrane-permeabilized myocytes by using a range of Ca2+ concentrations.
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In studies using gene transfer, TnI function has been examined by comparing Ca2+-activated tension in myocytes expressing cTnI vs. the ssTnI isoform (18). Increased Ca2+ sensitivity of tension and decreased myofilament cooperativity are observed in ssTnI-expressing compared with cTnI-expressing adult cardiac myocytes. These results provide direct evidence that TnI isoform expression prominently contributes to the differences in Ca 2+-activated myofilament tension observed during cardiac development, a finding subsequently confirmed in experiments with transgenic mice (7). An important question arising from these results is whether one or more domain(s) within the TnI protein are responsible for these isoform-specific differences in myofilament tension generation.
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Identification of TnI domains influencing myofilament Ca2+ sensitivity
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Functional domains within TnI have been widely studied using biochemical approaches. Multiple TnC and actin binding domains within TnI have been localized to the carboxy region of the protein with these approaches (6, 15). The minimum sequence necessary to inhibit actomyosin ATPase activity has been similarly defined using biochemical analysis of TnI fragments, and this carboxy portion of TnI is designated the inhibitory peptide (IP) region (Fig. 3A; Ref. 6). Binding studies indicate that the IP region toggles between actin and TnC in the absence and presence of Ca2+, respectively. Secondary actin and TnC binding regions lying further downstream in TnI also have been identified using TnI fragments (15). Thus sequence differences within the IP and other actin/TnC binding regions within the carboxy portion of TnI may be responsible for isoform-specific differences in Ca2+-activated force between cTnI- and ssTnI-expressing myocytes.
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FIGURE 3. Functional domains present in TnI isoforms and chimeras (A) and comparisons of the Ca2+-activated tension relationships observed in myocytes expressing the cardiac (cTnI) or slow skeletal (ssTnI) isoforms (B–D). A: the alignment of TnI isoforms and chimeras used to define functional domains in the intact myofilaments of adult cardiac myocytes. The hatched region within cTnI and N-card/slow-C TnI chimeras designates the 32-amino acid extension present in cTnI that is absent from the ssTnI isoform. IP indicates the inhibitory peptide region defined in earlier biochemical studies (6). Shown below the aligned proteins are the regions identified as contributing to Ca2+ sensitivity of tension, protein kinase A (PKA), and pH sensitivity using these isoforms and chimeras (16–18). B: representative comparison of tension-pCa (–log[Ca2+]) relationships in membrane-permeabilized myocytes expressng cTnI or ssTnI after gene transfer. In myocytes expressing ssTnI, there is increased Ca2+ sensitivity of tension and decreased cooperativity compared with myocytes expressing cTnI. The arrow indicates the shift in myofilament Ca2+ sensitivity caused by ssTnI expression. C: comparison of the tension-pCa relationships observed with altered pH (16–18) and PKA-mediated phosphorylation (19) in cTnI- and ssTnI-expressing myocytes. Dotted lines indicate tension relationship observed with acidic pH (left) or in response to PKA-mediated phosphorylation of TnI (right). Arrows between each pair of curves indicate shifts in the tension-pCa relationship observed in myocytes expressing cTnI or ssTnI with changes in pH (left) and with phosphorylation (right). A considerably greater decrease in myofilament Ca2+ sensitivity of tension is observed in cTnI-expressing myocytes with acidic pH or in response to phosphorylation by PKA compared with ssTnI-expressing myocytes. These results, along with those obtained with myocytes expressing each TnI chimera, form the basis for the functional domains shown in A.
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To test this hypothesis, two chimeras of the cTnI and ssTnI isoforms were designed and individually expressed in adult myocytes following gene transfer (16, 17). One chimera, N-slow/card-C TnI, was formed by joining the amino terminal 68 amino acids from ssTnI with the carboxy terminal 110 amino acids from cTnI (Fig. 3A
). A second chimera, N-card/slow-C TnI, was constructed from the amino terminal 100 amino acids from cTnI and the carboxy terminal 120 amino acids from ssTnI. Interestingly, the functional phenotypes of myocytes expressing these chimeras did not follow predictions derived from earlier biochemical results. Instead, the Ca2+ sensitivity of tension in myocytes expressing N-slow/card-C TnI decreased relative to cTnI, and N-card/slow-C TnI expression increased myofilament Ca2+ sensitivity of tension above that shown for adult cardiac myocytes expressing ssTnI (16, 17). Thus a single domain in the carboxy terminus of TnI could not fully explain these results. At least two isoform-specific functional domains within TnI were required to explain the chimera phenotypes. The predicted carboxy terminal domain was clearly present, with the sequence present in ssTnI causing a greater increase in Ca2+ of tension than the cTnI sequence. Also, the leftward shift in Ca2+ sensitivity of tension caused by N-card/slow-C TnI vs. ssTnI, as well as the decreased sensitivity observed with N-slow/card-C TnI vs. cTnI-expressing myocytes, provided evidence for a second isoform-specific, myofilament Ca2+ sensitivity domain within the amino terminal portion of TnI (Fig. 3A). In contrast to domain(s) within the carboxy region, the amino terminal domain was not predicted from biochemical findings. Results with TnI chimeras indicate that the amino terminal domain of cTnI results in increased Ca2+ sensitivity of tension compared with the amino terminal domain of ssTnI.
Further studies are now needed to identify the amino acids involved in defining isoform-specific differences in myofilament Ca2+ sensitivity within the amino and carboxy terminal domains of TnI. Several regions reported to bind actin and/or TnC in the carboxy portion of TnI encompass isoform-specific amino acid sequences within TnI. These sequence differences could contribute to isoform-specific differences in myofilament Ca2+ sensitivity. One or more amino acid regions in the amino terminal region of TnI may also contribute to the isoform-specific function of this domain. In the future, myocytes expressing unique TnIs containing well-defined isoform-specific sequence substitutions can be used to advance our understanding of the molecular switch functions of TnI in working myocytes and/or hearts under physiological and pathophysiological conditions.
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TnI acts as a pH sensor in the myofilaments
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Cellular acidosis is an important pathophysiological condition in which TnI significantly influences the Ca2+-activated tension response in cardiac myocytes (10). Acidosis develops during myocardial ischemia and is thought to be an important contributor to the contractile dysfunction associated with ischemia (10). Adult cardiac muscle develops more severe contractile dysfunction in response to acidosis than fetal/neonatal myocardium (13). Expression of the ssTnI isoform in developing myocardium has been proposed to provide a significant protective effect on force in fetal/neonatal hearts during ischemia based on the correlation between ssTnI expression and relative pH insensitivity in fetal myocardium (see references in Ref. 18). This correlation has given rise to the idea that TnI isoforms significantly influence the myofilament pH response in myocardium. To test this hypothesis directly, the tension response to acidosis was compared in adult cardiac myocytes expressing cTnI and ssTnI. Compared with myocytes expressing cTnI, expression of ssTnI significantly blunted the acidosis-insuced decrease in Ca2+ sensitivity of tension (18). These results demonstrate that TnI acts as an important pH sensor in an isoform-specific manner. In other experiments, myocytes expressing the N-slow/card-C TnI chimera responded to acidic pH in a manner comparable with cTnI-expressing myocytes and myocytes expressing ssTnI or N-card/slow-C TnI responded to acidic pH in a equivalent manner (Fig. 3C; Refs. 16 and 17). This chimera analysis provides evidence that the isoform-specific, pH-sensitive domain lies in the carboxy portion of TnI.
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cTnI is a phosphoprotein
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The regulatory properties of cTnI can also be modified by PKA-mediated phosphorylation of Ser23/Ser24 (11, 14) located within the unique 32-amino acid extension of cTnI. Phosphorylation of myosin binding protein C and both serines on TnI have been implicated in causing decreased myofilament Ca2+ sensitivity of tension, but the contribution of each protein to the functional response has been controversial (14). The amino terminal extension of cTnI is absent from ssTnI, and no detectable PKA-mediated phosphorylation of ssTnI is observed following gene transfer of this isoform into adult myocytes (19). The Ca2+ sensitivity of tension also does not change significantly in response to PKA in the ssTnI-expressing adult cardiac myocytes (Fig. 3C). These results, together with comparable findings in transgenic mice expressing ssTnI (7), provide conclusive evidence that phosphorylated cTnI is the major myofilament phosphoprotein contributing to the PKA-mediated decrease in myofilament Ca2+ sensitivity of tension.
The mechanism whereby PKA-mediated cTnI phosphorylation decreases the Ca2+ sensitivity of tension is not well understood. Phosphorylation of cTnI may cause direct conformational changes in the amino terminal extension to inhibit cTnI interactions with TnC and lead to reduced myofilament Ca2+ sensitivity (1). Alternatively, PKA-mediated cTnI phosphorylation may decrease myofilament Ca2+ sensitivity via long-range conformational changes in downstream TnI domains that interact with TnC (14). Evidence to support both of these possibilities has been obtained using purified TnI (1, 2, 11). To gain insight into this problem, the magnitude shift in Ca2+ sensitivity of tension caused by PKA phosphorylation was measured in myocytes expressing the N-card/slow-C TnI chimera. This protein was phosphorylated to levels comparable with cTnI in permeabilized myocytes treated with PKA and in intact myocytes stimulated with isoproterenol, and the Ca2+-activated myofilament tension response to PKA was comparable in myocytes expressing either cTnI or N-card/slow-C TnI (19). Thus isoform-specific differences between cTnI and N-card/slow-C TnI that were previously shown to influence baseline myofilament Ca2+ sensitivity did not influence the magnitude of the Ca2+-sensitive functional response to PKA. Further studies also indicated that the decreased Ca2+ sensitivity of tension observed in response to PKA phosphorylation of cTnI was independent from the pH-sensitive decrease in sensitivity previously localized to the carboxy portion of TnI. These findings are consistent with the idea that PKA phosphorylation causes a direct conformational change in the amino terminus of cTnI, although further experiments are needed to exclude the possibility that isoform-independent conformational changes in the carboxy portion of TnI contribute to the PKA-mediated myofilament functional response.
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Future directions
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Gene transfer of TnI isoforms and chimeras into adult cardiac myocytes has opened an exciting path for investigating the relationship between TnI structure and myofilament function. New insights into TnI function and its response to important cellular conditions such as acidosis and/or modulation by phosphorylation illustrate the usefulness of viral-based gene transfer for studying protein function within the contractile apparatus of adult cardiac myocytes. Future research could lead to the identification of precise isoform-specific functional domains within TnI. Charge and/or folding differences may be important for the isoform-dependent effects on basal as well as pH- and phosphorylation-dependent changes in performance. These hypotheses can now be tested to determine the motifs or sequences that lead to TnI isoform-dependent differences in function. In the future, the development of genetically engineered TnIs may offer a new approach to redesign sarcomeric function in failing or cardiomyopathic hearts.
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References
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Al-Hillawi E, Bhandari DG, Trayer HR, and Trayer IP. The effects of phosphorylation of cardiac troponin-I on its interactions with actin and cardiac troponin-C. Eur J Biochem 228: 962–970, 1995.[ISI][Medline]
-
Chandra M, Dong W, Pan B, Cheung HC, and Solaro RJ. Effects of protein kinase A phosphorylation on signaling between cardiac troponin I and the N-terminal domain of cardiac troponin C. Biochemistry 36: 13305–13311, 1997.[Medline]
-
Chapelle JP. Cardiac troponin I and troponin T: recent players in the field of myocardial markers. Clin Chem Lab Med 37: 11–20, 1999.[ISI][Medline]
-
Chen HH, Mack LM, Kelly R, Ontell M, Kochanek S, and Clemens PR. Persistence in muscle of an adenoviral vector that lacks all viral genes. Proc Natl Acad Sci USA 94: 1645–1650, 1997.[Abstract/Free Full Text]
-
Ebus JP, Stienen GJ, and Elzinga G. Influence of phosphate and pH on myofibrillar ATPase activity and force in skinned cardiac trabeculae from rat. J Physiol (Lond) 476: 501–516, 1994.[Abstract/Free Full Text]
-
Farah CS and Reinach FC. The troponin complex and regulation of muscle contraction. FASEB J 9: 755–767, 1995.[Abstract]
-
Fentzke RC, Buck SH, Patel JR, Lin H, Wolska BM, Stojanovic MO, Martin AF, Solaro RJ, Moss RL, and Leiden JM. Impaired cardiomyocyte relaxation and diastolic function in transgenic mice expressing slow skeletal troponin I in the heart. J Physiol (Lond) 517: 143–157, 1999.[Abstract/Free Full Text]
-
Kimura A, Harada H, Park JE, Nishi H, Satoh M, Takahashi M, Hiroi S, Sasaoka T, Ohbuchi N, Nakamura T, Koyanagi T, Hwang TH, Choo JA, Chung KS, Hasegawa A, Nagai R, Okazaki O, Nakamura H, Matsuzaki M, Sakamoto T, Toshima H, Koga Y, Imaizumi T, and Sasazuki T. Mutations in the cardiac troponin I gene associated with hypertrophic cardiomyopathy. Nat Genet 16: 379–382, 1997.[ISI][Medline]
-
Murphy AM, Kogler H, Georgakopoulos D, McDonough JL, Kass DA, Van Eyk JE, and Marban E. Transgenic mouse model of stunned myocardium. Science 287: 488–491, 2000.[Abstract/Free Full Text]
-
Orchard CH and Kentish JC. Effects of changes of pH on the contractile function of cardiac muscle. Am J Physiol Cell Physiol 258: C967–C981, 1990.[Abstract/Free Full Text]
-
Perry SV. Troponin I: inhibitor or facilitator. Mol Cell Biochem 190: 9–32, 1999.[ISI][Medline]
-
Schiaffino S and Reggiani C. Molecular diversity of myofibrillar proteins: gene regulation and functional significance. Physiol Rev 76: 371–423, 1996.[Abstract/Free Full Text]
-
Solaro RJ, Lee JA, Kentish JC, and Allen DG. Effects of acidosis on ventricular muscle from adult and neonatal rats. Circ Res 63: 779–787, 1988.[Abstract/Free Full Text]
-
Solaro RJ and Van Eyk J. Altered interactions among thin filament proteins modulate cardiac function. J Mol Cell Cardiol 28: 217–230, 1996.[ISI][Medline]
-
Tripet B, Van Eyk JE, and Hodges RS. Mapping of a second actin-tropomyosin and a second troponin C binding site within the C terminus of troponin I, and their importance in the Ca2+-dependent regulation of muscle contraction. J Mol Biol 271: 728–750, 1997.[ISI][Medline]
-
Westfall MV, Albayya FP, and Metzger JM. Functional analysis of troponin I regulatory domains in the intact myofilament of adult single cardiac myocytes. J Biol Chem 274: 22508–22516, 1999.[Abstract/Free Full Text]
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Westfall MV, Albayya FP, Turner II, and Metzger JM. Chimera analysis of troponin I domains that influence Ca(2+)-activated myofilament tension in adult cardiac myocytes. Circ Res 86: 470–477, 2000.[Abstract/Free Full Text]
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Westfall MV, Rust EM, and Metzger JM. Slow skeletal troponin I gene transfer, expression, and myofilament incorporation enhances adult cardiac myocyte contractile function. Proc Natl Acad Sci USA 94: 5444–5449, 1997.[Abstract/Free Full Text]
-
Westfall MV, Turner II, Albayya F, and Metzger JM. Troponin I chimera analysis of the cardiac myofilament tension response to protein kinase A. Am J Physiol Cell Physiol 280: C324–C332, 2001.[Abstract/Free Full Text]
-
Zakhary DR, Moravec CS, Stewart RW, and Bond M. Protein kinase A (PKA)-dependent troponin-I phosphorylation and PKA regulatory subunits are decreased in human dilated cardiomyopathy. Circulation 99: 505–510, 1999.[Abstract/Free Full Text]
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Introduction
Changes in TnI function...
