Cardiac Troponin I Threonine 144

Role in Myofilament Length–Dependent Activation

Kittipong Tachampa, Helen Wang,Gerrie P. Farman, Pieter P. de Tombe

Myofilament length–dependent activation is the maincellular mechanism responsible for the Frank–Starlinglaw of the heart. All striated muscle display length-dependent activation properties, but it is most pro-nounced in cardiac muscle and least in slow skeletalmuscle. Cardiac muscle expressing slow skeletal troponin(ssTn)I instead of cardiac troponin (cTn)I displays re-duced myofilament length–dependent activation. Theinhibitory region of troponin (Tn)I differs by a singleresidue, proline at position 112 in ssTnI versusthreonine at position 144 in cTnI. Here we testedwhether this substitution was important for myofila-ment length–dependent activation; using recombinanttechniques, we prepared wild-type cTnI, ssTnI, and 2mutants: cTnIThr>Pro and ssTnIPro>Thr. Purified proteinswere complexed with recombinant cardiac TnT/TnCand exchanged into skinned rat cardiac trabeculae.Force–Ca2� relationships were determined to derivemyofilament Ca2� sensitivity (EC50) at 2 sarcomerelengths: 2.0 and 2.2 �m (n�7). Myofilament length-dependent activation was indexed as �EC50, the differ-ence in EC50 between sarcomere lengths of 2.0 and2.2 �m. Incorporation of ssTnI compared with cTnIinto the cardiac sarcomere reduced �EC50 from1.26�0.30 to 0.19�0.04 �mol/L. A similar reductionalso could be observed when Tn contained cTnIThr>Pro

(�EC50�0.24�0.04 �mol/L), whereas the presence ofssTnIPro>Thr increased �EC50 to 0.94�0.12 �mol/L.These results suggest that Thr144 in cardiac TnI modu-lates cardiac myofilament length–dependent activation.

The Frank–Starling “Law of the Heart” describes therelationship between ventricular end-systolic pressure

and end-systolic volume. It is well established that thecellular basis for this phenomenon is the modulation ofmyofilament calcium sensitivity with sarcomere length.1

Although all mammalian striated muscle display myofilamentlength–dependent activation properties, it is most pronounced

in cardiac muscle and least in slow skeletal muscle.1 Themolecular mechanisms that underlie myofilament length–dependent activation are incompletely understood. The mam-malian heart expresses slow skeletal troponin (ssTn)I duringdevelopment and, in many species, also during the earlyneonatal state.2 Replacement of endogenous cardiac troponin(cTn)I by ssTnI by transgenesis has been shown to besufficient to reduce myofilament-length dependence.1 Thoseresults indicate that TnI plays a pivotal role in modulating theresponse of the cardiac sarcomere to changes in sarcomerelength and, moreover, that the extent of this modulationdepends on the structure of TnI. We found in preliminaryexperiments that a specific region of cTnI, located betweenthe inhibitory region and the C terminus of the molecule, maybe of specific importance for myofilament length–dependentactivation. Inspection of the sequence differences betweencTnI and ssTnI in the inhibitory region of TnI reveals asubstitution of a single residue, threonine, in cTnI (144) by aproline in ssTnI (112). Accordingly, in the present study, weinvestigated the role of Thr144 in length dependence. Wefound that the presence of threonine at position 144 (in cTnI)or 112 (in ssTnI) is sufficient to impart length dependenceonto the cardiac sarcomere.

Materials and MethodsAn expanded Materials and Methods section is available in theonline data supplement at http://circres.ahajournals.org.

Exchange of Recombinant cTn Into SkinnedRat TrabeculaeRight ventricular rat trabeculae were dissected, chemically perme-abilized with Triton X-100, and attached to T clips as described.3

Exogenous troponin (Tn) was exchanged for endogenous Tn byslight modification of previously described methods.3

Confocal Analysis of Tn ExchangeRecombinant TnT in the present study included an NH2 terminusmyc tag to allow for confocal analysis of recombinant Tn exchangeand colocalization with actin (model 2100 LSM, Bio-Rad). Previ-ously, we have demonstrated that the presence of the myc tag doesnot affect myofilament function.4

Measurement of Isometric TensionThe measurement of steady-state isometric tension at varied freeCa2� was conducted as described.3 Briefly, sarcomere length was setat 2.2 �m by laser diffraction. Trabeculae were activated over arange of free [Ca2�] to measure steady-state isometric tension. Onlymuscles that maintained �80% maximal tension were included foranalysis.

Data Processing and Statistical AnalysisForce–Ca2� relationships were fit to a modified Hill equation.3

Statistical analyses were performed by ANOVA. P�0.05 wasconsidered statistically significant; data are presented means�SEM.

ResultsWe used the “whole Tn exchange technique” to introduceeither wild-type or truncated cTnI into the cardiac sarcomere.As we reported previously,3,4 this procedure caused no majoralterations in the structure and properties of the fiber bundles.That is, exchanged skinned muscles retained a clearly detect-

Original received March 23, 2007; resubmission received October 4,2007; revised resubmission received October 17, 2007; accepted October18, 2007.

From the Department of Physiology and Biophysics and Center forCardiovascular Research, University of Illinois, Chicago.

Correspondence to Pieter P. de Tombe, Department of Physiology andBiophysics, University of Illinois, 835 S Wolcott, Chicago IL 60612.E-mail pdetombe@uic.edu

(Circ Res. 2007;101:1081-1083.)© 2007 American Heart Association, Inc.

Circulation Research is available at http://circres.ahajournals.orgDOI: 10.1161/CIRCRESAHA.107.165258

1081

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able laser diffraction pattern. In addition, maximum calcium-saturated force-development reduction resulting from Tnexchange was �10% in all 4 groups (Figure 1A). The amountof recombinant Tn replacement of endogenous Tn in the 4groups was 75% to 98% (Figure 1B). The principal aim of thepresent study was to examine the role of a subdomain of cTnIon myofilament length–dependent activation. Differentialexchange for recombinant Tn in only part of the thin filamentcould, potentially, artificially affect the contractile responseto changes in sarcomere length. Figure 1C demonstrates thatthe Tn exchange occurred uniformly along the thin filamentin the 4 groups studied. That is, the confocal signals obtainedfor actin (red) and myc-TnT (green) were striated in appear-ance and, moreover, the signals colocalized as shown by themerged images displayed in the bottom images (yellow).

To characterize myofilament length–dependent activationproperties following Tn exchange, force–Ca2� relationshipswere determined in the skinned cardiac trabeculae at 2sarcomere lengths (Figure 2). Examples of original forcerecordings as well as summarized Hill fit parameters arepresented in the online data supplement. Exchange for Tncontaining cTnI resulted in force–Ca2� relationships similarto those obtained in nonexchanged muscles,1 albeit with areduced level of cooperativity; importantly, myofilamentlength–dependent activation was not affected by recombinantTn exchange (see on-line data supplement). Exchange forcTnIThr�Pro Tn resulted in a reduction in overall Ca2� sensitiv-ity concomitant with a significant reduction of the influenceof sarcomere length on Ca2� sensitivity, similar to the resultobtained on ssTnI Tn exchange. Exchange for ssTnIPro�Thr Tninduced an increase in overall myofilament Ca2� sensitivity,an enhanced response to changes in sarcomere length and adecrease in cooperative activation, albeit only at the long

sarcomere length. To more directly quantify myofilamentlength–dependent activity, we computed �EC50, the differ-ence between myofilament Ca2� sensitivity recorded at the 2sarcomere lengths as indexed by EC50. As shown in Figure 3,average �EC50 was similar in cTnI and ssTnIPro�Thr Tn-exchanged muscles. Likewise, average �EC50 in both ssTnIand cTnIThr�Pro Tn-exchanged muscles was significantly re-duced, indicating very little if any myofilament length–dependent activation properties. The level of cooperativity ata sarcomere length of 2.2 �m was significantly higher in cTnITn exchange (Thr or Pro at position 144) compared withssTnI Tn exchange (Thr or Pro at position 112).

Figure 1. Recombinant Tn exchange. A,Maximum Ca2�-saturated force develop-ment in exchanged skinned trabeculaerelative to preexchange force. B, Amountof recombinant Tn exchange in the 4groups, as determined by SDS-PAGEanalysis. C, The distribution of recombi-nant Tn exchange in skinned trabeculaein the 4 groups was analyzed by confo-cal imaging for actin (green) and myc-TnT (red). The merged images are shownin yellow.

Figure 2. Impact of Tn exchange on force–Ca2� relationships.Following recombinant Tn exchange, force–Ca2� relationshipswere determined in skinned trabeculae at sarcomere lengthsof 2.0 �m (open circles) and 2.2 �m (closed circles) in the 4groups (Figure 1) as indicated.

1082 Circulation Research November 26, 2007

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DiscussionIn the present study, we found that presence of a threonineresidue at position 144 in cTnI significantly modulatedmyofilament length–dependent properties of the cardiac sar-comere. The molecular mechanisms responsible for thechange in Ca2� affinity on a change in sarcomere length arenot well understood.5 A role for interfilament spacing in thephenomenon5 is not supported by direct x-ray diffractionmeasurements.1 Consistent with this notion, the change inlattice spacing between sarcomere lengths 2.0 and 2.2 �mwas not different in the 4 groups studied (online datasupplement). What could be the molecular mechanism bywhich the Thr144 residue confers length-dependent proper-ties onto the sarcomere? Thr144 is located in the middle ofthe inhibitory region of TnI.2 Phosphorylation of this residuereduces the affinity of cTnI for cTnC and depresses slidingvelocity in the motility assay, albeit this effect is mostpronounced when serines 43/45 are also phosphorylated.2

Protein kinase A–mediated phosphorylation of cTnI is asso-ciated with an increase in myofilament length–dependentactivation.1 Thr144 phosphorylation may also affect thisparameter, but this awaits further study. A partial structure ofcTn has been determined by crystallography.6 Unfortunately,the structure of the inhibitory region of cTnI in the complexis still unresolved; hence few molecular cues exist as to thepotential structural role of Thr144 in cTnI. The generalmolecular mechanism2,7 of cardiac muscle activation involvesbinding of a Ca2� ion to the regulatory lobe of cTnC, amovement of cTnI away from cTnT toward cTnC, movementof tropomyosin into the actin groove, followed by binding ofmyosin to the exposed actin sites to form active cyclingcross-bridges. End-to-end interactions between tropomyosinmolecules along the thin filament are believed to aid in thecooperative spread of activation. Support exists for additionalbinding between Tn subunits and actin, including domainswithin cTnI2; binding of myosin heads has been shown topromote further activation of the thin filament, as well asincrease the binding affinity of cTnC to Ca2�. The cardiacthin filament may be less activated than the skeletal thinfilament, even under conditions of Ca2� saturation.7 Howsarcomere length and cTnI-Thr144 affects any of theseprocesses cannot be determined from the present study. It ispossible that Thr144 residue itself is a length sensor or thatabsence of Thr144 simply masks the length sensing mecha-nisms. Modulation of Ca2� sensitivity by sarcomere length

may involve regulation of Ca2� transduction by Tn possiblyvia the cooperative spread of thin filament activation com-municated by tropomyosin, and this phenomenon may requirethe presence of Thr144 in TnI. Introduction of Pro144 in cTnInot only interrupts this process but also reduces the gain ofCa2� transduction as evidenced by the reduction in myofila-ment Ca2� sensitivity and the nonparallel changes in cooper-ativity on Tn exchange. The recombinant Tns used here areunphosphorylated, and this may explain the observed changesin cooperative activation.

Myofilament-length dependence is affected by mechanicalstrain on titin, a large sarcomeric protein known to makemultiple interactions with both thin and thick filament pro-teins.8 It is possible that titin strain affects the interactionbetween cTnI and actin only in the presence of Thr144 withincTnI. Arguing against this hypothesis is the lack of anysignificant changes in passive force development among the4 groups of Tn-exchanged skinned trabeculae (online datasupplement). Finally, the finding that presence of cTnI-Pro144 in Tn greatly diminishes length-dependent propertiesof the sarcomere provides for a novel investigative avenue todetermine the molecular mechanisms that underlie myofila-ment length–dependent activation.

AcknowledgmentsWe thank Dr Katherine Sheehan for help with the confocal imagesand Drs Tom Irving and David Gore for assistance with the x-raydiffraction experiments. Use of the Advanced Photon Source wassupported by the US Department of Energy, Basic Energy Sciences,Office of Science, under contract no. W-31-109-ENG-38. BioCAT isa NIH-supported Research Center (RR-08630). The content of thisreport is the sole responsibility of the authors and does not neces-sarily reflect the official views of the National Center for ResearchResources or the NIH.

Sources of FundingThis study was supported by AHA pre-doctoral Fellowship0615597Z and NIH grants PO1-HL62426, RO1-HL75494.

DisclosuresNone.

References1. Konhilas JP, Irving TC, De Tombe PP. Frank-Starling law of the heart and

the cellular mechanisms of length-dependent activation. PflugersArch. 2002;445:305–310.

2. Kobayashi T, Solaro RJ. Calcium, thin filaments, and the integrativebiology of cardiac contractility. Annu Rev Physiol. 2005;67:39–67.

3. Sumandea MP, Pyle WG, Kobayashi T, de Tombe PP, Solaro RJ. Identi-fication of a functionally critical protein kinase C phosphorylation residueof cardiac troponin T. J Biol Chem. 2003;278:35135–35144.

4. Chandra M, Rundell VL, Tardiff JC, Leinwand LA, De Tombe PP, SolaroRJ. Ca(2�) activation of myofilaments from transgenic mouse heartsexpressing R92Q mutant cardiac troponin T. Am J Physiol. 2001;280:H705–713.

5. Fuchs F, Smith SH. Calcium, cross-bridges, and the Frank-Starling rela-tionship. News Physiol Sci. 2001;16:5–10.

6. Vassylyev DG, Takeda S, Wakatsuki S, Maeda K, Maeda Y. Crystalstructure of troponin C in complex with troponin I fragment at 2.3-Aresolution. Proc Natl Acad Sci U S A. 1998;95:4847–4852.

7. Gordon AM, Homsher E, Regnier M. Regulation of contraction in striatedmuscle. Physiol Rev. 2000;80:853–924.

8. Granzier HL, Labeit S. The giant protein titin: a major player in myocardialmechanics, signaling, and disease. Circ Res. 2004;94:284–295.

KEY WORDS: skinned muscle � rat � Frank–Starling relationship

Figure 3. Impact of Thr144 on myofilament-length dependence.Myofilament length–dependent activation was indexed as�EC50, the difference in EC50 between sarcomere lengths 2.0and 2.2 �m. Control represents unexchanged skinned trabecu-lae. The presence of Thr144 in TnI was associated with a signifi-cantly enhanced cardiac myofilament-length dependence.

Tachampa Troponin I and Length Dependence 1083

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CARDIAC TROPONIN-I THREONINE-144: ROLE IN MYOFILAMENT LENGTH DEPENDENT ACTIVATION.

Kittipong Tachampa, Helen Wang, Gerrie P. Farman, Pieter P. de Tombe.

Department of Physiology and Biophysics and Center for Cardiovascular Research, University of Illinois, Chicago IL 60612

ON-LINE DATA SUPPLEMENT Correspondence: Pieter P. de Tombe Department of Physiology and Biophysics University of Illinois, 835 S. Wolcott Chicago IL 60612 Phone:312-355-0259;312-996-1414(FAX). Email:pdetombe@uic.edu

METHODS

cDNA constructs

Cardiac and slow skeletal TnI mutations (cTnIThr>Pro and ssTnIPro>Thr) were

constructed by site-directed mutagenesis (Statagene QuickChange) of murine

TnI cDNA clones according to manufacturer’s protocols. Resultant mutagenesis

products were transformed into XL-1 Blue and purified prior to sequence

verification of the mutated codons.

Troponin expression, purification and troponin complex reconstitution

The recombinant troponins were expressed and purified as previously

described1. The expression and purification of the recombinant murine cardiac

TnT containing an NH2-terminal myc-tag and murine TnC was carried out as

previously described with slight modification of the purification protocol1. Briefly,

reconstitution and purification of intact troponin complex was carried out by

sequential dialysis to remove urea and decreases salt of an equimolar amount of

purified troponin components and purification using Resource-Q (Amersham).

Resultant fractions were analyzed by 12% SDS-PAGE. The fractions containing

pure troponin dialyzed 3 times at 4°C against exchange buffer: (in mM) KCl

(200), MgCl2 (5), EGTA (5), DTT (1), MOPS (20), pH=6.5; aliquots were stored at

-80°C until use.

Exchange of recombinant cardiac troponin into skinned rat trabeculae

Right ventricular rat trabeculae were dissected, chemically permeabilized with

Triton X-100, and attached to T-clips as described1. Exogenous troponin was

exchanged for endogenous troponin by slight modification of previously

described methods1. Briefly, skinned trabeculae were transferred to a 96-well

microtiter plate and incubated in 13µM recombinant cardiac troponin in relaxing

solution overnight at 4°C. The extent of recombinant cTn exchange was

determined after isometric tension measurements by immunoblot analysis of

control versus exchanged fiber bundles using anti-TnT (clone JLT-12, Sigma).

The presence of the 9-amino acid Myc tag at the N terminus of recombinant

cTnT (29) allowed us to separate it from the endogenous mouse cTnT on a 15%

SDS-PAGE (acrylamide:bis-acrylamide ratio 200:1).

Confocal analysis of troponin exchange

Recombinant TnT in the present study included an NH2-terminus myc-tag to

allow for confocal analysis of recombinant troponin exchange and co-localization

with actin. Following troponin exchange skinned trabeculae were fixed in 3.7%

formaldehyde in PBS for 20 minutes followed by washing twice with PBS for 10

minutes. The fixed samples were then incubated with mouse c-myc(9E10)

mouse monoclonal antibody(Santa Cruz Biotech) followed by chicken antimouse

IgG Alexa Fluor®488 (Molecular Probes) and treated with rhodamine-phalloidin.

Confocal images were acquired with a model 2100 LSM (Biorad). Previously,

our group has demonstrated that the presence of the myc-tag does not affect

myofilament function2.

Measurement of isometric tension

The measurement of steady state isometric tension at varied free Ca2+ was

conducted as previously described1. Briefly, sarcomere length was set at 2.2 µm

by laser diffraction. Trabeculae were activated over a range of free [Ca2+] to

measure steady-state isometric tension. Only muscles that maintained greater

then 80% maximal tension were included for analysis.

Data Processing and Statistical Analysis

Force-Ca2+ relationships were fit to a modified Hill equation1. Statistical analyses

were performed ANOVA. p<0.05 was considered statistically significant; data are

presented mean+SEM.

X-ray Diffraction Experiments

The overall experimental arrangement, sample preparation, and protocol have

been described in detail previously 3. Briefly, experiments were performed on the

BioCAT undulator based beamline at the Advanced Photon Source, Argonne

National Laboratory. For the X-ray studies, skinned trabeculae were mounted

between two hooks in a small trough that allowed for collection of the X-ray

patterns. Sarcomere length was adjusted off-line by viewing of the striation

pattern using a long working distance objective [40x] of an inverted microscope

equipped with a CCD video camera. Low angle X-ray diffraction patterns were

collected on a CCD-based X-ray detector. Spacings between the 1,0 and 1,1

equatorial reflections in the diffraction pattern were converted to d10 lattice

spacings using Bragg’s Law.

RESULTS

Online table 1: Summary of Hill Fit data

Online table 1: The isometric tension of control (no exchange) and recombinant

cTn-treated (exchanged) fiber bundles were determined at varying Ca2+

concentrations and at 2 SL’s. Hill coefficients and EC50 were derived from

modified Hill equation. N represents the number of skinned trabeculae used for

each group. Data are presented as mean±SEM.

Parameter Control

(no exchange

n=5)

cTnI

n=7

ssTnIPro>Thr

n=7

ssTnI

n=7

cTnIThr>Pro

n=7

SL (µm) 2.0 2.2 2.0 2.2 2.0 2.2 2.0 2.2 2.0 2.2

Hill

coefficient

4.4±0.5 3.9±0.5 1.9±0.2 2.7±0.2 2.1±0.4 2.2±0.3 1.9±0.3 1.9±0.2 1.7±0.2 3.0±0.3

EC50

(µm)

4.8±0.5 3.7±0.2 4.1±0.4 2.9±0.4 3.1±0.3 2.1±0.4 1.9±0.3 1.7±0.2 2.8±0.3 2.5±0.3

∆EC50

(µm)

1.1±0.3 1.3±0.3 0.9±0.1 0.2±0.04 0.3±0.06

Online figure 1. Substitution of endogenous cTn by recombinant cTn

complex: Representative SDS-PAGE & western blot demonstrating the

efficiency of recombinant troponin complex exchange in skinned rat cardiac

trabeculae. Detection of cTnT is by an cTnT antibody. Endogenous cTnT

migrates faster than TnT-cmyc, thus allowing quatitation of exchange. Lane 1 is a

representative trabecula that was used in mechanic measurements for the native

control group (non-exchanged muscle). Lane 2,3,4, and 5 are representative

fibers bundle from the cTnI-wt, ssTnIPro>Thr, cTnIThr>Pro, and ssTnI exchange

group, respectively.

1 2 3 4 5

MycTnT

TnT

7

0.1 1 10 1000.0

0.5

1.0

Calcium (µµµµM)

No

rmal

ized

fo

rce

Online figure 2a: Representative raw traces of force development (top) at various

Ca2+ concentrations and force-Ca2+ relationship (bottom) for cTnI exchange.

cTnI

30 mN

30 mN

30 mN

50 µµµµM

50 µµµµM

0.001µµµµM 1.2 µµµµM

2.25 µµµµM 3.6 µµµµM 4.5 µµµµM

8.2 µµµµM

8

0.1 1 10 1000.0

0.5

1.0

Calcium (µµµµM)

No

rmal

ized

fo

rce

Online figure 2b: Representative raw traces of force development (top) at various

Ca2+ concentrations and force-Ca2+ relationship (bottom) for ssTnIPro>Thr

exchange.

ssTnIPro>Thr

30 mN

30 mN

30 mN

50 µµµµM

50 µµµµM

0.001 µµµµM 0.97 µµµµM

2.25 µµµµM 3.6 µµµµM 4.5 µµµµM

5.9 µµµµM

9

0.1 1 10 1000.0

0.5

1.0

Calcium (µµµµM)

No

rmal

ized

fo

rce

Online figure 2c: Representative raw traces of force development (top) at various

Ca2+ concentrations and force-Ca2+ relationship (bottom) for ssTnI exchange.

ssTnI

30 mN

30 mN

30 mN

50 µµµµM

50 µµµµM

0.001 µµµµM 0.97 µµµµM

2.25 µµµµM 3.6 µµµµM 4.5 µµµµM

5.9 µµµµM

10

0.1 1 10 1000.0

0.5

1.0

Calcium (µµµµM)

No

rmal

ized

fo

rce

Online figure 2d: Representative raw traces of force development (top) at various

Ca2+ concentrations and force-Ca2+ relationship (bottom) for cTnIThr>Pro

exchange.

cTnIThr>Pro

30 mN

30 mN

30 mN

50 µµµµM

50 µµµµM

0.001 µµµµM 1.6 µµµµM

2.1 µµµµM 3.6 µµµµM 4.5 µµµµM

5.9 µµµµM

11

cTnI ssTnIPro>Thr ssTnI cTnIThr>Pro

0.00

0.15

0.30

∆∆ ∆∆F

pas

s (m

N/m

m2 )

Online figure 3: Difference in passive force between SL 2.0 µm and 2.2 µm in

relaxed skinned rat cardiac trabeculae following exchange with recombinant

troponin containing cardiac TnI, slow skeletal TnI or the mutants. There were no

significant differences between the groups.

cTnI ssTnIPro>Thr ssTnI cTnIThr>Pro

0

5

10

∆∆ ∆∆ l

atti

ce s

pac

ing

(nm

)

Online figure 4: Difference in myofilament lattice spacing between SL 2.0 µm and

2.2 µm in relaxed skinned rat cardiac trabeculae following exchange with

recombinant troponin containing cardiac TnI, slow skeletal TnI or the mutants.

There were no significant differences between the groups.

12

RERERENCES

1. Sumandea MP, Pyle WG, Kobayashi T, de Tombe PP, Solaro RJ. Identification of a functionally critical protein kinase C phosphorylation residue of cardiac troponin T. The Journal of biological chemistry. 2003;278(37):35135-35144.

2. Chandra M, Rundell VL, Tardiff JC, Leinwand LA, De Tombe PP, Solaro RJ. Ca(2+) activation of myofilaments from transgenic mouse hearts expressing R92Q mutant cardiac troponin T. American journal of physiology. 2001;280(2):H705-713.

3. Irving TC, Konhilas J, Perry D, Fischetti R, de Tombe PP. Myofilament lattice spacing as a function of sarcomere length in isolated rat myocardium. American journal of physiology. 2000;279(5):H2568-H2573.

Kittipong Tachampa, Helen Wang, Gerrie P. Farman and Pieter P. de TombeDependent Activation−Cardiac Troponin I Threonine 144: Role in Myofilament Length

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