Increased Cross-bridge Cycling Kinetics after Exchange of C-terminal Truncated Troponin I in Skinned Rat Cardiac Muscle

The precise mechanism of cardiac troponin I (cTnI) proteolysis in myocardial stunning is not fully understood. Accordingly, we determined the effect of cTnI C terminus truncation on chemo-mechanical transduction in isolated skinned rat trabeculae. Recombinant troponin complex (cTn), containing either mouse cTnI-(1–193) or human cTnI-(1–192) was exchanged into skinned cardiac trabeculae; Western blot analysis confirmed that 60–70% of the endogenous cTn was replaced by recombinant Tn. Incorporation of truncated cTnI induced significant reductions (∼50%) in maximum force and cooperative activation as well as increases (∼50%) in myofilament Ca2+ sensitivity and tension cost. Similar results were obtained with either mouse or human truncated cTn. Presence of truncated cTnI increased maximum actin-activated S1 ATPase activity as well as its Ca2+ sensitivity in vitro. Partial exchange (50%) for truncated cTnI resulted in similar reductions in maximum force and cooperativity; tension cost was increased in proportion to truncated cTnI content. In vitro, to determine the molecular mechanism responsible for the enhanced myofilament Ca2+ sensitivity, we measured Ca2+ binding to cTn as reported using a fluorescent probe. Incorporation of truncated cTnI did not affect Ca2+ binding affinity to cTn alone. However, when cTn was incorporated into thin filaments, cTnI truncation induced a significant increase in Ca2+ binding affinity to cTn. We conclude that cTnI truncation induces depressed myofilament function. Decreased cardiac function after ischemia/reperfusion injury may directly result, in part, from proteolytic degradation of cTnI, resulting in alterations in cross-bridge cycling kinetics. The precise mechanism of cardiac troponin I (cTnI) proteolysis in myocardial stunning is not fully understood. Accordingly, we determined the effect of cTnI C terminus truncation on chemo-mechanical transduction in isolated skinned rat trabeculae. Recombinant troponin complex (cTn), containing either mouse cTnI-(1–193) or human cTnI-(1–192) was exchanged into skinned cardiac trabeculae; Western blot analysis confirmed that 60–70% of the endogenous cTn was replaced by recombinant Tn. Incorporation of truncated cTnI induced significant reductions (∼50%) in maximum force and cooperative activation as well as increases (∼50%) in myofilament Ca2+ sensitivity and tension cost. Similar results were obtained with either mouse or human truncated cTn. Presence of truncated cTnI increased maximum actin-activated S1 ATPase activity as well as its Ca2+ sensitivity in vitro. Partial exchange (50%) for truncated cTnI resulted in similar reductions in maximum force and cooperativity; tension cost was increased in proportion to truncated cTnI content. In vitro, to determine the molecular mechanism responsible for the enhanced myofilament Ca2+ sensitivity, we measured Ca2+ binding to cTn as reported using a fluorescent probe. Incorporation of truncated cTnI did not affect Ca2+ binding affinity to cTn alone. However, when cTn was incorporated into thin filaments, cTnI truncation induced a significant increase in Ca2+ binding affinity to cTn. We conclude that cTnI truncation induces depressed myofilament function. Decreased cardiac function after ischemia/reperfusion injury may directly result, in part, from proteolytic degradation of cTnI, resulting in alterations in cross-bridge cycling kinetics. In experiments described here, we investigated the functional significance of losing the C terminus region of cardiac TnI as found in myocardial stunning. Myocardial stunning is a form of postischemic dysfunction that persists after restoration of normal coronary flow in the absence of irreversible damage. It has become increasingly evident that myocardial stunning may contribute significantly to the morbidity associated with coronary artery disease (1Bolli R. Marban E. Physiol. Rev. 1999; 79: 609-634Crossref PubMed Scopus (872) Google Scholar). As such, elucidation of molecular mechanisms underlying myocardial stunning is critical to aid in the development of novel therapeutic strategies. Proteolytic degradation of cardiac cTn I (cTnI) 2The abbreviations used are: cTnI, cardiac troponin I; Tn, troponin; cTn, cardiac troponin; cTnT, cardiac troponin T; cTnC, cardiac troponin C; MOPS, 3-N-morpholinopropane-sulfonic acid; WT, wild type; IAANS, 2-(4′-iodoacetamido)anilinonaphthalene-6-sulfonic acid. has emerged has a potential cellular mechanism underlying the depressed contractile function seen in myocardial stunning, possibly as the result of Ca2+ activation of the protease calpain-I upon reperfusion (1Bolli R. Marban E. Physiol. Rev. 1999; 79: 609-634Crossref PubMed Scopus (872) Google Scholar) and/or increased mechanical load on the heart (2Feng J. Schaus B.J. Fallavollita J.A. Lee T.C. Canty Jr., J.M. Circulation. 2001; 103: 2035-2037Crossref PubMed Scopus (239) Google Scholar). However, these findings are species-dependent (i.e. no degradation of cTnI was observed in larger animal models of myocardial stunning, such as the pig (3Thomas S.A. Fallavollita J.A. Lee T.C. Feng J. Canty Jr., J.M. Circ. Res. 1999; 85: 446-456Crossref PubMed Scopus (76) Google Scholar) or dog (4Colantonio D.A. Van Eyk J.E. Przyklenk K. Cardiovasc. Res. 2004; 63: 217-225Crossref PubMed Scopus (32) Google Scholar)). In addition, although application of activated calpain-I to rodent skinned myocardium results in depressed myofilament function (5Gao W.D. Atar D. Liu Y. Perez N.G. Murphy A.M. Marban E. Cir. Res. 1997; 80: 393-399Crossref PubMed Scopus (354) Google Scholar, 6Papp Z. van der Velden J. Stienen G.J. Cardiovasc. Res. 2000; 45: 981-993Crossref PubMed Scopus (72) Google Scholar), this effect is reported to be correlated with the degradation of cTnI in some (5Gao W.D. Atar D. Liu Y. Perez N.G. Murphy A.M. Marban E. Cir. Res. 1997; 80: 393-399Crossref PubMed Scopus (354) Google Scholar) but not all studies (6Papp Z. van der Velden J. Stienen G.J. Cardiovasc. Res. 2000; 45: 981-993Crossref PubMed Scopus (72) Google Scholar). Importantly, cTnI degradation has been reported in both in human cardiac tissue obtained during coronary artery bypass surgery (7McDonough J.L. Labugger R. Pickett W. Tse M.Y. MacKenzie S. Pang S.C. Atar D. Ropchan G. Van Eyk J.E. Circulation. 2001; 103: 58-64Crossref PubMed Scopus (107) Google Scholar) and in explanted hearts with transplantation (8Neagoe C. Kulke M. del Monte F. Gwathmey J.K. de Tombe P.P. Hajjar R.J. Linke W.A. Circulation. 2002; 106: 1333-1341Crossref PubMed Scopus (291) Google Scholar). Finally, overexpression of truncated rat cTnI in transgenic mice results in a recapitulation of the “stunned phenotype” (i.e. reduced cardiac function in vivo and reduced myofilament force in vitro) (9Murphy A.M. Kogler H. Georgakopoulos D. McDonough J.L. Kass D.A. Van Eyk J.E. Marban E. Science. 2000; 287: 488-491Crossref PubMed Scopus (215) Google Scholar). Hence, post-translational alterations in myofilament proteins, particularly C-terminal truncations of cTnI, probably play a central role in the depressed myocardial function that is seen in myocardial stunning. A proteolytic alteration in cTnI has the potential to significantly alter thin filament activation processes and also, as we show here, affect cross-bridge stability and cycling kinetics. The proteolytic damage to cTnI following ischemia/reperfusion has been identified as cleavage of 17 amino acid residues from the C terminus of cTnI. In addition to the inhibitory region of cTnI, biochemical and biophysical studies have identified the C terminus of the molecule as an additional regulatory domain with significant binding affinity to actin (10Kobayashi T. Solaro R.J. Annu. Rev. Physiol. 2005; 67: 39-67Crossref PubMed Scopus (280) Google Scholar) and required for the maximum inhibition of the cardiac thin filament. During diastole, several regions of cTnI hold the cardiac sarcomere in the relaxed state by binding to actin and in concert with TNT maintaining the position of tropomyosin on the outer domain of actin blocking the formation of strong binding cross-bridges (blocked state) (11Kobayashi T. Yang X. Walker L.A. Van Breemen R.B. Solaro R.J. J. Mol. Cell. Cardiol. 2005; 38: 213-218Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar, 12Geeves M.A. Holmes K.C. Adv. Protein Chem. 2005; 71: 161-193Crossref PubMed Scopus (315) Google Scholar). However, the molecular mechanisms by which truncation of cTnI at its C terminus affect myofilament functions are not well understood. In vitro analysis has shown that deletion at the C terminus induces an increase in solution ATPase activity as well as decreased force development, as determined in the motility assay (13Rarick H.M. Tu X.H. Solaro R.J. Martin A.F. J. Biol. Chem. 1997; 272: 26887-26892Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar, 14Foster D.B. Noguchi T. VanBuren P. Murphy A.M. Van Eyk J.E. Circ. Res. 2003; 93: 917-924Crossref PubMed Scopus (66) Google Scholar). However, in contrast to the effects of stunning on myofilament response to Ca2+, the presence of truncated cTnI in reconstituted preparations induces an increase in apparent myofilament Ca2+ sensitivity, both in solution and in the motility assay (14Foster D.B. Noguchi T. VanBuren P. Murphy A.M. Van Eyk J.E. Circ. Res. 2003; 93: 917-924Crossref PubMed Scopus (66) Google Scholar, 15Narolska N.A. Piroddi N. Belus A. Boontje N.M. Scellini B. Deppermann S. Zaremba R. R.J. C. K. D.B. Murphy A.M. van Eyk J.E. C. C. van der Velden J. Stienen G.J. Circ. Res. PubMed Scopus Google Scholar). myofilament Ca2+ sensitivity, but not decreased has been in human skinned and exchanged with truncated human cTnI N.A. Piroddi N. Belus A. Boontje N.M. Scellini B. Deppermann S. Zaremba R. R.J. C. K. D.B. Murphy A.M. van Eyk J.E. C. C. van der Velden J. Stienen G.J. Circ. Res. PubMed Scopus Google Scholar). In experiments reported here, we exchanged endogenous cTn in skinned rat cardiac with a recombinant complex containing either wild cTnI or truncated cTnI to myocardial stunning and determined chemo-mechanical transduction in skinned rat cardiac trabeculae. We found increases in cross-bridge and myofilament Ca2+ sensitivity and reductions in maximum force development and of In addition, in vitro actin experiments confirmed increased cross-bridge and increased Ca2+ of Ca2+ binding to a in the affinity induced by the presence of truncated cTnI but when was reconstituted into the thin filament. novel into the molecular mechanisms that the of truncated cTnI on myofilament function. In addition, that in normal cardiac the C terminus of cTnI may play an role in the of cooperative myofilament activation in the heart a with the thin filament. terminus truncated and human cardiac TnI were by from and human cTnI to the The and were used to a at in mouse cTnI to human cTnI-(1–192) was obtained using and were into and to of the The of cardiac were as described T. Solaro R.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). and cTn recombinant were and as described T. Solaro R.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, M. L.A. Tombe P.P. Solaro R.J. J. Physiol. 2001; PubMed Google Scholar, T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). The and of the recombinant human cardiac TNT containing an was as described with of the M. L.A. Tombe P.P. Solaro R.J. J. Physiol. 2001; PubMed Google Scholar, T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). and of complex was by to and of an of cTn and using M. L.A. Tombe P.P. Solaro R.J. J. Physiol. 2001; PubMed Google Scholar, T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). were by The containing cTn were at exchange MOPS, were at of Recombinant cTn into were from of after with to T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, Martin A.F. de Tombe P.P. J. Physiol. 2005; PubMed Scopus Google Scholar). was exchanged for endogenous by of described T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). were to a and in recombinant cardiac in solution at The of the skinned has been described in T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, Martin A.F. de Tombe P.P. J. Physiol. 2005; PubMed Scopus Google Scholar). of and ATPase of state tension and ATPase activity a of Ca2+ was as described T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, Martin A.F. de Tombe P.P. J. Physiol. 2005; PubMed Scopus Google Scholar). was at by was to which was measured in a As in Ca2+ activation induced force development and of in the the of which during state force were activated a of to tension and ATPase In some the of force following a was measured during a at maximum Ca2+ as described Martin A.F. de Tombe P.P. J. Physiol. 2005; PubMed Scopus Google Scholar). that tension were for the mechanical was in and for biochemical of cTn by Western TNT in the an terminus to for of the of cTn exchange by Western M. L.A. Tombe P.P. Solaro R.J. J. Physiol. 2001; PubMed Google Scholar, T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). has that the presence of this not affect myofilament function M. L.A. Tombe P.P. Solaro R.J. J. Physiol. 2001; PubMed Google Scholar). binding was measured by of to of a for the cTn complex and of a for reconstituted thin as described T. Solaro R.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). The MOPS, with Ca2+ were at the were for cTn ATPase assay was used (13Rarick H.M. Tu X.H. Solaro R.J. Martin A.F. J. Biol. Chem. 1997; 272: 26887-26892Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar, 14Foster D.B. Noguchi T. VanBuren P. Murphy A.M. Van Eyk J.E. Circ. Res. 2003; 93: 917-924Crossref PubMed Scopus (66) Google Scholar). the were as Tn, MOPS, and of The was by the addition of The ATPase activity was determined from the of determined using the to a was and the was by the addition of of The ATPase of S1 was from all and were to a M. L.A. Tombe P.P. Solaro R.J. J. Physiol. 2001; PubMed Google Scholar, T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar, Martin A.F. de Tombe P.P. J. Physiol. 2005; PubMed Scopus Google tension cost was determined by to the were either as or or analysis of as was are Recombinant cTn used the cTn exchange to either wild or truncated cTnI into the cardiac As we reported M. L.A. Tombe P.P. Solaro R.J. J. Physiol. 2001; PubMed Google Scholar, T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar), this no alterations in the and of the (i.e. exchanged skinned a In addition, maximum Ca2+ force development was not significantly and wild skinned not a and Western blot analysis of the recombinant cardiac cTn exchange The absence of 17 amino at the C terminus induced a of cTnI-(1–193) as with as is apparent both in the shown in A and in the cTnI Western blot shown in and with In addition, the presence of a acid at the terminus of recombinant for the of from endogenous by and by Western blot M. L.A. Tombe P.P. Solaro R.J. J. Physiol. 2001; PubMed Google Scholar, T. de Tombe P.P. Solaro R.J. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar), as C. analysis on endogenous cTnI was exchanged for and for cTnI-(1–193) in such as in and of C-terminal on the effect of cTnI truncation on myofilament function by of the and in skinned cardiac exchanged with containing either wild or C terminus cTnI. As in of cTnI-(1–193) was on with an of maximum tension development and an increase in myofilament Ca2+ sensitivity, by the of the In addition, was a significant in of myofilament activation of the on the was reduced to a in the cTnI-(1–193) As a result, was an increase in the of to as apparent from the increase in the of the activity the presence of cTnI-(1–193) in the sarcomere is associated with a significant increased tension cost. The and tension cost obtained in are in transduction obtained in skinned cardiac exchanged with cardiac troponin containing wild cTnI truncated cTnI a or human truncated cTnI for are cost in a In myocardial stunning, proteolytic degradation of TnI at the C terminus a of the TnI in the cardiac to determine the of myofilament dysfunction on the of cTnI-(1–193) exchanged into the we exchange experiments with a of wild and cTnI-(1–193) cTn. As shown in of cTnI-(1–193) did not affect all of myofilament Ca2+ activation (i.e. cTnI-(1–193) reduced maximum tension and to a similar to that seen with exchange with cTn containing the the increase in myofilament Ca2+ sensitivity was observed in the exchanged with cTn containing cost was the that increased in proportion to cTnI-(1–193) content. In addition to of tension we determined cross-bridge a to determine the of force following a As by the force obtained in a shown in in which the were to an the was significantly enhanced in the presence of was in the cTnI-(1–193) are with the increase in tension cost as determined from the of force and ATPase activity and We determined the effects of truncation of cTnI on the or the cTn into the rat cardiac The in show as was the with all mouse cTn, the presence of human cTnI-(1–192) was associated with a significant increases in myofilament Ca2+ sensitivity and with maximum tension development and were significantly reduced in the exchanged with human cTn containing to a that observed with mouse cTn. exchange with cTn containing either mouse or human truncated TnI similar effects on myofilament chemo-mechanical transduction was depressed following exchange with human truncated cardiac TnI as with wild cardiac troponin exchange are as of cTnI on presence of truncated cTnI resulted in significant increases in maximum actin-activated activity and apparent Ca2+ sensitivity In contrast to the skinned results and the was not by cTnI of cTnI on cTn into the molecular mechanisms underlying the effect of C cTnI on myofilament we measured the affinity of Ca2+ binding to cTn. Ca2+ of either cTn in or cTn reconstituted thin Ca2+ binding was using a to The presence of cTnI-(1–193) in cardiac cTn did not affect Ca2+ binding However, when cTn with its in reconstituted thin (i.e. actin cTnI-(1–193) induced a increase in Ca2+ binding Similar results were obtained in are in the to novel into the molecular mechanisms underlying the role of TnI truncation in the contractile dysfunction seen in myocardial stunning. The and novel findings of were that the presence of both and of truncated cTnI in the cardiac sarcomere resulted in decreased myofilament decreased of Ca2+ and increased myofilament Ca2+ truncated cTnI induced an increased of cross-bridge in proportion to the of in vitro reconstituted actin-activated increased maximum activity and Ca2+ sensitivity but cooperativity; and in vitro Ca2+ binding affinity that these effects a troponin and the thin filament. are with a molecular of cardiac activation by Ca2+ in which an additional the C terminus of cTnI and actin functions to the mechanism may result in both reduced myofilament Ca2+ sensitivity and an increase in the of cross-bridges and enhanced cooperative The results of the are with studies the of C terminus truncation of cTnI on myofilament function. The of amino acid residues from the C terminus of cTnI is reported to result in increased Ca2+ sensitivity in solution ATPase activity (13Rarick H.M. Tu X.H. Solaro R.J. Martin A.F. J. Biol. Chem. 1997; 272: 26887-26892Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar, 14Foster D.B. Noguchi T. VanBuren P. Murphy A.M. Van Eyk J.E. Circ. Res. 2003; 93: 917-924Crossref PubMed Scopus (66) Google Scholar). In addition, reduced force development has been but this was on an using a in vitro motility assay (14Foster D.B. Noguchi T. VanBuren P. Murphy A.M. Van Eyk J.E. Circ. Res. 2003; 93: 917-924Crossref PubMed Scopus (66) Google Scholar). using a of chemo-mechanical transduction in skinned cardiac we that the presence of truncated cTnI in the cardiac sarcomere induces reduced maximum myofilament force development and force was not reported by N.A. Piroddi N. Belus A. Boontje N.M. Scellini B. Deppermann S. Zaremba R. R.J. C. K. D.B. Murphy A.M. van Eyk J.E. C. C. van der Velden J. Stienen G.J. Circ. Res. PubMed Scopus Google Scholar). In that recombinant human cTn was exchanged for endogenous cTn in human or of with that exchange with cTn containing human truncated cTnI in the was associated with a of maximum force human which that the of cTnI truncation on maximum force on residues cTnI at the C In addition, associated with of force in or in the by N.A. Piroddi N. Belus A. Boontje N.M. Scellini B. Deppermann S. Zaremba R. R.J. C. K. D.B. Murphy A.M. van Eyk J.E. C. C. van der Velden J. Stienen G.J. Circ. Res. PubMed Scopus Google Scholar) may have of a of maximum Finally, the of human myocardium obtained in the of cardiac transplantation that are not when using myocardium isolated from E. P. J. Mol. Cell. Cardiol. Full Text Full Text PDF PubMed Scopus Google Scholar). to be the of cTnI truncation on maximum force rodent cardiac and human cardiac and this additional As in myocardial stunning W.D. T. D. J. Physiol. PubMed Scopus Google Scholar), truncation of cTnI resulted in increased tension a that is directly to the of from actin Tombe P.P. J. 2003; PubMed Scopus Google Scholar). result that truncation of the C terminus of cTnI of the cross-bridge by the of the the In a cross-bridge the of to the of myofilament force development on the of the to the of the and the the of force development, is to Hence, the increase in tension cost observed in the is with both the increase in and the in maximum force In addition, at the molecular this effect of the C terminus of cTnI to the cross-bridge to be a (i.e. a regulatory the increase in tension cost was directly to the of cTnI in the sarcomere N.A. Piroddi N. Belus A. Boontje N.M. Scellini B. Deppermann S. Zaremba R. R.J. C. K. D.B. Murphy A.M. van Eyk J.E. C. C. van der Velden J. Stienen G.J. Circ. Res. PubMed Scopus Google Scholar) did not an increase in cross-bridge kinetics. However, cross-bridge in that was by the of Ca2+ activation and using solution in the we measured the of Incorporation of truncated cTnI into the sarcomere induces a increase in myofilament Ca2+ Ca2+ activation may become determined by thin filament cross-bridge kinetics. this is the that alterations in cTn Ca2+ binding affinity not affect Tombe P.P. Belus A. Piroddi N. Scellini B. Walker Martin A.F. C. C. J. Physiol. PubMed Scopus Google Scholar). is that the effect of C terminus truncation of cTnI on cross-bridge human and rodent of cTnI was associated with a increase in myofilament Ca2+ sensitivity, with in vitro studies (13Rarick H.M. Tu X.H. Solaro R.J. Martin A.F. J. Biol. Chem. 1997; 272: 26887-26892Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar, 14Foster D.B. Noguchi T. VanBuren P. Murphy A.M. Van Eyk J.E. Circ. Res. 2003; 93: 917-924Crossref PubMed Scopus (66) Google Scholar, 15Narolska N.A. Piroddi N. Belus A. Boontje N.M. Scellini B. Deppermann S. Zaremba R. R.J. C. K. D.B. Murphy A.M. van Eyk J.E. C. C. van der Velden J. Stienen G.J. Circ. Res. PubMed Scopus Google Scholar). In contrast to the results in skinned cooperative activation in the reconstituted in vitro was not by cTnI truncation A significant the is a in the sarcomere that may cross-bridge formation to a regulatory with this is the of that we observed in the reconstituted truncated cTnI may affect cooperative activation of significant S1 mechanical such as both in the motility assay (14Foster D.B. Noguchi T. VanBuren P. Murphy A.M. Van Eyk J.E. Circ. Res. 2003; 93: 917-924Crossref PubMed Scopus (66) Google Scholar) and in the skinned N.A. Piroddi N. Belus A. Boontje N.M. Scellini B. Deppermann S. Zaremba R. R.J. C. K. D.B. Murphy A.M. van Eyk J.E. C. C. van der Velden J. Stienen G.J. Circ. Res. PubMed Scopus Google Scholar) and has not been the increased myofilament Ca2+ sensitivity was the result of a effect of the truncated cTnI on cTn Ca2+ binding not to be the we show for the in that Ca2+ binding affinity to cTn in solution was not by cTnI this required of cTn into the thin filament of actin and Hence, is the the C terminus of cTnI and the thin filament that myofilament Ca2+ the that the role of the C terminus of cTnI is inhibitory in binding to the C terminus of actin may the thin filament in the state M.A. Holmes K.C. Adv. Protein Chem. 2005; 71: 161-193Crossref PubMed Scopus (315) Google Scholar), the cross-bridge the this with actin the affinity of TnI for (14Foster D.B. Noguchi T. VanBuren P. Murphy A.M. Van Eyk J.E. Circ. Res. 2003; 93: 917-924Crossref PubMed Scopus (66) Google Scholar). As a result, a cooperative Ca2+ may be required to cardiac cTn in the presence of the thin filament. of the C terminus of cTnI, the Ca2+ binding affinity to cardiac cTn in the sarcomere to that of a that by is not cooperative and T. Solaro R.J. Annu. Rev. Physiol. 2005; 67: 39-67Crossref PubMed Scopus (280) Google Scholar). The result of this these molecular mechanisms may be a of cooperative activation in the heart that both a development, R. M. G. J. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar, Physiol. Rev. PubMed Scopus Google Scholar), and of contractile a of increased myofilament Ca2+ sensitivity be to contractile in the of reduced maximum force However, the in cooperative activation result in contractile activation a of Ca2+, resulting in cross-bridge cycling that persists well into and during Tombe P.P. J. 2003; PubMed Scopus Google Scholar). at the of an enhanced of of reduced cellular of particularly such as that following the that myocardial stunning. It be that cTnI the C terminus region significantly affect Ca2+ has been well that of the and results in Ca2+ Myocardial stunning is a that complex of cellular alterations to functional and the of of cTnI is but of the alterations found in stunning. of and in and have been reported to be in myocardial stunning M.Y. A.M. G. 2005; PubMed Scopus (95) Google Scholar). these affect myofilament chemo-mechanical transduction in myocardial stunning be determined from the The of recombinant cTn exchange the to the effect of a on myofilament chemo-mechanical In truncation of cardiac TnI at the C terminus by 17 amino acid such as may in myocardial stunning, induces myofilament function in the form of depressed force development and cooperative activation and increased cross-bridge and myofilament Ca2+ The increased Ca2+ affinity an cardiac cTn and the thin the of this inhibitory to in the C terminus of cardiac TnI and may to the of Ca2+ required to the cardiac of the molecular in depressed chemo-mechanical transduction in myocardial stunning aid in the development of novel therapeutic to cardiac function in this