Enhanced Ca2+ binding of cardiac troponin reduces sarcomere length dependence of contractile activation independently of strong crossbridges

Calcium sensitivity of the force-pCa relationship depends strongly on sarcomere length (SL) in cardiac muscle and is considered to be the cellular basis of the Frank-Starling law of the heart. SL dependence may involve changes in myofilament lattice spacing and/or myosin crossbridge orientation to increase probability of binding to actin at longer SLs. We used the L48Q cardiac troponin C (cTnC) variant, which has enhanced Ca2+ binding affinity, to test the hypotheses that the intrinsic properties of cTnC are important in determining 1) thin filament binding site availability and responsiveness to crossbridge activation and 2) SL dependence of force in cardiac muscle. Trabeculae containing L48Q cTnC-cTn lost SL dependence of the Ca2+ sensitivity of force. This occurred despite maintaining the typical SL-dependent changes in maximal force (Fmax). Osmotic compression of preparations at SL 2.0 μm with 3% dextran increased Fmax but not pCa50 in L48Q cTnC-cTn exchanged trabeculae, whereas wild-type (WT)-cTnC-cTn exchanged trabeculae exhibited increases in both Fmax and pCa50. Furthermore, crossbridge inhibition with 2,3-butanedione monoxime at SL 2.3 μm decreased Fmax and pCa50 in WT cTnC-cTn trabeculae to levels measured at SL 2.0 μm, whereas only Fmax was decreased with L48Q cTnC-cTn. Overall, these results suggest that L48Q cTnC confers reduced crossbridge dependence of thin filament activation in cardiac muscle and that changes in the Ca2+ sensitivity of force in response to changes in SL are at least partially dependent on properties of thin filament troponin.

Download Full-text

The effect of altered temperature on Ca2(+)-sensitive force in permeabilized myocardium and skeletal muscle. Evidence for force dependence of thin filament activation.

The Journal of General Physiology ◽

10.1085/jgp.96.6.1221 ◽

1990 ◽

Vol 96 (6) ◽

pp. 1221-1245 ◽

Cited By ~ 69

Author(s):

N K Sweitzer ◽

R L Moss

Keyword(s):

Skeletal Muscle ◽

Cardiac Muscle ◽

Thin Filament ◽

Calcium Sensitivity ◽

Differential Sensitivity ◽

Fiber Types ◽

Tension Development ◽

Maximum Tension ◽

Filament Activation ◽

Muscle Types

The effect of changes in temperature on the calcium sensitivity of tension development was examined in permeabilized cellular preparations of rat ventricle and rabbit psoas muscle. Maximum force and Ca2+ sensitivity of force development increased with temperature in both muscle types. Cardiac muscle was more sensitive to changes in temperature than skeletal muscle in the range 10-15 degrees C. It was postulated that the level of thin filament activation may be decreased by cooling. To investigate this possibility, troponin C (TnC) was partially extracted from both muscle types, thus decreasing the level of thin filament activation independent of temperature and, at least in skeletal muscle fibers, decreasing cooperative activation of the thin filament as well. TnC extraction from cardiac muscle reduced the calcium sensitivity of tension less than did extraction of TnC from skeletal muscle. In skeletal muscle the midpoint shift of the tension-pCa curve with altered temperature was greater after TnC extraction than in control fibers. Calcium sensitivity of tension development was proportional to the maximum tension generated in cardiac or skeletal muscle under all conditions studied. Based on these results, we conclude that (a) maximum tension-generating capability and calcium sensitivity of tension development are related, perhaps causally, in fast skeletal and cardiac muscles, and (b) thin filament activation is less cooperative in cardiac muscle than in skeletal muscle, which explains the differential sensitivity of the two fiber types to temperature and TnC extraction. Reducing thin filament cooperativity in skeletal muscle by TnC extraction results in a response to temperature similar to that of control cardiac cells. This study provides evidence that force levels in striated muscle influence the calcium binding affinity of TnC.

Download Full-text

Skeletal and Cardiac Muscle Contractile Activation: Tropomyosin “Rocks and Rolls”

Physiology ◽

10.1152/physiologyonline.2001.16.2.49 ◽

2001 ◽

Vol 16 (2) ◽

pp. 49-55 ◽

Cited By ~ 14

Author(s):

A. M. Gordon ◽

M. Regnier ◽

E. Homsher

Keyword(s):

Cardiac Muscle ◽

Thin Filament ◽

Sarcomere Length ◽

Force Development ◽

Cross Bridge ◽

Functional Differences ◽

Filament Structure ◽

Muscle Types ◽

Contractile Activation ◽

Muscle Contractile

Changes in thin filament structure induced by Ca2+ binding to troponin and subsequent strong cross-bridge binding regulate additional strong cross-bridge attachment, force development, and dependence of force on sarcomere length in skeletal and cardiac muscle. Variations in activation properties account for functional differences between these muscle types.

Download Full-text

Effect of length and cross-bridge attachment on Ca2+ binding to cardiac troponin C

AJP Cell Physiology ◽

10.1152/ajpcell.1987.253.1.c90 ◽

1987 ◽

Vol 253 (1) ◽

pp. C90-C96 ◽

Cited By ~ 110

Author(s):

P. A. Hofmann ◽

F. Fuchs

Keyword(s):

Cardiac Muscle ◽

Cardiac Troponin ◽

Sarcomere Length ◽

Troponin C ◽

Saturation Curve ◽

Cross Bridge ◽

Cardiac Troponin C ◽

Length Dependence ◽

Length Relationship ◽

Cross Bridges

The sensitivity of skinned cardiac muscle bundles to Ca2+ is a function of sarcomere length. Ca2+ sensitivity is increased as fiber length is extended along the ascending limb of the force-length curve and it has been suggested that this phenomenon makes a major contribution to the steep force-length relationship that exists in living cardiac muscle. To gain greater insight into the mechanism behind the length dependence of Ca2+ sensitivity isotopic measurements of Ca2+ binding to detergent-extracted bovine, ventricular muscle bundles were made under conditions in which troponin C was the only major Ca2+ binding species. Experiments were designed to determine whether 1) Ca2+-troponin C affinity varies in the sarcomere length range corresponding to the ascending limb of the force-length curve, and 2) Ca2+ binding correlates with length per se or with changes in the number of length-dependent cross-bridge attachments. Measurements were made of Ca2+ binding in the rigor and relaxed states. The latter state was produced by suppressing actin-myosin interaction with the phosphate analogue, sodium vanadate. After vanadate treatment it is possible to obtain a complete Ca2+ saturation curve in the presence of physiological MgATP concentrations and at constant sarcomere length. The results show that the binding of Ca2+ to the regulatory site of cardiac troponin C is length dependent but this length dependence is actually a dependence on the number of attached cross bridges.

Download Full-text

Contributions of Ca 2+ -Independent Thin Filament Activation to Cardiac Muscle Function

Biophysical Journal ◽

10.1016/j.bpj.2015.09.028 ◽

2015 ◽

Vol 109 (10) ◽

pp. 2101-2112 ◽

Cited By ~ 17

Author(s):

Yasser Aboelkassem ◽

Jordan A. Bonilla ◽

Kimberly J. McCabe ◽

Stuart G. Campbell

Keyword(s):

Cardiac Muscle ◽

Muscle Function ◽

Thin Filament ◽

Filament Activation

Download Full-text

Phosphorylation of Cardiac Troponin I at Tyrosine 26 Decreases Thin Filament Calcium Sensitivity

Biophysical Journal ◽

10.1016/j.bpj.2013.11.3993 ◽

2014 ◽

Vol 106 (2) ◽

pp. 723a

Author(s):

Hussam Salhi

Keyword(s):

Cardiac Troponin ◽

Thin Filament ◽

Troponin I ◽

Calcium Sensitivity ◽

Cardiac Troponin I

Download Full-text

Abstract 226: Elucidating the Mechanism of Reduced Length-dependent Activation Due to a Dilated Cardiomyopathy-associated Mutation in Tropomyosin

Circulation Research ◽

10.1161/res.121.suppl_1.226 ◽

2017 ◽

Vol 121 (suppl_1) ◽

Author(s):

Joseph D Powers ◽

Farid Moussavi-Harami ◽

Maria Razumova ◽

Jil Tardiff ◽

Michael Regnier

Keyword(s):

Dilated Cardiomyopathy ◽

Thin Filament ◽

Calcium Sensitivity ◽

Experimental Chamber ◽

Binding Inhibition ◽

Intact Muscle ◽

Filament Activation ◽

State Force ◽

Length Dependent Activation ◽

Subcellular Level

At the subcellular level, the Frank-Starling law of the heart is described by an increase in calcium sensitivity and force with increased sarcomere length (SL). We examine how this relationship is affected by a dilated cardiomyopathy-associated mutation in tropomyosin (D230N, denoted Tm D230N ) by measuring contractility of intact and permeabilized cardiac muscle preparations at short (2.0 μm) and long (2.3 μm) SL. Transgenic mouse hearts containing the Tm D230N mutation have significantly dilated hearts and reduced cardiac output by ~6 months of age. Intact trabeculae were electrically stimulated and paced at 1 Hz with oxygenated solution (30°C) circulating through the experimental chamber, and permeabilized preparations were bathed in solutions (15°C) of progressively increased [Ca 2+ ] for measures of steady-state force. For intact muscle we found that the Tm D230N mutation results in significantly reduced twitch forces at SL 2.0 and 2.3 μm relative to wild-type (WT). Also, WT trabeculae displayed a significant increase in twitch force upon increase in SL (as expected) but Tm D230N trabeculae did not, demonstrating a loss of SL dependence of contraction. In permeabilized preparations, maximal activation (pCa 4.5) of both WT and Tm D230N preparations exhibited significant SL-dependent increases in force. However, at submaximal Ca 2+ (pCa 5.8), where the heart operates, WT preparations had significant increases in force with increasing length (comparing SL 2.0 to 2.3 μm), while this length-dependence of force augmentation in Tm D230N was absent. The increase in pCa 50 (pCa that produces half-maximal force) going from SL 2.0 to 2.3 μm was significantly less for Tm D230N preparations compared to WT, owing to a significantly smaller increase in pCa 50 at SL 2.3 μm (the pCa 50 at SL 2.0 μm was not significantly different between WT and Tm D230N ). These results suggest that the Tm D230N mutation limits an increase in the Ca 2+ sensitivity of contraction as the muscle lengthens by damping thin filament activation. To further examine length-dependent effects of the Tm D230N mutation, future experiments will test conditions that augment cross-bridge binding/inhibition, and other models of dilated cardiomyopathy that inhibit thin filament activation. Funding: HL111197

Download Full-text