Length-dependent effects on cardiac contractile dynamics are different in cardiac muscle containing α- or β-myosin heavy chain

Cellular Phenotypic Transformation in Heart Failure Caused by Coronary Heart Disease and Dilated Cardiomyopathy: Delineating at Single-Cell Level

Heart failure (HF) is known as the final manifestation of cardiovascular diseases. Although cellular heterogeneity of the heart is well understood, the phenotypic transformation of cardiac cells in progress of HF remains obscure. This study aimed to analyze phenotypic transformation of cardiac cells in HF through human single-cell RNA transcriptome profile. Here, phenotypic transformation of cardiomyocytes (CMs), endothelial cells (ECs), and fibroblasts was identified by data analysis and animal experiments. Abnormal myosin subunits including the decrease in Myosin Heavy Chain 6, Myosin Light Chain 7 and the increase in Myosin Heavy Chain 7 were found in CMs. Two disease phenotypes of ECs named inflammatory ECs and muscularized ECs were identified. In addition, myofibroblast was increased in HF and highly associated with abnormal extracellular matrix. Our study proposed an integrated map of phenotypic transformation of cardiac cells and highlighted the intercellular communication in HF. This detailed definition of cellular transformation will facilitate cell-based mapping of novel interventional targets for the treatment of HF.

Developmental increase in β-MHC enhances sarcomere length-dependent activation in the myocardium

The expression of β-myosin heavy chain (β-MHC) in the guinea pig heart increases during postnatal development. Reda et al. show that this increase in β-MHC enhances length-mediated increases in myofilament Ca²⁺ sensitivity and sarcomere length–dependent changes in contractile function.

Shifts in myosin heavy chain (MHC) isoforms in cardiac myocytes have been shown to alter cardiac muscle function not only in healthy developing hearts but also in diseased hearts. In guinea pig hearts, there is a large age-dependent shift in MHC isoforms from 80% α-MHC/20% β-MHC at 3 wk to 14% α-MHC/86% β-MHC at 11 wk. Because kinetic differences in α- and β-MHC cross-bridges (XBs) are known to impart different cooperative effects on thin filaments, we hypothesize here that differences in α- and β-MHC expression in guinea pig cardiac muscle impact sarcomere length (SL)–dependent contractile function. We therefore measure steady state and dynamic contractile parameters in detergent-skinned cardiac muscle preparations isolated from the left ventricles of young (3 wk old) or adult (11 wk old) guinea pigs at two different SLs: short (1.9 µm) and long (2.3 µm). Our data show that SL-dependent effects on contractile parameters are augmented in adult guinea pig cardiac muscle preparations. Notably, the SL-mediated increase in myofilament Ca²⁺ sensitivity (ΔpCa₅₀) is twofold greater in adult guinea pig muscle preparations (ΔpCa₅₀ being 0.11 units in adult preparations but only 0.05 units in young preparations). Furthermore, adult guinea pig cardiac muscle preparations display greater SL-dependent changes than young muscle preparations in (1) the magnitude of length-mediated increase in the recruitment of new force-bearing XBs, (2) XB detachment rate, (3) XB strain-mediated effects on other force-bearing XBs, and (4) the rate constant of force redevelopment. Our findings suggest that increased β-MHC expression enhances length-dependent activation in the adult guinea pig cardiac myocardium.

Regulatory light chain phosphorylation augments length-dependent contraction in PTU-treated rats

Contraction of cardiac muscle is regulated by sarcomere length and proteins that comprise the sarcomeric filaments. Breithaupt et al. find that phosphorylation of myosin regulatory light chain augments length-dependent activation of contraction when β-cardiac myosin heavy chain predominates.

Force production by actin–myosin cross-bridges in cardiac muscle is regulated by thin-filament proteins and sarcomere length (SL) throughout the heartbeat. Prior work has shown that myosin regulatory light chain (RLC), which binds to the neck of myosin heavy chain, increases cardiac contractility when phosphorylated. We recently showed that cross-bridge kinetics slow with increasing SLs, and that RLC phosphorylation amplifies this effect, using skinned rat myocardial strips predominantly composed of the faster α-cardiac myosin heavy chain isoform. In the present study, to assess how RLC phosphorylation influences length-dependent myosin function as myosin motor speed varies, we used a propylthiouracil (PTU) diet to induce >95% expression of the slower β-myosin heavy chain isoform in rat cardiac ventricles. We measured the effect of RLC phosphorylation on Ca²⁺-activated isometric contraction and myosin cross-bridge kinetics (via stochastic length perturbation analysis) in skinned rat papillary muscle strips at 1.9- and 2.2-µm SL. Maximum tension and Ca²⁺ sensitivity increased with SL, and RLC phosphorylation augmented this response at 2.2-µm SL. Subtle increases in viscoelastic myocardial stiffness occurred with RLC phosphorylation at 2.2-µm SL, but not at 1.9-µm SL, thereby suggesting that RLC phosphorylation increases β-myosin heavy chain binding or stiffness at longer SLs. The cross-bridge detachment rate slowed as SL increased, providing a potential mechanism for prolonged cross-bridge attachment to augment length-dependent activation of contraction at longer SLs. Length-dependent slowing of β-myosin heavy chain detachment rate was not affected by RLC phosphorylation. Together with our previous studies, these data suggest that both α- and β-myosin heavy chain isoforms show a length-dependent activation response and prolonged myosin attachment as SL increases in rat myocardial strips, and that RLC phosphorylation augments length-dependent activation at longer SLs. In comparing cardiac isoforms, however, we found that β-myosin heavy chain consistently showed greater length-dependent sensitivity than α-myosin heavy chain. Our work suggests that RLC phosphorylation is a vital contributor to the regulation of myocardial contractility in both cardiac myosin heavy chain isoforms.

Cardiomyopathy mutation (F88L) in troponin T abolishes length dependency of myofilament Ca2+ sensitivity

The F88L mutation in cardiac troponin T (TnT_F88L) is associated with hypertrophic cardiomyopathy. Reda and Chandra reveal that it abolishes length-mediated increase in myofilament Ca²⁺ sensitivity and attenuates cooperative mechanisms governing length-dependent activation.

Recent clinical studies have revealed a new hypertrophic cardiomyopathy–associated mutation (F87L) in the central region of human cardiac troponin T (TnT). However, despite its implication in several incidences of sudden cardiac death in young and old adults, whether F87L is associated with cardiac contractile dysfunction is unknown. Because the central region of TnT is important for modulating the muscle length–mediated recruitment of new force-bearing cross-bridges (XBs), we hypothesize that the F87L mutation causes molecular changes that are linked to the length-dependent activation of cardiac myofilaments. Length-dependent activation is important because it contributes significantly to the Frank–Starling mechanism, which enables the heart to vary stroke volume as a function of changes in venous return. We measured steady-state and dynamic contractile parameters in detergent-skinned guinea pig cardiac muscle fibers reconstituted with recombinant guinea pig wild-type TnT (TnT_WT) or the guinea pig analogue (TnT_F88L) of the human mutation at two different sarcomere lengths (SLs): short (1.9 µm) and long (2.3 µm). TnT_F88L increases pCa₅₀ (−log [Ca²⁺]_free required for half-maximal activation) to a greater extent at short SL than at long SL; for example, pCa₅₀ increases by 0.25 pCa units at short SL and 0.17 pCa units at long SL. The greater increase in pCa₅₀ at short SL leads to the abolishment of the SL-dependent increase in myofilament Ca²⁺ sensitivity (ΔpCa₅₀) in TnT_F88L fibers, ΔpCa₅₀ being 0.10 units in TnT_WT fibers but only 0.02 units in TnT_F88L fibers. Furthermore, at short SL, TnT_F88L attenuates the negative impact of strained XBs on force-bearing XBs and augments the magnitude of muscle length–mediated recruitment of new force-bearing XBs. Our findings suggest that the TnT_F88L-mediated effects on cardiac thin filaments may lead to a negative impact on the Frank–Starling mechanism.

Omecamtiv Mecarbil Abolishes Length-Mediated Increase in Guinea Pig Cardiac Myofiber Ca2+ Sensitivity

Omecamtiv mecarbil (OM) is a pharmacological agent that augments cardiac contractile function by enhancing myofilament Ca²⁺ sensitivity. Given that interventions that increase myofilament Ca²⁺ sensitivity have the potential to alter length-dependent activation (LDA) of cardiac myofilaments, we tested the influence of OM on this fundamental property of the heart. This is significant not only because LDA is prominent in cardiac muscle but also because it contributes to the Frank-Starling law, a mechanism by which the heart increases stroke volume in response to an increase in venous return. We measured steady-state and dynamic contractile indices in detergent-skinned guinea pig (Cavia porcellus) cardiac muscle fibers in the absence and presence of 0.3 and 3.0 μM OM at two different sarcomere lengths (SLs), short SL (1.9 μm) and long SL (2.3 μm). Myofilament Ca²⁺ sensitivity, as measured by pCa₅₀ (-log of [Ca²⁺]_free concentration required for half-maximal activation), increased significantly at both short and long SLs in OM-treated fibers when compared to untreated fibers; however, the magnitude of increase in pCa₅₀ was twofold greater at short SL than at long SL. A consequence of this greater increase in pCa₅₀ at short SL was that pCa₅₀ did not increase any further at long SL, suggesting that OM abolished the SL dependency of pCa₅₀. Furthermore, the SL dependency of rate constants of cross-bridge distortion dynamics (c) and force redevelopment (k_tr) was abolished in 0.3-μM-OM-treated fibers. The negative impact of OM on the SL dependency of pCa₅₀, c, and k_tr was also observed in 3.0-μM-OM-treated fibers, indicating that cooperative mechanisms linked to LDA were altered by the OM-mediated effects on cardiac myofilaments.

Hypertrophic cardiomyopathy mutation in cardiac troponin T (R95H) attenuates length-dependent activation in guinea pig cardiac muscle fibers

The central region of cardiac troponin T (TnT) is important for modulating the dynamics of muscle length-mediated cross-bridge recruitment. Therefore, hypertrophic cardiomyopathy mutations in the central region may affect cross-bridge recruitment dynamics to alter myofilament Ca²⁺ sensitivity and length-dependent activation of cardiac myofilaments. Given the importance of the central region of TnT for cardiac contractile dynamics, we studied if hypertrophic cardiomyopathy-linked mutation (TnT_R94H)-induced effects on contractile function would be differently modulated by sarcomere length (SL). Recombinant wild-type TnT (TnT_WT) and the guinea pig analog of the human R94H mutation (TnT_R95H) were reconstituted into detergent-skinned cardiac muscle fibers from guinea pigs. Steady-state and dynamic contractile measurements were made at short and long SLs (1.9 and 2.3 µm, respectively). Our results demonstrated that TnT_R95H increased pCa₅₀ (-log of free Ca²⁺ concentration) to a greater extent at short SL; TnT_R95H increased pCa₅₀ by 0.11 pCa units at short SL and 0.07 pCa units at long SL. The increase in pCa₅₀ associated with an increase in SL from 1.9 to 2.3 µm (ΔpCa₅₀) was attenuated nearly twofold in TnT_R95H fibers; ΔpCa₅₀ was 0.09 pCa units for TnT_WT fibers but only 0.05 pCa units for TnT_R95H fibers. The SL dependency of rate constants of cross-bridge distortion dynamics and tension redevelopment was also blunted by TnT_R95H Collectively, our observations on the SL dependency of pCa₅₀ and rate constants of cross-bridge distortion dynamics and tension redevelopment suggest that mechanisms underlying the length-dependent activation cardiac myofilaments are attenuated by TnT_R95HNEW & NOTEWORTHY Mutant cardiac troponin T (TnT_R95H) differently affects myofilament Ca²⁺ sensitivity at short and long sarcomere length, indicating that mechanisms underlying length-dependent activation are altered by TnT_R95H TnT_R95H enhances myofilament Ca²⁺ sensitivity to a greater extent at short sarcomere length, thus attenuating the length-dependent increase in myofilament Ca²⁺ sensitivity.

Cardiomyopathy-related mutation (A30V) in mouse cardiac troponin T divergently alters the magnitude of stretch activation in α- and β-myosin heavy chain fibers

The present study investigated the functional consequences of the human hypertrophic cardiomyopathy (HCM) mutation A28V in cardiac troponin T (TnT). The A28V mutation is located within the NH2 terminus of TnT, a region known to be important for full activation of cardiac thin filaments. The functional consequences of the A28V mutation in TnT remain unknown. Given how α- and β-myosin heavy chain (MHC) isoforms differently alter the functional effect of the NH2 terminus of TnT, we hypothesized that the A28V-induced effects would be differently modulated by α- and β-MHC isoforms. Recombinant wild-type mouse TnT (TnTWT) and the mouse equivalent of the human A28V mutation (TnTA30V) were reconstituted into detergent-skinned cardiac muscle fibers extracted from normal (α-MHC) and transgenic (β-MHC) mice. Dynamic and steady-state contractile parameters were measured in reconstituted muscle fibers. Step-like length perturbation experiments demonstrated that TnTA30V decreased the magnitude of the muscle length-mediated recruitment of new force-bearing cross bridges (ER) by 30% in α-MHC fibers. In sharp contrast, TnTA30V increased ER by 55% in β-MHC fibers. Inferences drawn from other dynamic contractile parameters suggest that directional changes in ER in TnTA30V + α-MHC and TnTA30V + β-MHC fibers result from a divergent impact on cross bridge-regulatory unit (troponin-tropomyosin complex) cooperativity. TnTA30V-mediated effects on Ca2+-activated maximal tension and instantaneous muscle fiber stiffness (ED) were also divergently affected by α- and β-MHC. Our study demonstrates that TnTA30V + α-MHC and TnTA30V + β-MHC fibers show contrasting contractile phenotypes; however, only the observations from β-MHC fibers are consistent with the clinical data for A28V in humans.

NEW & NOTEWORTHY

The differential impact of α- and β-myosin heavy chain (MHC) on contractile dynamics causes a mutant cardiac troponin T (TnTA30V) to differently modulate cardiac contractile function. TnTA30V attenuated Ca2+-activated maximal tension and length-mediated cross-bridge recruitment against α-MHC but augmented these parameters against β-MHC, suggesting divergent contractile phenotypes.

L71F mutation in rat cardiac troponin T augments crossbridge recruitment and detachment dynamics against α-myosin heavy chain, but not against β-myosin heavy chain

The N-terminal extension of human cardiac troponin T (TnT), which modulates myofilament Ca²⁺ sensitivity, contains several hypertrophic cardiomyopathy (HCM)-causing mutations including S69F. However, the functional consequence of S69F mutation is unknown. The human analog of S69F in rat TnT is L71F (TnT_L71F). Because the functional consequences due to structural changes in the N-terminal extension are influenced by the type of myosin heavy chain (MHC) isoform, we hypothesized that the TnT_L71F-mediated effect would be differently modulated by α- and β-MHC isoforms. TnT_L71F and wild-type rat TnT were reconstituted into de-membranated muscle fibers from normal (α-MHC) and propylthiouracil-treated rat hearts (β-MHC) to measure steady-state and dynamic contractile parameters. The magnitude of the TnT_L71F-mediated attenuation of Ca²⁺-activated maximal tension was greater in α- than in β-MHC fibers. For example, TnT_L71F attenuated maximal tension by 31% in α-MHC fibers but only by 10% in β-MHC fibers. Furthermore, TnT_L71F reduced myofilament Ca²⁺ sensitivity by 0.11 pCa units in α-MHC fibers but only by 0.05 pCa units in β-MHC fibers. TnT_L71F augmented rate constants of crossbridge recruitment and crossbridge detachment dynamics in α-MHC fibers but not in β-MHC fibers. Collectively, our data demonstrate that TnT_L71F induces greater contractile deficits against α-MHC than against β-MHC background.

Dilated Cardiomyopathy Mutation (R134W) in Mouse Cardiac Troponin T Induces Greater Contractile Deficits against α-Myosin Heavy Chain than against β-Myosin Heavy Chain

Many studies have demonstrated that depressed myofilament Ca²⁺ sensitivity is common to dilated cardiomyopathy (DCM) in humans. However, it remains unclear whether a single determinant-such as myofilament Ca²⁺ sensitivity-is sufficient to characterize all cases of DCM because the severity of disease varies widely with a given mutation. Because dynamic features dominate in the heart muscle, alterations in dynamic contractile parameters may offer better insight on the molecular mechanisms that underlie disparate effects of DCM mutations on cardiac phenotypes. Dynamic features are dominated by myofilament cooperativity that stem from different sources. One such source is the strong tropomyosin binding region in troponin T (TnT), which is known to modulate crossbridge (XB) recruitment dynamics in a myosin heavy chain (MHC)-dependent manner. Therefore, we hypothesized that the effects of DCM-linked mutations in TnT on contractile dynamics would be differently modulated by α- and β-MHC. After reconstitution with the mouse TnT equivalent (TnT_R134W) of the human DCM mutation (R131W), we measured dynamic contractile parameters in detergent-skinned cardiac muscle fiber bundles from normal (α-MHC) and transgenic mice (β-MHC). TnT_R134W significantly attenuated the rate constants of tension redevelopment, XB recruitment dynamics, XB distortion dynamics, and the magnitude of length-mediated XB recruitment only in α-MHC fiber bundles. TnT_R134W decreased myofilament Ca²⁺ sensitivity to a greater extent in α-MHC (0.14 pCa units) than in β-MHC fiber bundles (0.08 pCa units). Thus, our data demonstrate that TnT_R134W induces a more severe DCM-like contractile phenotype against α-MHC than against β-MHC background.

The effect of cardiomyopathy mutation (R97L) in mouse cardiac troponin T on the muscle length-mediated recruitment of crossbridges is modified divergently by α- and β-myosin heavy chain

Hypertrophic cardiomyopathy mutations in cardiac troponin T (TnT) lead to sudden cardiac death. Augmented myofilament Ca(2+) sensitivity is a common feature in TnT mutants, but such observations fail to provide a rational explanation for severe cardiac phenotypes. To better understand the mutation-induced effect on the cardiac phenotype, it is imperative to determine the effects on dynamic contractile features such as the muscle length (ML)-mediated activation against α- and β-myosin heavy chain (MHC) isoforms. α- and β-MHC are not only differentially expressed in rodent and human hearts, but they also modify ML-mediated activation differently. Mouse analog of human TnTR94L (TnTR97L) or wild-type TnT was reconstituted into de-membranated muscle fibers from normal (α-MHC) and transgenic (β-MHC) mouse hearts. TnTR97L augmented myofilament Ca(2+) sensitivity by a similar amount in α- and β-MHC fibers. However, TnTR97L augmented the negative impact of strained crossbridges on other crossbridges (γ) by 22% in α-MHC fibers, but attenuated γ by 21% in β-MHC fibers. TnTR97L decreased the magnitude of ML-mediated recruitment of crossbridges (ER) by 37% in α-MHC fibers, but increased ER by 35% in β-MHC fibers. We provide a mechanistic basis for the TnTR97L-induced effects in α- and β-MHC fibers and discuss the relevance to human hearts.

Myosin MgADP release rate decreases at longer sarcomere length to prolong myosin attachment time in skinned rat myocardium

Cardiac contractility increases as sarcomere length increases, suggesting that intrinsic molecular mechanisms underlie the Frank-Starling relationship to confer increased cardiac output with greater ventricular filling. The capacity of myosin to bind with actin and generate force in a muscle cell is Ca(2+) regulated by thin-filament proteins and spatially regulated by sarcomere length as thick-to-thin filament overlap varies. One mechanism underlying greater cardiac contractility as sarcomere length increases could involve longer myosin attachment time (ton) due to slowed myosin kinetics at longer sarcomere length. To test this idea, we used stochastic length-perturbation analysis in skinned rat papillary muscle strips to measure ton as [MgATP] varied (0.05-5 mM) at 1.9 and 2.2 μm sarcomere lengths. From this ton-MgATP relationship, we calculated cross-bridge MgADP release rate and MgATP binding rates. As MgATP increased, ton decreased for both sarcomere lengths, but ton was roughly 70% longer for 2.2 vs. 1.9 μm sarcomere length at maximally activated conditions. These ton differences were driven by a slower MgADP release rate at 2.2 μm sarcomere length (41 ± 3 vs. 74 ± 7 s(-1)), since MgATP binding rate was not different between the two sarcomere lengths. At submaximal activation levels near the pCa50 value of the tension-pCa relationship for each sarcomere length, length-dependent increases in ton were roughly 15% longer for 2.2 vs. 1.9 μm sarcomere length. These changes in cross-bridge kinetics could amplify cooperative cross-bridge contributions to force production and thin-filament activation at longer sarcomere length and suggest that length-dependent changes in myosin MgADP release rate may contribute to the Frank-Starling relationship in the heart.

Rat cardiac troponin T mutation (F72L)-mediated impact on thin filament cooperativity is divergently modulated by α- and β-myosin heavy chain isoforms

The primary causal link between disparate effects of human hypertrophic cardiomyopathy (HCM)-related mutations in troponin T (TnT) and α- and β-myosin heavy chain (MHC) isoforms on cardiac contractile phenotype remains poorly understood. Given the divergent impact of α- and β-MHC on the NH2-terminal extension (44-73 residues) of TnT, we tested if the effects of the HCM-linked mutation (TnTF70L) were differentially altered by α- and β-MHC. We hypothesized that the emergence of divergent thin filament cooperativity would lead to contrasting effects of TnTF70L on contractile function in the presence of α- and β-MHC. The rat TnT analog of the human F70L mutation (TnTF72L) or the wild-type rat TnT (TnTWT) was reconstituted into demembranated muscle fibers from normal (α-MHC) and propylthiouracil-treated (β-MHC) rat hearts to measure steady-state and dynamic contractile function. TnTF72L-mediated effects on tension, myofilament Ca(2+) sensitivity, myofilament cooperativity, rate constants of cross-bridge (XB) recruitment dynamics, and force redevelopment were divergently modulated by α- and β-MHC. TnTF72L increased the rate of XB distortion dynamics by 49% in α-MHC fibers but had no effect in β-MHC fibers; these observations suggest that TnTF72L augmented XB detachment kinetics in α-MHC, but not β-MHC, fibers. TnTF72L increased the negative impact of strained XBs on the force-bearing XBs by 39% in α-MHC fibers but had no effect in β-MHC fibers. Therefore, TnTF72L leads to contractile changes that are linked to dilated cardiomyopathy in the presence of α-MHC. On the other hand, TnTF72L leads to contractile changes that are linked to HCM in the presence of β-MHC.

The functional effect of dilated cardiomyopathy mutation (R144W) in mouse cardiac troponin T is differently affected by α- and β-myosin heavy chain isoforms

Given the differential impact of α- and β-myosin heavy chain (MHC) isoforms on how troponin T (TnT) modulates contractile dynamics, we hypothesized that the effects of dilated cardiomyopathy (DCM) mutations in TnT would be altered differently by α- and β-MHC. We characterized dynamic contractile features of normal (α-MHC) and transgenic (β-MHC) mouse cardiac muscle fibers reconstituted with a mouse TnT analog (TnTR144W) of the human DCM R141W mutation. TnTR144W did not alter maximal tension but attenuated myofilament Ca(2+) sensitivity (pCa50) to a similar extent in α- and β-MHC fibers. TnTR144W attenuated the speed of cross-bridge (XB) distortion dynamics (c) by 24% and the speed of XB recruitment dynamics (b) by 17% in α-MHC fibers; however, both b and c remained unaltered in β-MHC fibers. Likewise, TnTR144W attenuated the rates of XB detachment (g) and tension redevelopment (ktr) only in α-MHC fibers. TnTR144W also decreased the impact of strained XBs on the recruitment of new XBs (γ) by 30% only in α-MHC fibers. Because c, b, g, ktr, and γ are strongly influenced by thin filament-based cooperative mechanisms, we conclude that the TnTR144W- and β-MHC-mediated changes in the thin filament interact to produce a less severe functional phenotype, compared with that brought about by TnTR144W and α-MHC. These observations provide a basis for lower mortality rates of humans (β-MHC) harboring the TnTR141W mutant compared with transgenic mouse studies. Our findings strongly suggest that some caution is necessary when extrapolating data from transgenic mouse studies to human hearts.

Divergent effects of α- and β-myosin heavy chain isoforms on the N terminus of rat cardiac troponin T

Divergent effects of α– and β–myosin heavy chain (MHC) isoforms on contractile behavior arise mainly because of their impact on thin filament cooperativity. The N terminus of cardiac troponin T (cTnT) also modulates thin filament cooperativity. Our hypothesis is that the impact of the N terminus of cTnT on thin filament activation is modulated by a shift from α- to β-MHC isoform. We engineered two recombinant proteins by deleting residues 1–43 and 44–73 in rat cTnT (RcTnT): RcTnT_1–43Δ and RcTnT_44–73Δ, respectively. Dynamic and steady-state contractile parameters were measured at sarcomere length of 2.3 µm after reconstituting proteins into detergent-skinned muscle fibers from normal (α-MHC) and propylthiouracil-treated (β-MHC) rat hearts. α-MHC attenuated Ca²⁺-activated maximal tension (∼46%) in RcTnT_1–43Δ fibers. In contrast, β-MHC decreased tension only by 19% in RcTnT_1–43Δ fibers. Both α- and β-MHC did not affect tension in RcTnT_44–73Δ fibers. The instantaneous muscle fiber stiffness measurements corroborated the divergent impact of α- and β-MHC on tension in RcTnT_1–43Δ fibers. pCa₅₀ (-log of [Ca²⁺]_free required for half-maximal activation) decreased significantly by 0.13 pCa units in α-MHC + RcTnT_1–43Δ fibers but remained unaltered in β-MHC + RcTnT_1–43Δ fibers, demonstrating that β-MHC counteracted the attenuating effect of RcTnT_1–43Δ on myofilament Ca²⁺ sensitivity. β-MHC did not alter the sudden stretch–mediated recruitment of new cross-bridges (E_R) in RcTnT_1–43Δ fibers, but α-MHC attenuated E_R by 36% in RcTnT_1–43Δ fibers. The divergent impact of α- and β-MHC on how the N terminus of cTnT modulates contractile dynamics has implications for heart disease; alterations in cTnT and MHC are known to occur via changes in isoform expression or mutations.