Smooth, cardiac and skeletal muscle myosin force and motion generation assessed by cross-bridge mechanical interactions in vitro

Phosphorylation-dependent regulation is absent in a nonmuscle heavy meromyosin construct with one complete head and one head lacking the motor domain

To understand the domain requirements of phosphorylation-dependent regulation, we prepared three recombinant constructs of nonmuscle heavy meromyosin IIB containing 1) two complete heads, 2) one complete head and one head lacking the motor domain, and 3) one complete head and one head lacking both motor and regulatory domains. Steady-state ATPase measurements showed that phosphorylation did not alter the affinity for actin by more than a factor of 2 for any construct. Phosphorylation increased V(max) by a factor of 10 for construct 1 and 1.5-3 for construct 2 but had no effect for construct 3. Single turnover measurements, a better measure of slow rates inherent to unphosphorylated regulated myosins, showed that the single-headed construct 2, like construct 3 retains less than 1% of the regulatory properties of the double-headed construct 1 (300-fold activation). Therefore, a complete head cannot be down-regulated by a regulatory domain (without the motor domain) on the partner head. Two motor domains are required for regulation. This result is predicted by a structural model (Wendt, T., Taylor, D., Messier, T., Trybus, K. M., and Taylor, K. A. (1999) J. Cell Biol. 147, 1385-1390) showing interaction between the motor domains for unphosphorylated smooth muscle myosin, if motor-motor interaction is the basis for down-regulation.

Myocardial cross-bridge kinetics in transition to failure in Dahl salt-sensitive rats

The role of altered cross-bridge kinetics during the transition from cardiac hypertrophy to failure is poorly defined. We examined this in Dahl salt-sensitive (DS) rats, which develop hypertrophy and failure when fed a high-salt diet (HS). DS rats fed a low-salt diet were controls. Serial echocardiography disclosed compensated hypertrophy at 6 wk of HS, followed by progressive dilatation and impaired function. Mechanical properties of skinned left ventricular papillary muscle strips were analyzed at 6 wk of HS and then during failure (12 wk HS) by applying small amplitude (0.125%) length perturbations over a range of calcium concentrations. No differences in isometric tension-calcium relations or cross-bridge cycling kinetics or mechanical function were found at 6 wk. In contrast, 12 wk HS strips exhibited increased calcium sensitivity of isometric tension, decreased frequency of minimal dynamic stiffness, and a decreased range of frequencies over which cross bridges produce work and power. Thus the transition from hypertrophy to heart failure in DS rats is characterized by major changes in cross-bridge cycling kinetics and mechanical performance.

Loaded shortening and power output in cardiac myocytes are dependent on myosin heavy chain isoform expression

The purpose of this study was to examine the role of myosin heavy chain (MHC) in determining loaded shortening velocities and power output in cardiac myocytes. Cardiac myocytes were obtained from euthyroid rats that expressed alpha-MHC or from thyroidectomized rats that expressed beta-MHC. Skinned myocytes were attached to a force transducer and a position motor, and isotonic shortening velocities were measured at several loads during steady-state maximal Ca(2+) activation (P(pCa4.5)). MHC expression was determined after mechanical measurements using SDS-PAGE. Both alpha-MHC and beta-MHC myocytes generated similar maximal Ca(2+)-activated force, but alpha-MHC myocytes shortened faster at all loads and generated approximately 170% greater peak normalized power output. Additionally, the curvature of force-velocity relationships was less, and therefore the relative load optimal for power output (F(opt)) was greater in alpha-MHC myocytes. F(opt) was 0.31 +/- 0.03 P(pCa4.5) and 0.20 +/- 0.06 P(pCa4.5) for alpha-MHC and beta-MHC myocytes, respectively. These results indicate that MHC expression is a primary determinant of the shape of force-velocity relationships, velocity of loaded shortening, and overall power output-generating capacity of individual cardiac myocytes.

R403Q and L908V mutant beta-cardiac myosin from patients with familial hypertrophic cardiomyopathy exhibit enhanced mechanical performance at the single molecule level

Familial hypertrophic cardiomyopathy (FHC) is a disease of the sarcomere. In the beta-myosin heavy chain gene, which codes for the mechanical enzyme myosin, greater than 40 point mutations have been found that are causal for this disease. We have studied the effect of two mutations, the R403Q and L908V, on myosin molecular mechanics. In the in vitro motility assay, the mutant myosins produced a 30% greater velocity of actin filament movement (v(actin)). At the single molecule level, v(actin) approximately d/t(on), where d is the myosin unitary step displacement and t(on) is the step duration. Laser trap studies were performed at 10 microM MgATP to estimate d and t(on) for the normal and mutant myosin molecules. The increase in v(actin) can be explained by a significant decrease in the average t(on)'s in both the R403Q and L908V mutants (approximately 30 ms) compared to controls (approximately 40 ms), while d was not different for all myosins tested (approximately 7 nm). Thus the mutations affect the kinetics of the cross-bridge cycle without any effect on myosin's inherent motion and force generating capacity. Based on these studies, the primary signal for the hypertrophic response appears to be an apparent gain in function of the individual mutant myosin molecules.

2-deoxy-ATP enhances contractility of rat cardiac muscle

To investigate the kinetic parameters of the crossbridge cycle that regulate force and shortening in cardiac muscle, we compared the mechanical properties of cardiac trabeculae with either ATP or 2-deoxy-ATP (dATP) as the substrate for contraction. Comparisons were made in trabeculae from untreated rats (predominantly V1 myosin) and those treated with propylthiouracil (PTU; V3 myosin). Steady-state hydrolytic activity of cardiac heavy meromyosin (HMM) showed that PTU treatment resulted in >40% reduction of ATPase activity. dATPase activity was >50% elevated above ATPase activity in HMM from both untreated and PTU-treated rats. V(max) of actin-activated hydrolytic activity was also >50% greater with dATP, whereas the K(m) for dATP was similar to that for ATP. This indicates that dATP increased the rate of crossbridge cycling in cardiac muscle. Increases in hydrolytic activity were paralleled by increases of 30% to 80% in isometric force (F(max)), rate of tension redevelopment (k(tr)), and unloaded shortening velocity (V(u)) in trabeculae from both untreated and PTU-treated rats (at maximal Ca(2+) activation), and F-actin sliding speed in an in vitro motility assay (V(f)). These results contrast with the effect of dATP in rabbit psoas and soleus fibers, where F(max) is unchanged even though k(tr), V(u), and V(f) are increased. The substantial enhancement of mechanical performance with dATP in cardiac muscle suggests that it may be a better substrate for contractility than ATP and warrants exploration of ribonucleotide reductase as a target for therapy in heart failure.

Motion determination in actin filament fluorescence images with a spatio-temporal orientation analysis method

We present a novel approach of automatically measuring motion in series of microscopic fluorescence images. As a differential method, the three-dimensional structure tensor technique is used to calculate the displacement vector field for every image of the sequence, from which the velocities are subsequently derived. We have used this method for the analysis of the movement of single actin filaments in the in vitro motility assay, where fluorescently labeled actin filaments move over a myosin decorated surface. With its fast implementation and subpixel accuracy, this approach is, in general, very valuable for analyzing dynamic processes by image sequence analysis.

Single-molecule mechanics of R403Q cardiac myosin isolated from the mouse model of familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is an inherited cardiac disease that can result in sudden death in the absence of any overt symptoms. Many of the cases documented to date have been linked with missense mutations in the beta-myosin heavy chain gene. Here we present data detailing the functional impact of one of the most deadly mutations, R403Q, on myosin motor function. Experiments were performed on whole cardiac myosin purified from a mouse model of FHC to eliminate potential uncertainties associated with protein expression systems. The R403Q mutant myosin demonstrated 2.3-fold higher actin-activated ATPase activity, 2.2-fold greater average force generation, and 1.6-fold faster actin filament sliding in the motility assay. The force- and displacement-generating capacities of both the normal and mutant myosin were also characterized at the single molecule level in the laser trap assay. Both control and mutant generated similar unitary forces ( approximately 1 pN) and displacements ( approximately 7 nm) without any differences in event durations. On the basis of the distribution of mean unitary displacements, this mutation may possibly perturb the mechanical coordination between the 2 heads of cardiac myosin. Any of these observations could, alone or possibly in combination, result in abnormal power output and potentially a stimulus for the hypertrophic response.

Myosin heavy chain isoform expression in the failing and nonfailing human heart

In the heart, the relative proportions of the 2 forms of the motor protein myosin heavy chain (MyHC) have been shown to be affected by a wide variety of pathological and physiological stimuli. Hearts that express the faster MyHC motor protein, alpha, produce more power than those expressing the slower MyHC motor protein, beta, leading to the hypothesis that MyHC isoforms play a major role in the determination of cardiac contractility. We showed previously that a significant amount of alphaMyHC mRNA is expressed in nonfailing human ventricular myocardium and that alphaMyHC mRNA expression is decreased 15-fold in end-stage failing left ventricles. In the present study, we determined the MyHC protein isoform content of human heart samples of known MyHC mRNA composition. We demonstrate that alphaMyHC protein was easily detectable in 12 nonfailing hearts. alphaMyHC protein represented 7.2+/-3.2% of total MyHC protein (compared with approximately 35% of the MyHC mRNA), suggesting that translational regulation may be operative; in contrast, there was effectively no detectable alphaMyHC protein in the left ventricles of 10 end-stage failing human hearts.

Expression of the beta (slow)-isoform of MHC in the adult mouse heart causes dominant-negative functional effects

Alpha- and beta-myosin heavy chain (MHC), the two MHC isoforms expressed in the mammalian heart, differ quantitatively in their enzymatic activities. The MHC composition of the heart can change dramatically in response to numerous stimuli, leading to the hypothesis that changes in cardiac function can be caused by myosin isoform shifts. However, this hypothesis has remained unproven because the stimuli used to generate these shifts are complex and accompanied by many additional physiological changes, including alterations in cardiac mass and geometry. Adult mouse ventricles normally express only alpha-MHC (the faster motor). To determine whether genetic alteration of the MHC isoform composition in the adult mouse heart would result in changes in cardiac chamber mass and contractility, we established transgenic mouse lines that express a Myc-tagged beta-MHC molecule (the slower motor) in adult ventricular tissue, one of which expresses 12% of its myosin as the transgene. There is no evidence of hypertrophy, induction of hypertrophic markers, and no histopathology. Myofibrillar Ca(2+)-activated ATPase activity is decreased by 23%, and Langendorff preparations demonstrate a significant 15% decrease in systolic function in transgenic hearts. These results suggest that even small shifts in the myosin isoform composition of the myocardium can result in physiologically significant changes in cardiac contractility and could be relevant to cardiovascular disease.

Evidence for differential post-translational modifications of slow myosin heavy chain during murine skeletal muscle development

The contractile properties of muscle fibres are, in part, determined by the myosin heavy chain (MyHC) isoforms they express. Using monoclonal antibodies, we show that at least three forms of slow twitch MyHC accumulate sequentially during mouse fetal development and that slow MyHC maturation in slow fibres occurs before expression of the adult fast MyHCs in fast fibres. Expression of deletion derivatives of beta-cardiac MyHC cDNA shows that the slow MyHC epitopes that are detected in adult but not in young animals are located near the N-terminus. The same N-terminal region of various fast MyHC molecules contains a conserved epitope that can, on occasions, be observed when slow MyHC cDNA is expressed in non-muscle cells. The results raise the possibility that the N-terminal epitopes result from post-translational modification of the MyHC and that a sequence of slow and fast MyHC isoform post-translational modifications plays a significant role during development of murine muscle fibres.

Myosin molecular motor dysfunction in dystrophic mouse diaphragm

Cross-bridge properties and myosin heavy chain (MHC) composition were investigated in isolated diaphragm from 6-mo-old control (n = 12) and mdx (n = 12) mice. Compared with control, peak tetanic tension fell by 50% in mdx mice (P < 0.001). The total number of cross bridges per square millimeter (x10(9)), the elementary force per cross bridge, and the peak mechanical efficiency were lower in mdx than in control mice (each P < 0.001). The duration of the cycle and the rate constant for cross-bridge detachment were significantly lower in mdx than in control mice. In the overall population, there was a linear relationship between peak tetanic tension and either total number of cross bridges per square millimeter or elementary force per cross bridge (r = 0.996 and r = 0.667, respectively, each P < 0.001). The mdx mice presented a higher proportion of type IIA MHC (P < 0.001) than control mice and a reduction in type IIX MHC (P < 0.001) and slow myosin isoforms (P < 0.01) compared with control mice. We concluded that, in mdx mice, impaired diaphragm strength was associated with qualitative and quantitative changes in myosin molecular motors. It is proposed that reduced force generated per cross bridge contributed to diaphragm weakness in mdx mice.

In vitro motility speed of slow myosin extracted from single soleus fibres from young and old rats

1. Isolated soleus muscle fibres from aged rats contract more slowly than those from young rats. To determine whether this effect is due to a difference between the myosin molecules, we measured the rate at which actin filaments are driven over a myosin coated surface in the presence of ATP by using a novel in vitro motility assay where myosin is extracted from single muscle fibre segments. 2. Motility was dependent on the myosin density on the coverslip. In regions of high myosin density, actin motility was orientated parallel and anti-parallel to the direction of flow during myosin adhesion to the coverslip. In contrast, in regions of lower myosin density, actin motility was more random. The speed was about 20 % higher in the high density regions (P < 0.001). Further, the speed of filaments in the high density region, moving away or towards the fibre was less variable (P < 0.05) than that of more randomly moving filaments in the low density region. 3. The speed with myosin from slow soleus fibres of young adult rats (3-6 months old; v = 1.43 +/- 0.23 microm s-1; mean +/- s.d.) was faster (P < 0.001) than with myosin from aged rats (20-24 months old; v = 1.27 +/- 0.23 microm s-1). 4. No difference in myosin isoforms between young adult and aged fibres could be detected using electrophoretic and immunocytochemical techniques. Fibres of both ages expressed the beta/slow myosin heavy chain (MyHC) isoform and slow isoforms of essential and regulatory myosin light chains (MyLCs). 5. It is concluded that an age-related alteration in myosin contributes to the slowing of the maximum shortening velocity (V0) observed in soleus muscle fibres expressing the beta/slow MyHC isoform.

Comparison of cross-bridge cycling kinetics in neonatal vs. adult rat ventricular muscle

The developmental shift in contractile protein isoform expression in the rodent heart likely affects actin-myosin cross-bridge interactions. We compared the Ca2+ sensitivity for force generation and cross-bridge cycling kinetics in neonatal (postnatal days 0-3) and adult (day 84) rats. The force-pCa relationship was determined in Triton-X skinned muscle bundles activated at pCa 9.0 to 4.0. In strips maximally activated at pCa 4.0, the following parameters of cross-bridge cycling were measured: (1) rate of force redevelopment following rapid shortening and restretching (ktr); and (2) isometric stiffness at maximal activation and in rigor. The fraction of attached cross-bridges (alpha fs) and apparent rate constants for cross-bridge attachment (fapp) and detachment (gapp) were derived assuming a two-state model for cross-bridge cycling. Compared to the adult, the force-pCa curve for neonatal cardiac muscle was significantly shifted to the left. Neonatal cardiac muscle also displayed significantly smaller alpha fs, slower ktr and fapp; however, gapp was not significantly different between age groups. These data indicate that weaker force production in neonatal cardiac muscle involves, at least in part, less efficient cross-bridge cycling kinetics.

Kinetic differences at the single molecule level account for the functional diversity of rabbit cardiac myosin isoforms

1. Cardiac V3 myosin generates slower actin filament velocities and higher average isometric forces (in an in vitro motility assay) when compared with the V1 isoform. 2. To account for differences in V1 and V3 force and motion generation at the molecular level, we characterized the mechanics and kinetics of single V1 and V3 myosin molecules using a dual laser trap setup. 3. No differences in either unitary displacement (approximately 7 nm) or force (approximately 0.8 pN) were observed between isoforms; however, the duration of unitary displacement events was significantly longer for the V3 isoform at MgATP concentrations > 10 microM. 4. Our results were interpreted on the basis of a cross-bridge model in which displacement event durations were determined by the rates of MgADP release from, and MgATP binding to, myosin. 5. We propose that the release rate of MgADP from V3 myosin is half that of V1 myosin without any difference in their rates of MgATP binding; thus, kinetic differences between the two cardiac myosin isoforms are sufficient to account for their functional diversity.

Aging-dependent depression in the kinetics of force development in rat skinned myocardium

Normal aging of the rodent heart results in prominent prolongation of the twitch. We tested the hypothesis that increased expression of beta-myosin heavy chain (MHC), as occurs in the normal aging process in the rodent heart, contributes to the prolongation of the twitch by depressing the kinetics of cross-bridge interaction. Using 3-, 9-, 21-, and 33-mo-old male Fischer 344 x Brown Norway F1 hybrid rats, we examined both the rate of tension development (kCa) and unloaded shortening velocity in chemically skinned myocardium. Although kCa in all four age groups was dependent on the level of Ca2+ activation, both submaximal and maximal kCa were significantly slower in 9-, 21-, and 33-mo-old rats relative to 3-mo-old rats. Furthermore, unloaded shortening velocity was significantly reduced in 9-, 21-, and 33-mo-old rats compared with 3-mo-old rats. Collectively, these data strongly suggest that the aging-related increase in beta-MHC expression results in a progressive slowing of cross-bridge interaction kinetics in skinned myocardium, which most likely contributes to the overall aging-dependent reduction in myocardial functional capacity.

Mechanoenergetic alterations during the transition from cardiac hypertrophy to failure in Dahl salt-sensitive rats

BACKGROUND

The time course and mechanisms of altered mechanoenergetics and depressed cross-bridge cycling in hypertrophied and failing myocardium are uncertain.

METHODS AND RESULTS

We studied mechanoenergetics in Dahl salt-sensitive (DS) rats fed high-salt diet (HS) for 6 (HS-6) and 12 (HS-12) weeks to produce compensated hypertrophy and failure. The slope of the end-systolic pressure-volume relation (E'max) was similar in HS-6 and low-salt controls (LS-6), but reduced in HS-12 compared with controls (LS-12). Efficiency [1/slope of oxygen consumption (&f1;O2)-pressure-volume area (PVA) relation] was similar in HS-6 and LS-6 but higher in HS-12 versus LS-12 (59+/-16% versus 44+/-7%, P<0.05). Economy [1/slope of the force-time integral (FTI)-&f1;O2 relation] was similar in HS-6 and LS-6 but higher in HS-12 versus LS-12 (218+/-123 versus 74+/-39x10(3) g. s. mL O2-1. g; P<0.05). Compared with controls, myofibrillar ATPase activity was reduced by 24% in HS-6 and 44% in HS-12. V3 Isomyosin was increased in HS-6 (40+/-12% versus 9+/-8%; P<0.05) and further increased in HS-12 (76+/-10% versus 22+/-18%; P<0.05). Hypothyroid LS-12 rats had 100% V3 isomyosin, yet efficiency, economy, and ATPase values were intermediate between LS-12 and HS-12. HS-12 rats demonstrated increased troponin T3 isoform (17+/-2 versus 23+/-2%, P<0.05). There were no changes in troponin I or tropomyosin isoforms. However, the proportion of phosphorylated troponin T was reduced in HS-12 versus LS-12 hearts (P<.001).

CONCLUSIONS

In DS rats, the transition to failure is associated with depressed E'max and increased efficiency and economy. These findings are linked to myofibrillar ATPase activity and suggest that mechanisms other than isomyosin switching are important determinants of ventricular energetics. A troponin T isoform switch is one potential mechanism.

Cardiovascular physiology in the twentieth century: great strides and missed opportunities

In a broad sense, physiology is the study of the chemical and physical bases of life processes. Consequently, the evolution of our knowledge of cardiovascular functions is closely linked to the developments in many fields of science, including chemistry, physics, engineering, and biology. A cursory examination reveals that different "foundation" sciences predominated in different stages of the history of cardiovascular physiology. Today, cardiovascular physiology is poised to exploit new developments in all areas of scientific inquiry. However, cardiovascular physiologists have not always embraced the power of the multidisciplinary approach. In this brief overview of the history of cardiovascular physiology in the 20th century, the major focus is on some of the major advances in the field and the contributions of other disciplines to these developments. In addition, the forces that influenced cardiovascular science in this century and their impact on the evolution of the field in the new millennium are discussed.

Role of myosin heavy chain composition in kinetics of force development and relaxation in rat myocardium

1. The effects of ventricular myosin heavy chain (MHC) composition on the kinetics of activation and relaxation were examined in both chemically skinned and intact myocardial preparations from adult rats. Thyroid deficiency was induced to alter ventricular MHC isoform expression from approximately 80% alpha-MHC/20% beta-MHC in euthyroid rats to 100% beta-MHC, without altering the expression of thin-filament-associated regulatory proteins. 2. In single skinned myocytes, increased expression of beta-MHC did not significantly affect either maximal Ca2+-activated tension (P0) or the Ca2+ sensitivity of tension (pCa50). However, unloaded shortening velocity (V0) decreased by 80% due to increased beta-MHC expression. 3. The kinetics of activation and relaxation were examined in skinned multicellular preparations using the caged Ca2+ compound DM-nitrophen and caged Ca2+ chelator diazo-2, respectively. Myocardium expressing 100% beta-MHC exhibited apparent rates of submaximal and maximal tension development (kCa) that were 60% lower than in control myocardium, and a 2-fold increase in the half-time for relaxation from steady-state submaximal force. 4. The time courses of cell shortening and intracellular Ca2+ transients were assessed in living, electrically paced myocytes, both with and without beta-adrenergic stimulation (70 nM isoproterenol (isoprenaline)). Thyroid deficiency had no affect on either the extent of myocyte shortening or the resting or peak fura-2 fluorescence ratios. However, induction of beta-MHC expression by thyroid deficiency was associated with increased half-times for myocyte shortening and relengthening and increased half-time for the decay of the fura-2 fluorescence ratio. Qualitatively similar results were obtained in both the absence and the presence of beta-adrenergic stimulation although the beta-agonist accelerated the kinetics of the twitch and the Ca2+ transient. 5. Collectively, these data provide evidence that increased beta-MHC expression contributes significantly to the observed depression of contractile function in thyroid deficient myocardium by slowing the rates of both force development and force relaxation.

A 7-amino-acid insert in the heavy chain nucleotide binding loop alters the kinetics of smooth muscle myosin in the laser trap

Two smooth muscle myosin heavy chain isoforms differ by a 7-amino-acid insert in a flexible surface loop located near the nucleotide binding site. The non-inserted isoform is predominantly found in tonic muscle, while the inserted isoform is mainly found in phasic muscle. The inserted isoform has twice the actin-activated ATPase activity and actin filament velocity in the in vitro motility assay as the non-inserted isoform. We used the laser trap to characterize the molecular mechanics and kinetics of the inserted isoform ((+)insert) and of a mutant lacking the insert ((-)insert), analogous to the isoform found in tonic muscle. The constructs were expressed as heavy meromyosin using the baculovirus/insect cell system. Unitary displacement (d) was similar for both constructs (approximately 10 nm) but the attachment time (t(on) for the (-)insert was twice as long as for the (+)insert regardless of the [MgATP]. Both the relative average isometric force (Favg(-insert)/Favg(+insert) = 1.1 +/- 0.2 (mean +/- SE) using the in vitro motility mixture assay, and the unitary force (F approximately 1 pN) using the laser trap, showed no difference between the two constructs. However, as under unloaded conditions, t(on) under loaded conditions was longer for the (-)insert compared with the (+)insert construct at limiting [MgATP]. These data suggest that the insert in this surface loop does not affect the mechanics but rather the kinetics of the cross-bridge cycle. Through comparisons of t(on) from d measurements to various [MgATP], we conclude that the insert affects two specific steps in the cross-bridge cycle, that is, MgADP release and MgATP binding.

Relation between crossbridge structure and actomyosin ATPase activity in rat heart

Cardiac myofilaments contain proteins that regulate the interaction between actin and myosin. In the thick filament, there are several proteins that may contribute to the regulation of the contraction. The myosin binding protein C, or C protein, has 4 sites that can be phosphorylated by a Ca2+-calmodulin-controlled kinase, protein kinase A or protein kinase C. Using electron microscopy and optical diffraction, we examined the structure of thick filaments isolated from rat ventricles with either the alpha or beta isoform of myosin heavy chain (MHC) and the effect of specific phosphorylation of C protein on the structure. In thick filaments with alpha-MHC, crossbridges were clearly visible. Phosphorylation of C protein by protein kinase A extended the crossbridges from the backbone of the filament, changed their orientation, increased the degree of order of the crossbridges, and decreased the flexibility of the crossbridges. Crossbridges in filaments with beta-MHC were less ordered and apparently more flexible. Phosphorylation of C protein in beta-MHC-containing filaments did not extend the crossbridges and did not alter degree of order or flexibility. The relative flexibility of the crossbridges inferred from the optical diffraction pattern correlated well with the rate of ATP hydrolysis by actomyosin. These results suggest that (1) crossbridge flexibility is an important parameter in setting the rate of crossbridge cycling, and (2) C protein-mediated control of the position and flexibility of crossbridges may regulate actomyosin ATPase activity by modifying the kinetics of crossbridge cycling.

Comparison of unitary displacements and forces between 2 cardiac myosin isoforms by the optical trap technique: molecular basis for cardiac adaptation

To provide information on the mechanism of cardiac adaptation at the molecular level, we compared the unitary displacements and forces between the 2 rat cardiac myosin isoforms, V1 and V3. A fluorescently labeled actin filament, with a polystyrene bead attached, was caught by an optical trap and brought close to a glass surface sparsely coated with either of the 2 isoforms, so that the actin-myosin interaction took place in the presence of a low concentration of ATP (0.5 micromol/L). Discrete displacement events were recorded with a low trap stiffness (0.03 to 0.06 pN/nm). Frequency distribution of the amplitude of the displacements consisted of 2 gaussian curves with peaks at 9 to 10 and 18 to 20 nm for both V1 and V3, suggesting that 9 to 10 nm is the unitary displacement for both isoforms. The duration of the displacement events was longer for V3 than for V1. On the other hand, discrete force transients were recorded with a high trap stiffness (2.1 pN/nm), and their amplitude showed a broad distribution with mean values between 1 and 2 pN for V1 and V3. The durations of the force transients were also longer for V3 than for V1. These results indicate that both the unitary displacements and forces are similar in amplitude but different in duration between the 2 cardiac myosin isoforms, being consistent with the reports that the tension cost is higher in muscles consisting mainly of V1 than those consisting mainly of V3.

Electrophoretic separation and quantitation of cardiac myosin heavy chain isoforms in eight mammalian species

A protocol for sample preparation and gel electrophoresis is described that reliably results in the separation of the alpha- and beta-isoforms of cardiac myosin heavy chain (MHC-alpha and MHC-beta) in eight mammalian species. The protocol is based on a simple, nongradient denaturing gel. The magnitude of separation of MHC-alpha and MHC-beta achieved with this protocol is sufficient for quantitative determination of the relative amounts of these two isoforms in mouse, rat, guinea pig, rabbit, canine, pig, baboon, and human myocardial samples. The sensitivity of the protocol is sufficient for the detection of MHC isoforms in samples at least as small as 1 microgram. The glycerol concentration in the separating gel is an important factor for successfully separating MHC-alpha and MHC-beta in myocardial samples from different species. The effect of sample load on MHC-alpha and MHC-beta band resolution is illustrated. The results also indicate that inclusion of a homogenization step during sample preparation increases the amount of MHC detected on the gel for cardiac samples to a much greater extent than for skeletal muscle samples. Although the protocol described in this study is excellent for analyzing cardiac samples, it should be noted that the same protocol is not optimal for separating MHC isoforms expressed in skeletal muscle, as is illustrated.

Modulation of contractility in human cardiac hypertrophy by myosin essential light chain isoforms

Cardiac hypertrophy is an adaptive response that normalizes wall stress and compensates for increased workload. It is accompanied by distinct qualitative and quantitative changes in the expression of protein isoforms concerning contractility, intracellular Ca(2+)-homeostasis and metabolism. Changes in the myosin subunit isoform expression improves contractility by an increase in force generation at a given Ca(2+)-concentration (increased Ca(2+)-sensitivity) and by improving the economy of the chemo-mechanical transduction process per amount of utilised ATP (increased duty ratio). In the human atrium this is achieved by partial replacement of the endogenous fast myosin by the ventricular slow-type heavy and light chains. In the hypertrophic human ventricle the slow-type beta-myosin heavy chains remain unchanged, but the ectopic expression of the atrial myosin essential light chain (ALC1) partially replaces the endogenous ventricular isoform (VLC1). The ventricular contractile apparatus with myosin containing ALC1 is characterised by faster cross-bridge kinetics, a higher Ca(2+)-sensitivity of force generation and an increased duty ratio. The mechanism for cross-bridge modulation relies on the extended Ala-Pro-rich N-terminus of the essential light chains of which the first eleven residues interact with the C-terminus of actin. A change in charge in this region between ALC1 and VLC1 explains their functional difference. The intracellular Ca(2+)-handling may be impaired in heart failure, resulting in either higher or lower cytosolic Ca(2+)-levels. Thus the state of the cardiomyocyte determines whether this hypertrophic adaptation remains beneficial or becomes detrimental during failure. Also discussed are the effects on contractility of long-term changes in isoform expression of other sarcomeric proteins. Positive and negative modulation of contractility by short-term phosphorylation reactions at multiple sites in the myosin regulatory light chain, troponin-I, troponin-T, alpha-tropomyosin and myosin binding protein-C are considered in detail.

Human heart failure: determinants of ventricular dysfunction

Thin muscle strips were obtained from non-failing (NF) and failing (dilated cardiomyopathy (DCM)) hearts, using a new harvesting and dissection technique. The strips were used to carry out a myothermal and mechanical analysis so that contractile and excitation coupling phenomena in the NF and failing (DCM-F) preparations can be compared. Peak isometric force and rate of relaxation in DCM-F were reduced 46% (p < 0.02) while time to peak tension was increased 14% (p < 0.03). Initial, tension dependent, tension independent and the rate of tension independent heat liberation were reduced 62-70% in DCM-F (p < 0.03). The crossbridge force-time integral (FTIXBr) was calculated from these measurements and was shown to increase 40% while the amount and rate of calcium cycled per beat was reduced 70%. As a result of these changes in the contractile and excitation-contraction coupling systems in DCM-F, the force-frequency relationship was significantly blunted while the power output was markedly reduced. These fundamental alterations account for the substantial ventricular dysfunction found in the dilated cardiomyopathic failing heart.

The in vitro motility activity of beta-cardiac myosin depends on the nature of the beta-myosin heavy chain gene mutation in hypertrophic cardiomyopathy

Several mutations in the beta-myosin heavy chain gene cause hypertrophic cardiomyopathy. This study investigates (1) the in vitro velocities of translocation of fluorescently-labelled actin by beta-myosin purified from soleus muscle of 30 hypertrophic cardiomyopathy patients with seven distinct beta-myosin heavy chain gene mutations: Thr124Ile, Tyr162Cys, Gly256Glu, Arg403Gln, Val606Met, Arg870His, and Leu908Val mutations; and (2) motility activity of beta-myosin purified from cardiac and soleus muscle biopsies in the same patients. The velocity of translocation of actin by beta-myosin purified from soleus or cardiac muscle of 22 normal controls was 0.48 +/- 0.09 micron s-1. By comparison, the motility activity was reduced in all 30 patients with beta-myosin heavy chain gene mutations (range, 0.112 +/- 0.041 to 0.292 +/- 0.066 micron s-1. Notably, the Tyr162Cys and Arg403Gln mutations demonstrated significantly lower actin sliding velocities: 0.123 +/- 0.044, and 0.112 +/- 0.041 micron s-1, respectively. beta-myosin purified from soleus muscle from four patients with the Arg403Gln mutation had a similar actomyosin motility activity compared to beta-myosin purified from their cardiac biopsies (0.127 +/- 0.045 micron s-1 versus 0.119 +/- 0.068 micron s-1, respectively). Since these seven mutations lie in several distinct functional domains, it is likely that the mechanisms of their inhibitions of motility are different.

Myosin isoforms and functional diversity in vertebrate smooth muscle

The expression of fast and slow myosin isoforms in individual cells is associated with differences in shortening velocities and power output in fully differentiated vertebrate striated muscle. This paradigm in which shortening velocity is determined by the myosin isoform (and load) is inappropriate for smooth muscle. Smooth muscle tissues express multiple myosin heavy and light chain isoforms, and it is not currently possible to separate and identify chemically distinct native myosin hexamers (i.e., isoforms). It is not known if different isoforms are localized in subpopulations of cells or in specific cellular domains nor whether they combine preferentially to form a small number of native myosin hexamer isoforms. Potentially, thick filaments are aggregates of many different combinations of heavy and light chain isoforms that may or may not exhibit different kinetics. Shortening velocities in smooth muscle are regulated by Ca(2+)-dependent crossbridge phosphorylation of the myosin regulatory light chains. Much of the observed diversity in power output in smooth muscle may be attributed to regulatory mechanisms modulating crossbridge cycling rates rather than contractile protein isoform expression.

Active and passive forces of isolated myofibrils from cardiac and fast skeletal muscle of the frog

1. Force measurements in isolated myofibrils (15 degrees C; sarcomere length, 2.10 microns) were used in this study to determine whether sarcomeric proteins are responsible for the large differences in the amounts of active and passive tension of cardiac versus skeletal muscle. Single myofibrils and bundles of two to four myofibrils were prepared from glycerinated tibialis anterior and sartorius muscles of the frog. Skinned frog atrial myocytes were used as a model for cardiac myofibrils. 2. Electron microscope analysis of the preparations showed that: (i) frog atrial myocytes contained a small and variable number of individual myofibrils (from 1 to 7); (ii) the mean cross-sectional area and mean number of myosin filaments of individual cardiac myofibrils did not differ significantly from those of single skeletal myofibrils; and (iii) the total myofibril cross-sectional area of atrial myocytes was on average comparable to that of bundles of two to four skeletal myofibrils. 3. In maximally activated skeletal preparations, values of active force ranged from 0.45 +/- 0.03 microN for the single myofibrils (mean +/- S.E.M.; n = 16) to 1.44 +/- 0.24 microN for the bundles of two to four myofibrils (n = 9). Maximum active force values of forty-five cardiac myocytes averaged 1.47 +/- 0.10 microN and exhibited a non-continuous distribution with peaks at intervals of about 0.5 microN. The results suggest that variation in active force among cardiac preparations mainly reflects variability in the number of myofibrils inside the myocytes and that individual cardiac myofibrils develop the same average amount of force as single skeletal myofibrils. 4. The mean sarcomere length-resting force relation of atrial myocytes could be superimposed on that of bundles of two to four skeletal myofibrils. This suggests that, for any given amount of strain, individual cardiac and skeletal sarcomeres bear essentially the same passive force. 5. The length-passive tension data of all preparations could be fitted by an exponential equation. Equation parameters obtained for both types of myofibrils were in reasonable agreement with those reported for larger preparations of frog skeletal muscle but were very different from those estimated for multicellular frog atrial preparations. It is concluded that myofibrils are the major determinant of resting tension in skeletal muscle; structures other than the myofibrils are responsible for the high passive stiffness of frog cardiac muscle.

In vitro actin filament sliding velocities produced by mixtures of different types of myosin

Using in vitro motility assays, we examined the sliding velocity of actin filaments generated by pairwise mixings of six different types of actively cycling myosins. In isolation, the six myosins translocated actin filaments at differing velocities. We found that only small proportions of a more slowly translating myosin type could significantly inhibit the sliding velocity generated by a myosin type that translocated filaments rapidly. In other experiments, the addition of noncycling, unphosphorylated smooth and nonmuscle myosin to actively translating myosin also inhibited the rapid sliding velocity, but to a significantly reduced extent. The data were analyzed in terms of a model derived from the original working cross-bridge model of A.F. Huxley. We found that the inhibition of rapidly translating myosins by slowly cycling was primarily dependent upon only a single parameter, the cross-bridge detachment rate at the end of the working powerstroke. In contrast, the inhibition induced by the presence of noncycling, unphosphorylated myosins required a change in another parameter, the transition rate from the weakly attached actomyosin state to the strongly attached state at the beginning of the cross-bridge power stroke.

Smooth muscle and skeletal muscle myosins produce similar unitary forces and displacements in the laser trap

Purified smooth muscle myosin in the in vitro motility assay propels actin filaments at 1/10 the velocity, yet produces 3-4 times more force than skeletal muscle myosin. At the level of a single myosin molecule, these differences in force and actin filament velocity may be reflected in the size and duration of single motion and force-generating events, or in the kinetics of the cross-bridge cycle. Specifically, an increase in either unitary force or duty cycle may explain the enhanced force-generating capacity of smooth muscle myosin. Similarly, an increase in attached time or decrease in unitary displacement may explain the reduced actin filament velocity of smooth muscle myosin. To discriminate between these possibilities, we used a laser trap to measure unitary forces and displacements from single smooth and skeletal muscle myosin molecules. We analyzed our data using mean-variance analysis, which does not rely on scoring individual events by eye, and emphasizes periods in the data with constant properties. Both myosins demonstrated multiple but similar event populations with discrete peaks at approximately +11 and -11 nm in displacement, and 1.5 and 3.5 pN in force. Mean attached times for smooth muscle myosin were longer than for skeletal-muscle myosin. These results explain much of the difference in actin filament velocity between these myosins, and suggest that an increased duty cycle is responsible for the enhanced force-generating capacity of smooth over skeletal-muscle myosin.

Functional analysis of the mutations in the human cardiac beta-myosin that are responsible for familial hypertrophic cardiomyopathy. Implication for the clinical outcome

More than 30 missense mutations in the beta-cardiac myosin heavy chain gene have been shown to be responsible for familial hypertrophic cardiomyopathy. To clarify the effects of these point mutations on myosin motor function, we expressed wild-type and mutant human beta-cardiac myosin heavy chains in insect cells with human cardiac light chains. The wild-type myosin was well purified with similar enzymatic and motor activities to those of the naturally isolated V3 cardiac myosin. Arg249-->Gln and Arg453-->Cys mutations resulted in decreased actin translocating activity (61 and 23% of the wild-type, respectively) with decreased intrinsic ATPase activity. Arg403-->Gln mutation greatly decreased actin translocating activity (27% of wild type) with a 3.3-fold increased dissociation constant for actin, while intrinsic ATPase activity was unchanged. Val606-->Met mutation only mildly affected the actin translocating activity as well as ATPase activity of myosin. The degree of deterioration by each mutation was closely correlated with the prognosis of the affected kindreds, indicating that myosin dysfunction caused by the point mutations is responsible for the pathogenesis of the disease. Structure/function relationship of myosin is discussed.

Molecular motor mechanics in the contracting heart. V1 versus V3 myosin heavy chain

The amount of iron in the low molecular weight pool (LMW) increases during no-flow ischemia and is thought to be essential to oxygen radical-derived damage upon reperfusion. Applying three short ischemic periods (5 min) preconditioning before 15 min ischemia results in an improved contractility compared to a direct 15 min ischemic insult. This raises the question whether preconditioning leads to a decrease in hte LMW iron pool. We therefore investigated the change in in hte LMW iron pool during ischemic insult after applying preconditioning. It is assumed that an increase in LMW iron is dependent on the accumulation of reduction equivalents derived from the anaerobic glycolysis. Therefore the glycogen content was also reduced by administration by anoxia and glucagon administration to study the effect on the LMW iron pool.

Cardiac troponin T isoform expression correlates with pathophysiological descriptors in patients who underwent corrective surgery for congenital heart disease

BACKGROUND

This study examined cardiac troponin T (cTnT) isoform expression in patients who had undergone surgery at Duke University Medical Center (Durham, NC) between December 1, 1993, and January 31, 1995, to correct congenital heart defects. The human heart expresses four cTnT isoforms (cTnT1 through cTnT4) whose sequence differences result from combinatorial alternative splicing of two exons. We have previously shown that cTnT4 is expressed at higher levels in severely failing hearts from transplant patients. In this study, we tested the hypothesis that congenital heart defects that have a more negative effect on myocardial function increase cTnT4 expression. We used the presence or absence of drug treatment for heart failure or congested circulation before surgery and the duration of inotropic support after corrective surgery as indicators of the pathophysiological state of the heart just before surgery.

METHODS AND RESULTS

Right atrial appendage tissue was collected from 34 patients, 6 days to 35 years old (median age, 3.4 months). The amounts of the cTnT1 through cTnT4 isoforms, measured as a percentage of total cTnT, were determined from Western blots probed with MAb13-11, a cTnT-specific monoclonal antibody. We found that cTnT4 expression correlated positively with the duration of inotropic support and was higher in patients who received drug treatment before surgery than in those who did not. Furthermore, we found that the percent of cTnT4 was significantly higher in hearts with congenital defects that caused congestive failure than in hearts with tetralogy of Fallot.

CONCLUSIONS

These findings suggest that in patients with congenital cardiac defects, cTnT4 expression is modulated by heart failure and is increased in hearts that are more hemodynamically stressed.

Different cardiac myosin isoforms exhibit equal force-generating ability in vitro

We measured forces generated by myosin molecules and a single actin filament using an optical trap system. The force per unit length of actin filament did not differ significantly between cardiac myosin isoforms. V1 and V3. This indicates that the ability to generate force is equal between V1 and V3, despite their difference in the unloaded sliding velocity past actin.

Thiophosphorylation independently activates each head of smooth muscle myosin in vitro

To determine whether thiophosphorylation of the 20-kDa myosin light chain activates each head of smooth muscle myosin independently of the head with which it is paired, chicken gizzard smooth muscle myosin was randomly thiophosphorylated, producing a mixture of unphosphorylated and singly and doubly thiophosphorylated myosin. Thiophosphorylation levels were measured by glycerol-urea gels, and the activity of this myosin was determined by actin-activated adenosinetriphosphatase measurements and in an in vitro motility assay, where the velocity of actin filaments moving over a myosin-coated surface is measured. Activity at each thiophosphorylation level was similar to that previously observed for mixtures of unphosphorylated and doubly thiophosphorylated myosin (D. E. Harris, S. S. Work, R. K. Wright, N. R. Alpert, and D. M. Warshaw. J. Muscle Res. Cell Motil. 15: 11-19, 1994). All doubly thiophosphorylated myosin was then formed into filaments and removed from randomly thiophosphorylated myosin by centrifugation. The remaining myosin (mixture of unphosphorylated and singly phosphorylated myosin), which could not polymerize because of their conformation, retained approximately 70% activity compared with mixtures of unphosphorylated and doubly thiophosphorylated myosin. Thus a thiophosphorylated smooth muscle myosin head can produce substantial biochemical and mechanical activity, even when it is paired with an unphosphorylated partner.

Control of beta-myosin heavy chain expression in systemic hypertension and caloric restriction in the rat heart

In the rat left ventricle, both pressure overload induced by abdominal aortic constriction (Abcon) and caloric restriction (CR) induce an increase in the steady-state level of the beta-myosin heavy chain (MHC) protein and mRNA. Both models also induce a concomitant decrease in the alpha-MHC protein and mRNA. The goals of this study were to 1) determine if the changes in MHC expression in the models are due to altered transcription and 2) identify the relative levels of some key factors interacting with the regulatory regions of these genes. Female Sprague-Dawley rats were randomly assigned to the following groups: 1) normal control (NC), 2) Abcon, and 3) CR. After 5 wk of experimental manipulations, myocardial nuclei were isolated. These nuclei were used for 1) nuclear run-on assays or 2) nuclear extract, which was prepared and used for gel mobility shift assays (GMSAs). Nuclear run-on assays demonstrated that the increase in beta-MHC mRNA and protein expression in both Abcon and CR can be at least partially attributed to increased transcription. The concomitant decrease in alpha-MHC content can similarly be attributed to a decrease in transcription of this gene. Furthermore, GMSAs demonstrate that nuclear extract from each group interact differently with certain elements known to be important for expression in vitro. CR nuclear extracts have a 25.6 +/- 7.2% decrease (P < 0.05 vs. NC) in interaction with a thyroid-responsive element, a potential repressor of beta-MHC transcription.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac V1 and V3 myosins differ in their hydrolytic and mechanical activities in vitro

The two mammalian cardiac myosin heavy chain isoforms, alpha and beta, have 93% amino acid homology, but hearts expressing these myosins exhibit marked differences in their mechanical activities. To further understand the function of these cardiac myosins as molecular motors, we compared the ability of these myosins to hydrolyze ATP and to both translocate actin filaments and generate force in an in vitro motility assay. V1 myosin has twice the actin-activated ATPase activity and three times the actin filament sliding velocity when compared with V3 myosin. In contrast, the force-generating ability of these myosins is quite different when the total force produced by a small population of myosin molecules (> 50) is examined. V1 myosin produces only one half the average cross-bridge force of V3 myosin. With discrete areas of primary structural heterogeneity known to exist between alpha and beta heavy chains, the differences we report in the hydrolytic and mechanical activities of the motors are explored in the context of potential structural and kinetic differences between the V1 and V3 myosins.

Effect of unphosphorylated smooth muscle myosin on caldesmon-mediated regulation of actin filament velocity

The effect of smooth muscle myosin at different levels of light chain phosphorylation on caldesmon-mediated movement of actin filaments was investigated using an in vitro motility assay. Myosin at different levels of phosphorylation was obtained by mixing different proportions of fully phosphorylated and unphosphorylated myosin in monomeric form, while keeping the total myosin concentration constant. The average velocity of actin filaments containing tropomyosin was 1.20 +/- 0.046 microns s-1 at 30 degrees C with fully phosphorylated myosin. This velocity was not altered when the percentage of unphosphorylated myosin coated on the nitrocellulose surface was increased to 80%; further increases lowered the velocity. When the actin filaments with caldesmon bound at stoichiometric levels were used, filament velocity was unaffected until 50% of the myosin was unphosphorylated, but further increases in the percentage of unphosphorylated myosin induced a decrease in the velocity, and at 95% unphosphorylated myosin, filament movement had ceased. The decreased filament velocity in the presence of caldesmon was also observed when phosphorylated myosin was mixed with myosin rod instead of unphosphorylated myosin, but was not observed when the 38 kDa caldesmon C-terminal fragment, which lacks the myosin-binding domain, was used instead of intact caldesmon. These data indicate that the decreased filament velocity in the presence of caldesmon reflects the mechanical load produced by the tethering of actin to myosin through the interaction of the caldesmon N-terminal domain and the myosin S-2 region. The tethering effect mediated by caldesmon may play a role in smooth muscle contraction when a large number of myosin heads are dephosphorylated, as in force maintenance.

Signal transduction and regulation in smooth muscle

Smooth muscle cells in the walls of many organs are vital for most bodily functions, and their abnormalities contribute to a range of diseases. Although based on a sliding-filament mechanism similar to that of striated muscles, contraction of smooth muscle is regulated by pharmacomechanical as well as by electromechanical coupling mechanisms. Recent studies have revealed previously unrecognized contractile regulatory processes, such as G-protein-coupled inhibition of myosin light-chain phosphatase, regulation of myosin light-chain kinase by other kinases, and the functional effects of smooth muscle myosin isoforms. Abnormalities of these regulatory mechanisms and isoform variations may contribute to diseases of smooth muscle, and the G-protein-coupled inhibition of protein phosphatase is also likely to be important in regulating non-muscle cell functions mediated by cytoplasmic myosin II.

Passive and active tension in single cardiac myofibrils

Single myofibrils were isolated from chemically skinned rabbit heart and mounted in an apparatus described previously (Fearn et al., 1993; Linke et al., 1993). We measured the passive length-tension relation and active isometric force, both normalized to cross sectional area. Myofibrillar cross sectional area was calculated based on measurements of myofibril diameter from both phase-contrast images and electron micrographs. Passive tension values up to sarcomere lengths of approximately 2.2 microns were similar to those reported in larger cardiac muscle specimens. Thus, the element responsible for most, if not all, passive force of cardiac muscle at physiological sarcomere lengths appears to reside within the myofibrils. Above 2.2 microns, passive tension continued to rise, but not as steeply as reported in multicellular preparations. Apparently, structures other than the myofibrils become increasingly important in determining the magnitude of passive tension at these stretched lengths. Knowing the myofibrillar component of passive tension allowed us to infer the stress-strain relation of titin, the polypeptide thought to support passive force in the sarcomere. The elastic modulus of titin is 3.5 x 10(6) dyn cm-2, a value similar to that reported for elastin. Maximum active isometric tension in the single myofibril at sarcomere lengths of 2.1-2.3 microns was 145 +/- 35 mN/mm2 (mean +/- SD; n = 15). This value is comparable with that measured in fixed-end contractions of larger cardiac specimens, when the amount of nonmyofibrillar space in those preparations is considered. However, it is about 4 times lower than the maximum active tension previously measured in single skeletal myofibrils under similar conditions (Bartoo et al., 1993).

ADP inhibits the sliding velocity of fluorescent actin filaments on cardiac and skeletal myosins

We studied the effect of MgADP on the mechanical interaction of actomyosin in cardiac and skeletal muscles using an in vitro motility assay. The sliding velocities of fluorescently labeled actin filaments on rat cardiac and skeletal myosins were measured at various MgATP and MgADP concentrations. The filament velocity depended on MgATP concentration according to classic Michaelis-Menten kinetics with apparent Michaelis constants (Km) of 43 and 137 mumol/L and maximum velocity of 5.6 and 8.6 microns/s for cardiac and skeletal myosins, respectively. The presence of 2 mmol/L MgADP decreased the filament velocity and shifted the substrate concentration dependence of the velocity toward higher MgATP concentrations, yielding the inhibition constants of 194 and 478 mumol/L for cardiac and skeletal myosins, respectively. The activation energies determined by the temperature dependence of the velocity were 61 and 83 kJ/mol for rat V1 and rabbit cardiac myosins, which were similar to those of the dissociation rate constant of actomyosin-ADP complex reported in a solution study. The inhibition of the velocity by MgADP can be explained by the crossbridge scheme in which MgADP competes with MgATP for the substrate site on myosin molecules. In cardiac myosin, addition of a concentration of MgADP as low as 25 mumol/L significantly inhibited the velocity in the presence of 2 mmol/L MgATP, suggesting that increased intracellular MgADP may reduce the rate of crossbridge detachment, resulting in a decreased ATP consumption and an increased economy of force production under ischemic conditions. The present results support the idea that MgADP may be a physiologically important modulator of contraction in cardiac muscle.