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.

Strong binding of myosin modulates length-dependent Ca2+ activation of rat ventricular myocytes

Reductions in sarcomere length (SL) and concomitant increases in interfilament lattice spacing have been shown to decrease the Ca2+ sensitivity of tension in myocardium. We tested the idea that increased lattice spacing influences the SL dependence of isometric tension by reducing the probability of strong interactions of myosin crossbridges with actin, thereby decreasing cooperative activation of the thin filament. Single ventricular myocytes were isolated by enzymatic digestion of rat hearts and were subsequently rapidly skinned. Maximal tension and Ca2+ sensitivity of tension (ie, pCa50) were measured in the absence and presence of N-ethylmaleimide-modified myosin subfragment 1 (NEM-S1) at both short and long SLs. NEM-S1, a strong-binding non-tension-generating derivative of the myosin head, was applied to single skinned myocytes to cooperatively promote strong binding of endogenous myosin crossbridges. Compared with control myocytes at SL of approximately 1.90 microm, application of NEM-S1 markedly increased submaximal Ca2+-activated tensions and thereby increased Ca2+ sensitivity; ie, pCa50 increased from 5.40+/-0.02 to 5.52+/-0.02 pCa units in the presence of NEM-S1. Furthermore, NEM-S1 treatment reversibly eliminated the SL dependence of the Ca2+ sensitivity of tension, in that the DeltapCa50 between short and long lengths was 0. 02+/-0.01 pCa units in the presence of NEM-S1 compared with a DeltapCa50 of 0.10+/-0.01 pCa units in control myocytes. From these results we conclude that the decrease in the Ca2+ sensitivity of tension at short SL results predominantly from decreased cooperative activation of the thin filament due to reductions in the number of strong-binding crossbridges.

Cross-bridge interaction kinetics in rat myocardium are accelerated by strong binding of myosin to the thin filament

To determine the ability of strong-binding myosin cross-bridges to activate the myocardial thin filament, we examined the Ca2+ dependence of force and cross-bridge interaction kinetics at 15 degrees C in the absence and presence of a strong-binding, non-force-generating derivative of myosin subfragment-1 (NEM-S1) in chemically skinned myocardium from adult rats. Relative to control conditions, application of 6 microM NEM-S1 significantly increased Ca2+-independent tension, measured at pCa 9.0, from 0.8 +/- 0.3 to 3.7 +/- 0.8 mN mm-2. Furthermore, NEM-S1 potentiated submaximal Ca2+-activated forces and thereby increased the Ca2+ sensitivity of force, i.e. the [Ca2+] required for half-maximal activation (pCa50) increased from pCa 5.85 +/- 0.05 to 5.95 +/- 0.04 (change in pCa50 (dpCa50) = 0.11 +/- 0.02). The augmentation of submaximal force by NEM-S1 was accompanied by a marked reduction in the steepness of the force-pCa relationship for forces less than 0.50 Po (maximum Ca2+-activated force), i.e. the Hill coefficient (n2) decreased from 4.72 +/- 0.38 to 1.54 +/- 0.07. In the absence of NEM-S1, the rate of force redevelopment (ktr) was found to increase from 1.11 +/- 0.21 s-1 at submaximal [Ca2+] (pCa 6.0) to 9.28 +/- 0.41 s-1 during maximal Ca2+ activation (pCa 4.5). Addition of NEM-S1 reduced the Ca2+ dependence of ktr by eliciting maximal values at low levels of Ca2+, i.e. ktr was 9.38 +/- 0.30 s-1 at pCa 6.6 compared to 9.23 +/- 0.27 s-1 at pCa 4. At intermediate levels of Ca2+, ktr was less than maximal but was still greater than values obtained at the same pCa in the absence of NEM-S1. NEM-S1 dramatically reduced both the extent and rate of relaxation from steady-state submaximal force following flash photolysis of the caged Ca2+ chelator diazo-2. These data demonstrate that strongly bound myosin cross-bridges increase the level of thin filament activation in myocardium, which is manifested by an increase in the rate of cross-bridge attachment, potentiation of force at low levels of free Ca2+, and slowed rates of relaxation.

Cooperative mechanisms in the activation dependence of the rate of force development in rabbit skinned skeletal muscle fibers

Regulation of contraction in skeletal muscle is a highly cooperative process involving Ca(2+) binding to troponin C (TnC) and strong binding of myosin cross-bridges to actin. To further investigate the role(s) of cooperation in activating the kinetics of cross-bridge cycling, we measured the Ca(2+) dependence of the rate constant of force redevelopment (k(tr)) in skinned single fibers in which cross-bridge and Ca(2+) binding were also perturbed. Ca(2+) sensitivity of tension, the steepness of the force-pCa relationship, and Ca(2+) dependence of k(tr) were measured in skinned fibers that were (1) treated with NEM-S1, a strong-binding, non-force-generating derivative of myosin subfragment 1, to promote cooperative strong binding of endogenous cross-bridges to actin; (2) subjected to partial extraction of TnC to disrupt the spread of activation along the thin filament; or (3) both, partial extraction of TnC and treatment with NEM-S1. The steepness of the force-pCa relationship was consistently reduced by treatment with NEM-S1, by partial extraction of TnC, or by a combination of TnC extraction and NEM-S1, indicating a decrease in the apparent cooperativity of activation. Partial extraction of TnC or NEM-S1 treatment accelerated the rate of force redevelopment at each submaximal force, but had no effect on kinetics of force development in maximally activated preparations. At low levels of Ca(2+), 3 microM NEM-S1 increased k(tr) to maximal values, and higher concentrations of NEM-S1 (6 or 10 microM) increased k(tr) to greater than maximal values. NEM-S1 also accelerated k(tr) at intermediate levels of activation, but to values that were submaximal. However, the combination of partial TnC extraction and 6 microM NEM-S1 increased k(tr) to virtually identical supramaximal values at all levels of activation, thus, completely eliminating the activation dependence of k(tr). These results show that k(tr) is not maximal in control fibers, even at saturating [Ca(2+)], and suggest that activation dependence of k(tr) is due to the combined activating effects of Ca(2+) binding to TnC and cross-bridge binding to actin.

Effects of endurance exercise on isomyosin patterns in fast- and slow-twitch skeletal muscles

Although endurance training has been shown to profoundly affect the oxidative capacity of skeletal muscle, little information is available concerning the impact of endurance training on skeletal muscle isomyosin expression across a variety of muscle fiber types. Therefore, a 10-wk running program (1 h/day, 5 days/wk, 20% grade, 1 mile/h) was conducted to ascertain the effects of endurance training on isomyosin expression in the soleus, vastus intermedius (VI), plantaris (PLAN), red and white medial gastrocnemius (RMG and WMG), and red and white vastus lateralis muscles (RVL and WVL). Evidences of training were noted by the presence of a resting and a submaximal exercise bradycardia, as well as an enhancement in peak O2 consumption in the trained rodents relative to the nontrained controls. No evidence for skeletal muscle hypertrophy was observed subsequent to training when muscle weight was normalized to body weight. Shifts in the isomyosin profile of the trained VI, RMG, RVL, and PLAN were seen relative to the nontrained controls. Specifically, training affected the slow myosin (SM) composition of the VI by decreasing the relative content of the SM2 isoform by 14% while increasing that of the SM1 isoform (P less than 0.05). In addition, training elicited various degrees of a fast to slower myosin transformation in the RMG, RVL, and PLAN. All three muscles showed a significant reduction in the fast myosin 2 isoform (P less than 0.05), with significant increases in intermediate myosin in the RVL and PLAN along with elevations in SM2 in the RMG and PLAN (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

PKA accelerates rate of force development in murine skinned myocardium expressing alpha- or beta-tropomyosin

In myocardium, protein kinase A (PKA) is known to phosphorylate troponin I (TnI) and myosin-binding protein-C (MyBP-C). Here, we used skinned myocardial preparations from nontransgenic (NTG) mouse hearts expressing 100% alpha-tropomyosin (alpha-Tm) to examine the effects of phosphorylated TnI and MyBP-C on Ca2+ sensitivity of force and the rate constant of force redevelopment (k(tr)). Experiments were also done using transgenic (TG) myocardium expressing approximately 60% beta-Tm to test the idea that the alpha-Tm isoform is required to observe the mechanical effects of PKA phosphorylation. Compared with NTG myocardium, TG myocardium exhibited greater Ca2+ sensitivity of force and developed submaximal forces at faster rates. Treatment with PKA reduced Ca2+ sensitivity of force in NTG and TG myocardium, had no effect on maximum k(tr) in either NTG or TG myocardium, and increased the rates of submaximal force development in both kinds of myocardium. These results show that PKA-mediated phosphorylation of myofibrillar proteins significantly alters the static and dynamic mechanical properties of myocardium, and these effects occur regardless of the type of Tm expressed.

Left ventricular functional capacity in the endurance-trained rodent

Cardiac myosin P-light chain phosphorylation [P-LC(P)] has been proposed to augment myocardial force production. This study was undertaken to examine the potential for cardiac myosin P-LC(P) for both equivalent heart rate and work load in exercising endurance-trained and nontrained rodents. A 10-wk training protocol elicited a significant reduction in submaximal running O2 uptake while enhancing peak O2 uptake (-17 and 10%, respectively, P less than 0.05). Left ventricular functional index during submaximal exercise, obtained with a high-fidelity Millar ultraminiature pressure transducer, indicated that the trained animals were able to maintain peak left ventricular pressure (LVP) in comparison to their sedentary counterparts, even though both heart rate and rate of LVP development were significantly reduced (P less than 0.05). When expressed on the basis of equivalent submaximal heart rate, peak LVP was augmented in the trained animals. Cardiac myosin P-LC(P) was examined under two conditions known to produce disparate responses in trained vs. sedentary animals. For an equivalent work load, we observed parallel increases in P-LC(P) (20%) and systolic pressure (17%) in both groups, even though the trained animals exhibited significantly lower heart rates (P less than 0.05). For an equivalent heart rate, training evoked a significant increase in systolic pressure (26%, P less than 0.05) and caused a slight increase in P-LC(P) relative to the nontrained controls. Cardiac myosin adenosinetriphosphatase was reduced approximately 10% in the trained animals (P less than 0.05), commensurate with a 2.0-fold increase in the V3 (low adenosinetriphosphatase) isomyosin.(ABSTRACT TRUNCATED AT 250 WORDS)

Isomyosin distributions in rodent muscles: effects of altered thyroid state

In this study we examined the effects of 6-8 wk of thyroid hormone manipulation on striated muscle isomyosin expression in adult female rats. Animals were randomly assigned to one of three groups: 1) euthyroid controls, 2) thyroid deficient (propylthiouracil treated), and 3) hyperthyroid (triiodothyronine treated). Thyroid deficiency resulted in a marked increase in the low-adenosinetriphosphatase V3 isoform by 20- and 49-fold in the left and right ventricle, respectively. Conversely, hyperthyroidism induced a modest (3-11%) but significant increase in the high-adenosinetriphosphatase V1 isoform in both ventricles. The thyroid-deficient rats exhibited significant increases in slow myosin in both soleus (8%) and red gastrocnemius (24%), with concomitant reductions in intermediate myosin in both muscles. Interestingly, while the slow-myosin isoform was decreased in both the soleus (-19%) and the red gastrocnemius (-43%) of the hyperthyroid group, the intermediate-myosin isoform was affected differentially in the two muscles, with a fivefold increase in the former vs. a 16% decrease in the latter. Furthermore, hyperthyroidism increased the fast myosins in the red gastrocnemius while exerting no effect on the same isoforms in the white gastrocnemius. Collectively these data suggest both different specificity and sensitivity among the myosin genes of different striated muscle types in response to thyroid hormone.

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.

In vivo left ventricular functional capacity is compromised in cMyBP-C null mice

Cardiac myosin binding protein-C (cMyBP-C) is a thick filament-associated protein that binds tightly to myosin and has a potential role for modulating myocardial contraction. We tested the hypothesis that cMyBP-C 1) contributes to the enhanced in vivo contractile state following beta-adrenergic stimulation and 2) is necessary for myocardial adaptation to chronic increases in afterload. In vivo pressure-volume relations demonstrated that left ventricular (LV) systolic and diastolic function were compromised under basal conditions in cMyBP-C(-/-) compared with WT mice. Moreover, whereas beta-adrenergic treatment significantly improved ejection fraction, peak elastance, and the time to peak elastance in WT mice, these functional indexes remained unchanged in cMyBP-C(-/-) mice. Morphological and functional changes were measured through echocardiography in anesthetized mice following 5 wk of aortic banding. Adaptation to pressure overload was diminished in cMyBP-C(-/-) mice as characterized by a lack of an increase in posterior wall thickness, increased LV diameter, deterioration of fractional shortening, and prolonged isovolumic relaxation time. These results suggest that the absence of cMyBP-C significantly diminishes in vivo LV function and markedly attenuates the increase in LV contractility following beta-adrenergic stimulation or adaptation to pressure overload.

Phosphorylation of rodent cardiac myosin light chain 2: effects of exercise

The purpose of this study was to ascertain whether the degree of cardiac myosin light chain 2 (P-light chain) phosphorylation occurs as a function of changes in cardiovascular functional state as induced by 1) treadmill exercise (20-27 m/min, 0% grade for 20, 30, 45 min) (phase I) and 2) pharmacological intervention (phase II) in adult female Sprague-Dawley rats. It was hypothesized that cardiac myosin phosphorylation is regulated in accordance with time-dependent sustained elevations in myocardial work demands requiring alterations in either heart rate or left ventricular pressure development. Exercise heart rates (HR) and double products (HR x DP) were equivalent among the three exercise groups and were significantly elevated in comparison with the normal-rest (NR) group (P less than 0.05). In phase II, isoproterenol elicited higher HR, although the atenolol group exhibited a marked reduction in HR, mean arterial pressure, and double product relative to NR (P less than 0.05). Percent myosin P-light chain phosphorylation exhibited both a HR- and a work load-dependent modulation in P-light chain levels (-9% to +23% change) in the two phases of the study (P less than 0.05). These data are consistent with the view that the above responses are associated with modulations in intracellular calcium concentrations commensurate with the alterations in HR and left ventricular pressure. Also, elevations in P-light chain phosphorylation could serve to augment left ventricular pressure development under these functional states.

Myocardial functional correlates of cardiac myosin light chain 2 phosphorylation

In vitro and in situ studies have proposed a potentiation of submaximal force production after myosin light chain 2 (P-light chain) phosphorylation in mammalian striated muscle. The purpose of this study was to ascertain the relationship between the augmentation in left ventricular pressure development and cardiac myosin P-light chain phosphorylation at different times during and after submaximal treadmill exercise involving adult female Sprague-Dawley rats. In vivo hemodynamic measurements were monitored with an indwelling high-fidelity solid-state pressure transducer. Exercise heart rate, peak left ventricular (LV) pressure, and rate of LV pressure development/relaxation (LV +/- dP/dt) were significantly elevated compared with a normal sedentary group (P less than 0.001). Peak LV pressure remained significantly elevated throughout 20 min of postexercise recovery (P less than 0.01), and heart rate, LV end-diastolic pressure, and LV +/- dP/dt returned rapidly to preexercise values. Corresponding to these in vivo hemodynamic changes, increased levels of P-light chain phosphorylation were observed during both exercise (16%, P less than 0.01) and subsequent recovery periods (14%, P less than 0.02) compared with the NC group. A quasi-temporal relationship was observed between postexercise peak LV pressure potentiation and P-light chain phosphorylation. These results demonstrate that cardiac myosin P-light chain phosphorylation is associated, in part, with the augmentation of peak LV pressure observed during both exercise and recovery.

Exercise capacity of rats remains unaffected by a chronic pressure overload

OBJECTIVE

The aim was to determine if the adaptive responses of the myocardium to a chronic pressure overload affected cardiovascular performance when evaluated under conditions of increased functional demand.

METHODS

Selected female rats were made hypertensive by abdominal aortic constriction. After eight weeks of aortic constriction, cardiovascular responses and work performance were measured during a maximal treadmill exercise bout.

RESULTS

Aortic constriction increased mean arterial pressure and the relative quantity of the slow ATPase myosin isoform, V3, relative to untreated controls (p < 0.05). Both groups had similar oxygen consumptions (VO2), heart rates (HR), and oxygen pulses (VO2/HR) at rest and throughout the exercise test. Both groups reached their VO2 max at the same exercise duration and exercise intensity (40.4 m.min-1). Soleus citrate synthase activity was not different between the groups.

CONCLUSIONS

These similarities in work capacity, VO2, oxygen pulse, and muscle oxidative capacity suggest (1) that cardiovascular and exercise capacity can both be maintained in spite of the presence of a chronic pressure overload; (2) that after two months of aortic constriction the heart appears to be in a compensated stage of adaptation; and (3) that the cardiac myosin isoenzyme profile may have little direct effect on cardiovascular functional capacity.

Effect of thyroid state on cardiac myosin P-light chain phosphorylation during exercise

This study ascertained the effects of thyroid deficiency (TD) and hyperthyroidism (H) on in vivo cardiovascular functional capacity in the context of cardiac myosin light chain 2 phosphorylation [P-LC(P)], a proposed modulator of myocardial function, at rest and during exercise. Compared with normal controls (NC), Ca2(+)-regulated myofibril adenosinetriphosphatase was reduced by 39% in TD and increased by 9% in H rats. This response was associated with a 20-fold increase in the V3 isoform and an 11% increase in the V1 isoform in TD and H rats, respectively. Submaximal treadmill exercise elicited significant elevations in all myocardial functional indexes examined in H rats compared with the NC group, whereas the opposite occurred for the TD group. Despite the marked contrast in cardiac function among the three groups, intrinsic levels of P-LC(P) were similar at rest among the groups and were significantly reduced in both TD and H groups relative to controls during exercise. These data suggest that although thyroid state exerts a profound impact on intrinsic myocardial functional state, it exerts little control over cellular processes regulating P-LC(P) during rest and exercise.

Cardiac biochemical and functional adaptations to exercise in sympathectomized neonatal rats

This study was undertaken to examine the influence of guanethidine monosulfate-induced sympathectomy on exercise-induced adaptations of cardiac contractile protein and on acute hemodynamic responses to exercise involving female neonatal rats. Four groups of rats were studied: 1) normal sedentary (NS), 2) normal trained (NT), 3) sympathectomized sedentary (SS), and 4) sympathectomized trained (ST). The 9-wk running program, which began at 20 days of age, induced increases in whole-body maximal O2 consumption and skeletal-muscle citrate synthase activity in both NT and ST groups compared with NS (P less than 0.05). Submaximal exercise tests demonstrated circulatory adaptations for NT, SS, and ST groups compared with NC; however, the ST group demonstrated the greatest degree of altered cardiac function (decreased heart rate, left ventricular pressure, and contractility index) during exercise. Also, significant reductions in both myosin- and Ca2+-regulated myofibril adenosinetriphosphatase (ATPase) activity and increases in the relative content of the low ATPase myosin isozyme, V3, occurred in the hearts of the two trained groups (P less than 0.05). These findings suggest that chronic exercise involving normal and sympathectomized neonatal rats improves cardiac function without compromising maximal exercise capacity. Also, the exercise-related adaptation involving myosin isozyme shifts are exaggerated when involvement of the sympathetic nervous system is reduced during training.

Identification of basic residues involved in activation and calmodulin binding of rabbit smooth muscle myosin light chain kinase

It is postulated that basic residues in the regulatory region of myosin light chain kinase are important for conferring autoinhibition by binding to the catalytic core. To investigate this proposal, 10 basic amino acids within the regulatory region of rabbit smooth muscle myosin light chain kinase (Lys961-Lys979) were replaced either singularly or in combination with acidic or nonpolar residues by site-directed mutagenesis. All active mutant kinases were dependent on Ca2+/calmodulin for catalytic activity. None of the mutants was active in the absence of Ca2+/calmodulin, suggesting that the autoinhibitory region has not been defined completely. Charge reversal mutants at Arg974, Arg975, and Lys976 resulted in loss of high affinity binding of calmodulin and increased the concentration of calmodulin required for half-maximal activation (KCaM). The charge reversal mutant at Lys979 also increased KCaM but to a lesser extent. Charge reversal mutants at Lys965 and Arg967 resulted in an inactive myosin light chain kinase that could not be proteolytically activated. When these residues were mutated to Ala, the expressed kinase was dependent upon Ca2+/calmodulin for activity and exhibited a decrease in KCaM. Charge reversal mutants in Lys961 and Lys962 also had decreased KCaM values. These basic residues amino-terminal of the calmodulin binding domain may play an important role in the activation of the kinase.