Variations in cross-bridge attachment rate and tension with phosphorylation of myosin in mammalian skinned skeletal muscle fibers. Implications for twitch potentiation in intact muscle

Effects of a R133W beta-tropomyosin mutation on regulation of muscle contraction in single human muscle fibres

A novel R133W beta-tropomyosin (beta-Tm) mutation, associated with muscle weakness and distal limb deformities, has recently been identified in a woman and her daughter. The muscle weakness was not accompanied by progressive muscle wasting or histopathological abnormalities in tibialis anterior muscle biopsy specimens. The aim of the present study was to explore the mechanisms underlying the impaired muscle function in patients with the beta-Tm mutation. Maximum force normalized to fibre cross-sectional area (specific force, SF), maximum velocity of unloaded shortening (V0), apparent rate constant of force redevelopment (ktr) and force-pCa relationship were evaluated in single chemically skinned muscle fibres from the two patients carrying the beta-Tm mutation and from healthy control subjects. Significant differences in regulation of muscle contraction were observed in the type I fibres: a lower SF (P<0.05) and ktr (P<0.01), and a faster V0 (P<0.05). The force-pCa relationship did not differ between patient and control fibres, indicating an unaltered Ca2+ activation of contractile proteins. Collectively, these results indicate a slower cross-bridge attachment rate and a faster detachment rate caused by the R133W beta-Tm mutation. It is suggested that the R133W beta-Tm mutation induces alteration in myosin-actin kinetics causing a reduced number of myosin molecules in the strong actin-binding state, resulting in overall muscle weakness in the absence of muscle wasting.

Investigation of thin filament near-neighbour regulatory unit interactions during force development in skinned cardiac and skeletal muscle

Ca(2+)-dependent activation of striated muscle involves cooperative interactions of cross-bridges and thin filament regulatory proteins. We investigated how interactions between individual structural regulatory units (RUs; 1 tropomyosin, 1 troponin, 7 actins) influence the level and rate of demembranated (skinned) cardiac muscle force development by exchanging native cardiac troponin (cTn) with different ratio mixtures of wild-type (WT) cTn and cTn containing WT cardiac troponin T/I + cardiac troponin C (cTnC) D65A (a site II inactive cTnC mutant). Maximal Ca(2+)-activated force (F(max)) increased in less than a linear manner with WT cTn. This contrasts with results we obtained previously in skeletal fibres (using sTnC D28A, D65A) where F(max) increased in a greater than linear manner with WT sTnC, and suggests that Ca(2+) binding to each functional Tn activates < 7 actins of a structural regulatory unit in cardiac muscle and > 7 actins in skeletal muscle. The Ca(2+) sensitivity of force and rate of force redevelopment (k(tr)) was leftward shifted by 0.1-0.2 -log [Ca(2+)] (pCa) units as WT cTn content was increased, but the slope of the force-pCa relation and maximal k(tr) were unaffected by loss of near-neighbour RU interactions. Cross-bridge inhibition (with butanedione monoxime) or augmentation (with 2 deoxy-ATP) had no greater effect in cardiac muscle with disruption of near-neighbour RU interactions, in contrast to skeletal muscle fibres where the effect was enhanced. The rate of Ca(2+) dissociation was found to be > 2-fold faster from whole cardiac Tn compared with skeletal Tn. Together the data suggest that in cardiac (as opposed to skeletal) muscle, Ca(2+) binding to individual Tn complexes is insufficient to completely activate their corresponding RUs, making thin filament activation level more dependent on concomitant Ca(2+) binding at neighbouring Tn sites and/or crossbridge feedback effects on Ca(2+) binding affinity.

O-linked N-acetylglucosaminylation is involved in the Ca2+ activation properties of rat skeletal muscle

O-Linked N-acetylglucosaminylation termed O-GlcNAc is a dynamic cytosolic and nuclear glycosylation that is dependent both on glucose flow through the hexosamine biosynthesis pathway and on phosphorylation because of the existence of a balance between phosphorylation and O-GlcNAc. This glycosylation is a ubiquitous post-translational modification, which probably plays an important role in many aspects of protein functions. We have previously reported that, in skeletal muscle, proteins of the glycolytic pathway, energetic metabolism, and contractile proteins were O-GlcNAc-modified and that O-Glc-NAc variations could control the muscle protein homeostasis and be implicated in the regulation of muscular atrophy. In this paper, we report O-N-acetylglucosaminylation of a number of key contractile proteins (i.e. myosin heavy and light chains and actin), which suggests that this glycosylation could be involved in skeletal muscle contraction. Moreover, our results showed that incubation of skeletal muscle skinned fibers in N-acetyl-d-glucosamine, in a concentration solution known to inhibit O-GlcNAc-dependent interactions, induced a decrease in calcium sensitivity and affinity of muscular fibers, whereas the cooperativity of the thin filament proteins was not modified. Thus, our results suggest that O-GlcNAc is involved in contractile protein interactions and could thereby modulate muscle contraction.

Postactivation potentiation in a human muscle: effect on the rate of torque development of tetanic and voluntary isometric contractions

Postactivation potentiation (PAP), a mechanism by which the torque of a muscle twitch is increased following a conditioning contraction, is well documented in muscular physiology, but little is known about its effect on the maximal rate of torque development and functional significance during voluntary movements. The objective of this study was to investigate the PAP effect on the rate of isometric torque development of electrically induced and voluntary contractions. To that purpose, the electromechanical responses of the thumb adductor muscles to a single electrical stimulus (twitch), a train of 15 pulses at 250 Hz (HFT(250)), and during ballistic (i.e., rapid torque development) voluntary contractions at torque levels ranging from 10 to 75% of maximal voluntary contraction (MVC) were recorded before and after a conditioning 6-s MVC. The results showed that the rate of torque development was significantly (P < 0.001) increased after the conditioning MVC, but the effect was greater for the twitch ( approximately 200%) compared with the HFT(250) ( approximately 17%) or ballistic contractions (range: 9-24%). Although twitch potentiation was maximal immediately after the conditioning MVC, maximal potentiation for HFT(250) and ballistic contractions was delayed to 1 min after the 6-s MVC. Furthermore, the similar degree of potentiation for the rate of isometric torque development between tetanic and voluntary ballistic contractions indicates that PAP is not related to the modality of muscle activation. These observations suggest that PAP may be considered as a mechanism that can influence our contractions during daily tasks and can be utilized to improve muscle performance in explosive sports.

Regulation of contraction kinetics in skinned skeletal muscle fibers by calcium and troponin C

The influences of [Ca(2+)] and Ca(2+) dissociation rate from troponin C (TnC) on the kinetics of contraction (k(Ca)) activated by photolysis of a caged Ca(2+) compound in skinned fast-twitch psoas and slow-twitch soleus fibers from rabbits were investigated at 15 degrees C. Increasing the amount of Ca(2+) released increased the amount of force in psoas and soleus fibers and increased k(Ca) in a curvilinear manner in psoas fibers approximately 5-fold but did not alter k(Ca) in soleus fibers. Reconstituting psoas fibers with mutants of TnC that in solution exhibited increased Ca(2+) affinity and approximately 2- to 5-fold decreased Ca(2+) dissociation rate (M82Q TnC) or decreased Ca(2+) affinity and approximately 2-fold increased Ca(2+) dissociation rate (NHdel TnC) did not affect maximal k(Ca). Thus the influence of [Ca(2+)] on k(Ca) is fiber type dependent and the maximum k(Ca) in psoas fibers is dominated by kinetics of cross-bridge cycling over kinetics of Ca(2+) exchange with TnC.

Myosin light chain phosphorylation inhibits muscle fiber shortening velocity in the presence of vanadate

We have shown that myosin light chain phosphorylation inhibits fiber shortening velocity at high temperatures, 30 degrees C, in the presence of the phosphate analog vanadate. Vanadate inhibits tension by reversing the transition to force-generating states, thus mimicking a prepower stroke state. We have previously shown that at low temperatures vanadate also inhibits velocity, but at high temperatures it does not, with an abrupt transition in inhibition occurring near 25 degrees C (E. Pate, G. Wilson, M. Bhimani, and R. Cooke. Biophys J 66: 1554-1562, 1994). Here we show that for fibers activated in the presence of 0.5 mM vanadate, at 30 degrees C, shortening velocity is not inhibited in dephosphorylated fibers but is inhibited by 37 +/- 10% in fibers with phosphorylated myosin light chains. There is no effect of phosphorylation on fiber velocity in the presence of vanadate at 10 degrees C. The K(m) for ATP, defined by the maximum velocity of fibers partially inhibited by vanadate at 30 degrees C, is 20 +/- 4 microM for phosphorylated fibers and 192 +/- 40 microM for dephosphorylated fibers, showing that phosphorylation also affects the binding of ATP. Fiber stiffness is not affected by phosphorylation. Inhibition of velocity by phosphorylation at 30 degrees C depends on the phosphate analog, with approximately 12% inhibition in fibers activated in the presence of 5 mM BeF(3) and no inhibition in the presence of 0.25 mM AlF(4). Our results show that myosin phosphorylation can inhibit shortening velocity in fibers with large populations of myosin heads trapped in prepower stroke states, such as occurs during muscle fatigue.

Structural changes in troponin in response to Ca2+ and myosin binding to thin filaments during activation of skeletal muscle

Contraction of skeletal and cardiac muscle is regulated by Ca2+ -dependent structural changes in troponin that control the interaction between myosin and actin. We measured the orientations of troponin domains in skeletal muscle fibers using polarized fluorescence from bifunctional rhodamine probes on the C and E helices of troponin C. The C helix, in the regulatory head domain, tilts by approximately 30 degrees when muscle is activated in physiological conditions, with a Ca2+ -sensitivity similar to that of active force. Complete inhibition of active force did not affect C-helix orientation, and binding of rigor myosin heads did not affect its orientation at saturating [Ca2+]. The E helix, in the IT arm of troponin, tilted by approximately 10 degrees on activation, and this was reduced to only 3 degrees when active force was inhibited. Binding of rigor myosin heads produced a larger tilt of the E helix. Thus, in situ, the regulatory head acts as a pure Ca2+ -sensor, whereas the IT arm is primarily sensitive to myosin head binding. The polarized fluorescence data from active muscle are consistent with an in vitro structure of the troponin core complex in which the D and E helices of troponin C are collinear. The present data were used to orient this structure in the fiber and suggest that the IT arm is at approximately 30 degrees to the filament axis in active muscle. In relaxed muscle, the IT arm tilts to approximately 40 degrees but the D/E helix linker melts, allowing the regulatory head to tilt through a larger angle.

X-ray diffraction studies of the thick filament in permeabilized myocardium from rabbit

Low angle x-ray diffraction patterns from relaxed permeabilized rabbit cardiac trabeculae and psoas muscle fibers were compared. Temperature was varied from 25 degrees C to 5 degrees C at 200 mM and 50 mM ionic strengths (mu), respectively. Effects of temperature and mu on the intensities of the myosin layer lines (MLL), the equatorial intensity ratio I(1,1)/I(1,0), and the spacing of the filament lattice are similar in both muscles. At 25 degrees C, particularly at mu = 50 mM, the x-ray patterns exhibited up to six orders of MLL and sharp meridional reflections, signifying that myosin heads (cross-bridges) are distributed in a well-ordered helical array. Decreasing temperature reduced MLL intensities but increased I(1,1)/I(1,0). Decreases in the MLL intensities indicate increasing disorder in the distribution of cross-bridges on the thick filaments surface. In the skeletal muscle, order/disorder is directly correlated with the hydrolysis equilibrium of ATP by myosin, [M.ADP.P(i)]/[M.ATP]. Similar effects of temperature on MLL and similar biochemical ATP hydrolysis pathway found in both types of muscles suggest that the order/disorder states of cardiac cross-bridges may well be correlated with the same biochemical and structural states. This implies that in relaxed cardiac muscle under physiological conditions, the unattached cross-bridges are largely in the M.ADP.P(i) state and with the lowering of the temperature, the equilibrium is increasingly in favor of [M.ATP] and [A.M.ATP]. There appear to be some differences in the diffraction patterns from the two muscles, however. Mainly, in the cardiac muscle, the MLL are weaker, the I(1,1)/I(1,0) ratio tends to be higher, and the lattice spacing D(10), larger. These differences are consistent with the idea that under a wide range of conditions, a greater fraction of cross-bridges is weakly bound to actin in the myocardium.

Use of thin filament reconstituted muscle fibres to probe the mechanism of force generation

The technique of selective removal of the thin filament by gelsolin in bovine cardiac muscle fibres, and reconstitution of the thin filament from isolated proteins is reviewed, and papers that used reconstituted preparations are discussed. By comparing the results obtained in the absence/presence of regulatory proteins tropomyosin (Tm) and troponin (Tn), it is concluded that the role of Tm and Tn in force generation is not only to expose the binding site of actin to myosin, but also to modify actin for better stereospecific and hydrophobic interaction with myosin. This conclusion is further supported by experiments that used a truncated Tm mutant and the temperature study of reconstituted fibres. The conclusion is consistent with the hypothesis that there are three states in the thin filament: blocked state, closed state, and open state. Tm is the major player to produce these effects, with Tn playing the role of Ca2+ sensing and signal transmission mechanism. Experiments that changed the number of negative charges at the N-terminal finger of actin demonstrates that this part of actin is essential to promote the strong interaction between actin and myosin molecules, in addition to the well-known weak interaction that positions the myosin head at the active site of actin prior to force generation.

Acceleration of stretch activation in murine myocardium due to phosphorylation of myosin regulatory light chain

The regulatory light chains (RLCs) of vertebrate muscle myosins bind to the neck region of the heavy chain domain and are thought to play important structural roles in force transmission between the cross-bridge head and thick filament backbone. In vertebrate striated muscles, the RLCs are reversibly phosphorylated by a specific myosin light chain kinase (MLCK), and while phosphorylation has been shown to accelerate the kinetics of force development in skeletal muscle, the effects of RLC phosphorylation in cardiac muscle are not well understood. Here, we assessed the effects of RLC phosphorylation on force, and the kinetics of force development in myocardium was isolated in the presence of 2,3-butanedione monoxime (BDM) to dephosphorylate RLC, subsequently skinned, and then treated with MLCK to phosphorylate RLC. Since RLC phosphorylation may be an important determinant of stretch activation in myocardium, we recorded the force responses of skinned myocardium to sudden stretches of 1% of muscle length both before and after treatment with MLCK. MLCK increased RLC phosphorylation, increased the Ca(2+) sensitivity of isometric force, reduced the steepness of the force-pCa relationship, and increased both Ca(2+)-activated and Ca(2+)-independent force. Sudden stretch of myocardium during an otherwise isometric contraction resulted in a concomitant increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force, i.e., stretch activation, to levels greater than pre-stretch force. MLCK had profound effects on the stretch activation responses during maximal and submaximal activations: the amplitude and rate of force decay after stretch were significantly reduced, and the rate of delayed force recovery was accelerated and its amplitude reduced. These data show that RLC phosphorylation increases force and the rate of cross-bridge recruitment in murine myocardium, which would increase power generation in vivo and thereby enhance systolic function.

The effect of myosin regulatory light chain phosphorylation on the frequency-dependent regulation of cardiac function

Although it has been suggested that in cardiac muscle the phosphorylation level of myosin regulatory light chain (RLC) correlates with frequency of stimulation, its significance in the modulation of the force-frequency and pressure-frequency relationships remains unclear. We examined the role of RLC phosphorylation on the force-frequency relation (papillary muscles), the pressure-frequency relation (Langendorff perfused hearts) and shortening-frequency relation (isolated cardiac myocytes) in nontransgenic (NTG) and transgenic mouse hearts expressing a nonphosphorylatable RLC protein (RLC(P-)). At 22 degrees C, NTG and RLC(P-) muscles showed a negative force-frequency relation. At 32 degrees C, at frequencies above 1 Hz, both groups showed a flat force-frequency relation. There was a small increase in RLC phosphorylation in NTG muscles when the frequency of stimulation was increased from 0.2 Hz to 4.0 Hz. However, the level of RLC phosphorylation in these isolated muscles was significantly lower compared to samples taken from NTG intact hearts. In perfused hearts, there was no difference in the slope of pressure-frequency relationship between groups, but the RLC(P-) group consistently developed a reduced systolic pressure and demonstrated a decreased contractility. There was no difference in the level of RLC phosphorylation in hearts paced at 300 and 600 bpm. In RLC(P-) hearts, the level of TnI phosphorylation was reduced compared to NTG. There was no change in the expression of PLB between groups, but expression of SERCA2 was increased in hearts from RLC(P-) compared to NTG. In isolated cardiac myocytes, there was no change in shortening-frequency relationship between groups. Moreover, there was no change in Ca(2+) transient parameters in cells from NTG and RLC(P-) hearts. Our data demonstrate that in cardiac muscle RLC phosphorylation is not an essential determinant of force- and pressure-frequency relations but the absence of RLC phosphorylation decreases contractility in force/pressure developing preparations.

Inorganic phosphate affects the pCa-force relationship more than the pCa-ATPase by increasing the rate of dissociation of force generating cross-bridges in skinned fibers from both EDL and soleus muscles of the rat

The effect of inorganic phosphate (Pi) on Ca2+ -activation of actomyosin ATPase activity and force in permeabilized (skinned) single extensor digitorum longus (EDL) and soleus muscle fibers of the rat were investigated. Increasing concentrations of Pi decreased force more than ATPase rate at all Ca2+ concentrations and this effect was more pronounced at submaximal Ca2+ -activation. Increasing Pi caused both the normalized pCa-ATPase and pCa-force relationship to be shifted to a higher Ca2+ concentration. At all Ca2+ concentrations ATPase was activated at a lower concentration of Ca2+ than force and this difference in Ca2+ concentration required for the activation of ATPase and force was greater in fast-twitch (EDL) than in slow twitch (soleus) muscle. Soleus muscle pCa-ATPase and pCa-force curves were more sensitive to Ca2+ (pCa50 = 5.97 and 5.89, respectively) than EDL (pCa50 = 5.68 and 5.54, respectively). Finally the shape of the pCa-ATPase and pCa-force curves was similar and not affected by Pi. Analysis shows that Pi increases the rate of dissociation of force generating myosin cross-bridges (ratio of ATPase/force (g(app at all Ca2+ concentration, especially at submaximal Ca2+ -activation levels. Pi effects on g(app) are discussed in terms Pi interacting with the isomerization high force AM*ADP states to form high force transitional AM*ADP*Pi* states which facilitate the dissociation of ADP from AM*ADP. Increasing Ca2+ during Ca2+ -activation of the fibers is associated with a progressive decrease in rate of dissociation of force generating myosin cross-bridges g(app).

Basal myosin light chain phosphorylation is a determinant of Ca2+ sensitivity of force and activation dependence of the kinetics of myocardial force development

It is generally recognized that ventricular myosin regulatory light chains (RLC) are approximately 40% phosphorylated under basal conditions, and there is little change in RLC phosphorylation with agonist stimulation of myocardium or altered stimulation frequency. To establish the functional consequences of basal RLC phosphorylation in the heart, we measured mechanical properties of rat skinned trabeculae in which approximately 7% or approximately 58% of total RLC was phosphorylated. The protocol for achieving approximately 7% phosphorylation of RLC involved isolating trabeculae in the presence of 2,3-butanedione monoxime (BDM) to dephosphorylate RLC from its baseline level. Subsequent phosphorylation to approximately 58% of total was achieved by incubating BDM-treated trabeculae in solution containing smooth muscle myosin light chain kinase, calmodulin, and Ca2+ (i.e., MLCK treatment). After MLCK treatment, Ca2+ sensitivity of force increased by 0.06 pCa units and maximum force increased by 5%. The rate constant of force development (ktr) increased as a function of Ca2+ concentration in the range between pCa 5.8 and pCa 4.5. When expressed versus pCa, the activation dependence of ktr appeared to be unaffected by MLCK treatment; however, when activation was expressed in terms of isometric force-generating capability (as a fraction of maximum), MLCK treatment slowed ktr at submaximal activations. These results suggest that basal phosphorylation of RLC plays a role in setting the kinetics of force development and Ca2+ sensitivity of force in cardiac muscle. Our results also argue that changes in RLC phosphorylation in the range examined here influence actin-myosin interaction kinetics differently in heart muscle than was previously reported for skeletal muscle.

Myofibrillar determinants of rate of relaxation in skinned skeletal muscle fibers

The influence of Ca2+ dissociation rate from TnC and decreased cross-bridge detachment rate on the time course of relaxation induced by flash photolysis of diazo-2 in rabbit skinned psoas fibers was investigated at 15 degrees C. A TnC mutant (M82Q TnC) that exhibited increased Ca2+ sensitivity caused by a decreased Ca2+ dissociation rate in solution also increased the Ca2+ sensitivity of force and decreased the rate of relaxation in fibers approximately 2-fold. In contrast, a TnC mutant (NHdel TnC) with decreased Ca2+ sensitivity caused by an increased Ca2+ dissociation rate in solution decreased Ca2+ sensitivity of force but did not accelerate relaxation. Decreasing the rate of cross-bridge kinetics by reducing [Pi] slowed relaxation -2-fold and led to two phases of relaxation, a linear phase followed by an exponential phase. In fibers, M82Q TnC further slowed relaxation in low [Pi] approximately 2-fold whereas NHdel TnC had no significant effect on relaxation. These results are consistent with the interpretation that the Ca2+ dissociation rate and cross-bridge detachment rate are similar in fast twitch skeletal muscle such that decreasing either rate slows relaxation but accelerating Ca2+ dissociation has little effect on relaxation.

Mutations of hydrophobic residues in the N-terminal domain of troponin C affect calcium binding and exchange with the troponin C-troponin I96-148 complex and muscle force production

Interactions between troponin C and troponin I play a critical role in the regulation of skeletal muscle contraction and relaxation. We individually substituted 27 hydrophobic Phe, Ile, Leu, Val, and Met residues in the regulatory domain of the fluorescent troponin C(F29W) with polar Gln to examine the effects of these mutations on: (a) the calcium binding and dynamics of troponin C(F29W) complexed with the regulatory fragment of troponin I (troponin I(96-148)) and (b) the calcium sensitivity of force production. Troponin I(96-148) was an accurate mimic of intact troponin I for measuring the calcium dynamics of the troponin C(F29W)-troponin I complexes. The calcium affinities of the troponin C(F29W)-troponin I(96-148) complexes varied approximately 243-fold, whereas the calcium association and dissociation rates varied approximately 38- and approximately 33-fold, respectively. Interestingly, the effect of the mutations on the calcium sensitivity of force development could be better predicted from the calcium affinities of the troponin C(F29W)-troponin I(96-148) complexes than from that of the isolated troponin C(F29W) mutants. Most of the mutations did not dramatically affect the affinity of calcium-saturated troponin C(F29W) for troponin I(96-148). However, the Phe(26) to Gln and Ile(62) to Gln mutations led to >10-fold lower affinity of calcium-saturated troponin C(F29W) for troponin I(96-148), causing a drastic reduction in force recovery, even though these troponin C(F29W) mutants still bound to the thin filaments. In conclusion, elucidating the determinants of calcium binding and exchange with troponin C in the presence of troponin I provides a deeper understanding of how troponin C controls signal transduction.

Coexistence of potentiation and low-frequency fatigue during voluntary exercise in human skeletal muscle

The role of muscle potentiation in overcoming low-frequency fatigue (LFF) as it developed during submaximal voluntary exercise was investigated in eight males (age 26.4 ± 0.7 years, mean ± SE) performing isometric leg extension at ~30% of maximal voluntary contraction for 60 min using a 0.5-duty cycle (1 s contraction, 1 s rest). At 5, 20, 40, and 60 min, exercise was interrupted for 3 min, and the maximum positive rate of force development (+dF/dtmax) and maximal twitch force (Pt) were measured in maximal twitch contractions at 0, 1, 2, and 3 min of rest (R0, R1, R2, R3); they were also measured at 15 min of recovery following the entire 60-min exercise period. These measures were compared with pre-exercise (PRE) as an indicator of potentiation. Force at low frequency (10 Hz) was also measured at R0, R1, R2, and R3, and at 15 min of recovery, while force at high frequency (100 Hz) was measured only at R0 and R3 and in recovery. Voluntary exercise increased twitch +dF/dtmax at R0 following 5, 20, 40, and 60 min of exercise, from 2553 ± 150 N/s at PRE to 39%, 41%, 42%, and 36% above PRE, respectively (P < 0.005). Twitch +dF/dtmax decayed at brief rest (R3) following 20, 40, and 60 min of exercise (P < 0.05). Pt at R0 following 5 and 20 min of exercise was above that at PRE (P < 0.05), indicating that during the early phase of moderate- intensity repetitive exercise, potentiation occurs in the relative absence of LFF. At 40 and 60 min of exercise, Pt at R0 was unchanged from PRE. The LFF (10 Hz) induced by the protocol was evident at 40 and 60 min (R0–R3; P < 0.05) and at 15 min following exercise (P < 0.05). High-frequency force was not significantly compromised by the protocol. Since twitch force was maintained, these results suggest that as exercise progresses, LFF develops, which can be compensated for by potentiation.Key words: excitability, myosin light chain, phosphorylation, isometric exercise.

Regulatory light chain phosphorylation increases eccentric contraction-induced injury in skinned fast-twitch fibers

During contraction, activation of Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) results in phosphorylation of myosin's regulatory light chain (RLC), which potentiates force and increases speed of force development over a wide range of [Ca(2+)]. We tested the hypothesis that RLC phosphorylation by MLCK mediates the extent of eccentric contraction-induced injury as measured by force deficit in skinned fast-twitch skeletal muscle fibers. Results indicated that RLC phosphorylation in single skinned rat psoas fibers significantly increased Ca(2+) sensitivity of isometric force; isometric force from 50 +/- 16 to 59 +/- 18 kN/m(2) during maximal Ca(2+) activation; peak absolute power output from 38 +/- 15 to 48 +/- 14 nW during maximal Ca(2+) activation; and the magnitude of contraction-induced force deficit during maximal (pCa 4.5) activation from 26 +/- 9.8 to 35 +/- 9.6%. We conclude that RLC phosphorylation increases force deficits following eccentric contractions, perhaps by increasing the number of force-generating cross-bridges.

Thin filament activation and unloaded shortening velocity of rabbit skinned muscle fibres

The unloaded shortening velocity of skinned rabbit psoas muscle fibres is sensitive to [Ca2+]. To determine whether Ca2+ affects the unloaded shortening velocity via regulation of crossbridge kinetics or crossbridge number, the shortening velocity was measured following changes in either [Ca2+] or the number of active thin filament regulatory units. The native troponin C (TnC) was extracted and replaced with either cardiac TnC (cTnC) or a mixture of cTnC and an inactive mutant cardiac TnC (CBMII TnC). The unloaded shortening velocity of the cTnC-replaced fibres was determined at various values of [Ca2+] and compared with different cTnC:CBMII TnC ratios at a saturating [Ca2+]. If Ca2+ regulates the unloaded shortening velocity via kinetic modulation, differences in the velocity-tension relationship between the cTnC fibres and the cTnC:CBMII TnC fibres would be apparent. Alternatively, Ca2+ control of the number of active crossbridges would yield similar velocity-tension relationships when comparing the cTnC and cTnC:CBMII TnC fibres. The results show a decline in the unloaded shortening velocity that is determined by the relative tension, defined as the level of thin filament activation, rather than the [Ca2+]. Furthermore, at lower levels of relative tension, the reduction in unloaded shortening is not the result of changes in any cooperative effects of myosin on Ca2+ binding to the thin filament. Rather, it may be related to a decrease in crossbridge-induced activation of the thin filament at the level of the individual regulatory unit. In summary, the results suggest that Ca2+ regulates the unloaded shortening velocity in skinned fibres by reducing the number of crossbridges able to productively bind to the thin filament without affecting any inherent property of the myosin.

Role of hydration in the closed-to-open transition involved in Ca2+ binding by troponin C

Troponin C (TnC) is the Ca(2+)-binding subunit of the troponin complex of vertebrate skeletal muscle. It consists of two structurally homologous domains, N and C, connected by an exposed alpha-helix. The C-domain has two high-affinity sites for Ca(2+) that also bind Mg(2+), whereas the N-domain has two low-affinity sites for Ca(2+). Previous studies using isolated N- and C-domains showed that the C-domain apo form was less stable than the N-domain. Here we analyzed the stability of isolated N-domain (F29W/N-domain) against urea and pressure denaturation in the absence and in the presence of glycerol using fluorescence spectroscopy. Increasing the glycerol concentration promoted an increase in the stability of the protein to urea (0-8 M) in the absence of Ca(2+). Furthermore, the ability to expose hydrophobic surfaces normally promoted by Ca(2+) binding or low temperature under pressure was partially lost in the presence of 20% (v/v) glycerol. Glycerol also led to a decrease in the Ca(2+) affinity of the N-domain in solution. From the ln K(obs) versus ln a(H)2(O), we obtained the number of water molecules (63.5 +/- 8.7) involved in the transition N <=>N:Ca(2) that corresponds to an increase in the exposed surface area of 571.5 +/- 78.3 A(2). In skinned fibers, the affinity for Ca(2+) was also reduced by glycerol, although the effect was much less pronounced than in solution. Our results demonstrate quantitatively that the stability of this protein and its affinity for Ca(2+) are critically dependent on protein hydration.

Mechanical defects of muscle fibers with myosin light chain mutants that cause cardiomyopathy

Familial hypertrophic cardiomyopathy is a disease caused by single mutations in several sarcomeric proteins, including the human myosin ventricular regulatory light chain (vRLC). The effects of four of these mutations (A13T, F18L, E22K, and P95A) in vRLC on force generation were determined as a function of Ca(2+) concentration. The endogenous RLC was removed from skinned rabbit psoas muscle fibers, and replaced with either rat wildtype vRLC or recombinant rat vRLC (G13T, F18L, E22K, and P95A). Compared to fibers with wildtype rat vRLC, the E22K mutant increased Ca sensitivity of force generation, whereas the G13T and F18L mutants decreased the Ca sensitivity, and the P95A mutant had no significant effect. None of the RLC mutants affected the maximal tension (observed at saturating Ca(2+) concentrations), except for F18L, which decreased the maximal tension to 69 +/- 10% of the wildtype value. Of the mutant RLCs, only F18L decreased the cooperativity of activation of force generation. These results suggest that the primary cause of familial hypertrophic cardiomyopathy, in some cases, is perturbation in the Ca sensitivity of force generation, in which Ca-sensitizing or Ca-desensitizing effects can lead to similar disease phenotypes.

Solution structure of heavy meromyosin by small-angle scattering

Elucidation of x-ray crystal structures for the S1 subfragment of myosin afforded atomic resolution of the nucleotide and actin binding sites of the enzyme. The structures have led to more detailed hypotheses regarding the mechanisms by which force generation is coupled to ATP hydrolysis. However, the three-dimensional structure of double-headed myosin consisting of two S1 subfragments has not yet been solved. Therefore, to investigate the overall shape and relative orientations of the two heads of myosin, we performed small-angle x-ray and neutron scattering measurements of heavy meromyosin containing all three light chains (LC(1-3)) in solution. The resulting small-angle scattering intensity profiles were best fit by models of the heavy meromyosin head-tail junction in which the angular separation between heads was less than 180 degrees. The S1 heads of the best fit models are not related by an axis of symmetry, and one of the two S1 heads is bent back along the rod. These results provide new information on the structure of the head-tail junction of myosin and indicate that combining scattering measurements with high resolution structural modeling is a feasible approach for investigating myosin head-head interactions in solution.

Engineering competitive magnesium binding into the first EF-hand of skeletal troponin C

The goal of this study was to examine the mechanism of magnesium binding to the regulatory domain of skeletal troponin C (TnC). The fluorescence of Trp(29), immediately preceding the first calcium-binding loop in TnC(F29W), was unchanged by addition of magnesium, but increased upon calcium binding with an affinity of 3.3 microm. However, the calcium-dependent increase in TnC(F29W) fluorescence could be reversed by addition of magnesium, with a calculated competitive magnesium affinity of 2.2 mm. When a Z acid pair was introduced into the first EF-hand of TnC(F29W), the fluorescence of G34DTnC(F29W) increased upon addition of magnesium or calcium with affinities of 295 and 1.9 microm, respectively. Addition of 3 mm magnesium decreased the calcium sensitivity of TnC(F29W) and G34DTnC(F29W) approximately 2- and 6-fold, respectively. Exchange of G34DTnC(F29W) into skinned psoas muscle fibers decreased fiber calcium sensitivity approximately 1.7-fold compared with TnC(F29W) at 1 mm [magnesium](free) and approximately 3.2-fold at 3 mm [magnesium](free). Thus, incorporation of a Z acid pair into the first EF-hand allows it to bind magnesium with high affinity. Furthermore, the data suggests that the second EF-hand, but not the first, of TnC is responsible for the competitive magnesium binding to the regulatory domain.

Determinants of relaxation rate in rabbit skinned skeletal muscle fibres

The influence of Ca(2+)-activated force, the rate of dissociation of Ca(2+) from troponin C (TnC) and decreased crossbridge detachment rate on the time course of relaxation induced by flash photolysis of diazo-2 in rabbit skinned psoas fibres was investigated at 15 degrees C. The rate of relaxation increased as the diazo-2 chelating capacity (i.e. free [diazo-2]/free [Ca(2+)]) increased. At a constant diazo-2 chelating capacity, the rate of relaxation was independent of the pre-photolysis Ca(2+)-activated force in the range 0.3-0.8 of maximum isometric force. A TnC mutant that exhibited increased Ca(2+) sensitivity caused by a decreased Ca(2+) dissociation rate in solution (M82Q TnC) also increased the Ca(2+) sensitivity of steady-state force and decreased the rate of relaxation in fibres by approximately twofold. In contrast, a TnC mutant with decreased Ca(2+) sensitivity caused by an increased Ca(2+) dissociation rate in solution (NHdel TnC) decreased the Ca(2+) sensitivity of steady-state force but did not accelerate relaxation. Decreasing the rate of crossbridge kinetics by reducing intracellular inorganic phosphate concentration ([P(i)]) slowed relaxation by approximately twofold and led to two phases of relaxation, a slow linear phase followed by a fast exponential phase. In fibres, M82Q TnC further slowed relaxation in low [P(i)] conditions by approximately twofold, whereas NHdel TnC had no significant effect on relaxation. These results are consistent with the interpretation that the Ca(2+)-dissociation rate and crossbridge detachment rate are similar in fast-twitch skeletal muscle, such that decreasing either rate slows relaxation, but accelerating Ca(2+) dissociation has little effect on relaxation.

Relaxation kinetics following sudden Ca(2+) reduction in single myofibrils from skeletal muscle

To investigate the roles of cross-bridge dissociation and cross-bridge-induced thin filament activation in the time course of muscle relaxation, we initiated force relaxation in single myofibrils from skeletal muscles by rapidly (approximately 10 ms) switching from high to low [Ca(2+)] solutions. Full force decay from maximal activation occurs in two phases: a slow one followed by a rapid one. The latter is initiated by sarcomere "give" and dominated by inter-sarcomere dynamics (see the companion paper, Stehle, R., M. Krueger, and G. Pfitzer. 2002. Biophys. J. 83:2152-2161), while the former occurs under nearly isometric conditions and is sensitive to mechanical perturbations. Decreasing the Ca(2+)-activated force preceding the start of relaxation does not increase the rate of the slow isometric phase, suggesting that cycling force-generating cross-bridges do not significantly sustain activation during relaxation. This conclusion is strengthened by the finding that the rate of isometric relaxation from maximum force to any given Ca(2+)-activated force level is similar to that of Ca(2+)-activation from rest to that given force. It is likely, therefore, that the slow rate of force decay in full relaxation simply reflects the rate at which cross-bridges leave force-generating states. Because increasing [P(i)] accelerates relaxation while increasing [MgADP] slows relaxation, both forward and backward transitions of cross-bridges from force-generating to non-force-generating states contribute to muscle relaxation.

Prolonged force increase following a high-frequency burst is not due to a sustained elevation of [Ca2+]i

A brief high-frequency burst of action potentials results in a sustained force increase in skeletal muscle. The present study investigates whether this force potentiation is the result of a sustained increase of the free myoplasmic [Ca2+] ([Ca2+]i). Single fibers from mouse flexor brevis muscles were stimulated with three impulses at 150 Hz (triplet) at the start of a 350-ms tetanus or in the middle of a 700-ms tetanus; the stimulation frequency of the rest of the tetanus ranged from 20 to 60 Hz. After the triplet, force was significantly (P < 0.05) increased between 17 and 20% when the triplet was given at the start of the tetanus and between 5 and 18% when the triplet was given in the middle (n = 7). However, during this potentiation, [Ca2+]i was not consistently increased. Hence, the increased force following a high-frequency burst is likely due to changes in the myofibrillar properties.

Kinetic effects of myosin regulatory light chain phosphorylation on skeletal muscle contraction

Kinetic analysis of contracting fast and slow rabbit muscle fibers in the presence of the tension inhibitor 2,3-butanedione monoxime suggests that regulatory light chain (RLC) phosphorylation up-regulates the flux of weakly attached cross-bridges entering the contractile cycle by increasing the actin-catalyzed release of phosphate from myosin. This step appears to be separate from earlier Ca(2+) regulated steps. Small step-stretches of single skinned fibers were used to study the effect of phosphorylation on fiber mechanics. Subdivision of the resultant tension transients into the Huxley-Simmons phases 1, 2(fast), 2(slow), 3, and 4 reveals that phosphorylation reduces the normalized amplitude of the delayed rise in tension (stretch activation response) by decreasing the amplitudes of phase 3 and, to a lesser extent, phase 2(slow). In slow fibers, the RLC P1 isoform phosphorylates at least 4-fold faster than the P2 isoform, complicating the role of RLC phosphorylation in heart and slow muscle. We discuss the functional relevance of the regulation of stretch activation by RLC phosphorylation for cardiac and other oscillating muscles and speculate how the interaction of the two heads of myosin could account for the inverse effect of Ca(2+) levels on isometric tension and rate of force redevelopment (k(TR)).

Thin filament near-neighbour regulatory unit interactions affect rabbit skeletal muscle steady-state force-Ca(2+) relations

The role of cooperative interactions between individual structural regulatory units (SUs) of thin filaments (7 actin monomers : 1 tropomyosin : 1 troponin complex) on steady-state Ca(2+)-activated force was studied. Native troponin C (TnC) was extracted from single, de-membranated rabbit psoas fibres and replaced by mixtures of purified rabbit skeletal TnC (sTnC) and recombinant rabbit sTnC (D27A, D63A), which contains mutations that disrupt Ca(2+) coordination at N-terminal sites I and II (xxsTnC). Control experiments in fibres indicated that, in the absence of Ca(2+), both sTnC and xxsTnC bind with similar apparent affinity to sTnC-extracted thin filaments. Endogenous sTnC-extracted fibres reconstituted with 100 % xxsTnC did not develop Ca(2+)-activated force. In fibres reconstituted with mixtures of sTnC and xxsTnC, maximal Ca(2+)-activated force increased in a greater than linear manner with the fraction of sTnC. This suggests that Ca(2+) binding to functional Tn can spread activation beyond the seven actins of an SU into neighbouring units, and the data suggest that this functional unit (FU) size is up to 10-12 actins. As the number of FUs was decreased, Ca(2+) sensitivity of force (pCa(50)) decreased proportionally. The slope of the force-pCa relation (the Hill coefficient, n(H)) also decreased when the reconstitution mixture contained < 50 % sTnC. With 15 % sTnC in the reconstitution mixture, n(H) was reduced to 1.7 +/- 0.2, compared with 3.8 +/- 0.1 in fibres reconstituted with 100 % sTnC, indicating that most of the cooperative thin filament activation was eliminated. The results suggest that cooperative activation of skeletal muscle fibres occurs primarily through spread of activation to near-neighbour FUs along the thin filament (via head-to-tail tropomyosin interactions).

Phosphorylation of the regulatory light chains of myosin affects Ca2+ sensitivity of skeletal muscle contraction

The role of phosphorylation of the myosin regulatory light chains (RLC) is well established in smooth muscle contraction, but in striated (skeletal and cardiac) muscle its role is still controversial. We have studied the effects of RLC phosphorylation in reconstituted myosin and in skinned skeletal muscle fibers where Ca2+ sensitivity and the kinetics of steady-state force development were measured. Skeletal muscle myosin reconstituted with phosphorylated RLC produced a much higher Ca2+ sensitivity of thin filament-regulated ATPase activity than nonphosphorylated RLC (change in -log of the Ca2+ concentration producing half-maximal activation = approximately 0.25). The same was true for the Ca2+ sensitivity of force in skinned skeletal muscle fibers, which increased on reconstitution of the fibers with the phosphorylated RLC. In addition, we have shown that the level of endogenous RLC phosphorylation is a crucial determinant of the Ca2+ sensitivity of force development. Studies of the effects of RLC phosphorylation on the kinetics of force activation with the caged Ca2+, DM-nitrophen, showed a slight increase in the rates of force development with low statistical significance. However, an increase from 69 to 84% of the initial steady-state force was observed when nonphosphorylated RLC-reconstituted fibers were subsequently phosphorylated with exogenous myosin light chain kinase. In conclusion, our results suggest that, although Ca2+ binding to the troponin-tropomyosin complex is the primary regulator of skeletal muscle contraction, RLC play an important modulatory role in this process.

The second EF-hand is responsible for the isoform-specific sorting of myosin essential light chain

It has been known that isoforms of myosin essential light chain (LC) exhibit the isoform-specific sorting within cardiac myocytes and fibroblasts. In order to analyze which domain of LC is responsible for the sorting, various chimeric cDNA constructs between human nonmuscle isoform (LC3nm) and chicken fast skeletal muscle isoform (LC3f) were generated and expressed in cultured chicken cardiac myocytes. If chimeras contained LC3f sequence at the place that was restricted by BssHII and PstI, they were preferentially sorted to sarcomeres and precisely localized at A-bands, and their incorporation levels into the A-bands were identical with that of the wild type LC3f. However, other chimeras were distributed throughout the cytoplasm like the wild type LC3nm. Comparison of amino acid sequences revealed that 12 amino acids are different between chicken LC3f and human LC3nm in the BssHII-PstI fragment, and these amino acids are located within the second EF-hand of LC. These results indicated that the second EF-hand is responsible for the isoform-specific sorting of LC. Although the second EF-hand is not included in the key contacts with myosin heavy chain, it is supposed that this domain is important for the relative disposition of neighboring domains. Thus, the 12 amino acids in the second EF-hand might play a key role for modulation of overall configuration of LC, thereby influencing the precise association of the key contacts.

Regulation of force development studied by photolysis of caged ADP in rabbit skinned psoas fibers

The present study examined the effects of Ca(2+) and strongly bound cross-bridges on tension development induced by changes in the concentration of MgADP. Addition of MgADP to the bath increased isometric tension over a wide range of [Ca(2+)] in skinned fibers from rabbit psoas muscle. Tension-pCa (pCa is -log [Ca(2+)]) relationships and stiffness measurements indicated that MgADP increased mean force per cross-bridge at maximal Ca(2+) and increased recruitment of cross-bridges at submaximal Ca(2+). Photolysis of caged ADP to cause a 0.5 mM MgADP jump initiated an increase in isometric tension under all conditions examined, even at pCa 6.4 where there was no active tension before ADP release. Tension increased monophasically with an observed rate constant, k(ADP), which was similar in rate and Ca(2+) sensitivity to the rate constant of tension re-development, k(tr), measured in the same fibers by a release-re-stretch protocol. The amplitude of the caged ADP tension transient had a bell-shaped dependence on Ca(2+), reaching a maximum at intermediate Ca(2+) (pCa 6). The role of strong binding cross-bridges in the ADP response was tested by treatment of fibers with a strong binding derivative of myosin subfragment 1 (NEM-S1). In the presence of NEM-S1, the rate and amplitude of the caged ADP response were no longer sensitive to variations in the level of activator Ca(2+). The results are consistent with a model in which ADP-bound cross-bridges cooperatively activate the thin filament regulatory system at submaximal Ca(2+). This cooperative interaction influences both the magnitude and kinetics of force generation in skeletal muscle.

Modulation of contractile activation in skeletal muscle by a calcium-insensitive troponin C mutant

Calcium controls the level of muscle activation via interactions with the troponin complex. Replacement of the native, skeletal calcium-binding subunit of troponin, troponin C, with mixtures of functional cardiac and mutant cardiac troponin C insensitive to calcium and permanently inactive provides a novel method to alter the number of myosin cross-bridges capable of binding to the actin filament. Extraction of skeletal troponin C and replacement with functional and mutant cardiac troponin C were used to evaluate the relationship between the extent of thin filament activation (fractional calcium binding), isometric force, and the rate of force generation in muscle fibers independent of the calcium concentration. The experiments showed a direct, linear relationship between force and the number of cross-bridges attaching to the thin filament. Further, above 35% maximal isometric activation, following partial replacement with mixtures of cardiac and mutant troponin C, the rate of force generation was independent of the number of actin sites available for cross-bridge interaction at saturating calcium concentrations. This contrasts with the marked decrease in the rate of force generation when force was reduced by decreasing the calcium concentration. The results are consistent with hypotheses proposing that calcium controls the transition between weakly and strongly bound cross-bridge states.

Familial hypertrophic cardiomyopathy mutations in the regulatory light chains of myosin affect their structure, Ca2+ binding, and phosphorylation

The effect of the familial hypertrophic cardiomyopathy mutations, A13T, F18L, E22K, R58Q, and P95A, found in the regulatory light chains of human cardiac myosin has been investigated. The results demonstrate that E22K and R58Q, located in the immediate extension of the helices flanking the regulatory light chain Ca(2+) binding site, had dramatically altered Ca(2+) binding properties. The K(Ca) value for E22K was decreased by approximately 17-fold compared with the wild-type light chain, and the R58Q mutant did not bind Ca(2+). Interestingly, Ca(2+) binding to the R58Q mutant was restored upon phosphorylation, whereas the E22K mutant could not be phosphorylated. In addition, the alpha-helical content of phosphorylated R58Q greatly increased with Ca(2+) binding. The A13T mutation, located near the phosphorylation site (Ser-15) of the human cardiac regulatory light chain, had 3-fold lower K(Ca) than wild-type light chain, whereas phosphorylation of this mutant increased the Ca(2+) affinity 6-fold. Whereas phosphorylation of wild-type light chain decreased its Ca(2+) affinity, the opposite was true for A13T. The alpha-helical content of the A13T mutant returned to the level of wild-type light chain upon phosphorylation. The phosphorylation and Ca(2+) binding properties of the regulatory light chain of human cardiac myosin are important for physiological function, and alteration any of these could contribute to the development of hypertrophic cardiomyopathy.

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.

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.

Biological function and site II Ca2+-induced opening of the regulatory domain of skeletal troponin C are impaired by invariant site I or II Glu mutations

To investigate the roles of site I and II invariant Glu residues 41 and 77 in the functional properties and calcium-induced structural opening of skeletal muscle troponin C (TnC) regulatory domain, we have replaced them by Ala in intact F29W TnC and in wild-type and F29W N domains (TnC residues 1-90). Reconstitution of intact E41A/F29W and E77A/F29W mutants into TnC-depleted muscle skinned fibers showed that Ca(2+)-induced tension is greatly reduced compared with the F29W control. Circular dichroism measurements of wild-type N domain as a function of pCa (= -log[Ca(2+)]) demonstrated that approximately 90% of the total change in molar ellipticity at 222 nm ([theta](222 nm)) could be assigned to site II Ca(2+) binding. With E41A, E77A, and cardiac TnC N domains this [theta](222 nm) change attributable to site II was reduced to < or =40% of that seen with wild type, consistent with their structures remaining closed in +Ca(2+). Furthermore, the Ca(2+)-induced changes in fluorescence, near UV CD, and UV difference spectra observed with intact F29W are largely abolished with E41A/F29W and E77A/F29W TnCs. Taken together, the data indicate that the major structural change in N domain, including the closed to open transition, is triggered by site II Ca(2+) binding, an interpretation relevant to the energetics of the skeletal muscle TnC and cardiac TnC systems.

Spaceflight effects on single skeletal muscle fiber function in the rhesus monkey

The purpose of this investigation was to understand how 14 days of weightlessness alters the cellular properties of individual slow- and fast-twitch muscle fibers in the rhesus monkey. The diameter of the soleus (Sol) type I, medial gastrocnemius (MG) type I, and MG type II fibers from the vivarium controls averaged 60 +/- 1, 46 +/- 2, and 59 +/- 2 microm, respectively. Both a control 1-G capsule sit (CS) and spaceflight (SF) significantly reduced the Sol type I fiber diameter (20 and 13%, respectively) and peak force, with the latter declining from 0.48 +/- 0.01 to 0.31 +/- 0.02 (CS group) and 0.32 +/- 0.01 mN (SF group). When the peak force was expressed as kiloNewtons per square meter (kN/m(2)), only the SF group showed a significant decline. This group also showed a significant 15% drop in peak fiber stiffness that suggests that fewer cross bridges were contracting in parallel. In the MG, SF but not CS depressed the type I fiber diameter and force. Additionally, SF significantly depressed absolute (mN) and relative (kN/m(2)) force in the fast-twitch MG fibers by 30% and 28%, respectively. The Ca(2+) sensitivity of the type I fiber (Sol and MG) was significantly reduced by growth but unaltered by SF. Flight had no significant effect on the mean maximal fiber shortening velocity in any fiber type or muscle. The post-SF Sol type I fibers showed a reduced peak power and, at peak power, an elevated velocity and decreased force. In conclusion, CS and SF caused atrophy and a reduced force and power in the Sol type I fiber. However, only SF elicited atrophy and reduced force (mN) in the MG type I fiber and a decline in relative force (kN/m(2)) in the Sol type I and MG type II fibers.

Cross-bridge kinetics in rat myocardium: effect of sarcomere length and calcium activation

We tested the hypotheses that Ca(2+) concentration ([Ca(2+)]) and sarcomere length (SL) modulate force development via graded effects on cross-bridge kinetics in chemically permeabilized rat cardiac trabeculae. Using sinusoidal length perturbations, we derived the transfer functions of stiffness over a range of [Ca(2+)] at a constant SL of 2.1 micrometer (n = 8) and at SL of 2.0, 2.1, and 2.2 micrometer (n = 4). We found that changes in SL affected only the magnitude of stiffness, whereas [Ca(2+)] affected the magnitude and phase-frequency relations. The data were fit to complex functions of two exponential processes. The characteristic frequencies (b and c) of these processes are indexes of cross-bridge kinetics, with b relating to cross-bridge attachment to and c to detachment from certain non-force-generating states. Both were significantly affected by [Ca(2+)], with an increase in b and c of 140 and 44%, respectively, over the range of [Ca(2+)] studied (P < 0.01). In contrast, SL had no effect on the characteristic frequencies (P > 0.6). We conclude that Ca(2+) activation modulates force development in rat myocardium, at least in part, via a graded effect on cross-bridge kinetics, whereas SL effects are mediated mainly by recruitment of cross bridges.

Fatigue after submaximal intensive stretch-shortening cycle exercise

OBJECTIVE

The aim of the present study was to examine some sites of neuromuscular fatigue after submaximal intensity stretch-shortening cycle exercise.

METHODS

Twelve male subjects performed consecutive sledge jumps at 60% of maximal height until exhaustion (mean duration 443.7 s +/- 304.9 s, mean +/- SD).

RESULTS

During the exercise, the blood lactate increased from 1.8 +/- 0.6 mmol x L(-1) (before exercise) to 6.1 +/- 1.7 mmol x L(-1) (P < 0.001) and serum creatine-kinase from 248 +/- 142 IU x L(-1) to 584 +/- 344 IU x L(-1) (P < 0.001). Electrical stimulation of the vastus lateralis and quadriceps femoris muscles to induce isometric knee extension resulted in decreased peak torque during single and double twitch after workout (from 22.1 +/- 6.3 Nm to 17.3 +/- 8.0 Nm, P < 0.05, and from 96.6 +/- 15.4 Nm to 76.2 x 19.8 Nm, P < 0.001, respectively), whereas there were no significant changes in contraction and relaxation times. Torque during 20-Hz stimulation decreased significantly (from 23.7 +/- 9.2 to 16.1 +/- 7.8 Nm, P < 0.01) but not at 100-Hz stimulation. During maximal voluntary isometric knee extensions, the rate of torque development was significantly (P < 0.01) more impaired than maximal torque (from 1619 +/- 390 Nm x s(-1) to 1,004 +/- 360 Nm x s(-1) and from 185 +/- 30.7 Nm to 151 +/- 32.3 Nm, respectively, both P < 0.001). At the same time, the muscle activation level increased by 15.8 +/- 24.1% (P < 0.05). The mean EMG amplitude of vastus lateralis during MVC increased by 34.9 +/- 39.2% (P > 0.05).

CONCLUSION

It was concluded that after submaximal stretch-shortening exercise, the low-frequency fatigue occurred, very likely caused by lower Ca2+ release per single action potential.

A thixotropic effect in contracting rabbit psoas muscle: prior movement reduces the initial tension response to stretch

Paired ramp stretches and releases ('triangular length changes', typically 0.04 +/- 0.09L0 s-1; mean +/- s.e.m.) were imposed on permeabilised rabbit psoas fibre segments under sarcomere length control. In actively contracting fibres, the tension response to stretch was biphasic; tension rose more rapidly during the first 0. 005L0 of the imposed stretch than thereafter. Tension also dropped in a biphasic manner during shortening, and at the end of the length change was reduced below the steady state. If a second triangular length change was imposed shortly after the first, tension rose less sharply during the initial phase of lengthening, i.e. the stiffness of the muscle during the initial phase of the response was reduced in the second stretch. This is a thixotropic effect. If a third triangular length change was imposed on the muscle, the response was the same as that to the second. The time required to recover the original tension response was measured by varying the interval between triangular length changes. Recovery to steady state occurred at a rate of approximately 1 s-1. The stiffness of the muscle during the initial phase of the response scaled with the developed tension in pCa (= -log10[Ca2+]) solutions ranging from 6.3 (minimal activation) to 4.5 (saturating effect). The relative thixotropic reduction in stiffness measured using paired length changes was independent of the pCa of the activating solution. The thixotropic behaviour of contracting skeletal muscle can be explained by a cross-bridge model of muscle contraction in which the number of attached cross-bridges is temporarily reduced following an imposed movement.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

The effect of removing the N-terminal extension of the Drosophila myosin regulatory light chain upon flight ability and the contractile dynamics of indirect flight muscle

The Drosophila myosin regulatory light chain (DMLC2) is homologous to MLC2s of vertebrate organisms, except for the presence of a unique 46-amino acid N-terminal extension. To study the role of the DMLC2 N-terminal extension in Drosophila flight muscle, we constructed a truncated form of the Dmlc2 gene lacking amino acids 2-46 (Dmlc2(Delta2-46)). The mutant gene was expressed in vivo, with no wild-type Dmlc2 gene expression, via P-element-mediated germline transformation. Expression of the truncated DMLC2 rescues the recessive lethality and dominant flightless phenotype of the Dmlc2 null, with no discernible effect on indirect flight muscle (IFM) sarcomere assembly. Homozygous Dmlc2(Delta2-46) flies have reduced IFM dynamic stiffness and elastic modulus at the frequency of maximum power output. The viscous modulus, a measure of the fly's ability to perform oscillatory work, was not significantly affected in Dmlc2(Delta2-46) IFM. In vivo flight performance measurements of Dmlc2(Delta2-46) flies using a visual closed-loop flight arena show deficits in maximum metabolic power (P(*)(CO(2))), mechanical power (P(*)(mech)), and flight force. However, mutant flies were capable of generating flight force levels comparable to body weight, thus enabling them to fly, albeit with diminished performance. The reduction in elastic modulus in Dmlc2(Delta2-46) skinned fibers is consistent with the N-terminal extension being a link between the thick and thin filaments that is parallel to the cross-bridges. Removal of this parallel link causes an unfavorable shift in the resonant properties of the flight system, thus leading to attenuated flight performance.

The force bearing capacity of frog muscle fibres during stretch: its relation to sarcomere length and fibre width

1. Single fibres isolated from the anterior tibialis muscle of Rana temporaria were tetanized (0.9-1.8 C) while a marked ( approximately 1 mm) segment was held at constant length by feedback control. Force enhancement was produced by applying a controlled stretch ramp to the fibre segment during the tetanus plateau, the steady force reached during stretch being used as a measure of the maximum force that the myosin cross-bridges can hold before they detach. 2. The amplitude of force enhancement during stretch did not vary in proportion to the isometric force as the sarcomere length was changed, maximum force enhancement being attained near 2.4 microm sarcomere length compared with 2.0 microm for the isometric force. 3. The influence of fibre width on the force enhancement-sarcomere length relationship was evaluated by normalizing force enhancement to the tetanic (pre-stretch) force in this way allowing for the differences in myofilament overlap at the various lengths. The amplitude of force enhancement (normalized to the tetanic force) increased by approximately 70 % as the relative width of the myofilament lattice was reduced from a nominal value of 1.05 at a sarcomere length of 1.8 microm to 0.85 at a sarcomere length of 2.8 microm. 4. Changes in fibre width equivalent to those produced by altering the sarcomere length were produced by varying the tonicity of the extracellular medium. Force enhancement, normalized to the control isometric force at each tonicity, exhibited a width dependence that agreed well with that described in the previous point. Stretch ramps applied to frog skinned muscle fibres during calcium-induced contracture likewise resulted in a greater force enhancement during stretch after reducing the fibre width by osmotic compression. 5. The results suggest that the strength of binding of the myosin cross-bridges, unlike the isometric force, varies with the lateral distance between the myofilaments.

Measured and modeled properties of mammalian skeletal muscle. I. The effects of post-activation potentiation on the time course and velocity dependencies of force production

Activation of mammalian fast-twitch skeletal muscle induces a persistent effect known as post-activation potentiation (PAP), classically defined as an increase in force production at sub-maximal levels of activation. The underlying mechanism is thought to be phosphorylation of the myosin regulatory light chain (MRLC), which leads to an increase in the rate constant for cross-bridge attachment (Sweeney et al., 1993). If true, this suggests the hypothesis that other contractile properties should be affected during PAP. Using a feline fast-twitch whole-muscle preparation (caudofemoralis) at 37 degrees C, we observed that PAP greatly increased tetanic forces during active lengthening decreased isometric tetanic rise times and delayed isometric tetanic force relaxation. The first two of these effects were length dependent with a greater effect occurring at shorter lengths. These findings confirmed that PAP has other functionally important effects beyond a simple increase in sub-maximal isometric forces. Furthermore, length was found to have an effect independent of PAP on the shortening half of the FV relationship (less force was produced at longer lengths) and on the rate of force relaxation during the later stages of isometric tetanic force decay (slower relaxation at longer lengths). All of these findings can be explained with a simplified, two-state model of cross-bridge dynamics that accounts for the interaction of both interfilament spacing and MRLC phosphorylation on the apparent rate constants for cross-bridge attachment and detachment. These findings are largely consistent with data collected previously from reduced preparations such as skinned fibers at cold, unphysiological temperatures (e.g. 5 degrees C). One finding that could not be explained by our model was that twitch fall times in the dispotentiated state were parabolically correlated with length, whereas in the potentiated state the relationship was linear. The time course of decay of this effect did not follow the time course of force dispotentiation, suggesting that there are other activation-dependent processes occurring in parallel with MRLC phosphorylation.

Abnormal cardiac structure and function in mice expressing nonphosphorylatable cardiac regulatory myosin light chain 2

A role for myosin phosphorylation in modulating normal cardiac function has long been suspected, and we hypothesized that changing the phosphorylation status of a cardiac myosin light chain might alter cardiac function in the whole animal. To test this directly, transgenic mice were created in which three potentially phosphorylatable serines in the ventricular isoform of the regulatory myosin light chain were mutated to alanines. Lines were obtained in which replacement of the endogenous species in the ventricle with the nonphosphorylatable, transgenically encoded protein was essentially complete. The mice show a spectrum of cardiovascular changes. As previously observed in skeletal muscle, Ca(2+) sensitivity of force development was dependent upon the phosphorylation status of the regulatory light chain. Structural abnormalities were detected by both gross histology and transmission electron microscopic analyses. Mature animals showed both atrial hypertrophy and dilatation. Echocardiographic analysis revealed that as a result of chamber enlargement, severe tricuspid valve insufficiency resulted in a detectable regurgitation jet. We conclude that regulated phosphorylation of the regulatory myosin light chains appears to play an important role in maintaining normal cardiac function over the lifetime of the animal.

Effects of myosin heavy chain isoform switching on Ca2+-activated tension development in single adult cardiac myocytes

Cardiac myosin heavy chain (MHC) isoforms are known to play a key role in defining the dynamic contractile behavior of the heart during development. It remains unclear, however, whether cardiac MHC isoforms influence other important features of cardiac contractility, including the Ca2+ sensitivity of isometric tension development. To address this question, adult rats were treated chemically to induce the hypothyroid state and cause a transition in the ventricular cardiac MHC isoform expression pattern from predominantly the alpha-MHC isoform to exclusively the beta-MHC isoform. We found a significant desensitization in the Ca2+ sensitivity of tension development in beta-MHC-expressing ventricular myocytes (pCa50=5. 51+/-0.03, where pCa is -log[Ca2+], and pCa50 is pCa at which tension is one-half maximal) compared with that in predominantly alpha-MHC-expressing myocytes (pCa50=5.68+/-0.05). No differences between the 2 groups were observed in the steepness of the tension-pCa relationship or in the maximum isometric force generated. Instantaneous stiffness measurements were made that provide a relative measure of changes in the numbers of myosin crossbridges attached to actin during Ca2+ activation. Results showed that the relative stiffness-pCa relationship was shifted to the right in beta-MHC-expressing myocytes compared with the alpha-MHC-expressing cardiac myocytes (pCa50=5.47+/-0.05 versus 5.76+/-0.05, respectively). We conclude that MHC isoform switching in adult cardiac myocytes alters the Ca2+ sensitivity of stiffness and tension development. These results suggest that the activation properties of the thin filament are in part MHC isoform dependent in cardiac muscle, indicating an additional role for MHC isoforms in defining cardiac contractile function.