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PubMed1995

Isomyosin patterns of single type IIB, IID and IIA fibres from rabbit skeletal muscle.

The present study demonstrates for the first time isomyosin patterns of the three fast-twitch fibre types IIB, IID/X, and IIA. Single muscle fibres were dissected from freeze-dried fibre bundles of rabbit adductor magnus, extensor digitorum longus, and psoas muscles. Pure fibre types, expressing only one myosin heavy chain isoform (MHCIIb, MHCIId/x, MHCIIa), were delineated by electrophoresis of fibre fragments under denaturing conditions. Pieces of the same fibres were then subjected to electrophoresis under non-denaturing conditions. A three-band pattern of fast isomyosins, representing the LC3f homodimer (FM1), the LC1f/LC3f heterodimer (FM2), and the LC1f homodimer (FM3), was detected in each of the three pure fibre types. Therefore, three isomyosins, different in their light chain complement, coexist in each pure fibre. The relative mobilities of the three bands, migrating in the order FM1 > FM2 > FM3, were the same in the three fibre types. The absolute electrophoretic mobilities of the MHCIIb-, MHCIId- and MHCIIa-based isomyosin triplets differed in the order MHCIIb triplets > MHCIId triplets > MHCIIa triplets. The proportions of FM1, FM2, and FM3 varied in type IIB, IID, and IIA fibres. FM2 was the dominant isomyosin in all three fibre types, but fibre type-related differences existed in the FM1 to FM3 ratio. This ratio was lowest in IIA fibres and highest in IIB fibres which agrees with our previous observations that the LC3f/(LC1f + LC3f) fraction is lowest in type IIA and highest in type IIB fibres.

Animals↗

PubMed2006

[Psoas abscesses. An up-dated perspective].

OBJECTIVE: To analyze the characteristics of abscesses of the psoas muscle and assess the differences between pyogenic and tuberculous abscesses. METHODS: Retrospective descriptive study of all patients with psoas abscess in our hospital over the period 1994 to 2004. RESULTS: Fourteen patients were studied (10 males), with a mean age of 42 years. Half of them had had an underlying disease. The most frequent clinical manifestations were abdominal pain (64%), fever (57%), and back pain (43%). All the abscesses were secondary. In 7 patients the origin was gastrointestinal, in 6 osteoarticular, and 1 was related with infection of an aortobifemoral bypass. Computed tomography was the diagnostic imaging method in all patients. Culture of drainage specimens was positive in 92% of patients undergoing this procedure. Causal microorganisms included Mycobacterium tuberculosis (5), Streptococcus intermedius (4), Staphylococcus aureus (3), Escherichia coli (3) and Bacteroides fragilis (2). Tuberculous abscesses originated in spondylitis and the clinical presentation was longer prior to diagnosis. Drainage was performed in 12 patients (8 percutaneous and 4 surgical). Mean duration of antimicrobial therapy was 4 weeks. The infection resolved in all patients. CONCLUSIONS: Psoas abscess commonly had a gastrointestinal and osteoarticular origin. We underscore the high percentage of tuberculous etiology, which had a more insidious clinical and analytical presentation and was usually secondary to spondylitis. Prolonged antimicrobial treatment associated with drainage was effective therapy.

Adolescent↗

PubMed1984

Computed tomography of paraspinal musculature in ankylosing spondylitis.

Computed tomography (CT) was used to delineate the paraspinal musculature in 14 patients with ankylosing spondylitis (AS). Abnormal atrophy of the erector spinae muscles and multifidi was demonstrated in all 8 patients with total bony ankylosis of the spine, but was not present in those with isolated syndesmophyte formation, vertebral squaring alone or sacroiliac joint ankylosis with normal spinal radiographs. There was a significant positive correlation between a CT score of paravertebral muscle wasting and clinical parameters of disease duration and restriction of spinal mobility. Wasting and asymmetry of the psoas muscles was seen in 3 patients with unilateral hip joint involvement. These findings suggest that a relationship exists between decreased or absent spinal movement and atrophy of the paraspinal musculature in AS.

Adult↗

PubMed1977

Cooperative interactions between calcium-binding sites on glycerinated muscle fibers. The influence of cross-bridge attachment.

A double isotope technique and EGTA buffers were used to measure the binding of Ca2+ to rabbit psoas muscle fibers extracted with detergent and glycerol. These experiments were designed to test the effect of rigor complex formation, determined by the degree of filament overlap, on the properties of the Ca2+-binding sites in the intact filament lattice. In the presence of 5 mM MgCl2 (no ATP), reduction of filament overlap was associated with a reduced binding of Ca2+ over the entire range of free Ca2+ concentrations (5.10(-8)-2.10(-5) M). With maximum filament overlap (sarcomere length 2.1-2.2 micrometer) the maximum bound Ca2+ was equivalent to 4 mol Ca2+/mol troponin and there was significant positive interaction between binding sites, as shown by Scatchard and Hill plots. With no filament overlap (sarcomere length 3.8-4.4 micrometer) the maximum bound Ca2+ was equivalent to 3 mumole Ca2+/mol troponin and graphical analysis indicated a single class of non-interacting sites. The data provide evidence that when cross-bridge attachments between actin and myosin filaments are formed not only does an additional Ca2+ binding site appear, but cooperative properties are imposed upon the binding sites.

Adenosine Triphosphate↗

PubMed1995

Evidence for increased low force cross-bridge population in shortening skinned skeletal muscle fibers: implications for actomyosin kinetics.

The dynamic characteristics of the low force myosin cross-bridges were determined in fully calcium-activated skinned rabbit psoas muscle fibers shortening under constant loads (0.04-0.7 x full isometric tension Po). The shortening was interrupted at various times by a ramp stretch (duration, 10 ms; amplitude, up to 1.8% fiber length) and the resulting tension response was recorded. Except for the earlier period of velocity transients, the tension response showed nonlinear dependence on stretch amplitude; i.e., the magnitude of the tension response started to rise disproportionately as the stretch exceeded a critical amplitude, as in the presence of inorganic phosphate (Pi). This result, as well as the result of stiffness measurement, suggests that the low force cross-bridges similar to those observed in the presence of Pi (presumably A.M.ADP.Pi) are significantly populated during shortening. The critical amplitude of the shortening fibers was greater than that of isometrically contracting fibers, suggesting that the low force cross-bridges are more negatively strained during shortening. As the load was reduced from 0.3 to 0.04 P0, the shortening velocity increased more than twofold, but the amount of the negative strain stayed remarkably constant (approximately 3 nm). This This insensitiveness of the negative strain to velocity is best explained if the dissociation of the low force cross-bridges is accelerated approximately in proportion to velocity. Along with previous reports, the results suggest that the actomyosin ATPase cycle in muscle fibers has at least two key reaction steps in which rate constants are sensitively regulated by shortening velocity and that one of them is the dissociation of the low force A.M.ADP.Pi cross-bridges. This step may virtually limit the rate of actomyosin ATPase turnover and help increase efficiency in fibers shortening at high velocities.

Actomyosin↗

PubMed1990

The psoas sign in normal subjects without bowel preparation: the influence of scoliosis on visualisation.

The lumbar spine films of 1494 subjects who were examined for low back pain or prior to job placement were studied to assess psoas muscle visualisation. The study group were almost all aged 18-39 years. None of the subjects had prior bowel preparation for a contrast examination. In the subjects without scoliosis only 36% had well seen psoas shadows on both sides. Taking the psoas shadows separately, 72% of left psoas shadows were well seen, while only 40% were seen on the right side. In the scoliotic group, the psoas shadow was seen bilaterally in 30% of subjects while in another 50% the muscle was seen only on the convex side of the curve. In no case was it seen only on the concave side. The direction of the curve to left or right did not influence visualisation. Thus in 80% of subjects with scoliosis of 5 degrees or more, the psoas was seen on the convex side, while in only 30% was it seen on the concave side.

Adolescent↗

PubMed1983

Kinetic properties and functional role of creatine phosphokinase in glycerinated muscle fibers--further evidence for compartmentation.

The following phenomena were observed when relative contraction and relaxation effects of ATP and creatine phosphate (CP) were studied in rabbit psoas muscle glycerinated fiber bundles containing native creatine phosphokinase (CPK) and ATPase activities: (1) nucleotide was absolutely necessary for contraction; (2) in the presence of a small amount of ADP (250 microM), physiological concentration of CP (10 mM) produced faster and stronger contraction and faster, more complete, relaxation than equimolar or higher concentrations of ATP; (3) if the nucleotide was in the form of ATP, the nucleotide Km for contraction was about 1.5 mM; (4) if the nucleotide was in the form of ADP, the nucleotide Km for contraction at physiological concentration of CP (10 mM) was 0.076 to 1.18 mM depending upon the order of addition of ADP and CP; (5) the apparent Km for CP for contraction was 2.67 mM independent of sequence of addition of ADP and CP.

Adenosine Diphosphate↗

PubMed2004

The ATP hydrolysis and phosphate release steps control the time course of force development in rabbit skeletal muscle.

The time course of isometric force development following photolytic release of ATP in the presence of Ca(2+) was characterized in single skinned fibres from rabbit psoas muscle. Pre-photolysis force was minimized using apyrase to remove contaminating ATP and ADP. After the initial force rise induced by ATP release, a rapid shortening ramp terminated by a step stretch to the original length was imposed, and the time course of the subsequent force redevelopment was again characterized. Force development after ATP release was accurately described by a lag phase followed by one or two exponential components. At 20 degrees C, the lag was 5.6 +/- 0.4 ms (s.e.m., n = 11), and the force rise was well fitted by a single exponential with rate constant 71 +/- 4 s(-1). Force redevelopment after shortening-restretch began from about half the plateau force level, and its single-exponential rate constant was 68 +/- 3 s(-1), very similar to that following ATP release. When fibres were activated by the addition of Ca(2+) in ATP-containing solution, force developed more slowly, and the rate constant for force redevelopment following shortening-restretch reached a maximum value of 38 +/- 4 s(-1) (n = 6) after about 6 s of activation. This lower value may be associated with progressive sarcomere disorder at elevated temperature. Force development following ATP release was much slower at 5 degrees C than at 20 degrees C. The rate constant of a single-exponential fit to the force rise was 4.3 +/- 0.4 s(-1) (n = 22), and this was again similar to that after shortening-restretch in the same activation at this temperature, 3.8 +/- 0.2 s(-1). We conclude that force development after ATP release and shortening-restretch are controlled by the same steps in the actin-myosin ATPase cycle. The present results and much previous work on mechanical-chemical coupling in muscle can be explained by a kinetic scheme in which force is generated by a rapid conformational change bracketed by two biochemical steps with similar rate constants -- ATP hydrolysis and the release of inorganic phosphate -- both of which combine to control the rate of force development.

Adenosine Triphosphate↗

PubMed1992

Effects of Ca2+ on the kinetics of phosphate release in skeletal muscle.

The process of phosphate dissociation during the muscle cross-bridge cycle has been investigated by photoliberation of inorganic phosphate (Pi) within skinned fibers of rabbit psoas muscle. This permitted a test of the idea that Ca2+ controls muscle contraction by regulating the Pi release step of the cycle. Photoliberation of Pi from structurally distinct "caged" Pi precursors initiated a rapid tension decline of up to 12% of active tension, and this was followed by a slower tension decline. The apparent rate constant of the fast phase, kPi, depended on both [Pi] and [Ca2+], whereas the slow phase generally occurred at 2-4 s-1. At maximal Ca2+, kPi increased in a nonlinear manner from 43 +/- 2 s-1 to 118 +/- 7 s-1, as Pi was raised from 0.9 to 12 mM. This was analyzed in terms of a three-state kinetic model in which a force-generating transition is coupled to Pi dissociation from the cross-bridge. As Ca(2+)-activated tension was reduced from maximal (Pmax) to 0.1 Pmax, (i) kPi decreased by up to 2.5-fold, (ii) the relative amplitude of the rapid phase increased 2-fold, and (iii) the relative amplitude of the slow phase increased about 6-fold. Changes in the rapid phase are compatible with Ca2+ influencing an apparent equilibrium constant for the force-generating transition. By comparison, kPi was faster than the rate constant of tension redevelopment, ktr, and was influenced less by Ca2+. Ca2+ effects on the caged Pi transient cannot account for the large effects of Ca2+ on actomyosin ATPase rates or cross-bridge cycling kinetics but may be a manifestation of reciprocal interactions between the thin filament and force-generating cross-bridges, and may represent Ca2+ regulation of the distribution of cross-bridges between non-force-and force-generating states.

Animals↗

PubMed1984

Different muscle-specific forms of rabbit skeletal muscle alpha-actinin.

The structures and functions of the two alpha-actinin isoforms [R. Kobayashi et al. (1983) Eur. J. Biochem. 133, 607-611] isolated from rabbit longissimus dorsi and psoas muscles were compared. One-dimensional and two-dimensional electrophoretic analyses showed that the two alpha-actinins were different from each other in their subunit chain weights and isoelectric points. The Stokes' radius of the longissimus dorsi and psoas alpha-actinins was 7.4 nm and 7.0 nm, respectively. The amino acid analyses showed that, although the two alpha-actinins are similar in their amino acid compositions, longissimus dorsi alpha-actinin contains more aspartic acid and isoleucine than psoas alpha-actinin but fewer glycine and valine residues. Analysis of the soluble tryptic peptides by two-dimensional mapping revealed that the two alpha-actinins had major differences. These data suggested that the two isoforms are the products of at least two different genes. Despite these differences, both alpha-actinins share a number of common properties. Both alpha-actinins contain a 55-kDa peptide resistant to trypsin. The two proteins show no differences in actomyosin turbidity assays. ATPase assays and F-actin binding assays of alpha-actinin activity. Immunological examination indicates that the two alpha-actinins share antigenic determinants in common.

Actinin↗

PubMed1980

Effect of electrical stimulation post mortem of bovine muscle on the binding of glycolytic enzymes. Functional and structural implications.

The extent of binding of glycolytic enzymes to the particulate fraction of homogenates was measured in bovine psoas muscle before and after electrical stimulation. In association with an accelerated glycolytic rate on stimulation, there was a significant increase in the binding of certain glycolytic enzymes, the most notable of which were phosphofructokinase, aldolase, glyceraldehyde 3-phosphate dehydrogenase and pyruvate kinase. From the known association of glycolytic enzymes with the I-band of muscle it is proposed that electrical stimulation of anaerobic muscle increases enzyme binding to actin filaments. Calculations of the extent of enzyme binding suggest that significant amounts of enzyme protein, particularly aldolase and glyceraldehyde 3-phosphate dehydrogenase, are associated with the actin filaments. The results also imply that kinetic parameters derived from considerations of the enzyme activity in the soluble state may not have direct application to the situation in the muscle fibre, particularly during accelerated glycolysis.

Animals↗

PubMed1991

X-ray diffraction studies on muscle regulation.

Using the intensity of the outer part of the second actin layer line as an indicator of thin filament conformation in vertebrate muscle we were able to identify the four different states of rest, and the three states induced by the presence of Ca2+ ions, rigor bridge attachment and actively cycling bridges, respectively. These findings are in qualitative agreement with a number of biochemical studies by Eisenberg and Greene and others, indicating that activation of the thin filament depends both on Ca2+ ions and crossbridge binding. Yet quantitatively, the biochemical data and our structural data are contradictory. Whereas the biochemical studies suggest a strong coupling between structural changes of the thin filament and the ATPase activity, the structural studies indicate that this is not necessarily the case. Troponin molecules also change their conformation upon activation depending on both Ca2+ ions and crossbridge binding as demonstrated by the early part of the time course of the thin filament meridional reflections in contracting frog muscle. Low ionic strength which has been shown by Brenner and collaborators to increase weakly binding crossbridges in relaxed rabbit psoas muscle does not influence the intensity of the second actin layer line in this muscle. Yet in contracting frog muscle the increase of the second actin layer line increases very rapidly in one step, suggesting that weak binding bridges which are attached to actin prior to force production may indeed influence the thin filament conformation. It therefore appears that weakly bound bridges in the low ionic strength state do not have the same effect on the thin filament conformation as weakly bound bridges in an actively contracting muscle. Arthropod muscles like the thin filament regulated lobster muscles differ from vertebrate muscle in not showing an increase of the second layer line during contraction, which may have to do with differences in crossbridge attachment. The myosin-regulated molluscan muscle ABRM shows a large increase on the second actin layer line upon phasic contraction and a much smaller increase in catch or rigor, indicating that actively cycling bridges influence the thin filament conformation differently than catch or rigor bridges. Several pieces of evidence which we have briefly outlined in this paper suggest that the thin filament conformational changes we have observed do not arise solely from tropomyosin movements and that conformational changes of actin domains should be considered.

Animals↗

PubMed2001

[Treatment of primary pyogenic abscess of the psoas: retrospective study of 18 cases].

The primitive abcess of psoas muscle is a rare affection which involves mainly the young. The authors report 18 cases of primitive pyogenic abcess of psoas collected during a period of six years in the urology service of UHC Ibn Rochd Casablanca. The purpose of this review is to discuss the different therapeutic attitudes. The study concerned 11 men and seven women, their mean age was 35 years. The mean delay of evolution was 45 days. The physical exam found a psoitisis (five cases) and a flunk swelling (14 cases). The ultrasound exam showed the collection which was hypoechogenic in seven cases and heterogenous in 11 cases. The CT scan showed an heterogenouse hypodense collection without enhansment in seven cases. The use of antibiotherapy alone (four cases) and percutaneous drainage (four cases) permitted the recovery just respectively in one and two cases. Fifteen patients (83%) were operated through a postero-lateral lumbotomy route without costal resection. The bacteriological exam of the pus found a Staphylococcis aureus (ten cases) and Escherichia coli (two cases). The evolution was favorable in 14 cases; one patient was dead at fifth day post operatively by heart infarction. For the authors, the surgical drainage remains the treatment of choice.

Adolescent↗

PubMed1995

Resolution of three structural states of spin-labeled myosin in contracting muscle.

We have used electron paramagnetic resonance (EPR) spectroscopy to detect ATP- and calcium-induced changes in the structure of spin-labeled myosin heads in glycerinated rabbit psoas muscle fibers in key physiological states. The probe was a nitroxide iodoacetamide derivative attached selectively to myosin SH1 (Cys 707), the conventional EPR spectra of which have been shown to resolve several conformational states of the myosin ATPase cycle, on the basis of nanosecond rotational motion within the protein. Spectra were acquired in rigor and during the steady-state phases of relaxation and isometric contraction. Spectral components corresponding to specific conformational states and biochemical intermediates were detected and assigned by reference to EPR spectra of trapped kinetic intermediates. In the absence of ATP, all of the myosin heads were rigidly attached to the thin filament, and only a single conformation was detected, in which there was no sub-microsecond probe motion. In relaxation, the EPR spectrum resolved two conformations of the myosin head that are distinct from rigor. These structural states were virtually identical to those observed previously for isolated myosin and were assigned to the populations of the M*.ATP and M**.ADP.Pi states. During isometric contraction, the EPR spectrum resolves the same two conformations observed in relaxation, plus a small fraction (20-30%) of heads in the oriented actin-bound conformation that is observed in rigor. This rigor-like component is a calcium-dependent, actin-bound state that may represent force-generating cross-bridges. As the spin label is located near the nucleotide-binding pocket in a region proposed to be pivotal for large-scale force-generating structural changes in myosin, we propose that the observed spectroscopic changes indicate directly the key steps in energy transduction in the molecular motor of contracting muscle.

Adenosine Triphosphate↗

PubMed1974

The site of force generation in muscle contraction as deduced from fluorescence polarization studies.

The fluorescent dye, N(-iodoacetylamino)-l-naphthylanine-5-sulfonic acid, labeled exclusively the myosin cross-bridges in rabbit glycerinated psoas muscle fibers, without impairing their function. Fluorescence polarization was used to study cross-bridge orientation in rigor, relaxation, and contraction, as a function of sarcomere length. At a length where no overlap between thick and thin filaments occurs, rigor-inducing, relaxation-inducing, and contraction-inducing solutions all induced the relaxation attitude. At lengths where overlap does exist, the slowly-hydrolyzing ATP analog, "alpha,beta-methylene ATP," induced the relaxation attitude. The data were consistent with the A. F. Huxley-Simmons model of force generation. Combined with our earlier results, the data indicated that torque was generated at the actin-myosin interface.

Animals↗

PubMed1997

Mechanical study of rat soleus muscle using caged ATP and X-ray diffraction: high ADP affinity of slow cross-bridges.

1. The cross-bridges in slow- and fast-twitch fibres (taken, respectively, from soleus and psoas muscles of rats) were examined in mechanical experiments using caged ATP and X-ray diffraction, to compare their binding of ATP and ADP. 2. Caged ATP was photolysed in rigor fibres. When ADP was removed from pre-photolysis fibres, the initial relaxation (+/- Ca2+) in soleus was as fast as that in psoas fibres, whereas the subsequent contraction (+Ca2+) was slower in soleus than in psoas. The ATPase rate during the steady-state contraction was also slower in soleus fibres. 3. When ADP was added to pre-photolysis fibres (+/- Ca2+), tension developed even in the initial phase, the overall tension development being biphasic. Both initial and late components of the Ca(2+)-free contraction were enhanced when ADP was added before photolysis, although pre-photolysis ADP was not a prerequisite for the late component. The effect of ADP was greater in soleus than in psoas fibres. Static experiments on rigor fibres revealed a higher ADP affinity in soleus fibres. 4. The intensity of the actin layer-line from ADP rigor soleus fibres decreased rapidly on photorelease of ATP. We conclude that, despite the tight ADP binding of the soleus cross-bridge, its isometric reaction is not rate limited by the 'off' rate of ADP.

Adenosine Diphosphate↗

PubMed1985

The effects of ADP and phosphate on the contraction of muscle fibers.

The products of MgATP hydrolysis bind to the nucleotide site of myosin and thus may be expected to inhibit the contraction of muscle fibers. We measured the effects of phosphate and MgADP on the isometric tensions and isotonic contraction velocities of glycerinated rabbit psoas muscle at 10 degrees C. Addition of phosphate decreased isometric force but did not affect the maximum velocity of shortening. To characterize the effects of ADP on fiber contractions, force-velocity curves were measured for fibers bathed in media containing various concentrations of MgATP (1.5-4 mM) and various concentrations of MgADP (1-4 mM). As the [MgADP]/[MgATP] ratio in the fiber increases, the maximum velocity achieved by the fiber decreases while the isometric tension increases. The inhibition of fiber velocities and the potentiation of fiber tension by MgADP is not altered by the presence of 12 mM phosphate. The concentration of both MgADP and MgATP within the fiber was calculated from the diffusion coefficient for nucleotides within the fiber, and the rate of MgADP production within the fiber. Using the calculated values for the nucleotide concentration inside the fiber, observed values of the maximum contraction velocity could be described, within experimental accuracy, by a model in which MgADP competed with MgATP and inhibited fiber velocity with an effective Ki of 0.2-0.3 mM. The average MgADP level generated by the fiber ATPase activity within the fiber was approximately 0.9 mM. In fatigued fibers MgADP and phosphate levels are known to be elevated, and tension and the maximum velocity of contraction are depressed. The results obtained here suggest that levels of MgADP in fatigued fibers play no role in these decreases in function, but the elevation of both phosphate and H+ is sufficient to account for much of the decrease in tension.

Adenosine Diphosphate↗

PubMed1990

Effects of partial extraction of light chain 2 on the Ca2+ sensitivities of isometric tension, stiffness, and velocity of shortening in skinned skeletal muscle fibers.

Various functional roles for myosin light chain 2 (LC2) have been suggested on the basis of numerous and predominantly in vitro biochemical studies. Using skinned fibers from rabbit psoas muscle, the present study examines the influence of partial removal of LC2 on isometric tension, stiffness, and maximum velocity of shortening at various levels of activation by Ca2+. Isometric tension, stiffness, and velocity of shortening were measured at pCa values between 6.6 and 4.5 (a) in a control fiber segment, (b) in the same fiber segment after partial removal of LC2, and (c) after recombination with LC2. The extraction solution contained 20 mM EDTA, 20 or 50 mM KCl, and either imidazole or PO4(2-) as a pH buffer (pH 7.0). The amount of LC2 extracted varied with the temperature, duration of extraction, and whether or not troponin C (0.5 mg/ml) was added to the extraction solution. Extraction of 20-40% LC2 resulted in increased active tensions in the range of pCa's between 6.6 and 5.7, but had no effect upon maximum tension. The tension-pCa relationship was left-shifted to lower [Ca2+] by as much as 0.2 pCa units after LC2 extraction. At low concentrations of Ca2+, an increase in stiffness proportional to the increase in tension was observed. Readdition of LC2 to these fiber segments resulted in a return of tension and stiffness to near control values. Stiffness during maximal activation was unaffected by partial extraction of LC2. LC2 extraction was shown to uniformly decrease (by 25-30%), the velocity of shortening during the high velocity phase but it did not significantly affect the low velocity phase of shortening. This effect was reversed by readdition of purified LC2 to the fiber segments. On the basis of these findings we conclude that LC2 may modulate the number of cross-bridges formed during Ca2+ activation and also the rate of cross-bridge detachment during shortening. These results are consistent with the idea that LC2 may modulate contraction via an influence upon the conformation of the S1-S2 hinge region of myosin.

Animals↗