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Query: UMLS:C0038454 (stroke)
 147,016 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

In an attempt to elucidate the effects of two major risk factors of heart failure in humans, high blood pressure and coronary artery disease, renal hypertension and coronary artery constriction were induced singularly and in combination in rats, and the functional, structural, and biochemical alterations of the myocardium were examined 12-13 wk later. Renal hypertension (RH), coronary narrowing (CN), and their association (NH) resulted in left ventricular failure demonstrated by a significant increase in left ventricular end-diastolic pressure, a decrease in +dP/dt and -dP/dt, and a reduction in stroke volume and cardiac output. Measurements of ventricular loading documented that RH was characterized by elevations in systolic and diastolic wall stress of 42 and 160%, respectively. Corresponding changes with NH were 80 and 315%. CN was accompanied by an augmentation of diastolic wall stress only (280%). The abnormalities in mural stress were coupled with reductions in systolic and diastolic wall thickness-to-chamber radius ratios of 39 and 29% after CN. These anatomic parameters were preserved with RH, whereas the systolic wall thickness-to-chamber radius ratio was reduced 31% with NH. Structurally, multiple foci of replacement fibrosis were found with each intervention. The sites of tissue injury and their volume percent in the myocardium were comparable with CN and RH but were significantly more numerous and occupied a larger fraction of the ventricular wall in the presence of NH. Biochemically, the calcium dose-response curve of myofibrillar Mg2+ adenosinetriphosphatase (ATPase) activity did not vary with CN, RH, and NH. In contrast, a marked decrease in Ca2+ myosin ATPase activity was found in NH rats in association with a shift in myosin isoenzymes from V1 to V3. In conclusion, multiple physiological, morphological, and biochemical factors may participate in the generation of the abnormalities in ventricular loading with hypertension and/or coronary artery stenosis.
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PMID:Effects of hypertension and coronary constriction on cardiac function, morphology, and contractile proteins in rats. 836 72

During muscle contraction, work is generated when a myosin cross-bridge attaches to an actin filament and exerts a force on it through some power-stroke distance, h. At the end of this power stroke, attached myosin heads are carried into regions where they exert a negative force on the actin filament (the drag stroke) and where they are released rapidly from actin by ATP binding. Although the length of the power stroke remains controversial, average distance traversed in the drag-stroke region can be determined when one knows both rate of cross-bridge dissociation and filament-sliding velocity. At maximum contraction velocity, the average force exerted in the drag stroke must balance that exerted in the power stroke. We discuss here a simple model of cross-bridge interaction that allows one to calculate the force exerted in the drag stroke and to relate this to the power-stroke distance h traversed by cross-bridges in the positive-force region. Both the rate at which myosin can be dissociated from actin and the velocity at which an actin filament can be translated have been measured for a series of myosin isozymes and for different substrates, producing a wide range of values for each. Nonetheless, we show here that the rate of myosin dissociation from actin correlates well with the velocity of filament sliding, providing support for the simple model presented and suggesting that the power stroke is approximately 10 nm in length.
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PMID:Determination of the myosin step size from mechanical and kinetic data. 846 Jan 56

Electron paramagnetic resonance spectroscopy of a spin probe attached to cys-707 on myosin cross-bridges was used to monitor the orientation of the myosin catalytic domain at the beginning and end of the working power stroke in active muscle. Elevated concentrations of orthophosphate and decreased pH were used to shift the population of cross-bridges from force-producing states into low force, pre-power-stroke states. The spectrum of probes in active fibers was not changed by conditions that reduced tension by 70%, indicating that the orientation of the catalytic domain was the same at the beginning and end of the power stroke. Thus the data show that the catalytic domain remains rigidly oriented on the actin filament during the power stroke.
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PMID:The myosin catalytic domain does not rotate during the working power stroke. 851 99

Current models for the action of linear biological motors may be grouped in two main categories. The conventional "bind and bend" models rely for their power stroke upon a structural change in the myosin headgroup (S1 fragment) which follows the binding of myosin to the F-actin filament. The more recent ratchet models demonstrate that directional motion of a particle along an asymmetrical ratchet is possible with a symmetrical but time-correlated stochastic drive. In this paper a new type of model is introduced which is deterministic like the "bind and bend" model but it requires no molecular structural changes to power the stroke. Like the ratchet models the motor is driven along the linear stator by tangential forces at the interface but the forces are electrostatic and controlled by the hydrolysis of ATP to ADP.
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PMID:Linear electric traction as an alternative model of the actin/myosin motor. 873 31

A. F. Huxley's suggestion in Nature (1992) that a structural modification in the myosin head driven by phosphate release can explain the rapid regeneration of the working stroke, which follows the quick recovery elicited by a step release of moderate size (3-6 nm per half-sarcomere), has been tested with a theoretical model. It is assumed that, in the shortening muscle, cross-bridges can undergo their work producing interaction in two ways distinct for the biochemical state and for the amount of filament sliding allowed. During shortening at low speed, as well as after a shortening step of moderate size, phosphate release from the cross-bridge in the AM-ADP-P state promotes a 100 s-1 structural change which resets the myosin head in a configuration that allows for a new complete working stroke in the AM-ADP state. In this case the total sliding distance for interaction is about 15 nm. With the increase in shortening velocity a progressively larger fraction of interacting cross-bridges remains in the AM-ADP-P state throughout the working stroke and the sliding distance for interaction is about 11 nm. Reattachment of detached cross-bridges occurs at moderate rate whichever is the pathway from which they originate. The model predicts satisfactorily the time course of the rapid regeneration of the working stroke in double step experiments, but fails to simulate the transition to the steady state response in staircase experiments, the maximum power output during steady shortening and the decrease in rate of energy liberation at high shortening velocities. These results strengthen the conclusion of our previous modelling work where we demonstrated that the condition necessary to fit the mechanical and energetic properties of shortening muscle is to assume two pathways for cross-bridge cycling distinct for the kinetics of detachment and reattachment.
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PMID:Simulation of the rapid regeneration of the actin-myosin working stroke with a tight coupling model of muscle contraction. 874 Apr 31

Based on the MHC isoform pattern, adult mammalian limb skeletal muscles contain two and, in some species, three types of fast fibers (Type IIa, IIx, and IIb), and one slow fiber (Type I). Slow muscles, such as the soleus, contain primarily the slow Type I fiber, whereas fast-twitch muscles are composed primarily of a mixture of the fast myosin isozymes. Force generation involves cross-bridge interaction and transition from a weakly bound, low-force state (AM-ADP-P(i)) to the strongly bound, high-force state (AM-ADP). This transition is thought to be rate limiting in terms of dP/dt, and the high-force state is the dominant cross-bridge form during a peak isometric contraction. Intact fast and slow skeletal muscles generate approximately the same amount of peak force (Po) of between 200 and 250 kN.m-2. However, the rate of transition from the low- to high-force state shows Ca2+ sensitivity and is 7-fold higher in fast-twitch, as compared to slow-twitch, skeletal muscle fibers. Fiber Vo or the maximal cross-bridge cycle rate is highly correlated with and thought to be dependent on the specific activity of the myosin or myofibrillar ATPase. The hierarchy for Vo is the Type IIb > IIx > IIa > I. This functional difference for the fast fiber types explains the higher Vo observed in the predominantly Type IIb SVL vs. the mixed fast Type IIa and IIb EDL muscle. A plot of Vo vs. species size demonstrates that an inverse relationship exists between Vo and body mass. From the standpoint of work capacity, the important property is power output. An analysis of individual muscles indicates that peak power is obtained at loads considerably below 50% of Po. Individuals with a high percentage of fast-twitch fibers generate a greater torque and higher power at a given velocity than those with predominantly slow-twitch fibers. In humans, mean peak power occurred in a ratio of 10:5:1 for the Type IIb, IIa, and I fibers. The in vivo measurement of the torque-velocity relationship and Vmax in human muscle is difficult because of limitations inherent in the equipment used and the inability to study the large limb muscles independently. Nevertheless, the in vivo torque-velocity relationships are similar to those measured in vitro in animals. This observation suggests that little central nervous system inhibition exists and that healthy subjects are able to achieve maximal activation of their muscles. Although peak isometric tension is not dependent on fiber type distribution, a positive correlation exists between the percentage of fast fibers and peak torque output at moderate-to-high angular isokinetic velocities. Consequently, peak power output is substantially greater in subjects possessing a predominance of fast fibers. The mechanical properties of slow and fast muscles do adapt to programs of regular exercise. Endurance exercise training has been shown to increase the Vo of the slow soleus by 20%. This increase could have been caused by either a small increase in all, or most, of the fibers, or to a conversion of a few fibers from slow to fast. Recently, the increase was shown to be caused by the former, as the individual slow Type I fibers of the soleus showed a 20% increase in Vo, but there was little or no change in the percentage of fast fibers. The increased Vo was correlated with, and likely caused by, an increased fiber ATPase. We hypothesize that the increased ATPase and cross-bridge cycling speed might be attributable to an increased expression of fast MLCs in the slow Type I fibers (Fig. 14.10). This hypothesis is based on the fact that light chains have been shown to be involved in the power stroke, and removal of light chains depresses force and velocity. Regular endurance exercise training had no effect on fiber size, but with prolonged durations of daily training it depressed Po and peak power. When the training is maintained over prolonged periods, it may even induce atrophy of the slow Type I and fast Type IIa fibers. (ABSTRACT TRUNCATED)
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PMID:Muscle mechanics: adaptations with exercise-training. 874 58

We have used electron paramagnetic resonance to study the orientation of myosin heads in the presence of nucleotides and nucleotide analogs, to induce equilibrium states that mimic intermediates in the actomyosin ATPase cycle. We obtained electron paramagnetic resonance spectra of an indane dione spin label (InVSL) bound to Cys 707 (SH1) of the myosin head, in skinned rabbit psoas muscle fibers. This probe is rigidly immobilized on the catalytic domain of the head, and the principal axis of the probe is aligned nearly parallel to the fiber axis in rigor (no nucleotide), making it directly sensitive to axial rotation of the head. On ADP addition, all of the heads remained strongly bound to actin, but the spectral hyperfine splitting increased by 0.55 +/- 0.02 G, corresponding to a small but significant axial rotation of 7 degrees. Adenosine 5'-(adenylylim-idodiphosphate) (AMPPNP) or pyrophosphate reduced the actomyosin affinity and introduced a highly disordered population of heads similar to that observed in relaxation. For the remaining oriented population, pyrophosphate induced no significant change relative to rigor, but AMPPNP induced a slight but probably significant rotation (2.2 degrees +/- 1.6 degrees), in the direction opposite that induced by ADP. Adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) relaxed the muscle fiber, completely dissociated the heads from actin, and produced disorder similar to that in relaxation by ATP. ATP gamma S plus Ca induced a weak-binding state with most of the actin-bound heads disordered. Vanadate had negligible effect in the presence of ADP, but in isometric contraction vanadate substantially reduced both force and the fraction of oriented heads. These results are consistent with a model in which myosin heads are disordered early in the power stroke (weak-binding states) and rigidly oriented later in the power stroke (strong-binding states), whereas transitions among the strong-binding states induce only slight changes in the axial orientation of the catalytic domain.
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PMID:Orientation of intermediate nucleotide states of indane dione spin-labeled myosin heads in muscle fibers. 874 17

Myosin couples ATP hydrolysis to the translocation of actin filaments to power many forms of cellular motility. A striking feature of the structure of the muscle myosin head domain is a 9-nm long "lever arm" that has been postulated to produce a 5-10-nm power stroke. This motion must be coupled to conformational changes around the actin and nucleotide binding sites. The linkage of these sites to the lever arm has been analyzed by site-directed mutagenesis of a conserved glycine residue (G699) found in a bend joining two helices containing the highly reactive and mobile cysteine residues, SH1 and SH2. Alanine mutagenesis of this glycine (G699A) dramatically alters the motor activity of skeletal muscle myosin, inhibiting the velocity of actin filament movement by > 100-fold. Analysis of the defect in the G699A mutant myosin is consistent with a marked slowing of the transition within the motor domain from a strong binding to a weak binding interaction with actin. This result is interpreted in terms of the role of this residue (G699) as a pivot point for motion of the lever arm. The recombinant myosin used in these experiments has been produced in a unique expression system. A shuttle vector containing a regulated muscle-specific promoter has been developed for the stable expression of recombinant myosin in C2C12 cells. The vector uses the promoter/enhancer region, the first two and the last five exons of an embryonic rat myosin gene, to regulate the expression of an embryonic chicken muscle myosin cDNA. Stable cell lines transfected with this vector express the unique genetically engineered myosin after differentiation into myotubes. The myosin assembles into myofibrils, copurifies with the endogenous myosin, and contains a complement of muscle-specific myosin light chains. The functional activity of the recombinant myosin is readily analyzed with an in vitro motility assay using a species-specific anti-S2 mAb to selectively assay the recombinant protein. This expression system has facilitated manipulation and analysis of the skeletal muscle myosin motor domain and is also amenable to a wide range of structure-function experiments addressing questions unique to the muscle-specific cytoarchitecture and myosin isoforms.
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PMID:Glycine 699 is pivotal for the motor activity of skeletal muscle myosin. 876 15

We have developed a new technique for measurements of piconewton forces and nanometer displacements in the millisecond time range caused by actin-myosin interaction in vitro by manipulating single actin filaments with a glass microneedle. Here, we describe in full the details of this method. Using this method, the elementary events in energy transduction by the actomyosin motor, driven by ATP hydrolysis, were directly recorded from multiple and single molecules. We found that not only the velocity but also the force greatly depended on the orientations of myosin relative to the actin filament axis. Therefore, to avoid the effects of random orientation of myosin and association of myosin with an artificial substrate in the surface motility assay, we measured forces and displacements by myosin molecules correctly oriented in single synthetic myosin rod cofilaments. At a high myosin-to-rod ratio, large force fluctuations were observed when the actin filament interacted in the correct orientation with a cofilament. The noise analysis of the force fluctuations caused by a small number of heads showed that the myosin head generated a force of 5.9 +/- 0.8 pN at peak and 2.1 +/- 0.4 pN on average over the whole ATPase cycle. The rate constants for transitions into (k+) and out of (k-) the force generation state and the duty ratio were 12 +/- 2 s-1, and 22 +/- 4 s-1, and 0.36 +/- 0.07, respectively. The stiffness was 0.14 pN nm-1 head-1 for slow length change (100 Hz), which would be approximately 0.28 pN nm-1 head-1 for rapid length change or in rigor. At a very low myosin-to-rod ratio, distinct actomyosin attachment, force generation (the power stroke), and detachment events were directly detected. At high load, one power stroke generated a force spike with a peak value of 5-6 pN and a duration of 50 ms (k(-)-1), which were compatible with those of individual myosin heads deduced from the force fluctuations. As the load was reduced, the force of the power stroke decreased and the needle displacement increased. At near zero load, the mean size of single displacement spikes, i.e., the unitary steps caused by correctly oriented myosin, which were corrected for the stiffness of the needle-to-myosin linkage and the randomizing effect by the thermal vibration of the needle, was approximately 20 nm.
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PMID:Multiple- and single-molecule analysis of the actomyosin motor by nanometer-piconewton manipulation with a microneedle: unitary steps and forces. 877 Feb 15

Current theories of muscle cross-bridge function suggest that force is generated by a change in the orientation of the myosin neck region. We attached a paramagnetic probe to a subunit in the neck region and measured the orientation of the probe using electron paramagnetic resonance spectroscopy. The angle of the probes on smooth myosin S1 were changed by 20 degrees +/- 4 degrees on addition of ADP (50% effect at 5 +/- 2 microM), but ADP produced little effect on skeletal S1. The orientation of smooth myosin, +ADP, resembled that of skeletal myosin, +/- ADP, suggesting that the release of ADP generates an extra rotation of the neck region in smooth muscle at the end of its power stroke.
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PMID:ADP release produces a rotation of the neck region of smooth myosin but not skeletal myosin. 878 43


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