Neutron source reconstruction using a generalized expectation-maximization algorithm on one-dimensional neutron images from the Z facility.

Magnetized Liner Inertial Fusion experiments have been performed at the Z facility at Sandia National Laboratories. These experiments use deuterium fuel, which produces 2.45 MeV neutrons on reaching thermonuclear conditions. To study the spatial structure of neutron production, the one-dimensional imager of neutrons diagnostic was fielded to record axial resolved neutron images. In this diagnostic, neutrons passing through a rolled edge aperture form an image on a CR-39-based solid state nuclear track detector. Here, we present a modified generalized expectation-maximization algorithm to reconstruct an axial neutron emission profile of the stagnated fusion plasma. We validate the approach by comparing the reconstructed neutron emission profile to an x-ray emission profile provided by a time-integrated pinhole camera.

Dimerization of native myosin LC2(RLC)-free subfragment 1 from adult rabbit skeletal muscle.

We reinvestigated whether the native myosin LC2-free-subfragment 1 (S1) dimer exists by using viscometry, capillary electrophoresis, and laser light scattering. We found that the intrinsic viscosity of the monomer is [eta]m = 6.7 cm3/g and its translation diffusion coefficient is (c = 0) = 4.43 x 10(-)7 cm2/s. For the dimer, [eta]d = 19.8 cm3/g and (c = 0) = 2.54 x 10(-)7 cm2/s. Using the Svedberg equation and introducing the values of the sedimentation coefficients (5.05 S for the monomer and 6.05 S for the dimer), we find the following molecular weights: Mr,m = 108 000 Da and Mr,d = 213 000 Da, which agree well with previous determinations. Capillary electrophoresis successfully separated S1(A1) and S1(A2), in a monomer buffer, and S1(A1) and S1(A2) and a heterodimer S1(A1)-S1(A2), in a dimer buffer. An interesting feature of the monomer-dimer equilibrium is the presence of temperature transitions, whose positions and widths depend upon the buffer conditions. At low temperatures, a pure dimer was observed, whereas at high temperatures only the monomer was present. The dimerization site on both myosin and S1 is extremely labile.

Hypotonically induced calcium increase and regulatory volume decrease in newborn rat cardiomyocytes.

The effect of cell swelling on intracellular calcium concentration ([Ca2+]i) was studied in newborn rat cardiomyocytes. Hypotonic cell swelling induced a fast and transient [Ca2+]i increase (hypotonically induced calcium increase, HICI; 388±47 nM, n=14). HICI was not inhibited by cyclopiazonic acid (CPA), an inhibitor of sarcoplasmic Ca2+-ATPase, nor ryanodine (an inhibitor of calcium-induced calcium release), whereas it was abolished (11±19 nM, n=5) in the absence of external calcium. Thus, HICI appeared to depend exclusively on entry of external calcium. Gadolinium ion (Gd3+), a generic inhibitor of stretch-activated cation channels (SACs), was unable to affect HICI (353±79 nM, n=6). Similarly, HICI was unaffected by internal Na+ depletion and external Na+ omission. These results suggest that neither Gd3+-sensitive SACs nor Na+-Ca2+ exchange is responsible for HICI. Conversely, HICI was inhibited by diltiazem (42±4 nM, n=3) and by membrane predepolarization (40±18 nM, n=5), suggesting an involvement of L-type voltage-activated calciumchannels. Cardiomyocyte swelling was followed by a regulatory volume decrease (RVD). The putative role of HICI in volume regulation was studied by removal of external calcium. This procedure significantly slowed RVD but did not abolish it. In conclusion, newborn rat cardiomyocytes exhibit an external-calcium-dependent HICI which contributes partially to the RVD.

Native myosin from adult rabbit skeletal muscle: isoenzymes and states of aggregation.

The globular heads of skeletal muscle myosin have been shown to exist as isoenzymes S1 (A1) and S1 (A2), and there are also isoforms of the heavy chains. Using capillary electrophoresis, we found two dominant isoenzymes of the whole native myosin molecule, in agreement with what has previously been found by various techniques for native and nondenatured myosin from adult rabbits. Findings about possible states of aggregation of myosin and its heads are contradictory. By analytical ultracentrifugation, we confirmed the existence of a tail-tail dimer. By laser light scattering, we found a head-head dimer in the presence of MgATP. Capillary electrophoresis coupled with analytical ultracentrifugation and laser light scattering led us to refine these results. We found tail-tail dimers in a conventional buffer. We found tail-tail and head-head dimers in the presence of 0.5 mM MgATP and pure head-head dimers in the presence of 6 mM MgATP. All the dimers were homodimers. Naming the dominant isoenzymes of myosin a and b, we observed tail-tail dimers with isoenzyme a (TaTa) and with isoenzyme b (TbTb) and also head-head dimers with isoenzyme a (HaHa) and with isoenzyme b (HbHb).

Effect of trimetazidine and verapamil on the cardiomyopathic hamster myosin phenotype.

1. In this study we investigated whether long-term trimetazidine (anti-ischaemic drug) therapy alters the ventricular myosin heavy chain (MHC) isoform composition in a model of cardiomyopathy. 2. MHC isoforms were analysed in the native state by electrophoresis in a pyrophosphate buffer. Myosin isoform patterns were studied in cardiac muscle from cardiomyopathic hamsters (CMH) of the BIO 14:6 strain during the time course of the disease and compared with those of healthy golden hamsters (F1B). The correlation between myosin profile and Ca2+-activated ATPase activity was determined from 220 days. 3. At the stage of insufficiency (350 days), CMH presented the most abnormal phenotype with 53% V1-24% V3 compared to 79% V1-7% V3 (P<0.001), in F1B. Trimetazidine was administered to cardiomyopathic hamsters from the early stage of active disease (30 days) to the congestive stages (220-350 days). Within 65 days, trimetazidine treatment, in CMH and F1B, reduced V1 to a low level (53% and 62%, respectively), which remained constant throughout the treatment. This level was similar to that in 220 and 350 days-old untreated-CMH. In sharp contrast, a standard calcium blocker, verapamil, administered to CMH in the same conditions resulted in a higher V1 (about 70%) and higher global myosin ATPase activity from 220 days. 4. Previous results in terms of hypertrophy and survival, compared to these results, suggest that verapamil and trimetazidine treatments reveal a dissociation between ventricular hypertrophy and isomyosin distribution. In addition, the shift in favour of V3 may not necessarily be an aggravating factor of the disease but an adaptative compensatory event.

Long-term therapy with trimetazidine in cardiomyopathic Syrian hamster BIO 14:6.

The cardiomyopathic Syrian hamster (CMH) of the strain BIO 14:6 is a model for both cardiac and skeletal muscle abnormalities. It has reduced longevity and noticeable hypertrophy of the heart and liver. At 220 days, CMHs display a total Ca2+ overload, 1.3-1.8-fold normal and a cytosolic Ca2+ concentration 2-4-fold higher than normal. Long-term oral treatment (18 mg/kg per day) with trimetazidine (anti-ischaemic drug), from age 30 to 350 days, was more efficient than the standard Ca2+ blocker verapamil. Trimetazidine increased the median survival time of CMH by 57% and the hypertrophy disappeared. The total Ca2+ level in CMHs reverted to that of normal Syrian hamsters (F1B). The cytosolic Ca2+ overload was limited to a factor of approximately 2. Therefore, trimetazidine possesses anti-Ca2+ properties and is effective in increasing survival and decreasing the heart and liver hypertrophy of CMH. This suggests that trimetazidine may be valuable in the prevention of congestive heart failure of similar aetiology.

Shape and length of myosin heads and radial compressive forces in muscle.

Irrespective of the exact shape and length of myosin heads, it is shown that radial compressive forces occur in contracting muscle, which tend to bring the myofilaments closer to each other. This is in contradiction with common observation: a muscle that shortens swells laterally. The presence of these forces is model-dependent and they appear only in the case of the traditional concepts of force generation. Here we present an overview and a historical account of the problem.

F-actin-myosin-subfragment-1 (S1) interactions. Identification of the refractory state of S1 with the S1 dimer.

Several conflicting experiments have been carried out concerning the existence of a refractory state for myosin subfragment 1 (S1; i.e. unable to bind to F-actin), in the presence of Mg-nucleotide compounds, at low temperature. Contradictory experiments have been published on the existence of a S1 dimer, under the same conditions. By taking into account some elementary, but crucially important, precautions in the preparation of myosin and S1, it is possible to maintain intact the head-to-head sites of dimerisation on both myosin and S1. Moreover, it has been shown that Mg2+ alone, at low temperature, is able to induce a reversible S1-S1 dimerisation [Morel, J. E. & Garrigos, M. (1982) Biochemistry 21, 2679-2686). Here, we have studied the intrinsic viscosity of S1, in the presence of 2 mM MgCl2, versus temperature, between 0.5-20 degrees C. At approximately 0.5-2.0 degrees C, the intrinsic viscosity of S1 was found to be 19.8 cm3/g. Above 2.5-3.0 degrees C, a steep decrease in the intrinsic viscosity was observed. Between 7 degrees C and 20 degrees C, a constant intrinsic viscosity of 6.7 cm3/g was measured. Therefore, there is a dramatic temperature transition between approximately 2.5-7.0 degrees C (width, 4.5 degrees C; midpoint, 5 degrees C): below 2.5 degrees C we observed the presence of a S1 dimer (confirmed by analytical ultracentrifugation performed at 1 degree C) and above 7-8 degrees C we observed the presence of a S1 monomer (confirmed by analytical ultracentrifugation performed at 8 degrees C). We have also studied the interactions of F-actin with S1, under the same conditions of temperature, and in the presence of 2 mM MgCl2, by using preparative ultracentrifugation. At 1-2 degrees C, S1, which is in the dimeric form, is unable to bind to F-actin. At 6-20 degrees C, F-actin and S1 bind stoichiometrically (1S1/1actin), with an equilibrium constant of 0.5 microM-1, under our experimental conditions. We also performed binding experiments at 3-5 degrees C. Although we were unable to clearly reach the asymptote corresponding to stoichiometry, by assuming a 1:1 ratio, we found a temperature transition in the equilibrium constants, between 3-5 degrees C (width, 2 degrees C; midpoint, 4 degrees C). These phenomena are comparable to those observed for the dimer. Thus, we have shown that there is also a temperature transition in the F-actin-S1 binding process. We conclude that the dimeric and the refractory states of S1 are identical.

Isometric tension exerted by a myofibril of the frog at 0 degree C: geometrical considerations.

For a number of years, the isometric force per myosin head has been taken as 1 pN by the authors who studied in vitro movement. This value was deduced from the value of the isometric tension exerted by a living or a skinned fibre at approximately 0 degree C and for the frog (1-3 kg cm-2). Starting from an isometric tension exerted by a single unit cell of approximately 10 kg cm-2, it has been shown that the isometric force per myosin head is (8 +/- 1)pN. The value of approximately 10 kg cm-2 was deduced from theoretical and semi-empirical reasonings independent of the mechanical roles of the crossbridges. Here, we discuss this latest value by using simple geometrical considerations.

Shape and length of myosin heads.

There is controversy concerning the shape and length of myosin heads. In the present paper we try to analyse the data and to draw clear conclusions in this field. When the myosin heads are isolated (S1) from the rest of the molecule, their length is approximately 12 nm and their shape is close to that of a prolate ellipsoid with an axial ratio approximately 2.3 (in solution) or close to that of a comma when attached to F-actin (with a length of 12-13 nm). When the myosin heads are observed on a whole molecule, their length is approximately 19 nm and they are pear-shaped. Here we suggest that all these observations are compatible. We believe that, for a whole myosin molecule, a large part of the head-rod joint (S1/S2 joint) is measured with the head, owing to a particularly heavy staining or shadowing of this joint. On the other hand, S1 is probably built up of a head part plus the S1/S2 joint, which is not revealed by the usual techniques (hydrodynamics, X-ray and neutron scattering). Finally, the comma shape would be related to a flexible part in the head region of S1, which is significantly bent when S1 is attached to F-actin, but which would be less bent for S1 in solution. A similar bending also occurs in crystalline S1.

Velocity-induced modifications in the crossbridge and/or the actin filament behavior during shortening of muscle fibers.

It has been shown experimentally that, in single muscle fibers, the force-velocity relation is more complicated than usually found (Edman, 1988a. Adv. expl. Med. Biol. 226, 643-652; 1988b, J. Physiol., Lond. 404, 301-321). In particular, there is a discontinuity in this relation for high loads (P/P0 greater than or equal to 0.8), i.e. for low velocities of shortening. Here the mathematical approach of the crossbridge behavior is used, independent of their mechanical role(s). This approach has been already presented in previous papers (Morel, 1984a,b, Prog. Biophys. molec. Biol. 44, 47-96, and references therein). It was found that, at P/P0 approximately 0.80, the force-velocity relation presents a reversal of curvature, which was compatible with the previous results obtained by Edman et al. (1976, Acta Physiol. Scand. 98, 143-156). However, by using extremely elegant techniques, Edman (1988a,b) found a different relation. Here, this problem is studied and it is shown that it is possible to fit the new relationship, provided it is assumed that shortening per se modifies the mechanical properties of the crossbridges and/or the actin filaments. For instance, the interval of attachment of a crossbridge increases with V and the constants of attachment f and detachment g decrease with V. Moreover, it is shown that this approach can predict the approximate constancy of the proportion of crossbridges attached to actin, irrespective of V. This is an old result presented by Podolsky et al. (1976, Proc. natn. Acad. Sci. U.S.A. 73, 813-817). This proportion is approximately 0.97.(ABSTRACT TRUNCATED AT 250 WORDS)

Behaviour of the crossbridges in stretched or compressed muscle fibres.

The maximum chord of the myosin heads is comparable to the closest surface-to-surface spacing between the myofilaments in a muscle at the slack length. Therefore, when the sarcomere length increases or when the fibre is compressed, the surface-to-surface myofilament spacing becomes lower than the head long axis. We conclude that, in stretched or compressed fibres, some crossbridges cannot attach, owing to steric hindrance. When the amount of compression is limited, this hindrance may be overcome by a tilting of the heads in the plane perpendicular to the filament axes; in this case, there is no consequence as concerns the crossbridge properties. In highly compressed fibres, the crossbridges become progressively hindered and all the crossbridges are hindered for an axis-to-axis spacing representing about 60% of the spacing observed under zero external osmotic pressure. In this case, both the isometric tension and the ATPase activity of the fibre are zero. In fibres stretched up to 3.77 microns (sarcomere length corresponding to the disappearance of the overlap between the thick and the thin filaments), the ratio of hindered crossbridges over the functional crossbridges may be estimated at about 55%. In stretched fibres, a noticeable proportion of crossbridges are sterically hindered and the crossbridges performance (e.g. constants of attachment and detachment) depends on filament spacing, i.e. on sarcomere length. Therefore, we think it is probably impossible to consider the crossbridges as independent force converters, since this idea requires that the crossbridge properties are independent of sarcomere length. In this connection, all the experiments performed on osmotically compressed fibres are of major importance for the understanding of the true mechanisms of muscle contraction.