Interferometric Scattering Microscopy for the Study of Molecular Motors

Our understanding of molecular motor function has been greatly improved by the development of imaging modalities, which enable real-time observation of their motion at the single-molecule level. Here, we describe the use of a new method, interferometric scattering microscopy, for the investigation of motor protein dynamics by attaching and tracking the motion of metallic nanoparticle labels as small as 20nm diameter. Using myosin-5, kinesin-1, and dynein as examples, we describe the basic assays, labeling strategies, and principles of data analysis. Our approach is relevant not only for motor protein dynamics but also provides a general tool for single-particle tracking with high spatiotemporal precision, which overcomes the limitations of single-molecule fluorescence methods.

Coupling between phosphate release and force generation in muscle actomyosin

Energetic, kinetic and oxygen exchange experiments in the mid-1980s and early 1990s suggested that phosphate (Pi) release from actomyosin-adenosine diphosphate Pi (AM.ADP.Pi) in muscle fibres is linked to force generation and that Pi release is reversible. The transition leading to the force-generating state and subsequent Pi release were hypothesized to be separate, but closely linked steps. Pi shortens single force-generating actomyosin interactions in an isometric optical clamp only if the conditions enable them to last 20-40 ms, enough time for Pi to dissociate. Until 2003, the available crystal forms of myosin suggested a rigid coupling between movement of switch II and tilting of the lever arm to generate force, but they did not explain the reciprocal affinity myosin has for actin and nucleotides. Newer crystal forms and other structural data suggest that closing of the actin-binding cleft opens switch I (presumably decreasing nucleotide affinity). These data are all consistent with the order of events suggested before: myosin.ADP.Pi binds weakly, then strongly to actin, generating force. Then Pi dissociates, possibly further increasing force or sliding.

A small-molecule inhibitor of skeletal muscle myosin II

We screened a small-molecule library for inhibitors of rabbit muscle myosin II subfragment 1 (S1) actin-stimulated ATPase activity. The best inhibitor, N-benzyl-p-toluene sulphonamide (BTS), an aryl sulphonamide, inhibited the Ca2+-stimulated S1 ATPase, and reversibly blocked gliding motility. Although BTS does not compete for the nucleotide-binding site of myosin, it weakens myosin's interaction with F-actin. BTS reversibly suppressed force production in skinned skeletal muscle fibres from rabbit and frog skin at micromolar concentrations. BTS suppressed twitch production of intact frog fibres with minimum alteration of Ca2+ metabolism. BTS is remarkably specific, as it was much less effective in suppressing contraction in rat myocardial or rabbit slow-twitch muscle, and did not inhibit platelet myosin II. The isolation of BTS and the recently discovered Eg5 kinesin inhibitor, monastrol, suggests that motor proteins may be potential targets for therapeutic applications.

Distribution and orientation of rhodamine-phalloidin bound to thin filaments in skeletal and cardiac myofibrils

Phalloidin staining of muscle does not reflect the known disposition of sarcomeric thin filaments. Quantitative image analysis and steady-state fluorescence polarization microscopy are used to measure the local intensity and orientation of tetramethyl rhodamine-labeled phalloidin (TR-phalloidin) in skinned myofibrils. TR-phalloidin staining of isolated skeletal myofibrils labeled while in rigor reveals fluorescence that is brighter at the pointed ends of the thin filaments and Z lines than it is in the middle of the filaments. In cardiac myofibrils, phalloidin staining is uniform along the lengths of the thin filaments in both relaxed and rigor myofibrils, except in 0.2-micron dark areas on either side of the Z line. Extraction of myosin or tropomyosin-troponin molecules does not change the nonuniform staining. To test whether long-term storage in glycerol changes the binding of phalloidin to thin filaments in myofibrils, minimally permeabilized (briefly skinned) myofibrils, or myofibrils stored in glycerol for at least 7 days (glycerol extraction) were compared. TR-phalloidin was well ordered throughout the sarcomere in briefly skinned skeletal and cardiac myofibrils, but TR-phalloidin bound to the Z line and pointed ends of thin filaments was randomly oriented in glycerol-extracted myofibrils, suggesting that the ends of the thin filaments become disordered after glycerol extraction. In relaxed skeletal myofibrils with sarcomere lengths greater than 3.0 microns, staining was nearly uniform all along the actin filaments. Exogeneous bare actin filaments polymerized from the Z line (Sanger et al., 1984: J. Cell Biol. 98:825-833) in and along the myofibril bind rhodamine phalloidin uniformly. Our results support the hypothesis that nebulin can block the binding of phalloidin to actin in skeletal myofibrils and nebulette can block phalloidin binding to cardiac thin filaments.

A maximum entropy analysis of protein orientations using fluorescence polarization data from multiple probes

Techniques have recently become available to label protein subunits with fluorescent probes at predetermined orientation relative to the protein coordinates. The known local orientation enables quantitative interpretation of fluorescence polarization experiments in terms of orientation and motions of the protein within a larger macromolecular assembly. Combining data obtained from probes placed at several distinct orientations relative to the protein structure reveals functionally relevant information about the axial and azimuthal orientation of the labeled protein segment relative to its surroundings. Here we present an analytical method to determine the protein orientational distribution from such data. The method produces the broadest distribution compatible with the data by maximizing its informational entropy. The key advantages of this approach are that no a priori assumptions are required about the shape of the distribution and that a unique, exact fit to the data is obtained. The relative orientations of the probes used for the experiments have great influence on information content of the maximum entropy distribution. Therefore, the choice of probe orientations is crucial. In particular, the probes must access independent aspects of the protein orientation, and two-fold rotational symmetries must be avoided. For a set of probes, a "figure of merit" is proposed, based on the independence among the probe orientations. With simulated fluorescence polarization data, we tested the capacity of maximum entropy analysis to recover specific protein orientational distributions and found that it is capable of recovering orientational distributions with one and two peaks. The similarity between the maximum entropy distribution and the test distribution improves gradually as the number of independent probe orientations increases. As a practical example, ME distributions were determined with experimental data from muscle fibers labeled with bifunctional rhodamine at known orientations with respect to the myosin regulatory light chain (RLC). These distributions show a complex relationship between the axial orientation of the RLC relative to the fiber axis and the azimuthal orientation of the RLC about its own axis. Maximum entropy analysis reveals limitations in available experimental data and supports the design of further probe angles to resolve details of the orientational distribution.

Tomographic 3D reconstruction of quick-frozen, Ca2+-activated contracting insect flight muscle

Motor actions of myosin were directly visualized by electron tomography of insect flight muscle quick-frozen during contraction. In 3D images, active cross-bridges are usually single myosin heads, bound preferentially to actin target zones sited midway between troponins. Active attached bridges (approximately 30% of all heads) depart markedly in axial and azimuthal angles from Rayment's rigor acto-S1 model, one-third requiring motor domain (MD) tilting on actin, and two-thirds keeping rigor contact with actin while the light chain domain (LCD) tilts axially from approximately 105 degrees to approximately 70 degrees. The results suggest the MD tilts and slews on actin from weak to strong binding, followed by swinging of the LCD through an approximately 35 degrees axial angle, giving an approximately 13 nm interaction distance and an approximately 4-6 nm working stroke.

Dynamic measurement of myosin light-chain-domain tilt and twist in muscle contraction

A new method is described for measuring motions of protein domains in their native environment on the physiological timescale. Pairs of cysteines are introduced into the domain at sites chosen from its static structure and are crosslinked by a bifunctional rhodamine. Domain orientation in a reconstituted macromolecular complex is determined by combining fluorescence polarization data from a small number of such labelled cysteine pairs. This approach bridges the gap between in vitro studies of protein structure and cellular studies of protein function and is used here to measure the tilt and twist of the myosin light-chain domain with respect to actin filaments in single muscle cells. The results reveal the structural basis for the lever-arm action of the light-chain domain of the myosin motor during force generation in muscle.

The ADP release step of the smooth muscle cross-bridge cycle is not directly associated with force generation

When smooth muscle myosin subfragment 1 (S1) is bound to actin filaments in vitro, the light chain domain tilts upon release of MgADP, producing a approximately 3.5-nm axial motion of the head-rod junction (Whittaker et al., 1995. Nature. 378:748-751). If this motion contributes significantly to the power stroke, rigor tension of smooth muscle should decrease substantially in response to cross-bridge binding of MgADP. To test this prediction, we monitored mechanical properties of permeabilized strips of chicken gizzard muscle in rigor and in the presence of MgADP. For comparison, we also tested psoas and soleus muscle fibers. Any residual bound ADP was minimized by incubation in Mg2+-free rigor solution containing 15 mM EDTA. The addition of 2 mM MgADP, while keeping ionic strength and free Mg2+ concentration constant, resulted in a slight increase in rigor tension in both gizzard and soleus muscles, but a decrease in psoas muscle. In-phase stiffness monitored during small (<0.1%) 500-Hz sinusoidal length oscillations decreased in all three muscle types when MgADP was added. The changes in force and stiffness with the addition of MgADP were similar at ionic strengths from 50 to 200 mM and were reversible. The results with gizzard muscle were similar after thiophosphorylation of the regulatory light chain of myosin. These results suggest that the axial motion of smooth muscle S1 bound to actin, upon dissociation of MgADP, is not associated with force generation. The difference between the present mechanical data and previous structural studies of smooth S1 may be explained if geometrical constraints of the intact contractile filament array alter the motions of the myosin heads.

Trading force for speed: why superfast crossbridge kinetics leads to superlow forces

Superfast muscles power high-frequency motions such as sound production and visual tracking. As a class, these muscles also generate low forces. Using the toadfish swimbladder muscle, the fastest known vertebrate muscle, we examined the crossbridge kinetic rates responsible for high contraction rates and how these might affect force generation. Swimbladder fibers have evolved a 10-fold faster crossbridge detachment rate than fast-twitch locomotory fibers, but surprisingly the crossbridge attachment rate has remained unchanged. These kinetics result in very few crossbridges being attached during contraction of superfast fibers (only approximately 1/6 of that in locomotory fibers) and thus low force. This imbalance between attachment and detachment rates is likely to be a general mechanism that imposes a tradeoff of force for speed in all superfast fibers.

Model-independent analysis of the orientation of fluorescent probes with restricted mobility in muscle fibers

The orientation of proteins in ordered biological samples can be investigated using steady-state polarized fluorescence from probes conjugated to the protein. A general limitation of this approach is that the probes typically exhibit rapid orientational motion ("wobble") with respect to the protein backbone. Here we present a method for characterizing the extent of this wobble and for removing its effects from the available information about the static orientational distribution of the probes. The analysis depends on four assumptions: 1) the probe wobble is fast compared with the nanosecond time scale of its excited-state decay; 2) the orientational distributions of the absorption and emission transition dipole moments are cylindrically symmetrical about a common axis c fixed in the protein; 3) protein motions are negligible during the excited-state decay; 4) the distribution of c is cylindrically symmetrical about the director of the experimental sample. In a muscle fiber, the director is the fiber axis, F. All of the information on the orientational order of the probe that is available from measurements of linearly polarized fluorescence is contained in five independent polarized fluorescence intensities measured with excitation and emission polarizers parallel or perpendicular to F and with the propagation axis of the detected fluorescence parallel or perpendicular to that of the excitation. The analysis then yields the average second-rank and fourth-rank order parameters (<P2> and <P4>) of the angular distribution of c relative to F, and <P2a> and <P2e>, the average second-rank order parameters of the angular distribution for wobble of the absorption and emission transition dipole moments relative to c. The method can also be applied to other cylindrically ordered systems such as oriented lipid bilayer membranes and to processes slower than fluorescence that may be observed using longer-lived optically excited states.

Orientation changes of fluorescent probes at five sites on the myosin regulatory light chain during contraction of single skeletal muscle fibres

Changes in the orientation of the myosin regulatory light chain (RLC) in single muscle fibres were measured using polarised fluorescence from acetamidotetramethylrhodamine (ATR). Mutants of chicken skeletal RLC containing single cysteine residues at positions 2, 73, 94, 126 and 155 were labelled with either the 5 or 6-isomer of iodo-ATR, giving ten different probes. The labelled RLCs were exchanged into demembranated fibres from rabbit psoas muscle without significant effect on active force generation. Fluorescence polarisation measurements showed that nine out of the ten probe dipoles were more perpendicular to the fibre axis in the absence of ATP (in rigor) than in either relaxation or active contraction. The orientational distribution of the RLC region of the myosin head in active contraction is closer to the relaxed than to the rigor orientation, and is not equivalent to a linear combination of the relaxed and rigor orientations. Rapid length steps were applied to the fibres to synchronise the motions of myosin heads attached to actin. In active contraction the fluorescence polarisation changed both during the step, indicating elastic distortion of the RLC region of the myosin head, and during the subsequent rapid force recovery that is thought to signal the working stroke. The peak change in fluorescence polarisation produced by an active release of 5 nm per half sarcomere indicates an axial tilt of less than 5 degrees for all ten probes, if all the myosin heads in the fibre respond to the length step. This tilting was towards the rigor orientation for all ten probes, and could be explained by 14% of the heads moving to the rigor orientation. An active stretch tilted the heads away from the rigor conformation by a similar extent.

Fluorescence polarization transients from rhodamine isomers on the myosin regulatory light chain in skeletal muscle fibers

Fluorescence polarization was used to examine orientation changes of two rhodamine probes bound to myosin heads in skeletal muscle fibers. Chicken gizzard myosin regulatory light chain (RLC) was labeled at Cys108 with either the 5- or the 6-isomer of iodoacetamidotetramethylrhodamine (IATR). Labeled RLC (termed Cys108-5 or Cys108-6) was exchanged for the endogenous RLC in single, skinned fibers from rabbit psoas muscle. Three independent fluorescence polarization ratios were used to determine the static angular distribution of the probe dipoles with respect to the fiber axis and the extent of probe motions on the nanosecond time scale of the fluorescence lifetime. We used step changes in fiber length to partially synchronize the transitions between biochemical, structural, and mechanical states of the myosin cross-bridges. Releases during active contraction tilted the Cys108-6 dipoles away from the fiber axis. This response saturated for releases beyond 3 nm/half-sarcomere (h.s.). Stretches in active contraction caused the dipoles to tilt toward the fiber axis, with no evidence of saturation for stretches up to 7 nm/h.s. These nonlinearities of the response to length changes are consistent with a partition of approximately 90% of the probes that did not tilt when length changes were applied and 10% of the probes that tilted. The responding fraction tilted approximately 30 degrees for a 7.5 nm/h.s. release and traversed the plane perpendicular to the fiber axis for larger releases. Stretches in rigor tilted Cys108-6 dipoles away from the fiber axis, which was the opposite of the response in active contraction. The transition from the rigor-type to the active-type response to stretch preceded the main force development when fibers were activated from rigor by photolysis of caged ATP in the presence of Ca2+. Polarization ratios for Cys108-6 in low ionic strength (20 mM) relaxing solution were compatible with a combination of the relaxed (200 mM ionic strength) and rigor intensities, but the response to length changes was of the active type. The nanosecond motions of the Cys108-6 dipole were restricted to a cone of approximately 20 degrees half-angle, and those of Cys108-5 dipole to a cone of approximately 25 degrees half-angle. These values changed little between relaxation, active contraction, and rigor. Cys108-5 showed very small-amplitude tilting toward the fiber axis for both stretches and releases in active contraction, but much larger amplitude tilting in rigor. The marked differences in these responses to length steps between the two probe isomers and between active contraction and rigor suggest that the RLC undergoes a large angle change (approximately 60 degrees) between these two states. This motion is likely to be a combination of tilting of the RLC relative to the fiber axis and twisting of the RLC about its own axis.

Fluorescence polarization of skeletal muscle fibers labeled with rhodamine isomers on the myosin heavy chain

Fluorescence polarization was used to examine orientational changes of Rhodamine probes in single, skinned muscle fibers from rabbit psoas muscle following either photolysis of caged nucleotides or rapid length changes. Fibers were extensively and predominantly labeled at SH1 (Cys-707) of the myosin heavy chain with either the 5- or the 6-isomer of iodoacetamidotetramethylrhodamine. Results from spectroscopic experiments utilizing the two Rhodamine isomers were quite similar. Following photolysis of either caged ATP or caged ADP, probes promptly reoriented toward the muscle fiber axis. Changes in the fluorescence polarization signals with transients elicited by the photolysis of caged ATP in the presence of saturating Ca2+ greatly preceded active force generation. Photolysis of caged ADP caused only a small, rapid decrease in force but elicited changes in the fluorescence polarization signals with time course and amplitude similar to those following photolysis of caged ATP. Fluorescence polarization signals were virtually unchanged by rapid length steps in both rigor and active muscle fibers. These results indicate that structural changes monitored by Rhodamine probes at SH1 are not associated directly with the force-generating event of muscle contraction. However, the fluorescence polarization transients were slightly faster than the estimated rate of cross-bridge detachment following photolysis of caged ATP, suggesting that the observed structural changes at SH1 may be involved in the communication pathway between the nucleotide- and actin-binding sites of myosin.

Structure and periodicities of cross-bridges in relaxation, in rigor, and during contractions initiated by photolysis of caged Ca2+

Ultra-rapid freezing and electron microscopy were used to directly observe structural details of frog muscle fibers in rigor, in relaxation, and during force development initiated by laser photolysis of DM-nitrophen (a caged Ca2+). Longitudinal sections from relaxed fibers show helical tracks of the myosin heads on the surface of the thick filaments. Fibers frozen at approximately 13, approximately 34, and approximately 220 ms after activation from the relaxed state by photorelease of Ca2+ all show surprisingly similar cross-bridge dispositions. In sections along the 1,1 lattice plane of activated fibers, individual cross-bridge densities have a wide range of shapes and angles, perpendicular to the fiber axis or pointing toward or away from the Z line. This highly variable distribution is established very early during development of contraction. Cross-bridge density across the interfilament space is more uniform than in rigor, wherein the cross-bridges are more dense near the thin filaments. Optical diffraction (OD) patterns and computed power density spectra of the electron micrographs were used to analyze periodicities of structures within the overlap regions of the sarcomeres. Most aspects of these patterns are consistent with time resolved x-ray diffraction data from the corresponding states of intact muscle, but some features are different, presumably reflecting different origins of contrast between the two methods and possible alterations in the structure of the electron microscopy samples during processing. In relaxed fibers, OD patterns show strong meridional spots and layer lines up to the sixth order of the 43-nm myosin repeat, indicating preservation and resolution of periodic structures smaller than 10 nm. In rigor, layer lines at 18, 24, and 36 nm indicate cross-bridge attachment along the thin filament helix. After activation by photorelease of Ca2+, the 14.3-nm meridional spot is present, but the second-order meridional spot (22 nm) disappears. The myosin 43-nm layer line becomes less intense, and higher orders of 43-nm layer lines disappear. A 36-nm layer line is apparent by 13 ms and becomes progressively stronger while moving laterally away from the meridian of the pattern at later times, indicating cross-bridges labeling the actin helix at decreasing radius.

Mechanical characterization of skeletal muscle myofibrils

A new instrument, based on a technique described previously, is presented for studying mechanics of micron-scale preparations of two to three myofibrils or single myofibrils from muscle. Forces in the nanonewton to micronewton range are measurable with 0.5-ms time resolution. Programmed quick (200-microseconds) steps or ramp length changes are applied to contracting myofibrils to test their mechanical properties. Individual striations can be monitored during force production and shortening. The active isometric force, force-velocity relationship, and force transients after rapid length steps were obtained from bundles of two to three myofibrils from rabbit psoas muscle. Contrary to some earlier reports on myofibrillar mechanics, these properties are generally similar to expectations from studies on intact and skinned muscle fibers. Our experiments provide strong evidence that the mechanical properties of a fiber result from a simple summation of the myofibrillar force and shortening of independently contracting sarcomeres.

Orientation changes in myosin regulatory light chains following photorelease of ATP in skinned muscle fibers

The orientation of the light-chain region of myosin heads in muscle fibers was followed by polarized fluorescence from an extrinsic probe during tension transients elicited by photolysis of caged ATP. Regulatory light chain from chicken gizzard myosin was covalently modified with iodoacetamidotetramethylrhodamine and exchanged into skinned fibers from rabbit psoas muscle without significant effect of the tension transients. Fluorescence polarization ratios Q parallel = (parallel I parallel-perpendicular I parallel)/ (parallel I parallel+perpendicular I parallel) and Q perpendicular = perpendicular I perpendicular - parallel I perpendicular)/ (perpendicular I perpendicular + parallel I perpendicular), where mIn denote fluorescence intensities for excitation (pre-subscript) and emission (post-subscript) parallel or perpendicular to the fiber axis, were simultaneously measured at 0.5 ms time resolution. Q perpendicular decreased and Q parallel increased promptly after ATP release in the presence or absence of CA2+, indicating changes in orientation of the light-chain region associated with ATP binding or cross-bridge detachment. Little further change in the Q signals accompanied either active tension development (+Ca2+) or the final relaxation (-Ca2+). The Q and tension transients slowed when liberated ATP concentration was reduced. Assuming that ATP is released at 118 s-1 (20 degrees C), the apparent second-order rate constants were 3-10 x 10(5) M-1 s-1 for Q parallel, 1-5 x 10(5) M-1 s-1 for Q perpendicular, and 0.5-2 x 10(5) M-1 s-1 for the convergence of tension traces starting from different rigor values. Fitting of model orientation distributions to the Q signals indicated that the angular disorder increases after ATP binding. This orientation change is specific to ATP because photo release of ADP caused much smaller changes in the Q signals.

Sliding distance per ATP molecule hydrolyzed by myosin heads during isotonic shortening of skinned muscle fibers

We measured isotonic sliding distance of single skinned fibers from rabbit psoas muscle when known and limited amounts of ATP were made available to the contractile apparatus. The fibers were immersed in paraffin oil at 20 degrees C, and laser pulse photolysis of caged ATP within the fiber initiated the contraction. The amount of ATP released was measured by photolyzing 3H-ATP within fibers, separating the reaction products by high-pressure liquid chromatography, and then counting the effluent peaks by liquid scintillation. The fiber stiffness was monitored to estimate the proportion of thick and thin filament sites interacting during filament sliding. The interaction distance, Di, defined as the sliding distance while a myosin head interacts with actin in the thin filament per ATP molecule hydrolyzed, was estimated from the shortening distance, the number of ATP molecules hydrolyzed by the myosin heads, and the stiffness. Di increased from 11 to 60 nm as the isotonic tension was reduced from 80% to 6% of the isometric tension. Velocity and Di increased with the concentration of ATP available. As isotonic load was increased, the interaction distance decreased linearly with decrease of the shortening velocity and extrapolated to 8 nm at zero velocity. Extrapolation of the relationship between Di and velocity to saturating ATP concentration suggests that Di reaches 100-190 nm at high shortening velocity. The interaction distance corresponds to the sliding distance while cross-bridges are producing positive (working) force plus the distance while they are dragging (producing negative forces). The results indicate that the working and drag distances increase as the velocity increases. Because Di is larger than the size of either the myosin head or the actin monomer, the results suggest that for each ATPase cycle, a myosin head interacts mechanically with several actin monomers either while working or while producing drag.

Compliance of thin filaments in skinned fibers of rabbit skeletal muscle

The mechanical compliance (reciprocal of stiffness) of thin filaments was estimated from the relative compliance of single, skinned muscle fibers in rigor at sarcomere lengths between 1.8 and 2.4 micron. The compliance of the fibers was calculated as the ratio of sarcomere length change to tension change during imposition of repetitive cycles of small stretches and releases. Fiber compliance decreased as the sarcomere length was decreased below 2.4 micron. The compliance of the thin filaments could be estimated from this decrement because in this range of lengths overlap between the thick and thin filaments is complete and all of the myosin heads bind to the thin filament in rigor. Thus, the compliance of the overlap region of the sarcomere is constant as length is changed and the decrease in fiber compliance is due to decrease of the nonoverlap length of the thin filaments (the I band). The compliance value obtained for the thin filaments implies that at 2.4-microns sarcomere length, the thin filaments contribute approximately 55% of the total sarcomere compliance. Considering that the sarcomeres are approximately 1.25-fold more compliant in active isometric contractions than in rigor, the thin filaments contribute approximately 44% to sarcomere compliance during isometric contraction.

Tilting of the light-chain region of myosin during step length changes and active force generation in skeletal muscle

Force generation and relative sliding between the myosin and actin filaments in muscle are thought to be caused by tilting of the head region of the myosin crossbridges between the filaments. Structural and spectroscopic experiments have demonstrated segmental flexibility of myosin in muscle, but have not shown a direct linkage between tilting of the myosin heads and either force generation or filament sliding. Here we use fluorescence polarization to detect changes in the orientation of the light-chain region of the head, the part most likely to tilt, and synchronized head movements by imposing rapid length steps. We found that the light-chain region of the myosin head tilts both during the imposed filament sliding and during the subsequent quick force recovery that is thought to signal the elementary force-generating event.

Fluorescence polarization from isomers of tetramethylrhodamine at SH-1 in rabbit psoas muscle fibers

We have used fluorescence polarization to examine orientational changes of the 5- and 6-isomers of acetamidotetramethylrhodamine (ATR) covalently bound to SH-1 (Cys-707 of the myosin heavy chain) in single, skinned fibers from rabbit psoas muscle after rapid length steps or photolysis of caged nucleotides. Similar results were obtained with both the 5- and 6-isomers of ATR. After the photolysis of caged ATP, large and rapid changes in the fluorescence polarization signals were observed and were complete well before appreciable force had been generated. Changes in the fluorescence polarization signals after the photolysis of caged ADP were similar to those after the photolysis of caged ATP, despite an almost negligible change in force. The fluorescence polarization signals remained almost constant after rapid length steps in both rigor and active muscle fibers. These results suggest that structural changes at SH-1 monitored by 5- or 6-ATR are not associated directly with the force-generating event of muscle contraction, but may be involved in the communication pathway between the nucleotide and actin-binding sites of myosin.

Transients of fluorescence polarization in skeletal muscle fibers labeled with rhodamine on the regulatory light chain

Structural changes of the myosin heads were correlated with mechanical events in the cross-bridge cycle by measuring fluorescence polarization signals at high time resolution from rhodamine probes bound to myosin regulatory light chains in skeletal muscle fibers. Motions of the cross-bridges were partially synchronized either by applying quick length changes to the fibers during active contractions or by activating the fibers from rigor by photolysis of caged ATP in the presence of Ca2+. With fibers in rigor, the fluorescence polarization values indicate that the probe dipoles are quite well ordered and are directed away from the muscle fiber axis. After photorelease of ATP from caged ATP, changes in polarization signals are consistent with broadening of the distribution of probe orientations. The signal deflections occur when ATP binds to actomyosin or when the cross-bridges detach, but the orientational distribution changes surprisingly little during active force development. In contrast, when staircases of quick releases are applied to labeled fibers during active contractions, the fluorescence polarization signals suggest a concerted rotation of the probes. The results indicate that the light chain region of myosin tilts during the quick release and/or during the tension recovery phase within the next few ms.

Structural changes in muscle crossbridges accompanying force generation

We have investigated the structure of the crossbridges in muscles rapidly frozen while relaxed, in rigor, and at various times after activation from rigor by flash photolysis of caged ATP. We used Fourier analysis of images of cross sections to obtain an average view of the muscle structure, and correspondence analysis to extract information about individual crossbridge shapes. The crossbridge structure changes dramatically between relaxed, rigor, and with time after ATP release. In relaxed muscle, most crossbridges are detached. In rigor, all are attached and have a characteristic asymmetric shape that shows strong left-handed curvature when viewed from the M-line towards the Z-line. Immediately after ATP release, before significant force has developed (20 ms) the homogeneous rigor population is replaced by a much more diverse collection of crossbridge shapes. Over the next few hundred milliseconds, the proportion of attached crossbridges changes little, but the distribution of the crossbridges among different structural classes continues to evolve. Some forms of attached crossbridge (presumably weakly attached) increase at early times when tension is low. The proportion of several other attached non-rigor crossbridge shapes increases in parallel with the development of active tension. The results lend strong support to models of muscle contraction that have attributed force generation to structural changes in attached crossbridges.

Flash and smash: rapid freezing of muscle fibers activated by photolysis of caged ATP

A new approach was used to study transient structural states of cross-bridges during activation of muscle fibers. Rabbit skinned muscle fibers were rapidly and synchronously activated from the rigor state by photolysis of caged ATP in the presence of Ca2+. At several different times during the switch from rigor to fully active tension development, the fibers were rapidly frozen on a liquid helium-cooled metal block, freeze-substituted, and examined in an electron microscope. The limits of structural preservation and resolution with this technique were analyzed. We demonstrate that the resolution of our images is sufficient to draw the following conclusions about cross-bridge structure. Rigor cross-bridges point away from the Z-line and most of them are wider near the thin filaments than near the backbone of the thick filaments. In contrast, cross-bridges in actively contracting fibers stretch between the thick and thin filaments at a variable angle, and are uniformly thin. Diffraction patterns computed from contracting muscle show layer lines both at 38 and 43 nm indicating that active cross-bridges contribute mass to both the actin- and myosin-based helical periodicities. The images obtained from fibers frozen 20 ms after release of ATP show a mixture of rigor and active type cross-bridge configurations. There is little evidence of cross-bridges with the rigor shape by 50 ms, and the difference in configurations between 50 and 300 ms after photolysis is surprisingly subtle.

Mechanics and structure of cross-bridges during contractions initiated by photolysis of caged Ca2+

Cross-bridge structure and mechanics were studied during development of skinned frog muscle fiber contractions initiated by photolysis of DM-nitrophen (a caged Ca2+). Stiffness rises earlier than tension following photo-release of Ca2+. A similar lead of stiffness in electrically stimulated fibers and the early rise of the I11/I10 ratio of equatorial X-ray reflections are thought to signal attachment of cross-bridges into states with lower force than in steady-state contraction. We investigated the structure of the early attachments by electron microscopy of fibers activated by photolysis of DM-nitrophen and then ultra-rapidly frozen and freeze substituted with tannic acid and OsO4. Sections from relaxed fibers show helical tracks of myosin heads on the thick filaments surface. Optical diffraction patterns show strong meridional intensities and layer lines up to the 6th order of 1/43 nm, indicating preservation and resolution of periodic structures smaller than 10 nm. Following photo-release of Ca2+, the 1/43 nm myosin layer line becomes less intense, and higher orders disappear. A approximately 1/36 nm layer line appears early (12-15 ms) and becomes stronger at later times. The 1/14.3 nm meridional spot weakens initially and recovers at a later time, while it broadens laterally. The 1/43 nm meridional spot is present during contraction, but the 2nd order meridional spot (1/21.5 nm) is weak or absent. These results are consistent with time resolved X-ray diffraction data on the periodic structures within the fiber. In sections along the 1,1 plane of activated fibers, the individual cross-bridges have a wide range of shapes and angles, perpendicular to the fiber axis or pointing toward or away from the Z-line. Fibers frozen at 13 ms, 33 ms, and 220 ms after photolysis all show surprisingly similar cross-bridges. Thus, a highly variable distribution of cross-bridge shapes and angles is established early in contraction.

Transients in orientation of a fluorescent cross-bridge probe following photolysis of caged nucleotides in skeletal muscle fibres

In muscle fibres labelled with iodoacetamidotetramethylrhodamine at Cys707 of the myosin heavy chain, the probes have been reported to change orientation when the fibre is activated, relaxed or put into rigor. In order to test whether these motions are indications of the cross-bridge power stroke, we monitored tension and linear dichroism of the probes in single glycerol-extracted fibres of rabbit psoas muscle during mechanical transients initiated by laser pulse photolysis of caged ATP and caged ADP. In rigor dichroism is negative, indicating average probe absorption dipole moments oriented more than 54.7 degrees away from the fibre axis. During activation from rigor induced by photoliberation of ATP from caged ATP in the presence of calcium, the dichroism reversed sign promptly (half-time 12.5 ms for 500 microM-ATP) upon release of ATP, but then changed only slightly during tension development 20 to 100 milliseconds later. During the onset of rigor following transfer of the fibre from an ATP-containing relaxing solution to a rigor medium lacking ATP, force generation preceded the change in dichroism. The dichroism change occurred slowly (half-time 47 s), because binding of ADP to sites within the muscle fibre limited its rate of diffusion out of the fibre. When ADP was introduced or removed, the dichroism transient was similar in time course and magnitude to that obtained after the introduction or removal of ATP. Neither adding nor removing ADP produced substantial changes in force. These results demonstrate that orientation of the rhodamine probes on the myosin head reflects mainly structural changes linked to nucleotide binding and release, rather than rotation of the cross-bridge during force generation.

Reversal of the cross-bridge force-generating transition by photogeneration of phosphate in rabbit psoas muscle fibres

1. Orthophosphate (P(i), 0.1-2.0 mM) was photogenerated within the filament lattice of isometrically contracting glycerinated fibres of rabbit psoas muscle at 10 and 20 degrees C. The P(i) was produced by laser flash photolysis of the photolabile compound 1-(2-nitrophenyl)ethylphosphate (caged P(i)). Caged P(i) caused a depression of tension that was much smaller than that caused by P(i). 2. Photolysis of caged P(i) produced a decline in isometric force composed of four phases: phase I, a lag phase (e.g. 1-4 ms at 10 degrees C) during which force did not change; phase II, an exponential decline by as much as 20% of the pre-pulse force; phase III, a partial force recovery (0-3% of the pre-pulse force); and phase IV, a further slow (0.5-3 s) decline to the steady value. Phases I, III and IV were largely independent of [P(i)] and are likely to be indirect effects caused by the caged P(i) photolysis. 3. Both the rate and amplitude of phase II depended markedly on [P(i)]. The amplitude of phase II was similar to the reduction of steady-state force by P(i). The rate of phase II increased with increasing temperature and [P(i)]. At high [P(i)] the rate began to saturate, and approached limits of 123 s-1 at 10 degrees C and 194 s-1 at 20 degrees C. 4. The rate of phase II was independent of sarcomere overlap, while the amplitude was proportional to tension at partial filament overlap. A control experiment using caged ATP showed that phase II was not produced by the photolytic by-products or the light pulse. The results suggest that phase II is associated with the force-generating transition of the cross-bridge cycle. 5. Sinusoidal length oscillations at 0.5 and 2 kHz were used to measure muscle stiffness during phase II. Stiffness declined in a single exponential phase, with the same time course as phase II of the tension transient. The change in stiffness was 83 +/- 6% (mean +/- S.E.M., n = 10, 0.5 kHz) of the change in tension when both signals were normalized to their pre-flash values. 6. Analysis of the data shows that two steps are involved in force generation and P(i) release. The non-force exerting AM-ADP-P(i) cross-bridge state first isomerizes to form a force-exerting cross-bridge state (AM'-ADP-P(i)). P(i) is then released to form a second force-generating state, AM'-ADP.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanical transients initiated by photolysis of caged ATP within fibers of insect fibrillar flight muscle

Kinetics of the cross-bridge cycle in insect fibrillar flight muscle have been measured using laser pulse photolysis of caged ATP and caged inorganic phosphate (Pi) to produce rapid step increases in the concentration of ATP and Pi within single glycerol-extracted fibers. Rapid photochemical liberation of 100 microM-1 mM ATP from caged ATP within a fiber caused relaxation in the absence of Ca2+ and initiated an active contraction in the presence of approximately 30 microM Ca2+. The apparent second order rate constant for detachment of rigor cross-bridges by ATP was between 5 x 10(4) and 2 x 10(5) M-1s-1. This rate is not appreciably sensitive to the Ca2+ or Pi concentrations or to rigor tension level. The value is within an order of magnitude of the analogous reaction rate constant measured with isolated actin and insect myosin subfragment-1 (1986. J. Muscle Res. Cell Motil. 7:179-192). In both the absence and presence of Ca2+ insect fibers showed evidence of transient cross-bridge reattachment after ATP-induced detachment from rigor, as found in corresponding experiments on rabbit psoas fibers. However, in contrast to results with rabbit fibers, tension traces of insect fibers starting at different rigor tensions did not converge to a common time course until late in the transients. This result suggests that the proportion of myosin cross-bridges that can reattach into force-generating states depends on stress or strain in the filament lattice. A steady 10-mM concentration of Pi markedly decreased the transient reattachment phase after caged ATP photolysis. Pi also decreased the amplitude of stretch activation after step stretches applied in the presence of Ca2+ and ATP. Photolysis of caged Pi during stretch activation abruptly terminated the development of tension. These results are consistent with a linkage between Pi release and the steps leading to force production in the cross-bridge cycle.

Sliding distance between actin and myosin filaments per ATP molecule hydrolysed in skinned muscle fibres

Muscle contraction is generally thought to be driven by tilting of the 19-nm-long myosin head, part of the thick filament, while attached to actin, part of the thin filament. This motion would produce about 12 nm of filament sliding. Recent estimates of the sliding distance per ATP molecule hydrolysed by actomyosin in vitro vary widely from 8 nm to greater than or equal to 200 nm. The latter value is incompatible with a power stroke incorporating a single tilting motion of the head. We have measured the isotonic sliding distance per ATP molecule hydrolysed during the interaction between myosin and actin in skinned muscle fibres. We directly estimated the proportion of simultaneously attached actomyosin complexes and their ATP use. We report here that at low loads the interaction distance is at least 40 nm. This distance corresponds to the length of the power stroke plus the filament sliding while actomyosin crossbridges bear negative drag forces. If the power stroke is 12 nm, then our results indicate the drag distance to be at least 28 nm. Our results could also be explained by multiple power strokes per ATP molecule hydrolysed.

Cross-bridge kinetics in the presence of MgADP investigated by photolysis of caged ATP in rabbit psoas muscle fibres

1. The interaction between MgADP and rigor cross-bridges in glycerol-extracted single fibres from rabbit psoas muscle has been investigated using laser pulse photolysis of caged ATP (P3-1(2-nitrophenyl)ethyladenosine 5'-triphosphate) in the presence of MgADP and following small length changes applied to the rigor fibre. 2. Addition of 465 microM-MgADP to a rigor fibre caused rigor tension to decrease by 15.3 +/- 0.7% (S.E.M., n = 24 trials in thirteen fibres). The half-saturation value for this tension reduction was 18 +/- 4 microM (n = 23, thirteen fibres). 3. Relaxation from rigor by photolysis of caged ATP in the absence of Ca2+ was markedly slowed by inclusion of 20 microM-2 mM-MgADP in the photolysis medium. 4. Four phases of tension relaxation occurred with MgADP in the medium: at, a quick partial relaxation (in pre-stretch fibres); bt, a slowing of relaxation or a rise in tension for 50-100 ms; ct, a sudden acceleration of relaxation; and dt, a final, nearly exponential relaxation. 5. Experiments at varied MgATP and MgADP concentrations suggested that phase at is due to MgATP binding to nucleotide-free cross-bridges. 6. Phase bt was abbreviated by including 1-20 mM-orthophosphate (Pi) in the photolysis medium, or by applying quick stretches before photolysis or during phase bt. These results suggest that phases bt and ct are complex processes involving ADP dissociation, cross-bridge reattachment and co-operative detachment involving filament sliding and the Ca(2+)-regulatory system. 7. Stretching relaxed muscle fibres to 3.2-3.4 microns striation spacing followed by ATP removal and release of the rigor fibre until tension fell below the relaxed level allowed investigation of the strain dependence of relaxation in the regions of negative cross-bridge strain. In the presence of 50 microM-2 mM-MgADP and either 10 mM-Pi or 20 mM-2,3-butanedione monoxime, relaxation following photolysis of caged ATP was 6- to 8-fold faster for negatively strained cross-bridges than for positively strained ones. This marked strain dependence of cross-bridge detachment is predicted from the model of A. F. Huxley (1957). 8. In the presence of Ca2+, activation of contraction following photolysis of caged ATP was slowed by inclusion of 20-500 microM-MgADP in the medium. An initial decrease in tension related to cross-bridge detachment by MgATP was markedly suppressed in the presence of MgADP. 9. Ten millimolar Pi partly suppressed active tension generation in the presence of MgADP.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics of contraction initiated by flash photolysis of caged adenosine triphosphate in tonic and phasic smooth muscles

Laser flash photolysis of caged adenosine triphosphate (ATP), in the presence of Ca2+, was used to examine the time course of isometric force development from rigor states in glycerinated tonic (rabbit trachealis) and phasic (guinea-pig ileum and portal vein) smooth muscles. Photolytic liberation of ATP from caged ATP initiated force development, at 20 degrees C, with half-time (t1/2) of 5.4 s in trachealis and 1.2-2.2 s in the phasic muscles. Prior to photolysis, some muscles were phosphorylated with ATP plus okadaic acid (an inhibitor of myosin light-chain phosphatase) or thiophosphorylated with ATP gamma S to fully activate the regulatory system, before turning on the contractile apparatus. In these prephosphorylated muscles, force development, after caged ATP photolysis, was more rapid than in the unphosphorylated muscles, but the t1/2 values for trachealis (0.8-1.1 s) were still longer than for ileum and portal-vein muscles (0.20-0.25 s). The results suggest that both the contractile machinery and the regulatory system are slower in the tonic than in the phasic smooth muscles. The time course of force development for each muscle type was sigmoidal, with an initial delay (td) of approximately 10% of the t1/2 value. Some possible chemical and mechanical origins of the delay are discussed.

Relaxation of muscle fibers with adenosine 5'-[gamma-thio]triphosphate (ATP[gamma S]) and by laser photolysis of caged ATP[gamma S]: evidence for Ca2+-dependent affinity of rapidly detaching zero-force cross-bridges

The relationship between the mechanical and biochemical states of the muscle cross-bridge cycle and the control of contraction were investigated by using the nucleotide analogs adenosine 5'-[gamma-thio]triphosphate (ATP[gamma S]) and caged ATP[gamma S] [the O-1(2-nitrophenyl)ethyl P3-ester of ATP[gamma S]]. ATP[gamma S] interacts with actomyosin in a manner similar to ATP but is hydrolyzed (by a factor of 500) more slowly. Generation of ATP[gamma S] by photolysis of caged ATP[gamma S] within a permeabilized fiber in rigor in the absence of Ca2+ relaxed tension and stiffness as occurs with ATP. The transient rise in tension prior to final relaxation observed with photolysis of caged ATP was absent with caged ATP[gamma S]. This result suggests that following detachment of a cross-bridge, ATP is normally hydrolyzed before force generation. In the presence of Ca2+, photolysis of caged ATP[gamma S] within rigor fibers caused tension to relax fully but significant stiffness remained. Stiffness also developed without concomitant tension when Ca2+ concentration was raised from less than 1 nM to 30 microM in the presence of ATP[gamma S]. The amplitude of the tension response to ramp stretches in the presence of Ca2+ and ATP[gamma S] increased with ramp stretch velocity, suggesting that the cross-bridges have detachment rate constants extending into the 10(3) s-1 range. The results provide evidence that the Ca2+-regulatory system can directly control attachment of cross-bridges into states before the power stroke.

Inositol trisphosphate, calcium and muscle contraction

The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P3 in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of 'caged' compounds. The participation of G-protein(s) in the release of intracellular Ca2+ was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca2+ in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca2+ storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca2+. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an alpha 1-agonist, is 1.5 +/- 0.26 s at 20 degrees C and 0.6 +/- 0.18 s at 30 degrees C; the latency of contraction after photolytic release of Ins(1,4,5)P3 from caged Ins(1,4,5)P3 is 0.5 +/- 0.12 s at 20 degrees C. The long latency of alpha 1-adrenergic Ca2+ release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) hydrolysis. GTP gamma S, a non-hydrolysable analogue of GTP, causes Ca2+ release and contraction in permeabilized smooth muscle. Ins(1,4,5)P3 has an additive effect during the late, but not the early, phase of GTP gamma S action, and GTP gamma S can cause Ca2+ release and contraction of permeabilized smooth muscles refractory to Ins(1,4,5)P3. These results suggest that activation of G protein(s) can release Ca2+ by, at least, two G-protein-regulated mechanisms: one mediated by Ins(1,4,5)P3 and the other Ins(1,4,5)P3-independent. The low Ins(1,4,5)P3 5-phosphatase activity and the slow time-course (seconds) of the contractile response to Ins(1,4,5)P3 released with laser flash photolysis from caged Ins(1,4,5)P3 in frog skeletal muscle suggest that Ins(1,4,5)P3 is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins(1,4,5)P3-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P3 are compatible with a physiological role of Ins(1,4,5)P3 as a messenger of pharmacomechanical coupling.

Cross-bridge kinetics, cooperativity, and negatively strained cross-bridges in vertebrate smooth muscle. A laser-flash photolysis study

The effects of laser-flash photolytic release of ATP from caged ATP [P3-1(2-nitrophenyl)ethyladenosine-5'-triphosphate] on stiffness and tension transients were studied in permeabilized guinea pig protal vein smooth muscle. During rigor, induced by removing ATP from the relaxed or contracting muscles, stiffness was greater than in relaxed muscle, and electron microscopy showed cross-bridges attached to actin filaments at an approximately 45 degree angle. In the absence of Ca2+, liberation of ATP (0.1-1 mM) into muscles in rigor caused relaxation, with kinetics indicating cooperative reattachment of some cross-bridges. Inorganic phosphate (Pi; 20 mM) accelerated relaxation. A rapid phase of force development, accompanied by a decline in stiffness and unaffected by 20 mM Pi, was observed upon liberation of ATP in muscles that were released by 0.5-1.0% just before the laser pulse. This force increment observed upon detachment suggests that the cross-bridges can bear a negative tension. The second-order rate constant for detachment of rigor cross-bridges by ATP, in the absence of Ca2+, was estimated to be 0.1-2.5 X 10(5) M-1s-1, which indicates that this reaction is too fast to limit the rate of ATP hydrolysis during physiological contractions. In the presence of Ca2+, force development occurred at a rate (0.4 s-1) similar to that of intact, electrically stimulated tissue. The rate of force development was an order of magnitude faster in muscles that had been thiophosphorylated with ATP gamma S before the photochemical liberation of ATP, which indicates that under physiological conditions, in non-thiophosphorylated muscles, light-chain phosphorylation, rather than intrinsic properties of the actomyosin cross-bridges, limits the rate of force development. The release of micromolar ATP or CTP from caged ATP or caged CTP caused force development of up to 40% of maximal active tension in the absence of Ca2+, consistent with cooperative attachment of cross-bridges. Cooperative reattachment of dephosphorylated cross-bridges may contribute to force maintenance at low energy cost and low cross-bridge cycling rates in smooth muscle.

Transient tension changes initiated by laser temperature jumps in rabbit psoas muscle fibres

1. A technique was developed to generate 2-8 degrees C step temperature perturbations (T-jumps) in single muscle fibres to study the thermodynamics of muscle contraction. A solid-state pulsed holmium laser emitting at 2.065 microns heated the fibre and surrounding solution in approximately 150 mus. The signal from a 100 microns thermocouple fed back to a heating wire maintained the elevated temperature after the laser pulse. 2. Tension of glycerol-extracted muscle fibres from rabbit psoas muscle did not change significantly following T-jumps when the fibre was relaxed. 3. In rigor, tension decreased abruptly on heating indicating normal (not rubber-like) thermoelasticity. The thermoelastic coefficient (negative ratio of relative length change to relative temperature change) of the fibre was estimated to be -0.021 at sarcomere lengths of 2.5-2.8 microns. Rigor tension was constant after the temperature step and returned to the original value on recooling. 4. In maximal Ca2+ activation, tension transients initiated by T-jumps had several phases. An immediate tension decrease suggests that thermoelasticity during contraction is similar to that in rigor. Active tension then recovered to the value before the T-jump with an apparent rate constant of approximately 400 s-1 (at 10-20 degrees C). This rate constant did not have an appreciable dependence on the final temperature. Finally, tension increased exponentially to a new higher level with a rate constant of approximately 20 s-1 at 20 degrees C. This rate constant increased with temperature with a Q10 of 1.4. 5. At submaximal Ca2+ activation the tension rise was followed by a decay to below the value before the T-jump. This decline was expected from the temperature dependence of steady pCa-tension curves. The final tension decline occurred on the 1-5 s time scale. 6. The value and amplitude dependence of the rate constant for the quick recovery following T-jumps were similar to those of the quick recovery following length steps during active contractions. The enthalpy change associated with the quick tension recovery following temperature-step perturbations was estimated to be positive suggesting that the recovery process is an endothermic reaction. Slower reaction steps on the 10-30 ms timescale, as well as reactions corresponding to the quick recovery, may contribute to the cross-bridge power stroke.

Measurement of sarcomere shortening in skinned fibers from frog muscle by white light diffraction

A new optical-electronic method has been developed to detect striation spacing of single muscle fibers. The technique avoids Bragg-angle and interference-fringe effects associated with laser light diffraction by using polychromatic (white) light. The light is diffracted once by an acousto-optical device and then diffracted again by the muscle fiber. The double diffraction reverses the chromatic dispersion normally obtained with polychromatic light. In frog skinned muscle fibers, active and passive sarcomere shortening were smooth when observed by white light diffraction, whereas steps and pauses occurred in the striation spacing signals obtained with laser illumination. During active contractions skinned fibers shortened at high rates (3-5 microns/s per half sarcomere, 0-5 degrees C) at loads below 5% of isometric tension. Compression of the myofibrillar lateral filament spacing using osmotic agents reduced the shortening velocity at low loads. A hypothesis is presented that high shortening velocities are observed with skinned muscle fibers because the cross-bridges cannot support compressive loads when the filament lattice is swollen.

Kinetics of smooth and skeletal muscle activation by laser pulse photolysis of caged inositol 1,4,5-trisphosphate

Inositol 1,4,5-trisphosphate (InsP3) can stimulate skinned smooth and skeletal muscle to contract by initiating Ca2+ release from the sarcoplasmic reticulum. Whether this process is an integral component of the in vivo muscle activation mechanism was tested by releasing InsP3 rapidly within skinned muscle fibers of rabbit main pulmonary artery and frog semitendinosus. InsP3 was liberated on laser pulse photolysis of a photolabile but biologically inactive precursor of InsP3 termed caged InsP3. Caged InsP3 is a mixture of compounds in which InsP3 is esterified with 1(2-nitrophenyl)diazoethane (probably at the P4- or P5-position). Photochemical release of InsP3 induced a full contraction in both muscles at physiological free Mg2+ concentrations, but only in the smooth muscle were the InsP3 concentration (0.5 microM) and the activation rate compatible with the in vivo physiological response. Endogenous InsP3-specific phosphatase activity was present in smooth muscle and had about 35-fold greater activity than that in the skeletal-muscle preparation. Caged InsP3 was not susceptible to phosphatases in either preparation.

Kinetics of the actomyosin ATPase in muscle fibers

Many characteristics expected from the cyclic ATPase mechanism of Scheme 1 are apparent in reactions measured directly in muscle fibers. ATP detaches rigor cross-bridges rapidly. Reattachment and force generation are also rapid compared to the overall cycling rate, but reversibility of many of the reactions allows significant population of detached states during contraction. ATP hydrolysis shows rapid, "burst" kinetics and is also readily reversible. Pi is released before ADP in the cycle. Pi release is slow in relaxed fibers but is promoted by the interaction between myosin and actin during contraction. Actomyosin kinetics differ in fibers from the ATPase reaction in solution in that Pi binds more readily to AM' X ADP in fibers, and complex, Ca2+-dependent kinetics are evident for ADP release. These properties suggest that the mechanical driving stroke of the cross-bridge cycle and events during physiological relaxation are closely linked to the product release steps. All of the reactions, except step 7a, in the main pathway for ATP hydrolysis, indicated in Scheme 1 by heavy arrows, are fast compared to the overall cycling rate in isometric contractions. Based on this finding, we expect step 7a (or isomerizations of the flanking states) to be relatively slow (approximately 3 s-1). But neither the rate-limiting reaction, nor the expected major dependence on mechanical load or shortening that would explain the Fenn effect, have actually been detected. Use of the pulse photolysis and oxygen exchange methods with structural and spectroscopic techniques and with perturbations of mechanical strain promise to reveal these aspects of the mechanism.

Oxygen exchange between Pi in the medium and water during ATP hydrolysis mediated by skinned fibers from rabbit skeletal muscle. Evidence for Pi binding to a force-generating state

Oxygen exchange between (18O4)Pi in the medium and water accompanies ATP hydrolysis catalyzed by the calcium-regulated MgATPase of vertebrate skeletal muscle. Exchange was observed in chemically skinned fibers from rabbit psoas muscle held isometrically and activated by 30 microM free Ca2+. The rate of exchange was approximately proportional to Pi concentration (up to 10 mM) and was characterized by an apparent second order rate constant greater than or equal to 475 M-1 S-1 (pH 7.1, ionic strength 0.2 M, 22 degrees C). Much less exchange occurred in the absence of Ca2+ or when ATP was replaced by ADP. It has been inferred from mechanical experiments that Pi can bind to a force-generating ADP-bound state of actomyosin with resultant suppression of force (Hibberd, M. G., Dantzig, J. A., Trentham, D. R., and Goldman, Y. E. (1985) Science 228, 1317-1319). The oxygen exchange results support this inference by providing direct evidence that Pi in the medium binds at the ATPase catalytic site in activated isometric fibers. The inter-relationship of these two effects involving Pi on mechanochemical coupling in muscle is discussed.

The stiffness of frog skinned muscle fibres at altered lateral filament spacing

When the surface membrane is removed from a frog muscle fibre the myofibrils swell, so that the spacing between the filaments increases by 10-30%. In this study, the stiffness of skinned fibres was measured when the lateral spacing of the filament lattice was reduced osmotically using polyvinyl pyrrolidone (PVP), a synthetic linear polymer. In the absence of PVP, the apparent stiffness of relaxed skinned fibres measured during ramp stretches was about 0.05-0.1 of the stiffness of intact fibres. The stiffness increased when the lattice spacing was decreased. The mechanical characteristics of resting stiffness in the presence of PVP (or bovine serum albumin) were similar to those of the short-range elastic component of resting intact fibres. However, when the spacing of skinned fibres was reduced osmotically to that of the intact lattice, the stiffness of skinned fibres was about 1.6 times higher. In the absence of PVP, Ca2+-activated skinned fibres were less stiff than are fully active intact fibres during isometric tetani. The skinned fibre force-extension relation was markedly curved. As the filament spacing was reduced, the stiffness of the Ca2+-activated skinned fibres increased, with little change in isometric tension, and the force-extension curve became more linear. Experiments at varied filament overlap and during feed-back control of sarcomere length, monitored by laser diffraction, showed that PVP increased the stiffness of the sarcomeres. The increase of active stiffness in the presence of PVP could be partially dissociated from the increase of resting stiffness by inclusion of Mg tripolyphosphate in the bathing solutions. It is concluded that the low stiffness and the non-linearity of the force-extension curve observed in fully activated skinned fibres are due primarily to the increase of filament separation that occurs when a fibre is skinned. The mechanism may be related to an increased angle between the subfragment-2 part of the cross-bridge and the backbone of the thick filament, perhaps leading to buckling of cross-bridges under compression.

Fine structure in near-field and far-field laser diffraction patterns from skeletal muscle fibers

Regions of muscle fibers that are many sarcomeres in length and uniform with regard to striation spacing, curvature, and tilt have been observed by light microscopy. We have investigated the possibility that these sarcomere domains can explain the fine structure in optical diffraction patterns of skeletal muscle fibers. We studied near-field and far-field diffraction patterns with respect to fiber translation and to masking of the laser beam. The position of diffracted light in the near-field pattern depends on sarcomere length and position of the diffracting regions within the laser beam. When a muscle fiber was translated longitudinally through a fixed laser beam, the fine structural lines in the near-field diffraction pattern moved in the same direction and by the same amount as the fiber movement. Translation of the muscle fiber did not result in fine structure movement in the far-field pattern. As the laser beam was incrementally masked from one side, some fine structural lines in both the near-field and far-field diffraction patterns changed in intensity while others remained the same. Eventually, all the fine structural lines broadened and decreased in intensity. Often a fine structural line increased in intensity or a dark area in the diffraction pattern became brighter as the laser beam was restricted. From these results we conclude that the fine structure in the laser diffraction pattern is due to localized and relatively uniform regions of sarcomeres (domains) and to cross interference among light rays scattered by different domains.