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At least 19 recordsLinked to original sources
PubMed1990

Optical properties of myofibril and actomyosin suspensions. 1. Angular dependence of light scattering by myofibril suspensions and its changes under myofibril contraction.

The angular dependence of light scattering by myofibril suspensions has been measured within the angle range of 0.05 degrees and 160 degrees at different suspension concentrations and wavelengths. Angular diagrams appear to be highly asymmetry, and extended forward in the direction of the primary light propagation. A suspension with the concentration of 0.1 mg/ml at 550 nm scatters, for example, no less than 90% light at angles of less than 6 degrees. The scattering diagram becomes less asymmetric upon increasing the suspension concentration and decreasing the wavelength of the light. Upon addition of MgATP in the presence of Ca2+, a myofibril suspension undergoes two kinds of optical change, namely, fast ones corresponding to myofibril contraction, and slow ones correlating with the subsequent aggregation of the contracted myofibrils. Fast changes are believed to be due to the increase of the refractive index of the contracting myofibrils, and slow changes to the increase in the size of the suspension particles resulting from aggregation. Light scattering by myofibril and actomyosin suspensions seems to be due not only to the particle as a whole, but also to its internal structure.

Actomyosin↗
PubMed1994

Ca(2+)- and Sr(2+)-sensitive ATPase activity of slow skeletal myofibrils in comparison with fast skeletal and cardiac myofibrils.

Ca(2+)- and Sr(2+)-sensitive ATPase of myofibrillar preparations from slow skeletal (chicken anterior latissimus dorsi), fast skeletal (chicken breast) and porcine cardiac ventricular muscles was investigated. Sr2+ at concentrations about 6 times higher than Ca2+ was required for activating the ATPase of slow skeletal or cardiac myofibrils, while Sr2+ about 30 times higher than Ca2+ was required for activating fast skeletal myofibrils. The cooperativity of activation of slow skeletal myofibrils was almost the same as that of cardiac myofibrils, but lower than that of fast skeletal myofibrils. Sr2+/Ca2+ sensitivity ratio of the desensitized (troponin-tropomyosin-depleted) fast skeletal myofibrils reconstituted with troponin-tropomyosin from slow skeletal, fast skeletal, and cardiac muscles were 13, 34, and 5, respectively. The troponin C-depleted fast skeletal myofibrils showed the same Sr2+/Ca2+ sensitivity ratio and the same cooperativity of activation as those of the intact fast skeletal myofibrils when reconstituted with fast skeletal troponin C, and as those of the intact slow skeletal myofibrils when reconstituted with slow skeletal troponin C.

Adenosine Triphosphatases↗
PubMed1992

Early steps of the Mg(2+)-ATPase of relaxed myofibrils. A comparison with Ca(2+)-activated myofibrils and myosin subfragment 1.

The early steps of the Mg(2+)-ATPase activity of relaxed rabbit psoas myofibrils were studied in a buffer of near-physiological ionic strength at 4 degrees C by the rapid flow quench technique. The initial ATP binding steps were studied by the ATP chase, and the cleavage and release of product steps by the Pi burst method. The data obtained were interpreted by [formula: see text] where M represents the myosin heads with or without actin interaction. This work is a continuation of our study on Ca(2+)-activated myofibrils [Houadjeto, M., Travers, F., & Barman, T. (1992) Biochemistry 31, 1564-1569]. Here the constants obtained with relaxed myofibrils were compared with those with activated myofibrils and myosin subfragment 1 (S1). We find that whereas Ca2+ increases 80X the release of products (k4), it has little effect upon the kinetics of the initial binding and cleavage steps. As with activated myofibrils and S1, the second-order binding constant for ATP (k2/K1) was about 1 microM-1 s-1 and the ATP was bound very tightly. With activated myofibrils, it was difficult to obtain an estimate for the koff for ATP(k-2) but it is much less than kcat. Here with relaxed myofibrils we estimate k-2 less than 8 x 10(-4) s-1, which is considerably smaller than kcat (0.019 s-1) and also previous estimates for this constant. The overall Kd for ATP to relaxed myofibrils is less than 8 x 10(-10) M. With S1 this Kd is about 10(-11) M.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1987

Irradiations of rabbit myofibrils with an ultraviolet microbeam. I. Effects of ultraviolet light on the myofibril components necessary for contraction.

Glycerinated rabbit psoas myofibrils, F-actin, and myofibril ghosts were irradiated with ultraviolet light (UV) to investigate how UV blocks myofibril contraction. Myofibril contraction is most sensitive to 270- and 290-nm wavelength light. We irradiated I and A bands separately with 270- and 290-nm wavelength light using a UV microbeam and constructed dose-response curves for blocking sarcomere contraction. For both wavelengths, irradiations of A bands required less energy per area to block contraction than did irradiations of I bands, suggesting that the primary effects of both 270- and 290-nm wavelength light in stopping myofibril contraction are on myosin. We investigated whether the primary effect of UV in blocking I-band contraction is the depolymerization of actin by comparing the relative sensitivities of I-band contraction, F-actin depolymerization, and thin filament depolymerization to 270- and 290-nm light. We also compared the dose of UV required to depolymerize F-actin in solution with the dose needed to block I-band contraction and the dose required to alter thin filament structure in myofibril ghosts. The results confirm that UV blocks I-band contraction by depolymerizing actin. We discuss how the results might be relevant to the hypothesis that an actomyosin-based system is involved in chromosome movement.

Actins↗
PubMed1987

Irradiations of rabbit myofibrils with an ultraviolet microbeam. II. Phalloidin protects actin in solution but not in myofibrils from depolymerization by ultraviolet light.

We tested whether phalloidin protects actin in myofibrils from depolymerization by ultraviolet light (UV). I bands in glycerinated rabbit psoas myofibrils were irradiated with a UV microbeam in the presence and absence of phalloidin. We used the retention of contractility of the irradiated I band as the assay for protection of actin by phalloidin, since previous experiments indicated that UV blocks contraction of an irradiated I band by depolymerizing the thin filaments. The I bands of myofibrils incubated in phalloidin were as sensitive to UV as control I bands, indicating that phalloidin did not protect the thin filaments. However, phalloidin did protect F-actin in solution from depolymerization by UV. This apparent contradiction between F-actin in myofibrils and F-actin in solution was resolved by observing unirradiated myofibrils that were stained with rhodamine-phalloidin. It was found that phalloidin does not bind uniformly to the thin filaments, though as the fluorescence image is observed over time the staining pattern changes until it does appear to bind uniformly. We conclude that phalloidin does not protect F-actin in myofibrils from depolymerization by UV because it does not bind uniformly to the filaments.

Actins↗
PubMedDate not supplied

[Aggregation of isolated myofibrils stimulated by their contraction. I. Origin of the second phase of optical changes during myofibril contraction].

Aggregation of contracted myofibrils was shown to be responsible for spontaneous: slow reduction of optical density of myofibril suspension on the final stage of their contraction. This effect faded with an increase in light wavelength, with increased angle of view of the photocell and diminished sizes of myofibrils.

Actomyosin↗
PubMed2002

Myofibril-bound serine protease and its endogenous inhibitor in mouse: extraction, partial characterization and effect on myofibrils.

The protein content of muscle is determined by the relative rates of synthesis and degradation. The balance between this process determines the number of functional contractile units within each muscle cell. Myofibril-bound protease, protease M previously reported in mouse skeletal muscle could be solubilized from the myofibrillar fraction by salt and acid treatment and partially purified by Mono Q and Superose 12 chromatography. Isolated protease M activity in vitro on whole myofibrils resulted in myosin, actin, troponin T, alpha-actinin and tropomyosin degradation. Protease M is serine type and was able to hydrolyze trypsin-type synthetic substrates but not those of chymotrypsin type. In gel filtration chromatography, protease M showed Mr 120.0 kDa. The endogenous inhibitor (MHPI) is a glycoprotein (110.0 kDa) that efficiently blocks the protease M-dependent proteolysis of myofibrillar proteins in a dose-dependent way, as shown by electrophoretic analysis and synthetic substrates assays. Protease M-Inhibitor system would be implicated in myofibrillar proteins turnover.

Animals↗
PubMed1976

Modulation of Ca2+ control of dog and rabbit cardiac myofibrils by Mg2+. Comparison with rabbit skeletal myofibrils.

Increases in free Mg2+ from 0.04 to 10.0 mM with constant pH 7.0 TO 0.10 M ionic strength, and 2 mM MgATP2- caused a rightward shift of the free Ca-relative ATPase relation for both cardiac skeletal myofibrils. The specific activity of cardiac myofibrillar ATPase over a wide range of free Ca2+ was, however, depressed in 0.04 vs. 1.0 mM Mg2+, whereas a similar decrease in free Mg2+ slightly enhanced skeletal myofibrillar ATPase. Lowering free Mg2+ from 1.0 to 0.04 mM caused similar increases in cardiac and skeletal myofibrillar bound calcium, which were largely attributable to increased calcium binding to myofibrillar myosin. Raising free Mg2+ from 1.0 to 10.0 mM caused only a slight decrease of skeletal myofibrillar bound calcium, and this change was attributable to myofibrillar myosin. The same increase in free Mg2+ caused cardiac myofibrils to bind increased amounts of calcium and this change was not attributable to myofibrillar myosin. By subtracting calcium bound to myofibrillar myosin, we were able to estimate calcium binding by myofibrillar troponin. The transition between basal and maximal ATPase in 1.0 and 10 mM Mg2+ was found to be assocciated with binding of an additional 2 mol/mol of either skeletal or cardiac myofibrillar troponin.

Adenosine Triphosphatases↗
PubMed2004

Evidence for myofibril remodeling as opposed to myofibril damage in human muscles with DOMS: an ultrastructural and immunoelectron microscopic study.

The myofibrillar and cytoskeletal alterations observed in delayed onset muscle soreness (DOMS) caused by eccentric exercise are generally considered to represent damage. By contrast our recent immunohistochemical studies suggested that the alterations reflect myofibrillar remodeling (Yu and Thornell 2002; Yu et al. 2003). In the present study the same human muscle biopsies were further analyzed with transmission electron microscopy and immunoelectron microscopy. We show that the ultrastructural hallmarks of DOMS, Z-disc streaming, Z-disc smearing, and Z-disc disruption were present in the biopsies and were significantly more frequent in biopsies taken 2-3 days and 7-8 days after exercise than in those from controls and 1 h after exercise. Four main types of changes were observed: amorphous widened Z-discs, amorphous sarcomeres, double Z-discs, and supernumerary sarcomeres. We confirm by immunoelectron microscopy that the main Z-disc protein alpha-actinin is not present in Z-disc alterations or in the links of electron-dense material between Z-discs in longitudinal register. These alterations were related to an increase of F-actin and desmin, where F-actin was present within the strands of amorphous material. Desmin, on the other hand, was seen in less dense regions of the alterations. Our results strongly support that the myofibrillar and cytoskeletal alterations, considered to be the hallmarks of DOMS, reflect an adaptive remodeling of the myofibrils.

Actinin↗
PubMed1990

Localization of anti-clathrin antibody in the sarcomere and sensitivity of myofibril structure to chloroquine suggest a role for clathrin in myofibril assembly.

Immunofluorescence microscopy has been used to demonstrate that X22, a monoclonal antibody specific for clathrin heavy chain, localizes in repetitive bands that appear soon after the fusion of skeletal myoblasts into multinucleate fibers. This organization has been found in cultures containing myotubes that develop in vitro from explants of newborn rat hindlimb cells and in myotubes derived from the L8E63 myogenic line. Bands were also prominent in skinned fibers prepared from adult rat soleus muscle and in cardiac myocytes grown in vitro from 4-day heart ventricles. Immunofluorescence banding was localized in the sarcomere as a doublet, with one element on either side of the Z line. Evidence that supports the conclusion that the reaction with X22 antibody is specific and indicative of the localization of clathrin in the sarcomere includes: (1) Identical titration of X22 antibody reactivity with the determinant in coated vesicles and in the sarcomere. (2) Conditions (eg., pH and Tris) that disrupt clathrin baskets or prevent its assembly likewise disrupt the localization of X22 in bands. (3) Chloroquine inhibits both the normal trafficking of clathrin in the cell and X22 banding in the sarcomere. (4) Immunoblot analysis of myotube lysates reveals a single band with an electrophoretic mobility identical to the 180,000-Da clathrin heavy chain. (5) The assembly of clathrin into sarcomeric bands occurs early in the development of the myofibrillar apparatus. Quantitation of the appearance of X22 banding in primary cultures of myotubes indicates that it precedes that of other myofibrillar proteins and that assembly takes place in the following order: X22, titin, myosin heavy chain, actin, and desmin. The assembly of myosin, titin, and actin into sarcomeric bands, as well as X22, is inhibited by chloroquine. Upon prolonged exposure to chloroquine previously assembled proteins are drastically reduced or no longer evident in the sarcomere. On the basis of these results and considering the role of clathrin in intracellular transport and its capacity to interact with actin and alpha-actinin, we suggest that clathrin may have diverse roles in the assembly, integrity, and functioning of the sarcomere and its integration with the sarcolemma. The early organization of X22 into bands further suggests that clathrin may also function early in the assembly of the contractile system.

Actins↗
PubMed1997

Active and passive forces of isolated myofibrils from cardiac and fast skeletal muscle of the frog.

1. Force measurements in isolated myofibrils (15 degrees C; sarcomere length, 2.10 microns) were used in this study to determine whether sarcomeric proteins are responsible for the large differences in the amounts of active and passive tension of cardiac versus skeletal muscle. Single myofibrils and bundles of two to four myofibrils were prepared from glycerinated tibialis anterior and sartorius muscles of the frog. Skinned frog atrial myocytes were used as a model for cardiac myofibrils. 2. Electron microscope analysis of the preparations showed that: (i) frog atrial myocytes contained a small and variable number of individual myofibrils (from 1 to 7); (ii) the mean cross-sectional area and mean number of myosin filaments of individual cardiac myofibrils did not differ significantly from those of single skeletal myofibrils; and (iii) the total myofibril cross-sectional area of atrial myocytes was on average comparable to that of bundles of two to four skeletal myofibrils. 3. In maximally activated skeletal preparations, values of active force ranged from 0.45 +/- 0.03 microN for the single myofibrils (mean +/- S.E.M.; n = 16) to 1.44 +/- 0.24 microN for the bundles of two to four myofibrils (n = 9). Maximum active force values of forty-five cardiac myocytes averaged 1.47 +/- 0.10 microN and exhibited a non-continuous distribution with peaks at intervals of about 0.5 microN. The results suggest that variation in active force among cardiac preparations mainly reflects variability in the number of myofibrils inside the myocytes and that individual cardiac myofibrils develop the same average amount of force as single skeletal myofibrils. 4. The mean sarcomere length-resting force relation of atrial myocytes could be superimposed on that of bundles of two to four skeletal myofibrils. This suggests that, for any given amount of strain, individual cardiac and skeletal sarcomeres bear essentially the same passive force. 5. The length-passive tension data of all preparations could be fitted by an exponential equation. Equation parameters obtained for both types of myofibrils were in reasonable agreement with those reported for larger preparations of frog skeletal muscle but were very different from those estimated for multicellular frog atrial preparations. It is concluded that myofibrils are the major determinant of resting tension in skeletal muscle; structures other than the myofibrils are responsible for the high passive stiffness of frog cardiac muscle.

Animals↗
PubMed2001

Properties of myofibril-bound calpain activity in longissimus muscle of callipyge and normal sheep.

Properties of the calpain bound to myofibrils in longissimus muscle from callipyge or noncallipyge sheep were examined after 0, 1, 3, and 10 d of postmortem storage at 4 degrees C. Western analysis has shown that most of this calpain is mu-calpain, although the sensitivity of the antibodies used in the earlier studies could not eliminate the possibility that up to 10% of the calpain was m-calpain. The calpain is bound tightly, and very little is removed by washing with the detergent Triton X-100; hence, it is not bound to phospholipids in the myofibril. Over 25% of total mu-calpain was bound to myofibrils from at-death muscle, and this increased to approximately 40% after 1 d postmortem. The amount of myofibril-bound mu-calpain increased only slightly between 1 and 10 d of postmortem storage. The percentage of autolyzed mu-calpain increases with time postmortem until after 10 d postmortem, when all myofibril-bound mu-calpain is autolyzed. The specific activity of the myofibril-bound calpain is very low and is only 6 to 13% as high as the specific activity of extractable mu-calpain from the same muscle. It is unclear whether this low specific activity is the result of unavailability of the active site of the myofibril-bound calpain to exogenous substrate. The myofibril-bound calpain degrades desmin, nebulin, titin, and troponin T in the myofibrils, and also releases undegraded alpha-actinin and undergoes additional autolysis when incubated with Ca2+; all these activities occurred slowly considering the amount of myofibril-bound calpain. Activity of the myofibril-bound calpain was partly (58 to 67%) inhibited by the calpain inhibitors, E-64 and iodoacetate; was more effectively inhibited by a broader-based protease inhibitor, leupeptin (84 to 89%); and was poorly inhibited (43 to 45%) by calpastatin. Release of undegraded alpha-actinin and autolysis are properties specific to the calpains, and it is unclear whether some of the myofibril-bound proteolytic activity originates from proteases other than the calpains or whether the active site of myofibril-bound calpain is shielded from the inhibitors. Activities and properties of the myofibril-bound calpain were identical in longissimus muscle from callipyge and normal sheep, although previous studies had indicated that the "normal" longissimus was much more tender than the callipyge longissimus. Hence, it seems unlikely that the myofibril-bound calpain has a significant role in postmortem tenderization of ovine longissimus.

Animals↗
PubMed1988

Effect of substitution of troponin C in cardiac myofibrils with skeletal troponin C or calmodulin on the Ca2+- and Sr2+-sensitive ATPase activity.

Troponin C was removed almost completely from the porcine cardiac myofibrils by the same extraction procedure using CDTA as that previously reported for the rabbit skeletal myofibrils (Morimoto, S. & Ohtsuki, I. (1987) J. Biochem. 101, 291-301), and the effects of substitution of troponin C in cardiac myofibrils with rabbit skeletal troponin C or bovine brain calmodulin were examined. While the ATPase activity of intact cardiac myofibrils or cardiac troponin C-reconstituted cardiac myofibrils was activated at only a little higher concentration of Sr2+ than Ca2+, the skeletal troponin C-substituted cardiac myofibrils, as well as intact rabbit skeletal myofibrils, required more than 10 times higher concentration of Sr2+ than Ca2+ for activation of the myofibrillar ATPase activity. However, the concentrations of Ca2+ and Sr2+ required for the activation of the ATPase activity of the skeletal troponin C-substituted cardiac myofibrils were both about 5 times higher than those of intact skeletal myofibrils. The skeletal troponin C-substituted cardiac myofibrils, as well as intact skeletal myofibrils, also showed higher cooperativity in the Ca2+-activation of the ATPase activity than intact or cardiac troponin C-reconstituted cardiac myofibrils. The ATPase activity of calmodulin-substituted cardiac myofibrils was activated at a several times lower concentration of Ca2+ or Sr2+ than that of calmodulin-substituted skeletal myofibrils, while the ratios of the concentration of Sr2+ to Ca2+ required for activation were almost the same in both cases.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1997

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.

Actin Cytoskeleton↗
PubMed1991

Post-translational incorporation of actin into myofibrils in vitro: evidence for isoform specificity.

The incorporation of actin into myofibrils has been examined in a cell-free system [Bouché et al.: Journal of Cell Biology 107:587-596, 1988; Goldfine et al.: Cellular and Molecular Biology of Muscle Development, 1989]. Actin was translated in a reticulocyte lysate in the presence of 35S-methionine (35S-actin) or purified from muscle and labeled with fluorescein-5-isothiocyanate (FITC-actin). Myofibrils were incubated with either 35S-actin or FITC-actin and then analyzed by gel electrophoresis or fluorescence microscopy. When myofibrils were incubated with FITC-actin monomer in the reticulocyte lysate buffer, strong fluorescent labeling was observed in Z-band regions and less so in I-bands. No fluorescence was detected in non-overlap regions of A-bands. Confocal microscopic analysis of these myofibrils indicated that FITC-actin was distributed evenly across the diameter of the myofibrils. These observations suggest that actin incorporation in the reticulocyte lysate buffer occurred at sites in the sarcomere which contain actin. In contrast, FITC-actin showed a variety of non-physiological incorporation patterns when incubated with myofibrils in the presence of an isotonic buffer (I-buffer). However, when ATP was added to I-buffer, FITC-actin showed a pattern of incorporation into myofibrils similar to that seen in the reticulocyte lysate buffer. Immunoblots indicated that actin of native size was released from myofibrils during incubation in the reticulocyte lysate buffer. No actin release was detected when the myofibrils were incubated in I-buffer lacking ATP. We used this system to compare the incorporation of actin isoforms into myofibrils. Both alpha- and beta-actins exhibited incorporation into the myofibrils but there was a three-fold greater incorporation of the alpha isoform. We propose that the differential affinities of actin isoforms for myofibrils and other cytoskeletal structures could provide a mechanism for actin isoform targeting within the cytoplasm.

Actins↗
PubMed1989

Polygons and adhesion plaques and the disassembly and assembly of myofibrils in cardiac myocytes.

Successive stages in the disassembly of myofibrils and the subsequent assembly of new myofibrils have been studied in cultures of dissociated chick cardiac myocytes. The myofibrils in trypsinized and dispersed myocytes are sequentially disassembled during the first 3 d of culture. They split longitudinally and then assemble into transitory polygons. Multiples of single sarcomeres, the cardiac polygons, are analogous to the transitory polygonal configurations assumed by stress fibers in spreading fibroblasts. They differ from their counterparts in fibroblasts in that they consist of muscle alpha-actinin vertices and muscle myosin heavy chain struts, rather than of the nonmuscle contractile protein isoforms of stress fiber polygons. EM sections reveal the vertices and struts in cardiac polygons to be typical Z and A bands. Most cardiac polygons are eliminated by day 5 of culture. Concurrent with the disassembly and elimination of the original myofibrils new myofibrils are rapidly assembled elsewhere in the same myocyte. Without exception both distal tips of each nascent myofibril terminate in adhesion plaques. The morphology and composition of the adhesion plaques capping each end of each myofibril are similar to those of the termini of stress fibers in fibroblasts. However, whereas the adhesion complexes involving stress fibers in fibroblasts consist of vinculin/nonmuscle alpha-actinin/beta- and gamma-actins, the analogous structures in myocytes involving myofibrils consist of vinculin/muscle alpha-actinin/alpha-actin. The addition of 1.7-2.0 microns sarcomeres to the distal tips of an elongating myofibril, irrespective of whether the myofibril consists of 1, 10, or several hundred tandem sarcomeres, occurs while the myofibril appears to remain linked to its respective adhesion plaques. The adhesion plaques in vitro are the equivalent of the in vivo intercalated discs, both in terms of their molecular composition and with respect to their functioning as initiating sites for the assembly of new sarcomeres. How 1.7-2.0 microns nascent sarcomeres can be added distally during elongation while the tips of the myofibrils remain inserted into submembranous adhesion plaques is unknown.

Actinin↗
PubMed1998

Calcium dependence of the apparent rate of force generation in single striated muscle myofibrils activated by rapid solution changes.

Single myofibrils or small groups of myofibrils were isolated from different types of striated muscle: rabbit psoas, frog tibialis anterior, frog atrial and ventricular muscle. The Ca2+ concentration of the solution perfusing the myofibrils was changed within few milliseconds by translating the interface between two flowing streams of solution across the preparations. In all types of myofibrils tested, the time course of force rise in response to maximal activation (pCa 4.75) was approximately monoexponential and nearly superimposable on that observed after a release-restretch protocol applied to the myofibril at the plateau of maximal contractions. This suggests that the kinetics of force development following rapid myofibril activation essentially reflects the kinetics of interaction between contractile proteins. The half time of force rise in response to maximal activation varied among different myofibril types; it was shortest in frog tibialis anterior myofibrils and longest in frog ventricular myofibrils. In all types of myofibril preparations tested the half time of force rise increased with decreasing Ca2+ levels in the activating solution. The finding provides support for a kinetic mechanism of force regulation by Ca2+ in all types of striated muscle. The extent of this Ca2+ effect, however, varied among the different myofibril preparations tested; at 15 degrees C for instance, it was smaller in frog tibialis anterior myofibrils than in the other preparations.

Animals↗