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• A Non-Cross-Bridge Stiffness in Activated Frog Muscle Fibers...

  A Non-Cross-Bridge Stiffness in Activated Frog Muscle Fibers Biophysical Journal, Volume 82, Issue 6, 1 June 2002, Pages 3118-3127 Maria A. Bagni, Giovanni Cecchi, Barbara Colombini and Francesco Colomo Abstract Force responses to fast ramp stretches of various amplitude and velocity, applied during tetanic contractions, were measured in single intact fibers from frog tibialis anterior muscle. Experiments were performed at 14°C at ∼2.1m sarcomere length on fibers bathed in Ringer’s solution containing various concentrations of 2,3-butanedione monoxime (BDM) to greatly reduce the isometric tension. The fast tension transient produced by the stretch was followed by a period, lasting until relaxation, during which the tension remained constant to a value that greatly exceeded the isometric tension. The excess of tension was termed “static tension,” and the ratio between the force and the accompanying sarcomere length change was termed “static stiffness.” The static stiffness was independent of the active tension developed by the fiber, and independent of stretch amplitude and stretching velocity in the whole range tested; it increased with sarcomere length in the range 2.1–2.8m, to decrease again at longer lengths. Static stiffness increased well ahead of tension during the tetanus rise, and fell ahead of tension during relaxation. These results suggest that activation increased the stiffness of some sarcomeric structure(s) outside the cross-bridges. Abstract | Full Text | PDF (133 kb)

• Effects of the Number of Actin-Bound S1 and Axial Force on X...

  Effects of the Number of Actin-Bound S1 and Axial Force on X-Ray Patterns of Intact Skeletal Muscle Biophysical Journal, Volume 90, Issue 3, 1 February 2006, Pages 975-984 P.J. Griffiths, M.A. Bagni, B. Colombini, H. Amenitsch, S. Bernstorff, S. Funari, C.C. Ashley and G. Cecchi Abstract Effects of the number of actin-bound S1 and of axial tension on x-ray patterns from tetanized, intact skeletal muscle fibers were investigated. The muscle relaxant, BDM, reduced tetanic M3 meridional x-ray reflection intensity (), M3 spacing (), and the equatorial / ratio in a manner consistent with a reduction in the fraction of S1 bound to actin rather than by generation of low-force S1-actin isomers. At complete force suppression, was 78% of its relaxed value. BDM distorted dynamic responses to sinusoidal length oscillations in a manner consistent with an increased cross-bridge contribution to total sarcomere compliance, rather than a changed S1 lever orientation in BDM. When the number of actin-bound S1 was varied by altering myofilament overlap, tetanic at low overlap was similar to that in high [BDM] (79% of relaxed ). Tetanic dependence on active tension in overlap experiments differed from that observed with BDM. At high BDM, tetanic approached its relaxed value (14.34nm), whereas tetanic at low overlap was 14.50nm, close to its value at full overlap (14.56nm). This difference in tetanic behavior was explicable by a nonlinear thick filament compliance which is extended by both active and passive tension. Abstract | Full Text | PDF (176 kb)

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Abstract

Using flash photolysis of caged ATP in skinned muscle fibers from rat psoas, we examined the inhibitory effects of 2,3-butanedione monoxime (BDM) on the contraction kinetics and the rate of ATP hydrolysis of the cross-bridges at approximately 10 degrees C. The hydrolysis rate was estimated from the stiffness records. The effects of BDM were compared with those of orthophosphate (P(i)) and of reduction in [Ca2+] (low Ca2+), and it was found that i) BDM and low Ca2+ inhibited ATPase activity to the same extent as they inhibited the steady tension, whereas P(i) inhibited ATPase activity much less than tension; ii) BDM and P(i) decreased tension per stiffness during the steady contraction more than did low Ca2+; iii) neither BDM nor low Ca2+ affected the initial relaxation of the fiber on release of ATP, but P(i) slightly slowed it; and iv) BDM hardly influenced the rate of contraction development after relaxation, although P(i) and low Ca2+ accelerated it. We concluded that BDM inhibits the Ca(2+)-regulated attachment of the cross-bridges and force-generation of the attached cross-bridges.