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First published online May 5, 2005
Journal of Experimental Biology 208, 1905-1913 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01573

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Contractile properties of mouse single muscle fibers, a comparison with amphibian muscle fibers

K. A. P. Edman

Department of Physiological Sciences, Biomedical Centre, F11, University of Lund, S-221 84 Lund, Sweden

View larger version (9K): [in a new window]   Fig. 1. Example records of fused isometric tetanus of an isolated muscle fiber of the mouse. (A) Standard `isometric' recording (fixed fiber ends). (B) Force production during length clamp of a fiber segment. Upper trace in each panel shows force; lower trace, segment length recording with sarcomere spacing indicated. A downward deflexion of segment length signal indicates shortening. An initial transient at the onset of the length-clamp manoeuvre in B caused the early tension step. Note, however, that the time required to reach plateau force is quite similar during contraction with fixed fiber ends and during segment length-clamp recording. The intrinsic shortening during the standard isometric recording in A was 5.0% of fibre length. Temperature, 23.5°C.

View larger version (12K): [in a new window]   Fig. 2. Load-clamp recordings at four different force levels during tetanus of a mouse single muscle fiber. (A-D) In each set of records traces represent (from the top): fiber length (puller signal), tetanic force and base line of force. Calibrations of time and force are the same in A-D. A downward deflection of puller signal (FL = fiber length) denotes shortening. Note the different calibrations of puller signal in A-D. The resting fiber length was 0.58 mm. Temperature, 23.5°C.

View larger version (13K): [in a new window]   Fig. 3. Biphasic force-velocity relationship. Force-velocity data collected from a mouse isolated muscle fiber are illustrated in a conventional diagram in (A) and as a semilogarithmic plot in (B). Data in A fitted by Eqn 1 (see Materials and methods) using the following numerical values for constants: k1=24.08 ; k2=0.85; a/P0*=0.27. Dashed line in A indicates a continuation of the hyperbola derived at low and intermediate loads. The straight lines in B are linear regressions of shortening velocity upon force on either side of the breakpoint of the force-velocity relationship. Note the distinct breakpoint of the force-velocity relationship near 0.8 P0. Temperature, 23.5°C.

View larger version (14K): [in a new window]   Fig. 4. Records of tetanic contractions in a single muscle fiber at optimum sarcomere length and along the descending (A) and ascending (B) limbs of the length-tension relationship. Records shown in A were derived by holding a discrete fiber segment at a constant length during the tetanus. For records below the slack length, the segment was initially shortened under segment length control to the desired sarcomere spacing. Records in B are standard isometric contractions (no segment-length control) of the same fiber as in A. For contractions at moderately reduced lengths, the fiber was allowed to shorten freely to the desired length by taking up a pre-set slack. For contractions below approximately 1.9 µm, the initial shortening phase was constrained by applying a controlled ramp movement of the electromagnetic puller. The latter manoeuvre made the fiber develop some tension during the initial shortening phase. The upper trace in each set of records shows force; the lower trace shows segment length recording with sarcomere spacing indicated. A downward deflexion of segment length signal indicates shortening. P0, measured isometric force at 2.45µm sarcomere length; L0, segment length at 2.45µm sarcomere length. Temperature, 24.1°C.

View larger version (16K): [in a new window]   Fig. 5. Tetanic force versus sarcomere length recorded in mouse single muscle fibers. (A) Force expressed in units of the tetanic force recorded near 2.4 µm sarcomere length. The straight line is linear regression of force upon sarcomere length for values between 2.35 and 4.00 µm sarcomere length. Asterisks along the ascending limb are calculated force values based on the assumptions described in text. (B) Schematic illustration of different degrees of overlap between thick (A) and thin (I) filaments. OS, single filament overlap; OD, double filament overlap; OC, compression of thick filaments when colliding with Z disks at short (<1.7 µm) sarcomere lengths. Reproduced, with permission, from Edman and Reggiani (1987). For further information, see text.