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Department of Pharmacology, University of Lund, Lund, Sweden
1. The relationship between active force and velocity of shortening was studied during tetanic contraction of isolated semitendinosus muscle fibres of the frog (0·5-2·0° C). Measurements were carried out with the fibre immersed in isotonic (1·00R) Ringer solution and in solutions that were made hypotonic by reduction of NaCl (osmolality 0·62 and 0·81 of normal Ringer) and hypertonic by addition of sucrose (osmolality 1·22 and 1·44 of normal Ringer).
2. The force—velocity relation was hyperbolic at loads lower than 80% of measured isometric force (P0) but exhibited a reversed curvature between 0·8P0 and P0. The maximum velocity of shortening was determined in two different ways: (i) by extrapolation to zero load from force—velocity data truncated at 0·8P0 (computer fitting of hyperbola, leastsquares method) and (ii) by recording the time required to take up the slack of the fibre after a quick release during tetanus.
3. Isometric force and maximum speed of shortening both changed inversely with the tonicity of the extracellular medium. Immersion of the fibre in 0·81R hypotonic solution caused active tension and shortening velocity to increase by 10 ± 1% (mean ± S.E. of mean, n = 14) and 12 ± 1%, respectively. Conversely, force and shortening velocity decreased by 12 ± 1% (n = 13) and 22 ± 2% when normal Ringer was replaced by 1·22R hypertonic solution. These changes doubled when the tonicity was altered from normal Ringer to 0·62R and 1·44R, respectively.
4. Changes in fibre cross-sectional area equivalent to those obtained in the 0·81R and 1·22R solutions (+11% and -13%, respectively) were produced by varying the sarcomere length within the range 2·0-2·5 µm in the normal Ringer solution. Maximum velocity of shortening remained very nearly constant under these conditions, indicating that the shortening velocity, like the isometric force, is not critically dependent on changes in myofilament lattice width over the range considered.
5. The results support the view that both shortening velocity and active force are modulated by changes of the intracellular ionic strength above and below the level that normally exists in the intact muscle fibre.
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