The force‐velocity relationship in vertebrate muscle fibres at varied tonicity of the extracellular medium

Abstract

The relationship between active force and velocity of shortening was studied during tetanic contraction of isolated semitendinosus muscle fibers of the frog [Rana temporaria] (0.5-2.0.degree. C). Measurements were carried out with the fiber immersed in isotonic Ringer solution (1.00 R) 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). The force-velocity relation was hyperbolic at loads lower than 80% of measured isometric force (P0) but exhibited a reversed curvature between 0.8 shortening was determined in 2 different ways: by extrapolation to 0 load from force-velocity data truncated at 0.8P0 and P0. The maximum velocity of P0 (computer fitting of hyperbola, least-squares method) and by recording the time required to take up the slack of the fiber after a quick release during tetanus. Isometric force and maximum speed of shortening both changed inversely with the tonicity of the extracellular medium. Immersion of the fiber in 0.81 R hypotonic solution caused active tension and shortening velocity to increase by 10 .+-. 1% (mean .+-. standard error of the mean, n = 14) and 12 .+-. , respectively. Force and shortening velocity decreased by 12 .+-. (n = 13) and 22 .+-. 2% when normal Ringer was replaced by 1.22 R hypertonic solution. These changes doubled when the tonicity was altered from normal Ringer to 0.62 R and 1.44 R, respectively. Changes in fiber cross-sectional area equivalent to those obtained in the 0.81 R and 1.22 R solutions (+ 1 and -13%, respectively) were produced by varying the sarcomere length within the range 2.0-2.5 .mu.m 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, was not critically dependent on changes in myofilament lattice width over the range considered. Both shortening velocity and active force were modulated by changes of the intracellular ionic strength above and below the level that normally exists in the intact muscle fiber.