title: The biphasic force-velocity relationship in frog muscle fibres and its evaluation in terms of cross-bridge function.
abstract: 1. The relationship between force and velocity of shortening was studied during fused tetani of single fibres isolated from the anterior tibialis muscle of Rana temporaria (1.5-3.3 degrees C; sarcomere length, 2.20 microns). Stiffness was measured as the change in force that occurred in response to a 4 kHz length oscillation of the fibre. 2. The results confirmed the existence of two distinct curvatures of the force-velocity relationship located on either side of a breakpoint in the high-force, low-velocity range. Reduction of the isometric force (P0) to 83.4 +/- 1.7% (mean +/- S.E.M., n = 5) of the control value by dantrolene did not affect the relative shape of the force-velocity relationship. The breakpoint between the two curvatures was located at 75.9 +/- 0.9% of P0 and 11.4 +/- 0.6% of maximum velocity of shortening (Vmax) in control Ringer solution and at 75.6 +/- 0.7% of P0 and 12.2 +/- 0.7% of Vmax in the presence of dantrolene. These results provide evidence that the transition between the two curvatures of the force-velocity relationship is primarily related to the speed of shortening, not to the actual force within the fibre. 3. The instantaneous stiffness varied with the speed of shortening forming a biphasic relationship with a breakpoint near 0.15 Vmax and 0.8 P0, respectively. The force/stiffness ratio (probably reflecting the average force per cross-bridge), increased with force during shortening. The increase of the force/stiffness ratio with force was less steep at forces exceeding 0.8 P0 than below this point. 4. A four-state cross-bridge model (described in the Appendix) was used to evaluate the experimental results. The model reproduces with great precision the characteristic features of the force-stiffness-velocity relationships recorded in intact muscle fibres.
authors.affiliation: Department of Pharmacology, University of Lund, Sweden. paul.edman@farm.lu.se
authors.forename: K A
authors.initials: KA
authors.lastname: Edman
authors.forename: A
authors.initials: A
authors.lastname: Månsson
authors.forename: C
authors.initials: C
authors.lastname: Caputo
mesh_terms: D000818:Animals; D003620:Dantrolene / Q000494:pharmacology; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D009991:Oscillometry; D011896:Rana temporaria; D012518:Sarcomeres / Q000502:physiology*; D013314:Stress, Mechanical; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D003620:Dantrolene