THE EFFECTS OF 2,3-BUTANEDIONE MONOXIME (BDM) ON THE FORCE-VELOCITY RELATION IN SINGLE MUSCLE-FIBERS OF THE FROG

The effects of 2,3-butanedione monoxime (BDM) on the force-velocity re lation were studied in single fibres from the anterior tibialis muscle of Rana temporaria (2.2 mu m sarcomere length, temperature 1.9-2.4 de grees C). BDM (1.0 and 1.8 mM) suppressed the maximum tetanic force (P -0) and the maximum speed of shortening (V-max), and increased the mai n curvature of the force-velocity relation. The biphasic shape of the force-velocity curve was maintained well in the presence of BDM, but t he interrelation between the two portions of the force-velocity relati on was significantly changed. Caffeine (0.5 mM) added in the presence of BDM increased the initial rate of rise of force during twitch and t etanus, increased the twitch amplitude, but did not affect the maximum tetanic force. The latter finding suggests that the contractile syste m was fully activated during tetanus in the presence of BDM. The resul ts support the view that BDM affects the cross-bridge function by exer ting a direct action upon the contractile apparatus. The decrease in t etanic force and the change of the force-velocity relation induced by BDM may be interpreted to show that a larger fraction of the attached crossbridges is in a state of low force production under the influence of BDM. This view is further supported by the observation that the in stantaneous stiffness of the muscle fibre is reduced proportionally le ss by BDM than the tetanic force.