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 relation were studied in single fibres from the anterior tibialis muscle of Rana temporaria (2.2 mu m sarcomere length, temperature 1.9-2.4 degrees 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 main curvature of the force-velocity relation. The biphasic shape of the force-velocity curve was maintained well in the presence of BDM, but the interrelation between the two portions of the force-velocity relation was significantly changed. Caffeine (0.5 mM) added in the presence of BDM increased the initial rate of rise of force during twitch and tetanus, increased the twitch amplitude, but did not affect the maximum tetanic force. The latter finding suggests that the contractile system was fully activated during tetanus in the presence of BDM. The results support the view that BDM affects the cross-bridge function by exerting a direct action upon the contractile apparatus. The decrease in tetanic 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 instantaneous stiffness of the muscle fibre is reduced proportionally less by BDM than the tetanic force.