2,3-Butanedione monoxime increases speed of relaxation in single muscle fibres of frog

The effects of 2,3-butanedione monoxime (BDM) on intracellular Ca2+ transient and cross-bridge function were studied in frog single fibres from the anterior tibialis muscle of Rana temporaria (sarcomere length, 2.2 microm; temperature, 2-4 degrees C). The fluorescent dye fluo-3 was used to monitor the intracellular free calcium concentration ([Ca2+]i) during isometric contractions. BDM (1-5 mM) reduced the amplitude of the Ca2+ transient during twitches, but this effect was too small to explain the marked inhibition of BDM on twitch force. [Ca2+]i reached at the end of 1-s tetanic stimulation was not significantly affected by BDM (1.0 and 1.8 mM) while the maximum tetanic tension was substantially reduced. The rate of relaxation during isometric tetanus was increased by BDM whereas the rate of decay of the Ca2+ transient was reduced in the presence of BDM. The results strongly suggest that BDM, under the experimental conditions used, mainly affects the contractile machinery resulting in altered performance of the cross-bridges. These effects of BDM were evaluated in terms of the cross-bridge model of Huxley (1957) which was fitted to the experimental force-velocity data in the presence and absence of BDM.