MULTIPLE ACTIONS OF 2,3-BUTANEDIONE MONOXIME ON CONTRACTILE ACTIVATION IN FROG TWITCH FIBERS

1. The effects of 2,3-butanedione monoxime (BDM) on various steps in the excitation-contraction coupling sequence, including action potential, charge movement and twitch tension, were studied in twitch fibres of Rana temporaria. 2. The resting potential of intact fibres in whole muscles bathed in 20 mM-BDM was the same as control. The resting potential also remained stable after more than 100 min in 20 mM-BDM. 3. The action potential was measured in intact fibres of fibre bundles with an intracellular microelectrode. Applications of 5 and 7.5 mM-BDM had no effect on its amplitude, whereas 10 and 20 mM suppressed its amplitude by about 4 and 10 %, respectively. Increasing concentrations of BDM prolonged the half-width and elevated the after-potential of the action potential progressively. The action potential was also measured in cut fibres mounted in a double Vaseline-gap chamber. Results were similar to those in intact fibres. 4. Charge movement was measured in intact fibres of halved muscles with the three-microelectrode voltage-clamp technique. The steady-state Q-V plot of the total charge measured in isotonic tetraethylammonium (TEA) Ringer solution with 20 mM-BDM appeared to be shifted about 10 mV in the depolarizing direction and to be slightly more shallow when compared with the control Q-V plot measured in hypertonic TEA Ringer solution with 350 mM-sucrose. After allowing for the voltage shift, 20 mM-BDM did not appear to affect the kinetics of both components of charge movement, but suppressed the maximum amount of total charge by about one-quarter. 5. Charge movement was also measured in cut fibres with the double Vaseline-gap voltage-clamp technique. In the presence of 20 mM-BDM, charge movement traces resembled those from intact fibres. Twenty millimolar BDM suppressed the maximum amount of total charge by about one-quarter, as in intact fibres. The steady-state Q-V plots from cut fibres were separated into Q-beta (early current) and Q-gamma (late hump current) components by least-squares fitting with a sum of two Boltzmann distribution functions. On average, 20 mM-BDM suppressed Q-beta and Q-gamma in roughly equal proportion, but did not affect the individual voltage distributions of Q-beta and Q-gamma. 6. Twitch tension was measured in single intact fibres stimulated extracellularly. BDM effectively reduced the peak amplitude, the time-to-peak and the half-width of twitch tension. The interaction of BDM with receptors appeared to follow more or less a simple 1:1 binding in fibres stretched to sarcomere lengths of about 3.6-mu-m. The half-blocking concentration was 0.43 mM. The dose-response relation of the binding was similar in fibres stretched to sarcomere lengths of about 3.0-mu-m but the interaction with receptors appeared to deviate slightly from a simple non-cooperative binding. 7. It is concluded that BDM affects the action potential, charge movement and twitch tension. However, its effect on twitch tension is much larger than that on the earlier steps in the excitation-contraction coupling sequence.