ALBERT

Notes: Summary The primary goal of this study was to determine the utility of 2,3-butanedione monoxime as a tool for determining and separating the chemical energy usage associated with force production from that of force-independent, or ‘activation’ processes in smooth and skeletal muscles. We determined the effects of 2,3-butanedione monoxime on force production, myosin light chain phosphorylation and high energy phosphate usage in intact and permeabilized smooth (rabbit taenia coli) and skeletal (mouse extensor digitorum longus) muscles. In the intact taenia coli, 2,3-butanedione monoxime depressed the tonic phase of the tetanus, contractures evoked by high potassium (90 mM) and by carbachol (10-5 M) and the small force response evoked by these agonists after treatment with D-600 (10-5 M). In the electrically stimulated intact taenia coli 2,3-butanedione monoxime (0–20 mM) caused a proportional inhibition of tetanic force output, myosin light chain phosphorylation and high energy phosphate usage (ED50 ∼ 7 mM for all these parameters). At 20 mM 2,3-butanedione monoxime, force and energy usage fell to near zero and the degree of myosin light chain phosphorylation decreased to resting values, indicating a shut-down of both force-dependent and force-independent energy usage at high concentrations of 2,3-butanedione monoxime. In permeabilized taenia coli, 2,3-butanedione monoxime had little or no depressant effects on force production, ATPase activity or calcium sensitivity. 2,3-butanedione monoxime had a very modest inhibitory effect on the in vitro motility of unregulated actin filaments interacting with thiophosphorylated myosin. In solution, 2,3-butanedione monoxime inhibited myosin light chain kinase, but not the phosphatase (SMP-IV). These results suggest that the major effect of 2,3-butanedione monoxime is not on the contractile proteins themselves, but rather on calcium delivery during excitation, thereby reducing the degree of activation of myosin light chain kinase and subsequent activation of myosin by light chain phosphorylation. Thus, 2,3-butanedione monoxime is not useful for the determination of the energetics of activation processes in smooth muscle because of its inhibition of both force-dependent and force-independent processes. In contrast, in the intact mouse extensor digitorum longus, 2,3-butanedione monoxime inhibits tetanic force production (ED50 ∼ 2 mM) to a much greater extent than myosin light chain phosphorylation. When 2,3-butanedione monoxime was used to manipulate force production in muscles at L0, it was found that ∼60% of the total energy usage was force-independent and the remainder was force-dependent. In the permeabilized extensor digitorum longus treated with 12 mM 2,3-butanedione monoxime, there was a decrease in calcium-activated force production and a decrease in calcium sensitivity. The effects of 2,3-butanedione monoxime were considerably greater in the intact than in the permeabilized mouse extensor digitorum longus. At 2,3-butanedione monoxime concentrations that block force production in the intact muscle, the effects on in vitro motility were small, yet far greater than those on smooth muscle myosin. These results suggest that 2,3-butanedione monoxime has a direct effect on the contractile proteins, but what cannot be ignored is the decrease in myosin light chain phosphorylation in the skeletal muscle, which, like the decreased force output, may result from a reduction in calcium release from the sarcoplasmic reticulum. For these reasons, the use of 2,3-butanedione monoxime to probe the components of energy usage during the contraction of skeletal muscle requires considerable caution and a full definition of its actions.