No abstract

No abstract

Amrinone is a bipyridine drug with characteristic effects on the force-velocity relationship of fast skeletal muscle. Here we combined in vitro motility assays, transient biochemical kinetics and optical tweezers studies to elucidate the mechanisms underlying the drug effects. Amrinone (1-2 mM) reduced the sliding velocity of heavy meromyosin (HMM) propelled actin filaments by 31.0 5 2.5% (n ¼ 15) at different ionic strengths of the assay solution (20 -160 mM). The drug also reduced (by 2 -18%) the sliding velocity of actin filaments propelled by subfragment 1 (S1). Stopped-flow studies of myofibrils, acto-HMM and acto-S1 showed no amrinone-induced reduction in the rate of MgATP induced actomyosin dissociation and optical tweezers studies detected no changes in the working stroke length. In contrast, the ADP affinity of acto-HMM (but not acto-S1) was increased about two-fold by 1 mM amrinone. Our results are consistent with inhibition of a strain-dependent MgADP-release step as the basis for amrinone induced reduction in sliding velocity. Modeling suggests that such an effect may also account for most other amrinone-induced changes of the force-velocity relationship of muscle (e.g. in isometric force and in shape of the force-velocity curve). Moreover, the results point to the possible importance of cooperative interactions between the two myosin heads in muscle contraction.