The Combined Effects of ADP, ATP, and Myosin Density on Cooperative Activation of Thin Filaments

Abstract

Cooperative activation of thin filaments has been achieved by decreasing [ATP], increasing [ADP], and increasing myosin densities; however, these observations have yet to be incorporated into a self-consistent description of thin filament regulation. It has been noted in previous skinned muscle fiber studies that contraction can be initiated with high [ADP] when Ca2+ is absent. Similarly, low [ATP] has been shown to activate thin filaments in Ca2+-free motility assays. Based on these observations, we hypothesize that high concentrations of ADP can cause thin filament cooperative activation in the absence of Ca2+. Using an in vitro motility assay, we determine the effects of ATP, ADP, and myosin density on the cooperative activation of myosin-based thin filament sliding, V. At 1mM [ATP], V is activated over an [ADP] range from ∼2 to 8mM and becomes increasingly inhibited at [ADP] >8mM. This biphasic effect of [ADP] on V is similar to the effects of [ATP] on cooperative thin filament activation previously measured in vitro. The observed effects of [ADP] and [ATP] on V are consistent with a model in which increasing [ADP] or decreasing [ATP] increase the probability that a myosin head is bound within a thin filament regulatory unit, cooperatively activating that regulatory unit (and possible adjacent ones) and increasing V. At sufficiently high [ADP] or sufficiently low [ATP], the thin filament becomes fully activated, and a further increase in [ADP] or decrease in [ATP] slows V by inhibiting detachment kinetics.