Regulation of force development studied by photolysis of caged ADP in rabbit skinned psoas fibers

The present study examined the effects of Ca²⁺ and strongly bound cross-bridges on tension development induced by changes in the concentration of MgADP. Addition of MgADP to the bath increased isometric tension over a wide range of [Ca²⁺] in skinned fibers from rabbit psoas muscle. Tension-pCa (pCa is -log [Ca²⁺]) relationships and stiffness measurements indicated that MgADP increased mean force per cross-bridge at maximal Ca²⁺ and increased recruitment of cross-bridges at submaximal Ca²⁺. Photolysis of caged to cause a 0.5 mM MgADP jump initiated an increase in isometric tension under all conditions examined, even at pCa 6.4 where there was no active tension before ADP release. Tension increased monophasically with an observed rate constant, k_ADP, which was similar in rate and Ca²⁺ sensitivity to the rate constant of tension re-development, k_tr, measured in the same fibers by a release-re-stretch protocol. The amplitude of the caged ADP tension transient had a bell-shaped dependence on Ca²⁺, reaching a maximum at intermediate Ca²⁺ (pCa 6). The role of strong binding cross-bridges in the ADP response was tested by treatment of fibers with a strong binding derivative of myosin subfragment 1 (NEM-S1). In the presence of NEM-S1, the rate and amplitude of the caged ADP response were no longer sensitive to variations in the level of activator Ca²⁺. The results are consistent with a model in which ADP-bound cross-bridges cooperatively activate the thin filament regulatory system at submaximal Ca²⁺. This cooperative interaction influences both the magnitude and kinetics of force generation in skeletal muscle.