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-bri dges on tension development induced by changes in the concentration of MgAD P. Addition of MgADP to the bath increased isometric tension over a wide ra nge of [Ca2+] in skinned fibers from rabbit psoas muscle. Tension-pCa (pCa is -log [Ca2+]) relationships and stiffness measurements indicated that MgA DP increased mean force per cross-bridge at maximal Ca2+ and increased recr uitment of cross-bridges at: submaximal Ca2+. Photolysis of caged ADP to ca use a 0.5 mM MgADP jump initiated an increase in isometric tension under al l conditions examined, even at pCa 6.4 where there was no active tension be fore ADP release. Tension increased monophasically with an observed rate co nstant, 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 transi ent had a bell-shaped dependence on Ca2+, reaching a maximum at intermediat e Ca2+ (pCa 6). The role of strong binding cross-bridges in the ADP respons e was tested by treatment of fibers with a strong binding derivative of myo sin subfragment 1 (NEM-SI). In the presence of NEM-S1, the rate and amplitu de 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 A DP-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.