Effects of phosphate and ADP on shortening velocity during maximal and submaximal calcium activation of the thin filament in skeletal muscle fibers

The effects of added phosphate and MgADP on unloaded shortening velocity during maximal and submaximal Ca²⁺ activation of the thin filament were examined in skinned single skeletal fibers from rabbit psoas muscle. During maximal Ca²⁺ activation, added phosphate (10-30 mM) had no effect on unloaded shortening velocity as determined by the slack-test technique. In fibers activated at submaximal concentrations of Ca²⁺ in the absence of added phosphate, plots of slack length versus duration of unloaded shortening were biphasic, consisting of an initial high velocity phase of shortening and a subsequent low velocity phase of shortening. Interestingly, in the presence of added phosphate, biphasic slack-test plots were no longer apparent. This result was obtained in control fibers over a range of submaximal Ca²⁺ concentrations and in maximally Ca²⁺ activated fibers, which were first treated to partially extract troponin C. Thus, under conditions that favor the appearance of biphasic shortening (i.e., low [Ca²⁺](i), troponin C extraction), added phosphate eliminated the low velocity component. In contrast, in fibers activated in the presence of 5 mM added MgADP, biphasic slack-test plots were apparent even during maximal Ca²⁺ activation. The basis of biphasic shortening is not known but if may be due to the formation of axially compressed cross-bridges that become strained to bear a tension that opposes the relative sliding of the myofilaments. The present findings could be explained if added phosphate and MgADP bind to cross-bridges in a strain-dependent manner. In this case, the results suggest that phosphate inhibits the formation of cross-bridges that bear a compressive strain. Added MgADP, on the other hand, may be expected to detain cross-bridges in strong binding states, thus promoting an increase in the population of cross- bridges bearing a compressive strain. Alterations in the population of strained cross-bridges by added phosphate and MgADP would alter the internal load within the fiber and thus affect the speed of fiber shortening.