Variations in cross-bridge attachment rate and tension with phosphorylation of myosin in mammalian skinned skeletal muscle fibers: Implications for twitch potentiation in intact muscle

The Ca²⁺ sensitivities of the rate constant of tension redevelopment (k_tr; Brenner, B., and E. Eisenberg. 1986. Proceedings of the National Academy of Sciences. 83: 3542-3546) and isometric force during steady-state activation were examined as functions of myosin light chain 2 (LC₂) phosphorylation in skinned single fibers from rabbit and rat fast-twitch skeletal muscles. To measure k_tr the fiber was activated with Ca²⁺ and steady isometric tension was allowed to develop; subsequently, the fiber was rapidly (> 1 ms) released to a shorter length and then reextended by ~200 nm per half sarcomere. This maneuver resulted in the complete dissociation of cross-bridges from actin, so that the subsequent redevelopment of tension was related to the rate of cross-bridge reattachment. The time course of tension redevelopment, which was recorded under sarcomere length control, was best fit by a first-order exponential equation (i.e., tension = C(1 - e^-kt) to obtain the value of k_a. In control fibers, k⁺ increased sigmoidally with increases in [Ca²⁺]_i maximum values of k_tr were obtained at pCa 4.5 and were significantly greater in rat superficial vastus lateralis fibers (26.1 ± 1.2 s^-1 at 15°C) than in rabbit psoas fibers (18.7 ± 1.0 s^-1). Phosphorylation of LC₂ was accomplished by repeated Ca²⁺ activations (pCa 4.5) of the fibers in solutions containing 6 µM calmodulin and 0.5 µM myosin light chain kinase, a protocol that resulted in an increase in LC₂ phosphorylation from ~10% in the control fibers to >80% after treatment. After phosphorylation, k_tr was unchanged at maximum or very low levels of Ca²⁺ activation. However, at intermediate levels of Ca²⁺ activation, between pCa 5.5 and 6.2, there was a significant increase in k_tr such that this portion of the fc_tr-pCa relationship was shifted to the left. The steady-state isometric tension-pCa relationship, which in control fibers was left shifted with respect to the K_tr-pCa relationship, was further left-shifted after LC₂ phosphorylation. Phosphorylation of LC₂ had no effect upon steady-state tension during maximum Ca²⁺ activation.