Variations in cross-bridge attachment rate and XXXX with phosphorylation of @entity3999 in mammalian skinned skeletal muscle fibers. Implications for twitch potentiation in intact muscle.

@entity35 @entity95 @entity2701 @entity3999 @entity141
The @entity141 sensitivities of the rate constant of @entity2701 redevelopment (ktr; 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 @entity3999 light chain 2 (LC2) phosphorylation in skinned single fibers from @entity95 and @entity35 fast-twitch skeletal muscles. To measure ktr the fiber was activated with @entity141 and steady isometric @entity2701 was allowed to develop; subsequently, the fiber was rapidly (less than 1 ms) released to a shorter length and then reextended by approximately 200 nm per half sarcomere. This maneuver resulted in the complete dissociation of cross-bridges from actin, so that the subsequent redevelopment of @entity2701 was related to the rate of cross-bridge reattachment. The time course of @entity2701 redevelopment, which was recorded under sarcomere length control, was best fit by a first-order exponential equation (i.e., @entity2701 = C(1 - e-kt) to obtain the value of ktr. In control fibers, ktr increased sigmoidally with increases in [ @entity141 ]; maximum values of ktr were obtained at pCa 4.5 and were significantly greater in @entity35 superficial vastus lateralis fibers (26.1 +/- 1.2 s-1 at 15 degrees C) than in @entity95 psoas fibers (18.7 +/- 1.0 s-1). Phosphorylation of LC2 was accomplished by repeated @entity141 activations (pCa 4.5) of the fibers in solutions containing 6 microM calmodulin and 0.5 microM @entity3999 light chain kinase, a protocol that resulted in an increase in LC2 phosphorylation from approximately 10% in the control fibers to greater than 80% after treatment. After phosphorylation, ktr was unchanged at maximum or very low levels of @entity141 activation. However, at intermediate levels of @entity141 activation, between pCa 5.5 and 6.2, there was a significant increase in ktr such that this portion of the ktr-pCa relationship was shifted to the left. The steady-state isometric @entity2701 -pCa relationship, which in control fibers was left shifted with respect to the ktr-pCa relationship, was further left-shifted after LC2 phosphorylation. Phosphorylation of LC2 had no effect upon steady-state @entity2701 during maximum @entity141 activation. In fibers from which troponin C was partially extracted to disrupt molecular cooperativity within the thin filament (Moss et al. 1985. Journal of General Physiology. 86:585-600), the effect of LC2 phosphorylation to increase the @entity141 sensitivity of steady-state isometric force was no longer evident, although the effect of phosphorylation to increase ktr was unaffected by this maneuver.(ABSTRACT TRUNCATED AT 400 WORDS)
