title: 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.
abstract: The Ca2+ sensitivities of the rate constant of tension 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 myosin light chain 2 (LC2) phosphorylation in skinned single fibers from rabbit and rat fast-twitch skeletal muscles. To measure ktr the fiber was activated with Ca2+ and steady isometric tension 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 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 ktr. In control fibers, ktr increased sigmoidally with increases in [Ca2+]; maximum values of ktr were obtained at pCa 4.5 and were significantly greater in rat superficial vastus lateralis fibers (26.1 +/- 1.2 s-1 at 15 degrees 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 microM calmodulin and 0.5 microM myosin 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 Ca2+ activation. However, at intermediate levels of Ca2+ 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 tension-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 tension during maximum Ca2+ 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 Ca2+ 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)
authors.affiliation: Department of Physiology, School of Medicine, University of Wisconsin, Madison 53706.
authors.forename: J M
authors.initials: JM
authors.lastname: Metzger
authors.forename: M L
authors.initials: ML
authors.lastname: Greaser
authors.forename: R L
authors.initials: RL
authors.lastname: Moss
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D005260:Female; D006652:Histological Techniques; D006706:Homeostasis; D008297:Male; D009119:Muscle Contraction; D009132:Muscles / Q000032:analysis / Q000378:metabolism / Q000502:physiology*; D009218:Myosins / Q000378:metabolism*; D009994:Osmolar Concentration; D010766:Phosphorylation; D011817:Rabbits; D051381:Rats; D011919:Rats, Inbred Strains; D012518:Sarcomeres / Q000502:physiology; D014336:Troponin / Q000032:analysis; D019209:Troponin C
chemical_list: D014336:Troponin; D019209:Troponin C; D009218:Myosins; D002118:Calcium