Striated Muscle Regulation of Isometric Tension by Multiple Equilibria

Cooperative activation of striated muscle by calcium is based on the movement of tropomyosin described by the steric blocking theory of muscle contraction. Presently, the Hill model stands alone in reproducing both myosin binding data and a sigmoidal-shaped curve characteristic of calcium activation (Hill TL (1983) Two elementary models for the regulation of skeletal muscle contraction by calcium. Biophys J 44: 383–396.). However, the free myosin is assumed to be fixed by the muscle lattice and the cooperative mechanism is based on calcium-dependent interactions between nearest neighbor tropomyosin subunits, which has yet to be validated. As a result, no comprehensive model has been shown capable of fitting actual tension data from striated muscle. We show how variable free myosin is a selective advantage for activating the muscle and describe a mechanism by which a conformational change in tropomyosin propagates free myosin given constant total myosin. This mechanism requires actin, tropomyosin, and filamentous myosin but is independent of troponin. Hence, it will work equally well with striated, smooth and non-muscle contractile systems. Results of simulations with and without data are consistent with a strand of tropomyosin composed of ∼20 subunits being moved by the concerted action of 3–5 myosin heads, which compares favorably with the predicted length of tropomyosin in the overlap region of thick and thin filaments. We demonstrate that our model fits both equilibrium myosin binding data and steady-state calcium-dependent tension data and show how both the steepness of the response and the sensitivity to calcium can be regulated by the actin-troponin interaction. The model simulates non-cooperative calcium binding both in the presence and absence of strong binding myosin as has been observed. Thus, a comprehensive model based on three well-described interactions with actin, namely, actin-troponin, actin-tropomyosin, and actin-myosin can explain the cooperative calcium activation of striated muscle

The Relationship Between Calcium Binding To Troponin C And Thin Filament Regulation Of Muscle Contraction (isometric Tension, Myofibrillar Atpase)

Endogenous troponin C (TnC) was extracted from rabbit fast skeletal muscle myofibrils and intact fibers by incubation with EDTA. Ca(\u272+)-induced myofibrillar ATPase and tension in fibers were reduced after extraction with EDTA but subsequent incubation with exogenous TnC restored Ca(\u272+) dependent activity to both preparations. Metal occupancy of the C-terminal Ca(\u272+)-Mg(\u272+) sites prevented the extraction of TnC consistent with previous suggestions that these sites function structurally. A fluorescent analogue of TnC (TnC(,DANZ)) which increases fluorescence intensity when Ca(\u272+) binds to the N-terminal Ca(\u272+)-specific sites was incorporated into TnC depleted fibers and myofibrils. Fluorescence and tension signals were recorded simultaneously from TnC(,DANZ) reconstituted fibers. The steady-state Ca(\u272+) sensitivity of the fluorescence change was found to be greater than the tension change. A similar relationship was found for simultaneously measured fluorescence and myofibrillar ATPase responses. Direct binding measurements demonstrated that Ca(\u272+) binding to either of the two Ca(\u272+)-specific sites produce the TnC(,DANZ) fluorescence change. The binding of Ca(\u272+) to the Ca(\u272+)-specific sites which was inferred from the fluorescence change was consistently greater than the resulting activity change. These results demonstrate that the change in activity correlates with Ca(\u272+) bound at more than one Ca(\u2723+)-specific site. Models for multiple binding within individual TnC molecules and among TnC molecules along the thin filament are shown to be consistent with the data