Ca2+ activation of troponin C-extracted vertebrate striated fast-twitch muscle fibers

To characterize the tension control in vertebrate striated muscle fibers, and to obtain insights into the cross-bridge mechanisms, Ca2+ activation on troponin C (TnC)-extracted skinned fibers was studied in standard (180 mM, physiological) and low (20-41 mM) ionic strength solutions. By tension measurement, TnC-extracted fibers had nearly lost their Ca2+ sensitivity in the standard ionic strength solutions, but surprisingly the fiber still exhibited significant tension on activation with Ca2+ in low ionic strength. Also, the presence of weak bridges (zero-force bridges) was inferred by stiffness measurements in Ca2+-free low ionic strength solution, and were found even after TnC extraction. The possibility is discussed that dual regulation by Ca2+ is present in the vertebrate muscle. One mechanism activates the thin filaments. The second may directly control the kinetic step for the transition between the weak and strong bridges, in the cross-bridge cycle in the fiber, and in this way may act as an additional Ca2+ switch.