C‐protein limits shortening velocity of rabbit skeletal muscle fibres at low levels of Ca2+ activation.

Abstract

SUMMARY1. Effects on maximum shortening velocity (Vmax) due to partial extraction of Cprotein were investigated in skinned fibres from rabbit psoas muscles. Up to 80 % of endogenous C-protein was extracted, as assessed by sodium dodecyl sulphatepolyacrylamide gel electrophoresis (SDS-PAGE) of fibre segments obtained before and after the extraction protocol. Vmax was obtained at 15°C by measuring the times required to take up various amounts of slack imposed at one end of the fibre.2. During maximal activation with Ca2+, Vmax in control fibres was 4'26 +0-16 (mean+ S.E.M., n = 7) muscle lengths per second (ML/s). Following extraction of approximately 40 % of endogenous C-protein, Vmax in these same fibres was 4-41 + 0-24 ML/s.3. At sufficiently low levels of submaximal activation, high-and low-velocity phases of unloaded shortening were observed. Partial extraction of C-protein significantly increased Vmax in the low-velocity phase but had no effect on the highvelocity phase. The effect on low-velocity Vmax was fully reversed by re-addition of purified C-protein. 4. At low levels of activation, the amount of shortening to the break-point between the high-and low-velocity phases was not significantly affected by C-protein extraction. Under control conditions the average break-point was 85-6 + 3-1 nm/halfsarcomere, while 84-1 + 3-1 nm/half-sarcomere was obtained following partial extraction of C-protein.5. These results are considered in terms of a model in which an internal load slows Vmax at low levels of activation once a given amount of active shortening has occurred. C-protein may contribute to this internal load either by binding to actin and myosin or by influencing mechanical properties of myosin cross-bridges.

Cardiac myosin binding protein-C modulates actomyosin binding and kinetics in the in vitro motility assay

“…These results are consistent with previous published experiments in muscle fibers where an increase in the unloaded shortening velocity was demonstrated with either chemical extraction [41] or transgenic deletion of cMyBP-C [13,42]. Compared to the skinned fiber studies, the motility assay allowed us to characterize the effect of cMyBP-C on unloaded shortening over the full physiologic calcium concentration range.…”

“…No change in maximal isometric force is observed with the addition of cMyBP-C, which is in agreement with results from fiber studies where cMyBP-C is either extracted [41] or transgenically knocked out [42]. If cMyBP-C decreases the rate of cross-bridge detachment as suggested above this would result in an increased cross-bridge attachment time [47].…”

Support for a trimeric collar of myosin binding protein C in cardiac and fast skeletal muscle, but not in slow skeletal muscle

“…Taken together, these data suggest that in cardiac and fast skeletal muscle, MyBPC may contribute to an internal load, perhaps by binding to actin via its N-terminal region, or else by influencing the mechanical properties of myosin cross-bridges [23][24][25]. This is consistent with the finding that cross-bridge mechanics and kinetics, rather than thin filament inactivation kinetics, are the principle determinants of relaxation rate in striated muscle [26].…”

Myosin-binding protein C displaces tropomyosin to activate cardiac thin filaments and governs their speed by an independent mechanism

“…Elimination of MyBP-C from muscle fibers by chemical extraction or genetic ablation results in changes to the muscle's Ca 2+ sensitivity of force production (40)(41)(42), to its shortening velocity (43,44), and to the kinetics of tension recovery after stretch (45). These alterations emphasize the physiological roles played by MyBP-C in both Ca 2+ -dependent muscle activation and in modulation of cardiac contractility.…”