C-PROTEIN LIMITS SHORTENING VELOCITY OF RABBIT SKELETAL-MUSCLE FIBERS AT LOW-LEVELS OF CA2+ ACTIVATION

1. Effects on maximum shortening velocity (V(max)) due to partial extraction of C-protein were investigated in skinned fibres from rabbit psoas muscles. Up to 80% of endogenous C-protein was extracted, as assessed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of fibre segments obtained before and after the extraction protocol. V(max) was obtained at 15-degrees-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+, V(max) 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, V(max) 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 V(max) in the low-velocity phase but had no effect on the high-velocity phase. The effect on low-velocity V(max) 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/half-sarcomere, 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 V(max) 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.