Ca2+ dependence of loaded shortening in rat skinned cardiac myocytes and skeletal muscle fibres.

This study examined the effects of activator Ca2+ on loaded shortening and power output in skinned rat cardiac myocyte preparations, and fast- and slow-twitch skeletal muscle fibres at 12 degrees C. Shortening velocities were slowed at nearly all relative loads when Ca2+ activation levels were reduced to approximately 70% maximal isometric force (P4.5) in cardiac myocyte preparations, as well as in fast-twitch and slow-twitch skeletal muscle fibres. Peak absolute power outputs declined significantly as Ca2+ activation levels were progressively reduced from maximal to 30% P4.5 in all three striated muscle types, with the greatest change in fast-twitch fibres. In cardiac myocyte preparations, even peak relative power output progressively fell when Ca2+ activation levels were lowered to approximately 70, 50 and 30% P4.5. Peak relative power output also progressively fell in fast-twitch fibres as Ca2+ activation levels were lowered from maximal down to 50% P4.5. However, in slow-twitch fibres, peak relative power output decreased only at 70% P4.5 and then remained unchanged with further reductions in Ca2+ activation levels. The greater Ca2+ dependence of peak relative power output in cardiac myocytes and fast-twitch fibres may arise from a shared mechanism such as cooperative inactivation of the thin filament, which is likely to be slowest in less cooperative slow-twitch fibres. During submaximal Ca2+ activations, the time course of shortening became markedly curvilinear during isotonic shortening in all three muscle types. The progressive slowdown in shortening velocity during isotonic contractions was greatest in fast-twitch fibres, consistent with the higher degree of cooperativity of Ca2+ activation in fast-twitch fibres. Additionally, fast-twitch and slow-twitch fibre stiffness decreased in concert with the curvature of length traces during loaded shortening. These results are consistent with the idea that cooperative inactivation of the thin filament occurs during loaded shortening and such a mechanism may contribute to the progressive slowing and overall Ca2+ dependence of loaded shortening velocity.