Effects of sarcomere length on the force—pCa relation in fast‐ and slow‐twitch skinned muscle fibres from the rat

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Abstract
Steady-state force-pCa [-log Ca concentration] relations were determined for mechanically skinned fibers of fast- and slow-twitch rat skeletal muscle, extensor digitorum longus (e.d.l.) and soleus, respectively, at varied sarcomere lengths and temperatures (3-35.degree. C). This relation was different for the 2 fiber types at all sarcomere lengths, with soleus fibers having lower Ca2+ concentration threshold for activation and requiring a wider Ca2+ concentration range to reach force saturation. An increase in sarcomere length within the range 2.2-3.6 .mu.m reversibly shifted the steady-state force-pCa curves for both fast- and slow-twitch fibers toward lower Ca2+ without affecting the steepness of the curves. The results are consistent with the idea that the sensitivity of the contractile apparatus to Ca2+ is increased with increasing sarcomere length. The change in apparent Ca2+ sensitivity (measured as the change in pCa corresponding to 50% Ca-activated force (.DELTA.pCa50)) was approximately proportional to the change in sarcomere length (.DELTA.SL) at all temperatures for both types of muscle fiber. The proportionality coefficient (.DELTA.pCa50/.DELTA.SL) was higher for slow-twitch fibers than for fast-twitch fibers by a factor of 1.5-2.0 at all temperatures investigated. The force-pCa relation was less affected by changes in sarcomere length at lower (3-5.degree. C) than at higher temperatures (22-35.degree. C). The sarcomere length-maximum force relation obtained with skinned fibers for sarcomere lengths between 2.10 and 4.05 .mu.m was similar for both fiber types at all temperatures despite large temperature-dependent variations in maximum Ca-activated force response. Maximum force decreased linearly to 0 between .apprx. 2.8 and 3.95 .mu.m; remained high and essentially constant between .apprx. 2.40 and 2.80 .mu.m; and decreased significantly below 2.40 .mu.m. Myofilament-generated tension oscillations produced by partially activated soleus skinned fibers are highly length-dependent, disappearing completely when sarcomere length is increased above 3.15 .mu.m.