Mechanical properties and myosin light chain composition of skinned muscle fibres from adult and new‐born rabbits.

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Abstract
The maximum velocity of shortening, Vmax, and stiffness were measured in skinned single fiber segments from psoas and soleus muscles of adult rabbits and psoas muscles of newborn rabbits, and the myosin light chain composition was determined in the same segments used in the mechanical studies. Vmax was obtained at 15.degree. C during maximal activation at pCa (-log[Ca]) 5.49 using a method involving measurement of the time required to take up various amounts of slack imposed on the segments. Stiffness was measured during activation at 10.degree. C by application of length steps complete in 0.6 ms. The myosin light chain composition of the segments was then determined by SDS(sodium dodecyl sulfate)-polyacrylamide gel electrophoresis. Only fast type light chains were present in the psoas fiber segments, although the relative amounts of myosin LC1f, LC2f and LC3f in these segments was somewhat variable. In most instances, the sum of the amounts of LC1f and LC3f present was equivalent to the amount of LC2f. Only slow type light chains were found in the soleus segments, and the sum of the amounts of LC1as and LC1bs was about equal to the amount of LC2s. The results indicate that there are no consistent relationships between Vmax, tension development or stiffness and LC1f/LC2f in the segments from adult and newborn psoas muscles or between these mechanical parameters and LC1as/LC2s or LC1bs/LC2s in the adult soleus segments. The psoas segments, which had light chains of the fast type, had Vmax values that were consistently higher than those of the soleus segments, which had light chains of the slow type. The stiffness values obtained in each of the 3 kinds of muscle were similar, suggesting that cross-bridge stiffness is similar in rabbit skeletal muscles of different type and age. Apparently, the amount of end compliance introduced by the connections to the fiber segments has a marked influence on the stiffness that is measured.