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

Abstract

1. The maximum velocity of shortening, Vmax, and stiffness were measured in skinned single fibre segments from psoas and soleus muscles of adult rabbits and psoas muscles of new‐born rabbits, and the myosin light chain composition was also determined in the same segments used in the mechanical studies. 2. Vmax was obtained at 15 degrees C during maximal activation at pCa 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 degrees C by application of length steps complete in 0.6 msec. The myosin light chain composition of the segments was then determined by SDS‐polyacrylamide gel electrophoresis. 3. Only fast type light chains were found to be present in the psoas fibre segments, though 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. 4. The results indicate that there are no consistent relationships between Vmax, tension development or stiffness and LC1f/LC2f in the segments from adult and new‐born psoas muscles, or between these mechanical parameters and LC1as/LC2s or LC1bs/LC2s in the adult soleus segments. However, 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. 5. The stiffness values obtained in each of the three kinds of muscle were similar, suggesting that cross‐bridge stiffness is similar in rabbit skeletal muscles of different type and age. Moreover, the results indicate that the amount of end compliance introduced by the connections to the fibre segments has a marked influence on the stiffness that is measured.

Slip of rabbit striated muscle in rigor or AMPPNP

“…Tre of fibres in rigor was much greater than that found with our earlier preparative technique (Tregear, 1988) and was comparable to the value found in isometric contraction [Tre of 650 ~N (Table 1) in a 75 ~m fibre implies 150 kN m-2; Julian et al (1981) obtained 160kN m -2 from isometrically-activated rabbit psoas at 15~…”

Three myosin heavy chain isoforms in type 2 skeletal muscle fibres

“…A number of studies on single fibres from mammalian and amphibian skeletal muscle showed that differences in MHCs but hot MLCs correlate with shortening velocity (Julian et al, 1981;Reiser et al, 1985;Lannergren, 1987). For instance, in the fast fibres of Xenopus skeletal muscle no significant correlation has been found between shortening velocity and the relative proportions of the two alkali light chains (Lannergren, 1987).…”

Effects on shortening velocity of rabbit skeletal muscle due to variations in the level of thin‐filament activation.

“…In addition, failure to identify the high-velocity phase of shortening would give rise to an over-estimate of the extension of series elements during tension development at low levels of activating Ca2+. Since the ordinal intercept of the straight line fitted to slack-test data is a measure of the extension of series elements (see Julian, Moss & Waller, 1981), lines fitted only to data in the low-velocity phase of shortening will yield an apparent and paradoxically greater extension of series elements (such as connexions to the apparatus) at low Ca2+ concentrations than at high (see Moss, 1982).…”