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Department of Physiology, School of Medicine, University of Wisconsin, Madison 53706.

1. Maximum shortening velocity (Vmax) was examined in skinned single fibres from rat slow-twitch soleus muscles at various degrees of Ca2+ activation of the thin filament, in the presence and absence of osmotic compression induced by 5% dextran. 2. At maximal levels of Ca2+ activation, Vmax remained constant as the extent of shortening was varied, with values averaging 1.43 +/- 0.05 muscle lengths/s (mean + S.E.M., n = 13). When thin filament activation was reduced by lowering the concentration of Ca2+, unloaded shortening consisted of an initial high-velocity phase for extents of shortening in the range 20-80 nm/half-sarcomere, and a subsequent low-velocity phase for greater extents of shortening. 3. In the absence of dextran, Vmax in the high-velocity phase of shortening was relatively invariant over a wide range of activation; however, at very low levels of activation, yielding tensions less than 5% of the peak value. Vmax declined precipitously. In contrast, fibres compressed radially with dextran to diameters comparable to those of intact fibres demonstrated a marked decrease in Vmax in the high-velocity phase when thin filament activation was varied over a wide range. These findings are consistent with the idea that cross-bridges in the expanded filament lattice of skinned fibres do not bear as great an axial compressive force as in intact fibres (Goldman & Simmons, 1986; Goldman, 1987) but instead buckle as the fibre shortens. 4. The value of Vmax in the low-velocity phase of shortening decreased as thin filament activation was reduced in both control and osmotically compressed fibres. The low-velocity phase, which occurred only at reduced levels of thin filament activation, may be a manifestation of a population of slowly dissociating crossbridges which with shortening become negatively strained and oppose contraction.

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