EFFECTS OF MYOFIBRILLAR BUNDLE DIAMETER ON THE UNLOADED SHORTENING VELOCITY OF SKINNED SKELETAL-MUSCLE FIBERS

Using both slack tests and force clamp experiments, the velocity of un loaded shortening (V-u; V-u(st), slack test; force V-u(fc), clamp) was determined for maximally Ca2+-activated myofibrillar bundles. These w ere obtained by mechanically splitting single muscle fibres of rat, ra bbit, crab and lobster skeletal muscles. A comparison was made between the V-u of thick (mammalian: 45-70 mu m mean diameter; crustacean: 90 -175 mu m) and thin (mammalian: 25-40 mu m; crustacean: 35-85 mu m) pr eparations of the same muscle fibre. The bundle diameter had opposite effects on V-u in mammalian and crustacean muscle fibres. The V-u of t hin mammalian bundles was about 0.6 times that of the thick ones, wher eas in crustacean preparations this ratio was about 1.5. The kinetics of stretch-induced delayed force increase of maximally Ca2+-activated fibres (stretch activation) appeared not to differ between the thick a nd thin bundles from any animal preparation. Control experiments showe d that the observed diameter effects on V-u are not due to differences in the chemical environment of the myofilaments. One possible explana tion is that the intrinsic physical factors of the myofibrils modify V -u differently during progressive shortening in mammalian and crustace an preparations. (C) Chapman & Hall Ltd.