Effects of myofibrillar bundle diameter on the unloaded shortening velocity of skinned skeletal muscle fibres.

Using both slack tests and force clamp experiments, the velocity of unloaded shortening (Vu; Vu(st), slack test; Vu(fc), force clamp) was determined for maximally Ca(2+)-activated myofibrillar bundles. These were obtained by mechanically splitting single muscle fibres of rat, rabbit, crab and lobster skeletal muscles. A comparison was made between the Vu of thick (mammalian: 45-70 microns mean diameter; crustacean: 90-175 microns) and thin (mammalian: 25-40 microns; crustacean: 35-85 microns) preparations of the same muscle fibre. The bundle diameter had opposite effects on Vu in mammalian and crustacean muscle fibres. The Vu of thin mammalian bundles was about 0.6 times that of the thick ones, whereas in crustacean preparations this ratio was about 1.5. The kinetics of stretch-induced delayed force increase of maximally Ca(2+)-activated fibres (stretch activation) appeared not to differ between the thick and thin bundles from any animal preparation. Control experiments showed that the observed diameter effects on Vu are not due to differences in the chemical environment of the myofilaments. One possible explanation is that the intrinsic physical factors of the myofibrils modify Vu differently during progressive shortening in mammalian and crustacean preparations.