The fast-twitch tibialis anterior muscle of the rabbit was stimulated (10 Hz, 8 h/day for 7 wk) to cause complete transformation of the fibers from type IIb to type IIa. The velocity of unloaded shortening of permeabilized single fiber segments dissected from control and chronically stimulated tibialis anterior muscles was measured by the slack test at 20 degrees C. The myosin isozymes in these segments were separated on pyrophosphate-containing polyacrylamide gels. Peptide mapping of the myosin chain was performed on the myosin bands cut from the gels. The velocity of unloaded shortening of the IIb fibers was significantly higher (2.50 +/- 0.09 fiber length/s; n = 6) than that of the IIa fibers (1.33 +/- 0.08 fiber lengths/s; n = 6). The two groups of fibers differed with respect to their alkali light chain complement, as assessed by nondenaturing gel analyses, and with respect to their myosin heavy chain complement, as demonstrated by peptide mapping. Thus two groups of fast-twitch muscle fibers that contain distinguishable myosin isozyme contents differ in their velocities of unloaded shortening by a factor of two.
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