ALTERED CA2-MUSCLE FIBERS FROM MYOD GENE-INACTIVATED MICE( SENSITIVITY OF TENSION IN SINGLE SKELETAL)

1. Single, fast glycolytic skeletal muscle fibres were isolated from w ild-type (MyoD(+/+)) and MyoD mutant mice (MyoD(-/-)), which lack a fu nctional copy of the MyoD gene. Fibres were chemically permeabilized t o permit manipulation and control of the ionic environment of the othe rwise intact myofilament apparatus. 2. Results show a fivefold greater variability in the [Ca2+] required for half-maximum tension generatio n among individual MyoD(-/-) fibres in comparison with controls (p < 0 .05). 3. Consistent with this finding, Western blot analysis showed a sevenfold greater variability in the isoform expression pattern of the thin filament regulatory protein troponin T in MyoD(-/-) compared wit h control fibres (p < 0.05). 4. Electrophoretic analysis of single-fib re segments indicated no apparent alteration in the isoform expression pattern of other regulatory and contractile proteins. In addition, ot her parameters of contractile function, including velocity of unloaded shortening, and maximum force production, were not significantly diff erent between MyoD(-/-) and MyoD(+/+) fibres. 5. These findings indica te that the thin filament structure-function relationship is altered d ue to the MyoD mutation and suggest that MyoD plays a role in establis hing and/or maintaining the differentiated phenotype of adult fast ske letal muscle fibres.