SENSITIVE DETECTION OF MYOSIN HEAVY-CHAIN COMPOSITION IN SKELETAL-MUSCLE UNDER DIFFERENT LOADING CONDITIONS

Different loading conditions were employed to study changes in myosin heavy chain (MHC) composition with the aid of a sensitive approach for separating and detecting MHC isoforms. Separation and detection of MHCs by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were achieved to a degree such that MHC composition is consistent with previous reports on functional and mRNA data. Neonatal MHC was detected at low levels in control, 14-day hindlimb unweighted (HU), and 28-day HU soleus muscles. Type IIa MHC remained unchanged in all groups, representing similar to 9% of total MHC present. Type IIb MHC was not detected in control but represented 3% of total MHC at both 14 and 28 days of HU. Type IIx MHC also was not detected in control but represented 6% of total MHC at 14 days HU, and increased to 14% of total MHC at 28 days HU (P < 0.05). Type I MHC decreased from 89% in control to 72% at 28 days HU (P < 0.05). Furthermore, the type I MHC band was separated into two bands of approximately equal content in all groups when low amounts of protein were loaded on gels. The decrease in type I MHC with HU could be attributed entirely to a decrease in the percentage of the band in the type I region with lower mobility, which corresponds to P-MHC. In addition, hypertrophied plantaris muscles demonstrated a fast-to-slow shift in MHC composition as evidenced by increased I, IIa, and IIx MHC and decreased IIb MHC expression.