Work overload induced changes in fast and slow skeletal muscle myosin heavy chain gene expression.

Work induced hypertrophy of the slow postural soleus and the fast phasic plantaris muscles was produced by tenotomy of the synergistic gastrocnemius muscle. Increases in weight of both muscles were associated with proportionately even larger increases in total RNA and mRNA levels. Alterations in levels of specific myosin heavy chain (MHC) isoform mRNAs were measured using the slot blot procedure with radioactively labelled oligonucleotides as probes. Type 1 MHC gene expression was unaffected in both muscles by work overload, whereas type 2a was deinduced in the soleus and type 2b was deinduced in the plantaris. The neonatal MHC gene was transiently reinduced in the plantaris.