Influence of mechanical loading on myosin heavy-chain protein and mRNA isoform expression.

The overall objective of the studies reported herein was to examine the effects of high-resistance training on myosin heavy-chain (MHC) protein and mRNA isoform expression. The findings from these studies can be summarized as follows: 1) there was a substantial increase in the fast type IIX MHC protein isoform content of the trained red and white medial gastrocnemius muscles, but this did not occur until after the eighth training session (i.e., 16 days); 2) single-fiber analyses demonstrated that many so-called fast type IIB fibers contained small amounts of the fast type IIX MHC protein isoform and that the high-resistance training program altered the bias of fast type IIB-type IIX MHC protein isoform distribution in these fibers but did not increase the number of fibers that could be categorized as exclusively fast type IIX fibers; 3) the high-resistance training program produced a rapid (i.e., after two training sessions) elevation in the fast type IIX MHC mRNA isoform and a corresponding repression of the fast type IIB MHC mRNA isoform; and 4) the dose-response study revealed that as few as 10 contractions (40 s) per training session were capable of elevating the expression of the fast type IIX MHC mRNA isoform by approximately 250%. These collective findings demonstrate that high-resistance training is a powerful modulator of MHC protein isoforms and that pretranslational mechanisms are very sensitive to even small amounts of high-resistance training.