INFLUENCE OF MECHANICAL LOADING ON MYOSIN HEAVY-CHAIN PROTEIN AND MESSENGER-RNA ISOFORM EXPRESSION

The overall objective of the studies reported herein was to examine th e effects of high-resistance training on myosin heavy-chain (MHC) prot ein and mRNA isoform expression. The findings from these studies can b e summarized as follows: 1) there was a substantial increase in the fa st type IIX MHC protein isoform content of the trained red and white m edial gastrocnemius muscles, but this did not occur until after the ei ghth training session (i.e., 16 days); 2) single-fiber analyses demons trated that many so-called fast type IIB fibers contained small amount s of the fast type IIX MHC protein isoform and that the high-resistanc e training program altered the bias of fast type IIB-type IIX MHC prot ein isoform distribution in these fibers but did not increase the numb er of fibers that could be categorized as exclusively fast type IIX fi bers; 3) the high-resistance training program produced a rapid (i.e., after two training sessions) elevation in the fast type IIX MHC mRNA i soform and a corresponding repression of the fast type IIB MHC mRNA is oform; and 4) the dose-response study revealed that as few as 10 contr actions (40 s) per training session were capable of elevating the expr ession of the fast type IIX MHC mRNA isoform by similar to 250%. These collective findings demonstrate that high-resistance training is a po werful modulator of MHC protein isoforms and that pretranslational mec hanisms are very sensitive to even small amounts of high-resistance tr aining.