Fiber type specific myogenic and proteolytic gene expression after acute resistance exercise in very old women

The purpose of this investigation was to analyze fiber type specific gene expression (GE) of select myogenic [myogenic differentiation factor (MyoD), muscle regulatory factor 4 (MRF4), and myogenin] and proteolytic markers [atrogin‐1, muscle RING finger‐1 (MuRF‐1), Forkhead box 3A (FOXO3A), cysteine‐dependent aspartate protease (caspase‐3)] and myostatin before and after resistance exercise (RE). Six old women (85 ± 1 y, 67 ± 4 kg) performed 3 × 10 knee extensions at 70% of 1‐repetition maximum. Muscle biopsies were taken from vastus lateralis pre and 4‐h post RE. Using real‐time RT PCR, mRNA from pooled myosin heavy chain (MHC)‐I and IIa fibers was amplified and normalized to GAPDH. Myogenic GE was not different between fiber types at baseline. With RE, only MHC‐I fibers (time × type) increased (p<0.05) myogenic GE; MyoD (125%), myogenin (278%), and MRF4 (100%). Both fiber types had a decrease (p<0.05) in myostatin mRNA (I: 61% IIa: 73%). Proteolytic GE was higher (p<0.05) in MHC‐I vs MHC‐IIa fibers at rest and 4‐h post RE for atrogin‐1, FOXO3A, and caspase‐3. With RE, both fiber types induced (p<0.05) atrogin‐1 mRNA (I: 143%, IIa: 118%), while only MHC‐IIa fibers (time × type) increased (p<0.05) MuRF‐1 expression (184%). In summary, after a bout of RE MHC‐I fibers increased myogenic mRNA levels, while the proteolytic mRNA induction after RE was more pronounced in MHC‐IIa fibers. These findings highlight intrinsic differences between MHC‐I and MHC‐IIa fibers at rest and in response to RE, which may contribute to MHC‐IIa selective atrophy with old age. Supported by NIH grant AG18409.