| Sarcopenia is the aging-related decrease in the quantity and quality of skeletal muscle tissue. Two important changes that are part of sarcopenia are a decrease in the rate of protein synthesis and a decrease in specific tension, or the amount of force a muscle can generate corrected for its cross-sectional area. The purpose of this study is to further elucidate the mechanisms behind these changes, by examining the aging-related changes in myosin heavy chain (MyHC) mRNA and protein content. MyHC protein is part of the molecular motor protein myosin and has been shown to have a decrease in its rate of protein synthesis and function in aging. I hypothesized that a decrease in the amount of MyHC mRNA will help explain, at least in part, for the decrease in MyHC protein synthesis in aging. Further, I hypothesized that a decrease in the concentration of the MyHC protein in skeletal muscle tissue of the elderly will help explain, at least in part, the decreases in specific tension seen in aging. A muscle biopsy was obtained from 16 healthy young (18--25 years) and 16 healthy old (65--80 years) men. None of the subjects had participated in a regular exercise program for at least six months prior to the study and all of the subjects signed an informed consent form before the study. MyHC mRNA was quantified using a real time, reverse-transcriptase, polymerase chain reaction. mRNA results were expressed relative to the type of muscle tissue (type I, IIA or IIX) contained in the biopsy. MyHC protein concentration was determined from single fibers by accurate volume measurements using confocal microscopy, aggressive protein extraction and quantification of protein by highly sensitive gel-electrophoresis. The main findings of the investigation were that contrary to my hypothesis, there were no differences between the young and old subjects in MyHC mRNA content. This supports the hypothesis that the decreases in protein synthesis are due to decreases in protein translation at the ribosome. No significant differences were detected between the young and old subjects for any specific isoform, or between isoforms, however when all of the fibers were pooled, there was a significant difference in MyHC protein content between the young and old subjects (Optical density values: Young type I 112 +/- 8.1, type IIA 96.3 +/- 7.8, type IIX 95.0 +/- 7.9; Old type I 101.2 +/- 6.2, type IIA 82.5 +/- 9.7, type IIX 80.3 +/- 13.5). This decrease helps explain the decreases in specific tension seen in the elderly. |
