The Effect of Aging on Myosin Heavy Chain Concentration in Skeletal Muscle of Young and Old Men

A decrease in specific tension (SpT), or force per unit cross-sectional area of skeletal muscle, in the elderly is well documented. This decrease may be due to a decrease in the number of acto-myosin interactions, a decrease in the force per acto-myosin interaction, or a combination of both of these. We hypothesized that a decrease in the concentration of the contractile proteins would be, at least in part, responsible for the decrease in SpT observed in the elderly. To test this hypothesis, the actin and myosin heavy chain (MyHC) concentration was determined from 73 single muscle fibers from a group of young men (18–25 years) and 72 muscle fibers from old men (65–80 years). The volume of fluorescent-labeled single muscle fibers was measured using confocal microscopy and the contractile protein content was determined using SDS-PAGE with Commassie Blue staining and soft laser densitometry. The main findings of the study were as follows: 1) An aging-related decrease in muscle fiber cross-sectional area, affecting primarily muscle fibers expressing fast MyHC isoforms (Young: 4,695 ± 170μm2 vs. Old: 4,332 ± 124 μm2); 2) A significant aging-related loss of MyHC, but not actin, per unit of cell volume (MyHC: Young: 95.6 ± 4.6 O. D. units vs. Old: 87.2 ± 5.3 O. D. units). There were no differences detected in actin concentration. These data demonstrate that the decrease in SpT in the elderly is, at least in part due to a decrease in the concentration of the molecular motor protein, myosin.