Ca2+ Sensitivity of Limb Muscle Fibers in Young and Older Adults

Abstract

Age‐induced declines in skeletal muscle contractile function have been attributed to multiple cellular factors, including a loss of peak force (PO), decreased Ca2+ sensitivity, and reduced shortening velocity (VO). However, changes in these cellular properties with aging remain unresolved, especially in older women, and the effect of submaximal Ca2+ on contractile function is unknown. Thus, we compared contractile properties of muscle fibers from 19 young (24±3 years; 11 women) and 21 older adults (77±7 years; 7 women) under maximal and submaximal Ca2+ and assessed the abundance of three proteins thought to influence Ca2+ sensitivity. Fast fibers showed marked atrophy with aging in both men (young=8,599 μm2; old=5,404 μm2) and women (young=4,909 μm2; old=3,174 μm2) that corresponded with a decreased absolute PO in men (young=1.48 mN; old=0.92 mN) and women (young=0.85 mN; old=0.60 mN). There were no differences in fast fiber size‐specific PO, indicating the age‐related decline in force was explained by differences in fiber size. Except for fiber size and absolute PO, no age or sex differences were observed in Ca2+ sensitivity, rate of force development (ktr), or VO. Submaximal Ca2+ depressed ktr and VO, but the effects were not altered by age in either sex. Contrary to rodent studies, regulatory light chain (RLC) and myosin binding protein‐C abundance and RLC phosphorylation were unaltered by age or sex. These data suggest the age‐induced decline in whole‐muscle contractile function is primarily due to the atrophy of fast fibers and that caution is warranted when extending results from rodent studies to humans.Support or Funding InformationNational Institute on Aging grant (R01AG048262) to Robert Fitts and Sandra Hunter and an American Heart Association postdoctoral fellowship (19POST34380411) to Christopher Sundberg