Sarcopenia, defined as muscle weakness and fiber atrophy, of respiratory muscles such as the diaphragm (DIAm) has not been well characterized. The DIAm is the main inspiratory muscle and knowledge of DIAm sarcopenia is important for establishing the effects of aging on respiratory function. We hypothesized that aging is associated with a loss of DIAm force and reduced fiber cross-sectional area (CSA), and that these changes vary across fiber types. DIAm sarcopenia was assessed in young (5 month; n=11) and old (23 month; n=12) wild-type mice reflecting ~100 and 75% survival, respectively. In addition, DIAm sarcopenia was evaluated in BubR1H/H mice (n=4) that display accelerated aging (~60% survival at 5 months) as a result of expression of a hypomorphic allele (H) of the mitotic checkpoint protein BubR1. Maximum specific force (normalized for CSA) of the DIAm was 34% less in old mice and 57% lower in BubR1H/H mice compared to young mice. Mean CSA of type IIx and/or IIb DIAm fibers was 27% smaller in old wild-type mice and 47% smaller in BubR1H/H mice compared to young mice. Mean CSA of type I or IIa fibers was not different between groups. Collectively these results demonstrate sarcopenia of the DIAm in aging wild-type mice and in BubR1H/H mice displaying accelerated aging. Sarcopenia may limit the ability of the DIAm to accomplish expulsive, non-ventilatory behaviors essential for airway clearance. As a result, these changes in the DIAm may contribute to respiratory complications with aging.

Overall, estrogen-based treatments were found to beneficially affect strength.

BackgroundEstrogens are associated with the loss of skeletal muscle strength in women with age. Ovarian hormone removal by ovariectomy in mice leads to a loss of muscle strength, which is reversed with 17β-estradiol replacement. Aging is also associated with an increase in antioxidant stress, and estrogens can improve antioxidant status via their interaction with estrogen receptors (ER) to regulate antioxidant gene expression. The purpose of this study was to determine if ER and antioxidant gene expression in skeletal muscle are responsive to changes in circulating estradiol, and if ERs regulate antioxidant gene expression in this tissue.Methodology/Principal FindingsAdult C57BL/6 mice underwent ovariectomies or sham surgeries to remove circulating estrogens. These mice were implanted with placebo or 17β-estradiol pellets acutely or chronically. A separate experiment examined mice that received weekly injections of Faslodex to chronically block ERs. Skeletal muscles were analyzed for expression of ER genes and proteins and antioxidant genes. ERα was the most abundant, followed by Gper and ERβ in both soleus and EDL muscles. The loss of estrogens through ovariectomy induced ERα gene and protein expression in the soleus, EDL, and TA muscles at both the acute and chronic time points. Gpx3 mRNA was also induced both acutely and chronically in all 3 muscles in mice receiving 17β-estradiol. When ERs were blocked using Faslodex, Gpx3 mRNA was downregulated in the soleus muscle, but not the EDL and TA muscles.Conclusions/SignificanceThese data suggest that Gpx3 and ERα gene expression are sensitive to circulating estrogens in skeletal muscle. ERs may regulate Gpx3 gene expression in the soleus muscle, but skeletal muscle regulation of Gpx3 via ERs is dependent upon muscle type. Further work is needed to determine the indirect effects of estrogen and ERα on Gpx3 expression in skeletal muscle, and their importance in the aging process.

To perform a range of ventilatory and nonventilatory behaviors, the diaphragm muscle (DIAm) must be able to generate sufficient forces throughout the lifespan. We hypothesized that sarcopenia impacts DIAm force generation and thus limits performance of expulsive, higher force, nonventilatory behaviors. Male and female mice (n = 79) at 6 and 24 mo of age (100 vs. 70-75% survival, respectively) were used to examine transdiaphragmatic pressure (Pdi) generation across motor behaviors in vivo and in vitro DIAm specific force. We found a significant effect of age on maximum Pdi (20-41% decline during tracheal occlusion and bilateral phrenic nerve stimulation), maximum DIAm specific force (30% decline), and DIAm fatigue resistance (15% increase). There were no differences between sexes in these age effects on DIAm performance. These results support our hypothesis that sarcopenia primarily impacts higher force, nonventilatory motor behaviors of the DIAm. Such functional limitations may have negative implications in the ability of the DIAm to generate forces needed for airway clearance in old age and thereby contribute to age-related respiratory complications.

Muscle weakness ensues when serum testosterone declines with age in men. Testosterone's female counterpart, estrogen, has also been implicated in age-related strength loss but these results are less conclusive. Our working hypothesis is that estrogens do benefit muscle strength, and that the underlying mechanism involves estrogen receptors to improve muscle quality more so than quantity.