Abstract
The diaphragm muscle (DIAm) is the primary inspiratory muscle in mammals and is active during ventilatory behaviors, but it is also involved in higher‐force behaviors such as those necessary for clearing the airway. Our laboratory has previously reported DIAm sarcopenia in rats and mice characterized by DIAm atrophy and a reduction in maximum specific force at 24 months of age. In Fischer 344 rats, these studies were limited to male animals, although in other studies, we noted a more rapid increase in body mass from 6 to 24 months of age in females (~140%) compared to males (~110%). This difference in body weight gain suggests a possible sex difference in the manifestation of sarcopenia. In mice, we previously measured transdiaphragmatic pressure (Pdi) to evaluate in vivo DIAm force generation across a range of motor behaviors, but found no evidence of sex‐related differences. The purpose of this study in Fischer 344 rats was to evaluate if there are sex‐related differences in DIAm sarcopenia, and if such differences translate to a functional impact on Pdi generation across motor behaviors and maximal Pdi (Pdimax) elicited by bilateral phrenic nerve stimulation. In both males and females, DIAm sarcopenia was apparent in 24‐month‐old rats with a ~30% reduction in both maximum specific force and the cross‐sectional area of type IIx and/or IIb fibers. Importantly, in both males and females, Pdi generated during ventilatory behaviors was unimpaired by sarcopenia, even during more forceful ventilatory efforts induced via airway occlusion. Although ventilatory behaviors were preserved with aging, there was a ~20% reduction in Pdimax, which likely impairs the ability of the DIAm to generate higher‐force expulsive airway clearance behaviors necessary to maintain airway patency.
Highlights
Sarcopenia, which is defined as the age-related loss of muscle-specific force and cross-sectional area, is observed in the diaphragm muscle (DIAm) (Gosselin et al 1994; Greising et al 2013, 2015a,b; Elliott et al 2016a,c) and may underlie the increased susceptibility to respiratory disease observed clinically among older individuals (Sharma and Goodwin 2006; Lowery et al 2013)
In a previous study in mice, we found that DIAm sarcopenia is associated with an age-related reduction in Pdimax as well as Pdi generated during maximal ventilatory efforts against an occluded airway (Greising et al 2015b)
Among the 24-month-old Fischer 344 (F344) rats, ~20% were excluded due to the presence of a tumor detected upon necropsy or a failure to thrive after the animals arrived
Summary
Sarcopenia, which is defined as the age-related loss of muscle-specific force and cross-sectional area, is observed in the diaphragm muscle (DIAm) (Gosselin et al 1994; Greising et al 2013, 2015a,b; Elliott et al 2016a,c) and may underlie the increased susceptibility to respiratory disease observed clinically among older individuals (Sharma and Goodwin 2006; Lowery et al 2013). The DIAm comprises different fiber types that vary in mechanical and fatigue properties (Sieck et al 1989a; Geiger et al 2000). Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society.
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