Impact of aging on diaphragm muscle function in male and female Fischer 344 rats

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 (Pdi_max ) 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 Pdi_max , which likely impairs the ability of the DIAm to generate higher-force expulsive airway clearance behaviors necessary to maintain airway patency.

Keywords: Aging; Sarcopenia; diaphragm muscle; fiber type; sex differences; transdiaphragmatic pressure.

Figure 1

Diaphragm muscle force–frequency curves (A) and maximal specific force, P ₀ (B), from young and old Fischer 344 rats show a ~30% age‐related reduction in P ₀ in both sexes. Force normalized to the maximal force (C) shows no evidence of a shift between young and old animals of either sex. *Significantly different from young rats for both sexes. Data are shown as means ± 95% CI.

Figure 2

Representative diaphragm muscle cross‐sections (A), fiber‐type proportion (B), average cross‐sectional areas for muscle fiber types (C), and relative contributions (D) from young and old Fischer 344 rats of both sexes. Data points are average cross‐sectional area from each animal, while the summarized data (black line) are the mean ± 95% CI across animals. *, Significantly different from young rats; †, significantly different from females within the same age group.

Figure 3

Representative transdiaphragmatic pressure tracings across motor behaviors (A) and during bilateral phrenic nerve stimulation (B). Esophageal and gastric pressure tracings during phrenic nerve stimulation are also shown, and were verified to deflect in the appropriate direction for all behaviors.

Figure 4

Transdiaphragmatic pressure generation across motor behaviors and during bilateral phrenic nerve stimulations in young (open) and old (gray) animals of both sexes (circle – female; square – male). There was no evidence of impairment in Pdi generation during ventilatory behaviors (eupnea, hypoxia–hypercapnia, sigh, and airway occlusion). By contrast, there was ~20% reduction in Pdi_max (nerve stimulation) in old age in both sexes. *, Significantly different from young rats for both sexes. Data are shown as mean ± 95% CI.