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 (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.

Impact of sarcopenia on diaphragm muscle fatigue

“…Post hoc analysis showed that there was a 29% reduction in the cross‐sectional areas of type IIx and/or IIb fibres in older rats ( P = 0.0001), with type I ( P > 0.99) and IIa fibre cross‐sectional areas unchanged ( P > 0.99; Figure d). The relative contribution of different fibre types to DIAm mass was defined as the product of the proportion and cross‐sectional areas of each fibre type, divided by the total of all fibre types (Elliott et al., ; Khurram et al., ). There was a statistically significant effect of fibre type ( F 2,48 = 297.2; P < 0.0001) and age × fibre type interaction ( F 2,48 = 37.7; P < 0.0001) to the relative contribution to DIAm mass, but no effect of age alone ( F 1,48 = 0.1; P > 0.99).…”

“…A ∼2‐mm‐wide strip was dissected from the mid‐costal portion of the left DIAm and suspended within a tissue bath, with the costal margin clamped and the central tendon tied with silk and attached to a force transducer (6350; Cambridge Technology, Cambridge, MA, USA). Optimal DIAm length ( L o ) and supramaximal stimulus settings were established in a manner identical to past rat studies (Ameredes et al., ; Elliott et al., ; Gosselin et al., ; Khurram et al., , ; Zhan, Watchko, Prakash, & Sieck, ). Electrical field stimulation was achieved via platinum plate electrodes placed on either side of the muscle, with stimulation current provided using a stimulator (701C; Aurora Scientific, Aurora, ON, Canada).…”

“…In adjacent sections, MyHC 2X (6H1, 1:1 dilution; Developmental Studies Hybridoma Bank) were incubated together, with the pattern of staining allowing the DIAm fibre types to be distinguished, in a manner extensively used and previously validated (Elliott et al., ; Prakash & Sieck, ; Schiaffino et al., ). Muscle cross‐sections were imaged using a ×20 oil‐immersion objective (NA 1.0) on an Olympus FV2000 laser confocal microscope (Olympus America, Melville, NY, USA) in a manner identical to past rat reports (Elliott et al., ; Khurram et al., , ).…”

Inspiratory pressure‐generating capacity is preserved during ventilatory and non‐ventilatory behaviours in young dystrophic mdx mice despite profound diaphragm muscle weakness

“…) or age‐related sarcopenia of the diaphragm (Khurram et al . ). Although force loss is severe in mdx diaphragm, ventilatory behaviours are accommodated, at least early in dystrophic disease.…”

Discharge properties of human diaphragm motor units with ageing

“…; Khurram et al . ). A larger action potential area in diaphragm motor units of middle‐aged and older groups compared to the young group is consistent with age‐related changes in the motor unit potentials observed in the genioglossus (Saboisky et al .…”