Sieck, Lucy

Aging selectively impacts the function of type IIx/IIb fibers in the diaphragm muscle. We show that sarcopenia reduces maximum specific force, maximum shortening velocity, and peak power output, whereas endurance during repetitive contractions is preserved. These findings explain why higher force airway clearance behaviors decline with age, whereas fatigue-resistant breathing behavior remains resilient.

Neuromotor control of diaphragm muscle (DIAm) involves size-dependent recruitment of phrenic motor neurons (PhMNs). To accomplish breathing, only smaller PhMNs innervating slow (type S) and fast, fatigue resistant (type FR) DIAm motor units are recruited. Thus, smaller PhMNs and type S and FR DIAm motor units are very active in sustaining ventilation throughout life. Larger PhMNs innervating more fatigable fast (type FF) motor units are very infrequently recruited for higher force, expulsive behaviors. Previously, we reported that smaller PhMNs have higher mitochondrial volume densities (MVD) reflecting their more frequent activation. Mitochondria are essential for maintaining energetic support and cellular homeostasis. We hypothesized that to sustain higher MVD, mitochondrial biogenesis is higher in smaller compared to larger PhMNs. In female and male Sprague Dawley rats (~3 months of age) PhMNs were retrogradely labeled by intrapleural injection of cholera toxin B (CTB). After 3 days, mitochondria in the cervical spinal cord were labeled by subdural injection of MitoTracker Red. The cervical spinal cord was collected and labeled PhMNs and mitochondria were imaged by high-resolution confocal microscopy and reconstructed in 3D. As previously reported, we found a significant negative correlation between MVD and PhMN somal surface area. By immunohistochemistry, we found an increased nuclear to cytosolic ratio of pCREBS1333 in smaller PhMNs compared to larger PhMNs. PGC1α and TFAM expression was also higher in smaller PhMNs. These findings support size-dependent differences in pCREBS133–PGC1α–TFAM pathway promoting mitochondrial biogenesis. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Sarcopenia is defined as age-related atrophy of muscle fibers as well as a decrease in specific force (force per cross-sectional area). In the rat diaphragm muscle (DIAm), sarcopenia primarily affects type IIx/IIb fibers that generate greater specific force, have the fastest maximum shortening velocities, but display greater fatigue with repeated activation. We hypothesized that as a result of sarcopenia, maximum shortening velocity and power output of the DIAm decrease with age. DIAm strips ~2-3 mm wide were excised from 6-month and 24-month female (n=4 per age group) and male (n=4 per age group) rats. DIAm strips were suspended in a tissue chamber containing Rees–Simpson solution maintained at 26oC, with the costal margin clamped and the central tendon attached to a Cambridge Dual-Mode force/length transducer. To determine maximum isometric specific force, DIAm strips were stimulated using platinum plate electrodes with supramaximal current (~250 mA) pulses (1 ms) at 75 Hz delivered in 1-s duration train. The muscle strips were then allowed to shorten over a 30-ms period against different percentages of maximum isometric load. Based on the load/velocity relationship, the maximum shortening velocity (Vmax) and peak power output were determined. Subsequently, the muscle strip was repetitively stimulated and allowed to shorten at 30% maximum load (approximating peak power output). In this protocol, the muscle strip was stimulated at 75 Hz an generated isometric force during the initial 225 ms of the 330 ms duration train the muscle. Thereafter, the load was reduced to 30% maximum and the muscle strip was allowed to shorten for the subsequent 105 ms. Finally, the DIAm strip was stretched to ~150% its resting length (corresponding to optimal sarcomere length) and rapidly frozen in melting isopentane. Transverse serial sections of DIAm fibers were cut, and fiber types identified by immunoreactivity to specific myosin heavy chain (MyHC) isoforms. As previously observed, the cross-sectional areas of type IIx/IIb DIAm fibers and maximum isometric specific force were reduced in 24-month compared to 6-month rats confirming sarcopenia. The maximum shortening velocity of the DIAm in 24-month rats was slower than in the 6-month rats and peak power output of the DIAm was reduced in 24-month rats. The duration during which peak power output could be maintained (i.e., endurance) of the DIAm was prolonged in 24-month compared to 6-month rats (120 s vs 60 s, respectively). These results support our hypothesis that aging affects maximum shortening velocity and peak power output of the DIAm. It is important to note that endurance of the DIAm is preserved, consistent with resilience of breathing motor function in old age. Research supported by NIH grant AG44615 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.