Exercise Preconditioning Attenuates Doxorubicin‐induced Diaphragm Contractile Dysfunction Independent of Phrenic Nerve‐stimulated Force Production

Abstract

Doxorubicin (DOX) is a highly effective anthracycline antibiotic used to treat a wide variety of cancers. Unfortunately, clinical use of DOX is often associated with respiratory muscle weakness characterized by fatigue, dyspnea and exercise intolerance. Investigation into the cause of DOX‐induced respiratory insufficiency has focused primarily on altered diaphragm muscle function; however, these symptoms of respiratory distress may also result from neural impairment. Recent preclinical evidence supports the postulate that neuromuscular function may be compromised following DOX administration, as soleus expression of key proteins required for neuromuscular transmission is negatively impacted. Interestingly, exercise preconditioning is a sufficient stimulus to prevent these adverse changes while also improving muscle quality and function. Although the functional impact of these findings is currently unclear, this data supports the premise that DOX‐induced neuromuscular toxicity contributes to the development of respiratory muscle dysfunction. To test this hypothesis, adult female Sprague‐Dawley rats were randomized into four experimental groups: 1) sedentary‐saline, 2) sedentary‐DOX, 3) exercise‐saline, or 4) exercise‐DOX. Exercise preconditioning consisted of treadmill running for 1 hour/day at 70% VO2Max for 10 days. 24 hours after the last bout of exercise, animals were injected with DOX (20 mg/kg I.P.) or saline (equal volume I.P.). Forty‐eight hours following saline or DOX treatment, the phrenic nerve was carefully isolated and remained innervated to the medial costal diaphragm to determine both phrenic nerve‐dependent diaphragm force and diaphragm force production independent of phrenic nerve activation. Our results demonstrate that maximal diaphragm muscle specific force production was significantly reduced in the sedentary‐DOX group in response to both phrenic nerve and direct diaphragm stimulation compared to both saline‐treated control groups. Importantly, exercise preconditioning in DOX treated animals significantly improved phrenic nerve‐independent diaphragm contractility, while phrenic nerve‐dependent contractile force remained depressed compared to sedentary DOX animals. These results reveal that DOX damages both the muscle contractile apparatus as well as neural input to the diaphragm, and that exercise training is sufficient to improve sarcomere contractility without identical improvements to neural function.