Changes in diaphragm contractility in cigarette smoking–exposed and smoking cessation rats are associated with alterations in mitochondrial morphology and homeostasis

Background: Cigarette smoking (CS) is one of the greatest risk factors for the pathogenesis and progression of chronic obstructive pulmonary disease (COPD). Although CS cessation is beneficial in preventing COPD progression, the effects of cessation on the diaphragm are unknown, as are the CS-induced mitochondrial changes in the diaphragm during COPD and CS-cessation. In this study, we examined the alterations in mitochondrial morphology and homeostasis, as well as changes in diaphragm contractility during CS exposure and after cessation.Methods: Rats were randomly divided into control, CS-exposure, and CS-cessation groups, including 3-month CS (S3), 6-month CS (S6), 6-month CS followed by 3 months cessation (S6N3), and age-matched control groups. The histological and functional changes in the lungs were examined to evaluate the CS-induced COPD model. The alterations in the diaphragm were further investigated, including contractile properties, the ultrastructure, and the expression of markers of mitochondrial homeostasis.Results: CS exposure caused histological disruption and functional depression in the lungs, and CS cessation failed to result in a significant recovery. CS induced a significant decline in diaphragmatic muscle contractility, accompanied with sever contractile dysfunction in extensor digitorum longus muscles, which was recovered after 3-month CS cessation. CS exposure in parallel disrupted the mitochondrial morphology in diaphragmatic muscle, including decreases in volume density and number density in the S6 group, which was significantly alleviated in the S6N3 group. The mitochondrial quality control was likely depressed in the S6 group, as indicated by the downregulation of Pink1 and Mfn1, markers for mitophagy and mitochondrial fusion/fission. Interestingly, the Mfn1 protein level was recovered after smoking cessation in the S6N3 group.Conclusions: Smoking cessation eased CS-induced diaphragmatic dysfunction and mitochondrial deregulation, but not the adverse changes in pulmonary structure. These diaphragmatic muscle changes are likely associated with deregulated mitochondrial homeostasis, including mitophagy and mitochondrial fusion/fission.