Functional, Ultrastructural, and Transcriptomic Changes in Rat Diaphragms with Different Durations of Cigarette Smoke Exposure

Aims: The aim of the study was to explore the functional and structural changes of the diaphragm and underlying mechanisms in response to 12 or 24 weeks of cigarette smoke (CS) exposure in rats. Materials and Methods: Rats were exposed to CS to develop a COPD model and the rats exposed to room air served as a control group. Rats were randomly divided into four groups: CS12W, CON12W, CS24W, and CON24W. Pulmonary function, lung histopathology, and the contractile properties and ultrastructure of diaphragm muscle were examined in these rats. The changes of transcriptomic profiling of diaphragm muscle were further compared between CS and control rats by the RNA Seq. Results: Both CS groups showed lower FEV0.3/FVC, elevated mean linear intercept (MLI), and reduced mean alveolar numbers (MAN) vs the control groups. The fatigue index (FI) of the diaphragm muscle from the CS12W group, but not CS24W, was significantly increased. Conversely, the force-frequency curves of the diaphragm muscle from the CS24W group, but not CS12W group, were significantly decreased. Consistently, mitochondrial number density (N A ) and volume density (Vv) were increased in the CS12W diaphragm muscle, while being decreased in the CS24W group. Furthermore, the diaphragm transcriptomic profiling results showed that genes regulating cell proliferation and energy metabolic activity were un-regulated and genes regulating protein degradation were down-regulated in the CS12W diaphragm, while CS24W diaphragm showed opposite changes. Conclusion: These observations suggested a transition of diaphragm muscle from initial compensatory to decompensatory changes in function, structure, and gene expression during the development of COPD.