Hypoxic Preconditioning Enhances Diaphragm Function Via Erk And Akt Signaling In COPD Mice

Abstract

Diaphragm experiences sustained hypoxia in chronic obstructive pulmonary disease (COPD). Prolonged low O2 exposure is linked with excessive reactive oxygen species formation, which may exacerbate muscle fatigue and compromise respiratory efficiency. Hypoxic preconditioning (HPC) represents a novel strategy that can effectively protect diaphragm against hypoxic stress caused by COPD. During HPC, muscles are treated with alternate high and low levels of O2, which can induce certain adaptive changes to hypoxia. However, the associated mechanism remains unresolved. PURPOSE: To determine whether ERK and AKT signaling contribute to the protective effect of HPC during hypoxia in COPD mice. METHODS: C57BL6 mice were exposed to cigarette smoke for two hours each day, five days a week for three months to develop COPD symptoms. When the smoking protocol was completed, mice were sacrificed, and their diaphragms were dissected out. Isolated muscle strip was mounted in a contractile chamber and treated with (n = 6) or non-treated with HPC (n = 5) or incubated with AKT inhibitor (MK 2206, 50 μM, n = 5) or ERK inhibitor (PD 98059, 100 μM, n = 5) prior to HPC. Muscle strips were electrically stimulated for five minutes during the middle of a 30-min hypoxia treatment (PO2 = 5 Torr). The contraction force at the end of contraction protocol was recorded and normalized by the initial force to indicate muscle function. Data were expressed as mean ± SE and analyzed using multi-way ANOVA. RESULTS: Our results show that HPC significantly improve muscle function during hypoxia (29 ± 2.8% for HPC vs. 6 ± 1.3% for control, p < 0.05). Either the inhibition of AKT or ERK diminished HPC protective effect on diaphragm (9 ± 2.6% for AKT inhibitor + HPC; 5 ± 1.3% for ERK inhibitor + HPC; 29 ± 2.8% for HPC, p < 0.05). CONCLUSION: We propose that HPC attenuates diaphragm fatigue during hypoxia through AKT and ERK activation in COPD mice.