Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCB007

Poster Communications

Acute hypoxic stress causes diaphragm muscle weakness in mice and rats

A. J. O'Leary1, C. Rieux1, L. Browne1, K. D. O' Halloran1

1. Physiology, University College Cork, Cork, Ireland.

Diaphragm weakness is a strong predictor of poor outcome in patients. Acute hypoxia (AH) is a feature of respiratory conditions such as acute respiratory distress syndrome and ventilator-associated lung injury. However, the effects of AH on the diaphragm are largely unknown despite the potential clinical relevance. Adult male C57BL6/J mice (n=8 per group) were exposed to 8hr of AH (FiO2 = 0.10) or normoxia. A separate group of mice (n=8) were administered N-acetyl cysteine (NAC; 200mg/kg, I.P.) immediately prior to AH. Ventilation was assessed using whole-body plethysmography. Oxygen consumption and carbon dioxide production were measured. Diaphragm muscle contractile performance was determined ex-vivo. Gene expression was examined at 1, 4, and 8hrs using qRT-PCR. Citrate synthase activity was measured following AH exposure using a spectrophotometric assay. Ventilation, metabolism and diaphragm contractile function was also determined in adult male Sprague Dawley rats exposed to normoxia (n=8) and AH (n=8). Data are mean±SD and were compared by ANOVA or Student t test; p values are reported. In mice, minute ventilation during AH was initially increased (p<0.001, ANOVA at 10mins), but quickly returned to normoxic levels for the duration of gas exposure. VCO2 production was reduced throughout AH exposure (p<0.0001). AH decreased diaphragm peak force (30±3 vs. 21±2 N/cm2, p=0.0334) and force-frequency relationship (p=0.0112), but increased endurance (p<0.0001). AH increased PGC1α (p<0.05), UCP-3 (p<0.01), FOXO-3 (p<0.05) and MuRF-1 (p<0.05) gene expression. Citrate synthase activity was increased (p<0.05) following AH. NAC pre-treatment prevented the AH-induced diaphragm weakness. In rats, AH increased minute ventilation (P<0.0001) throughout the 8hr exposure associated with a decline in VCO2 production (p<0.0001). AH caused diaphragm weakness in rat similar to mouse. Diaphragm weakness is reported in mechanically ventilated patients, which is primarily attributed to inactivity (unloading) of the muscle, although this is controversial. The potential role of hypoxia in the development and/or exacerbation of ICU-related weakness is unclear and perhaps underestimated. Our data reveals that AH is sufficient to cause diaphragm muscle weakness, which may relate to atrophy, as evident from increased pro-atrophy (MuRF-1) gene expression. Muscle weakness likely relates to direct hypoxic stress, as there was no persistent change in ventilation (muscle activity) in mouse compared with rat yet the muscle weakness was similar in the two species. Moreover, muscle weakness was prevented by antioxidant supplementation. These findings highlight a potentially critical role for hypoxia in diaphragm muscle dysfunction observed in acute respiratory patients.

Where applicable, experiments conform with Society ethical requirements