Muscle shortening increases sensitivity of fatigue to severe hypoxia in canine diaphragm

The effects of inspired O₂ on diaphragm tension development during fatigue were assessed using isovelocity (n = 6) and isometric (n = 6) muscle contractions performed during a series of exposures to moderate hypoxia [fraction of inspired O₂ (FI(O₂)) = 0.13], hyperoxia (FI(O₂) = 1), and severe hypoxia (FI(O₂) = 0.09). Muscle strips were created in situ from the canine diaphragm, attached to a linear ergometer, and electrically stimulated (30 Hz) to contract (contraction = 1.5 s/relaxation = 2 s) from optimal muscle length (L(o) = 8.9 cm). Isovelocity contractions shortened to 0.70L(o), resulting in a mean power output of 210 mW/cm². Fatigue trials of 35 min duration were performed while inspired O₂ was sequentially changed between the experimental mixtures and normoxia (FI(O₂) = 0.21) for 5-min periods. In this series, severe hypoxia consistently decreased isovelocity tension development by an average of 0.1 kg/cm² (P < 0.05), which was followed by a recovery of tension (P < 0.05) on return to normoxia. These responses were not consistently observed in isometric trials. Neither isovelocity nor isometric tension development was influenced by moderate hypoxia or hyperoxia. These results demonstrate that the in situ diaphragm is relatively insensitive to rapid changes in O₂ supply over a broad range and that the tension development of the shortening diaphragm appears to be more susceptible to severe hypoxia during fatigue. This may reflect a difference in either the metabolic or blood flow characteristics of shortening contractions of the diaphragm.