Episodic but not continuous hypoxia elicits long‐term facilitation of phrenic motor output in rats

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Abstract
1 Intermittent hypoxia elicits long-term facilitation (LTF) of phrenic motor output in anaesthetized rats. We tested the hypothesis that an equal cumulative duration of continuous hypoxia would not elicit phrenic LTF. 2 Integrated phrenic nerve activity was recorded in urethane-anaesthetized, vagotomized, paralysed and ventilated rats exposed to: (1) 3 × 3 min hypoxic episodes (inspired O2 fraction (FI,O2) = 0.11) separated by 5 min hyperoxia (FI,O2= 0.5; n= 6), (2) 9 min continuous hypoxia (n= 6), or (3) 20 min continuous hypoxia (n= 7). Isocapnia was maintained throughout the protocol. 3 Consistent with previous studies, phrenic amplitude was significantly elevated for at least 1 h following intermittent hypoxia (78 ± 15% 60 min post-hypoxia; P < 0.05) with an associated increase in burst frequency (11 ± 2.1 bursts min−1; P < 0.05). In contrast, 9 or 20 min continuous hypoxia did not elicit LTF of either phrenic amplitude (4.7 ± 5.1 and 10.1 ± 10.2% 60 min post-hypoxia, respectively; P > 0.05) or frequency (4.6 ± 1.3 and 5.1 ± 2 bursts min−1 60 min post-hypoxia, respectively; P > 0.05). 4 The results indicate that hypoxia-induced long-term facilitation of phrenic motor output is sensitive to the pattern of hypoxic exposure in anaesthetized rats.