Long‐term facilitation of expiratory and sympathetic activities following acute intermittent hypoxia in rats

Aim Acute intermittent hypoxia ( AIH ) promotes persistent increases in ventilation and sympathetic activity, referred as long‐term facilitation ( LTF ). Augmented inspiratory activity is suggested as a major component of respiratory LTF . In this study, we hypothesized that AIH also elicits a sustained increase in expiratory motor activity. We also investigated whether the expiratory LTF contributes to the development of sympathetic LTF after AIH . Methods Rats were exposed to AIH (10 × 6–7% O 2 for 45 s, every 5 min), and the cardiorespiratory parameters were evaluated during 60 min using in vivo and in situ approaches. Results In unanesthetized conditions ( n  = 9), AIH elicited a modest but sustained increase in baseline mean arterial pressure ( MAP , 104 ± 2 vs. 111 ± 3 mmHg, P  < 0.05) associated with enhanced sympathetic and respiratory‐related variabilities. In the in situ preparations ( n  = 9), AIH evoked LTF in phrenic (33 ± 12%), thoracic sympathetic (75 ± 25%) and abdominal nerve activities (69 ± 14%). The sympathetic overactivity after AIH was phase‐locked with the emergence of bursts in abdominal activity during the late‐expiratory phase. In anesthetized vagus‐intact animals, AIH increased baseline MAP (113 ± 3 vs. 122 ± 2 mmHg, P  < 0.05) and abdominal muscle activity (535 ± 94%), which were eliminated after pharmacological inhibition of the retrotrapezoid nucleus/parafacial respiratory group ( RTN / pFRG ). Conclusion These findings indicate that increased expiratory activity is also an important component of AIH ‐elicited respiratory LTF . Moreover, the development of sympathetic LTF after AIH is linked to the emergence of active expiratory pattern and depends on the integrity of the neurones of the RTN / pFRG .