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State‐Dependent Expiratory Abdominal Activity During Hypercapnia in Adult Unanaesthetized Rats

Expiration at rest is a passive process. Expiratory muscles (ex: abdominals) are recruited in situations where respiratory demand is increased, such as during hypercapnia, leading to emergence of active expiration (A.E.). The present study tested: (i) the hypothesis that A.E. occurs in a state‐dependent manner (sleep‐wake cycle) during hypercapnia exposure in adult rats and (ii) the functional effects of A.E. in the pulmonary ventilation (VE). Wistar rats (280–330g) were implanted with electrodes for electroencephalography (EEG) and electromyography (EMG) of the neck, diaphragm (DIA) and abdominal (ABD) muscles. A.E. events were considered as rhythmic contraction of the abdominal muscle interposed to diaphragm (i.e., the expiration period). VE was measured by the barometric method. At least eight days after surgery, animals were submitted to the experimental protocol. Animals were allowed to acclimate in the chamber (~30 min). After this period, EEG and EMG wires were connected and animals were exposed (~60 min) to room air and hypercapnia (7% CO2). EEG, EMG and VE were recorded. The results demonstrate that regardless of state, there was no A.E. during room air. During hypercapnia, A.E. occurred in both states, wakefulness and sleep. The DIA‐to‐ABD ratio indicated that the occurrence of A.E. was predominant during non‐REM sleep. When A.E. was present, the VE had a positive correlation (p<0.05) with peak amplitude of ABDEMG activity. The higher the amplitude of ABDEMG, the higher was VE values. Furthermore, in both wakefulness and sleep, VE was also higher (p<0.05) in periods with A.E. compared to periods in which there were no A.E. events These data indicate the predominant expression of A.E. during sleep and its functional role to support VE in conditions of increased respiratory demand.