Time-dependent alteration in the chemoreflex post-acute lung injury

Chemoreflex sensitization occurs in both male and female rats during recovery from acute lung injury

Introduction

Sex-specific patterns in respiratory conditions, such as asthma, COPD, cystic fibrosis, obstructive sleep apnea, and idiopathic pulmonary fibrosis, have been previously documented. Animal models of acute lung injury (ALI) have offered insights into sex differences, with male mice exhibiting distinct lung edema and vascular leakage compared to female mice. Our lab has provided evidence that the chemoreflex is sensitized in male rats during the recovery from bleomycin-induced ALI, but whether sex-based chemoreflex changes occur post-ALI is not known. To bridge this gap, the current study employed the bleomycin-induced ALI animal model to investigate sex-based differences in chemoreflex activation during the recovery from ALI.

Methods

ALI was induced using a single intra-tracheal instillation of bleomycin (bleo, 2.5 mg/Kg) (day 1). Resting respiratory frequency (fR) was measured at 1-2 days pre-bleo, day 7 (D7) post-bleo, and 1 month (1 mth) post-bleo. The chemoreflex responses to hypoxia (10% O2, 0% CO2) and normoxic-hypercapnia (21% O2, 5% CO2) were measured before bleo administration (pre-bleo) and 1 mth post-bleo using whole-body plethysmography. The apnea-hypopnea Index (AHI), post-sigh apneas, and sighs were measured at each time point.

Results

There were no significant differences in resting fR between male and female rats at the pre-bleo time point or in the increase in resting fR at D7 post-bleo. At 1 mth post-bleo, the resting fR was partially restored in both sexes but the recovery towards normal ranges of resting fR was significantly lower in male rats. The AHI, post-sigh apneas, and sighs were not different between male and female rats pre-bleo and 1 mth post-bleo. However, at D7 post-bleo, the male rats exhibited a higher AHI than female rats. Both male and female rats exhibited a sensitized chemoreflex in response to hypoxia and normoxic-hypercapnia with no significant differences between sexes.

Conclusion

A sex difference in resting ventilatory parameters occurs post ALI with a prolonged increase in resting fR and larger AHI in male rats. On the other hand, we did not find any sex differences in the chemoreflex sensitization that occurs at 1 mth post-bleo. This work contributes to a better understanding of sex-based variations in lung disorders.

The superior cervical ganglion is involved in chronic chemoreflex sensitization during recovery from acute lung injury

Introduction: Acute lung injury (ALI) initiates an inflammatory cascade that impairs gas exchange, induces hypoxemia, and causes an increase in respiratory rate (fR). This stimulates the carotid body (CB) chemoreflex, a fundamental protective reflex that maintains oxygen homeostasis. Our previous study indicated that the chemoreflex is sensitized during the recovery from ALI. The superior cervical ganglion (SCG) is known to innervate the CB, and its electrical stimulation has been shown to significantly sensitize the chemoreflex in hypertensive and normotensive rats. We hypothesized that the SCG is involved in the chemoreflex sensitization post-ALI. Methods: We performed a bilateral SCG ganglionectomy (SCGx) or sham-SCGx (Sx) in male Sprague Dawley rats 2 weeks before inducing ALI (Week -2 i.e., W-2). ALI was induced using a single intra-tracheal instillation of bleomycin (bleo) (day 1). Resting-fR, Vt (Tidal Volume), and V̇ E (Minute Ventilation) were measured. The chemoreflex response to hypoxia (10% O2, 0% CO2) and normoxic-hypercapnia (21% O2, 5% CO2) were measured before surgery on W (-3), before bleo administration on W0 and on W4 post-bleo using whole-body plethysmography (WBP). Results: SCGx did not affect resting fR, Vt and V̇E as well as the chemoreflex responses to hypoxia and normoxic hypercapnia in either group prior to bleo. There was no significant difference in ALI-induced increase in resting fR between Sx and SCGx rats at W1 post-bleo. At W4 post-bleo, there were no significant differences in resting fR, Vt, and V̇E between Sx and SCGx rats. Consistent with our previous study, we observed a sensitized chemoreflex (delta fR) in response to hypoxia and normoxic hypercapnia in Sx rats at W4 post-bleo. However, at the same time, compared to Sx rats, the chemoreflex sensitivity was significantly less in SCGx rats in response to either hypoxia or normoxic hypercapnia. Discussion: These data suggest that SCG is involved in the chemoreflex sensitization during ALI recovery. Further understanding of the underlying mechanism will provide important information for the long-term goal of developing novel targeted therapeutic approaches to pulmonary diseases to improve clinical outcomes.