Acute lung injury (ALI) induces inflammation that disrupts the normal alveolar-capillary endothelial barrier which impairs gas exchange to induce hypoxemia that reflexively increases respiration. The neural mechanisms underlying the respiratory dysfunction during ALI are not fully understood. The purpose of this study was to investigate the role of the chemoreflex in mediating abnormal ventilation during acute (early) and recovery (late) stages of ALI. We hypothesized that the increase in respiratory rate (fR) during post-ALI is mediated by a sensitized chemoreflex. ALI was induced in male Sprague-Dawley rats using a single intra-tracheal injection of bleomycin (Bleo: low-dose = 1.25 mg/Kg or high-dose = 2.5 mg/Kg) (day 1) and respiratory variables- fR, Vt (Tidal Volume), and VE (Minute Ventilation) in response to 10% hypoxia (10% O2, 0% CO2) and 5% hypercapnia/21% normoxia (21% O2, 5% CO2) were measured weekly from W0-W4 using whole-body plethysmography (WBP). Our data indicate sensitization (∆fR = 93 ± 31 bpm, p < 0.0001) of the chemoreflex at W1 post-ALI in response to hypoxic/hypercapnic gas challenge in the low-dose bleo (moderate ALI) group and a blunted chemoreflex (∆fR = −0.97 ± 42 bpm, p < 0.0001) at W1 post-ALI in the high-dose bleo (severe ALI) group. During recovery from ALI, at W3-W4, both low-dose and high-dose groups exhibited a sensitized chemoreflex in response to hypoxia and normoxic-hypercapnia. We then hypothesized that the blunted chemoreflex at W1 post-ALI in the high-dose bleo group could be due to near maximal tonic activation of chemoreceptors, called the “ceiling effect”. To test this possibility, 90% hyperoxia (90% O2, 0% CO2) was given to bleo treated rats to inhibit the chemoreflex. Our results showed no changes in fR, suggesting absence of the tonic chemoreflex activation in response to hypoxia at W1 post-ALI. These data suggest that during the acute stage of moderate (low-dose bleo) and severe (high-dose bleo) ALI, chemoreflex activity trends to be slightly sensitized and blunted, respectively while it becomes significantly sensitized during the recovery stage. Future studies are required to examine the molecular/cellular mechanisms underlying the time-course changes in chemoreflex sensitivity post-ALI.

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.

Background‐ Acute lung injury (ALI) induces inflammation that disrupts the normal alveolar‐capillary endothelial barrier which impairs gas exchange to induce hypoxemia that reflexively increases respiration. Increased respiratory rate (RR) usually develops within a few hours to a few days after ALI. This disorder of acute respiratory failure affects approximately 200,000 new cases each year in the US alone and has high morbidity and mortality. The neural mechanisms underlying the respiratory dysfunction after ALI are not fully understood. The purpose of this study involved investigating the role of carotid body chemoreceptor afferents in mediating abnormal ventilation in ALI. Methods and Results‐ ALI was induced in male SD rats (200‐250g) using a single intra‐tracheal injection of bleomycin (Bleo) (day 0) and respiratory variables‐ RR, TV (Tidal Volume), and MV (Minute Ventilation) in response to 10% hypoxia (10% O2, 0% CO2) and 5% hypercapnia (21% O2, 5% CO2) were measured weekly from W1‐W4 using whole‐ body plethysmography (WBP). MV at 21% O2 increased from baseline (W0) to W1 and W2 in bleo treated rats (p≤0.01 compared to saline treated sham rats). Changes in MV were due to a significant increase in RR while TV showed no changes. Chemoreflex was assessed by measuring the absolute difference between 21% O2 and hypoxia (10% O2, 0% CO2) & hypercapnia (21% O2, 5% CO2). Chemoreflex activation is a fundamental protective reflex that maintains oxygen homeostasis. Our data indicated a blunted chemoreflex at W1 and a sensitized chemoreflex in W3‐W4 during hypoxia and hypercapnia. We hypothesized that the blunted chemoreflex in W1 could be due to maximal pre‐activation of chemoreceptors, called the ‘ceiling effect’. To test this possibility, 90% hyperoxia (90% O2, 0% CO2) was given to bleo treated rats to inhibit the chemoreflex and our results showed no changes in RR, suggesting absence of the tonic activity at W1 post‐ALI. Conclusions‐ These data suggest that chemoreflex activation in response to hypoxia and hypercapnia is blunted at acute stage of lung injury but became sensitized at the recovery stage.

Background‐ Acute lung injury (ALI) induces inflammation that disrupts the normal alveolar‐capillary endothelial barrier which impairs gas exchange to induces hypoxemia that reflexively increases respiration. Increased respiratory rate (RR) usually develops within a few hours to a few days after ALI. The bleomycin (Bleo)‐induced lung injury rat model has been one of the most widely used animal models to study the mechanisms of respiratory failure post ALI. However, systemic and time‐dependent characterization of respiratory parameters in this model has not been done. In the current study, we aimed to use the plethysmography (DSI system) to characterize the changes in variable respiratory parameters at different stages of lung injury in the bleomycin‐induced ALI rat model. Methods and Results‐ ALI was induced in male SD rats (200‐250g) using a single intra‐tracheal injection of Bleo (day 0) and respiratory variables‐ Respiratory rate (RR), Tidal Volume (TV), Minute Ventilation (MV), enhanced pause (Penh), pause (PAU), ratio rate of achieving peak expiatory flow (Rpef), peak inspiratory flow (box) (PIFb), peak expiratory flow box (PEFb), inspiratory time (Ti), expiratory time (Te), expiratory flow at the point of 50%TV (EF50), end inspiratory pause (EIP), end expiratory pause (EEP), relaxation time (Tr) and time of brake (TB) were measured at room air for one hour weekly from W1 (acute phase of ALI) to W4 (recovery phase of ALI) using whole‐ body plethysmography (WBP). Our data showed that MV and RR were significantly elevated from baseline (W0) to W1‐W4 in Bleo‐treated rats whereas TV was decreased from baseline to W1‐W3 post Bleo (p≤0.01 compared to W0). Associated with increased RR, Ti, Te and Tr were significantly shorter at W1‐W4 post Bleo compared to baseline (W0). Penh and Rpef, measures of airway obstruction, were significantly increased at W2 and W1‐W4 post Bleo, respectively. PIFb and PEFb were significantly increased whereas EIP and EEP were significantly decreased in W1‐W4 post Bleo. EF50, a measurement of exhalation flow rate, was significantly increased at W1‐W4 post Bleo. Conclusions‐ These data suggest that a single intratracheal instillation of Bleo induced long‐lasting changes in respiratory function from acute to recovery phase of lung injury. Although blood gas returned to the normal level at W4 post Bleo, many respiratory parameters remained abnormal, indicating that pulmonary function might require a longer time to recover in this model.

Background/hypothesis of the study: Acute lung injury (ALI) increases respiratory rate (RR), induces hypoxemia. This stimulates carotid body (CB) chemoreflex to maintain oxygen homeostasis. Our previous study indicates a sensitized chemoreflex during the recovery of ALI. Chronic chemoreflex hyperactivity negatively affects the overall quality of life. The neural mechanisms underlying the sensitized CB chemoreflex during the recovery from ALI remain a gap in knowledge. Electrical stimulation of the superior cervical ganglion (SCG) sensitizes the CB chemoreflex in rats. We hypothesized that the SCG causes the CB chemoreflex sensitization post-ALI. Methods and Results: We performed a bilateral SCG ganglionectomy (SCGx) or sham-SCGx (Sx) in our male SD rats 2 weeks before inducing ALI (W-2). After 2 weeks of recovery, ALI was induced using a single intra-tracheal instillation of bleomycin (day 1) and measured the resting respiratory variables- RR, TV (Tidal Volume), and MV (Minute Ventilation). The chemoreflex response to 10% hypoxia and 5% normoxic hypercapnia were measured on W(-3) (pre-SCGx), W0 (post-SCGx/pre-ALI) and W4 (post-ALI) using whole-body plethysmography (WBP). Ganglionectomy did not alter the resting lung parameters and the chemoreflex response to either gas challenge in normal, healthy rats. An increase in resting RR at W1 post-ALI was not significantly different between Sx and SCGx rats. At W4 post-ALI, there were no significant differences in resting lung parameters between Sx and SCGx rats. Consistent with our previous study, the CB chemoreflex response to hypoxia and normoxic hypercapnia in Sx rats at W4 post-ALI was sensitized (p<0.01). However, the chemoreflex sensitivity significantly lowered in SCGx rats in response to hypoxia (p=0.01) and normoxic-hypercapnia (p=0.002). Conclusion(s): The CB chemoreflex sensitization during the recovery from the ALI may be mediated by the SCG. Further understanding of the underlying mechanism will provide important information to develop therapeutic approaches to pulmonary disease to improve clinical outcomes. This study was supported by: NIH Grant- RO1 HL-152160 and RO1 HL-126796; Lieberman Grant 2021- Dept. of Anesthesiology, UNMC; American Heart Association- Pre-doctoral fellowship, AHA. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.