Abstract

Background

We sought to elucidate the effects of dexmedetomidine, a selective 2-adrenergic receptor agonist, on the regulation of pulmonary inflammation in ventilator-induced lung injury (VILI) in a rat model.

Methods

A total of 64 adult male Sprague-Dawley rats were assigned to receive either standard ventilation (tidal volume 10 mL/kg; respiratory rate 50 breaths/minute), high-tidal volume ventilation (HV_T: tidal volume 20 mL/kg; respiratory rate 50 breaths/minute), HV_T plus dexmedetomidine (0.5, 2.5 or 5.0 mg/kg per hour), or HV_T plus dexmedetomidine (0.5, 2.5 or 5.0 mg/kg per hour) and yohimbine (the a2-adrenergic receptor antagonist) (n = 8 in each group). The doses of dexmedetomidine were chosen to correspond to 1, 5 and 10 times the clinical dose (0.5 mg/kg per hour). After maintaining ventilation for 4 hours, rats were sacrificed and pulmonary inflammatory changes as well as the upregulation of pulmonary inflammatory molecules were evaluated.

Results

Histological and arterial blood gas analyses confirmed that HV_T induced significant lung injury. HV_T also significantly increased the pulmonary concentrations of chemokines (e.g. macrophage inflammatory protein-2), cytokines (e.g. tumor necrosis factor-a, interleukin [IL]-1b, and IL-6), inducible nitric oxide synthase/nitric oxide, cyclooxygenase-2/prostaglandin E₂. Dexmedetomidine at the dose of 5.0 mg/kg per hour, but not at 0.5 and 2.5 mg/kg per hour, significantly attenuated the effects of HV_T. Moreover, these effects of dexmedetomidine were significantly attenuated by yohimbine.

Conclusion

Dexmedetomidine at clinically relevant doses had no significant effect in attenuating VILI. In contrast, dexmedetomidine at a dose approximately 10 times higher than the clinical dose significantly attenuated VILI. These effects of dexmedetomidine were mediated, at least in part, by the a2-adrenergic receptor.

Keywords

cyclooxygenase 2; cytokines; nitric oxide synthase type II; respiration, artificial;

1. Introduction

Mechanical ventilation may cause serious damage to both healthy and diseased lungs.¹ This type of lung injury is termed ventilator-induced lung injury (VILI).¹ Upregulation of pulmonary inflammatory molecules, including chemokines (e.g. macrophage inflammatory protein-2 [MIP-2]), inducible nitric oxide synthase (iNOS)/nitric oxide (NO), cyclooxygenase-2 (COX-2)/prostaglandin E₂ (PGE₂), and inflammatory cytokines (e.g. tumor necrosis factor-α [TNF-α], interleukin [IL]-1β, and IL-6) has been pro posed to play a crucial role in mediating the development of VILI.²⁻⁵ It has been shown that therapies aimed at attenuating this upregulation of pulmonary inflammatory molecules may be beneficial to the prevention and treatment of VILI.⁶

Dexmedetomidine, a selective agonist of α2- adrenergic receptors, was approved by the US Food and Drug Administration for clinical use as a shortterm (less than 24 hours) sedative agent for critically ill adult patients in late 1999.⁷ Unlike other sedative/analgesic agents, dexmedetomidine has been reported to have minimal effects on respiratory function.⁸ Patients who received dexmedetomidine were reported to be easily aroused.^8,9 The benefit of employing dexmedetomidine in reducing the duration of delirium and coma in mechanically ventilated ICU patients while providing adequate sedation has recently been reported.⁹ In addition to its sedative effects, dexmedetomidine has been shown to possess anxiolytic and analgesic effects that have made dexmedetomidine an effective adjunct to anesthesia and analgesia.⁸ Moreover, dexmedetomidine has been reported to reduce endotoxin-induced systemic inflammatory responses and acute organ injuries in septic rats.¹⁰ Dexme detomidine has also been shown to have significant effects on inhibiting sepsis-induced upregulation of inflammatory molecules, a crucial mechanism that contributes to lightening the physiologic aberrations and systemic inflammatory response seen in critically ill septic patients.¹¹ These data indicated that dexmedetomidine may possess potent anti-inflammatory activity.

Dexmedetomidine is used in patients receiving mechanical ventilation. Whether dexmedetomidine could have significant effects on decreasing VILI in patients receiving mechanical ventilation is a question with profound clinical implications. To elucidate this issue further, we performed a study in intact rats based on the hypothesis that dexmedetomidine attenuates the pulmonary inflammatory changes and upregulation of pulmonary inflammatory molecules following mechanical ventilation.

2. Methods

Sixty-four adult male Sprague-Dawley rats (BioLASCO Taiwan Co. Ltd., Taipei, Taiwan) weighing 200−250 g were used for the experiments. All animal studies were approved by the Animal Use and Care Committee of Mackay Memorial Hospital, and the care and handling of the animals were in accordance with Na tional Institutes of Health guidelines. All rats were fed with a standard laboratory chow and provided with water ad libitum until the day of the experiment.

2.1. Animal preparation

All rats were anesthetized with halothane followed by intraperitoneal injection of pentobarbital (20 mg/ kg; Sigma-Aldrich, St Louis, MO, USA), as in a previous report.¹⁰ Rats were then placed supinely on a heating pad and a rectal temperature probe was inserted. Body temperature was maintained at 37ºC throughout the experiment by a heating pad and heating lamp. Two polyethylene (PE-50; Becton Dickinson, Sparks, MD, USA) catheters containing heparinized isotonic saline were placed in the left femoral vein and right femoral artery. A tracheostomy was then performed and a 14-gauge intravenous (i.v.) catheter (B. Braun Melsungen AG, Melsungen, Germany) was inserted as a tracheostomy tube. Rats were then mechanically ventilated with a small animal ventilator (Rodent Model 683; Harvard Apparatus, South Natick, MA, USA). To facilitate me chanical ventilation, all rats were anesthetized with pentobarbital (5 mg/kg per hour, i.v.) and paralyzed with rocuronium (6 mg/kg per hour, i.v.; NV Organon, Oss, Netherlands) throughout the experiment. Rats were allowed to adapt to the stress of surgery for at least 30 minutes before the experiment began.

2.2. Ventilation protocols

This study employed a high-tidal volume ventilation (HV_T) protocol (tidal volume 20 mL/kg; respiratory rate 50 breaths/minute; fraction of inspired O₂ [FiO₂] 21%) to induce VILI. It also employed a standard tidal volume ventilation protocol (tidal volume 10 mL/kg; respiratory rate 50 breaths/minute; FiO₂ 21%) to serve as the control for ventilation (CV). The HV_T and CV protocols were modified from a previous report,¹² and each could achieve a peak airway pressure of approximately 40 and 20 cmH₂O, respectively. Peak airway pressure was measured with an Exactus II pressure gauge (Omron Health Care Inc., Vernon Hill, IL, USA).

2.3. Experimental protocols

The first set of rats were randomly allocated to receive CV or HV_T (denoted as the CV or HV_T groups; n= 8 in each group) to serve as the negative or positive control, respectively. The second set of rats were randomly allocated to receive HV_T plus de xmedetomidine (0.5, 2.5 or 5.0 μg/kg per hour and denoted as the HV_T -D(0.5), HV_T -D(2.5) or HV_T -D(5.0) groups (n= 8 in each group), respectively. To further elucidate the role of α2-adrenergic receptors, the third set of rats were randomly allocated to receive HV_T plus dexmedetomidine (0.5, 2.5 or 5.0 μg/kg per hour) in addition to yohimbine, a potent α2-adrenergic receptor antagonist (2 doses of 0.1 mg/kg; Sigma-Aldrich) and denoted as the HV_T -D(0.5)-Y, HV_T -D(2.5)-Y or HV_T -D(5.0)-Y groups (n= 8 in each group), respectively. Rats in the HV_T - D(0.5), HV_T -D(2.5), HV_T -D(5.0), HV_T -D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T -D(5.0)-Y groups received a loading dose of dexmedetomidine (1 μg/kg, intravenous infusion over 10 minutes) followed by dexmedetomidine infusion at different doses (i.e. 0.5, 2.5 and 5.0 μg/kg per hour, respectively) until the end of the experiment. Rats in the HV_T -D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T -D(5.0)-Y groups also received yohimbine (0.1mg/kg, intravenous infusion over 5 minutes) immediately after the loading dose of dexmedetomidine and 150 minutes after the experiment began. Rats in the CV group were ventilated with the CV protocol for 300 minutes before sacrifice. In contrast, rats in the HV_T, HV_T -D(0.5), HV_T - D(2.5), HV_T -D(5.0), HV_T -D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T -D(5.0)-Y groups were ventilated with the CV protocol for 60 minutes, which was then switched to the HV_T protocol for another 240 minutes to induce VILI. At the end of the experiment, all rats were sacrificed by injection of high dose pentobarbital (300 mg/kg, i.v.).

The duration for HV_T (i.e. 240 minutes) was determined from previous data in which HV_T for at least 180 minutes was needed before the development of VILI became clinically evident in rats.¹³ The dose of dexmedetomidine for clinical application (e.g. ICU sedation) is approximately 0.5 μg/kg per hour (0.2−0.7 μg/kg per hour).⁸ The doses of dexmedetomidine employed in this study were thus chosen accordingly to correspond to approximately 1, 5 and 10 times the clinical dose. The dosage and time points for yohimbine administration were determined according to a previous report.¹⁴

2.4. Arterial blood gas analysis

At the end of each experiment, blood (0.5 mL) was drawn from the right femoral artery. Arterial blood gas levels were immediately measured with a blood gas analyzer (Rapidlab 348; Bayer Healthcare LLC, East Walpole, MA, USA).

2.5. Tissue sample collection

At the end of each experiment, the left lung was removed, snap frozen in liquid nitrogen and then stored at −80ºC for subsequent analysis. The right lung was infused with 4% formaldehyde and then removed. 

2.6. Histological analysis

Formaldehyde-infused right lungs were then embedded in paraffin wax, serially sectioned, and stained with hematoxylin and eosin. Morphological characteristics were then evaluated under light microscopy to evaluate lung injury. According to the morphological characteristics, i.e. the edematous changes of the alveolar wall, swelling of the alveolar epithelium, and infiltration of polymorphonuclear leukocyte, the lung tissue injuries were classified as normal, mild, moderate, or severe inflammation by a pathologist who was blinded to the experiment.

2.7. NO, PGE2, TNF-α, IL-1β, IL-6, and MIP-2 assays

The lung tissues were homogenized, centrifuged, and the supernatants were transferred and centrifuged again to remove large proteins. Then the filtrates were analyzed for the concentrations of stable NO metabolites, nitrite (NO₂ −) and nitrate (NO₃ −), using a colorimetric assay kit (Cayman Chemical, Ann Arbor, MI, USA) that involves the Griess reaction. The filtrates were also analyzed for the concentrations of PGE₂, TNF-α, IL-1β, IL-6, and MIP-2, using enzyme-linked immunosorbent assay (ELISA) (PGE₂ Competitive ELISA Kit, TNF-α Colorimetric ELISA Kit, IL-1β Colorimetric ELISA Kit, and IL-6 Colorimetric ELISA Kit from Pierce Biotechnology Inc., Rockfold, IL, USA; and MIP-2 ELISA Kit from R&D Systems Inc., Minneapolis, MN, USA).

2.8. Reverse transcription-polymerase chain reaction (RT-PCR)

RNA was isolated and reverse transcription performed, as we have previously described.¹⁵ The complementary DNA encoding iNOS, COX-2, and β-actin (as an internal standard) were then amplified using PCR. The primer sequences for each enzyme were adapted from those previously published.^15,16 Amplification of iNOS/β-actin was performed using 35 cycles of 92ºC for 40 seconds, 57ºC for 40 seconds, 75ºC for 75 seconds with a final extension of product at 55ºC over 5 minutes. Amplification of COX-2/β-actin was performed using 35 cycles of 94ºC for 15 seconds, 55ºC for 15 seconds, 72ºC for 1 minute with a final extension of product at 72ºC for 7 minutes. PCRamplified samples were separated on 1% ethidium bromide-stained agarose gels. The Gel Doc 2000 System (Bio-Rad Laboratories, Hercules, CA, USA) was used to assay the PCR products. The cDNA band densities were quantified by using densitometric techniques with Scion Image for Windows (Scion Corp., Frederic, MD, USA).

2.9. Statistical analysis

The homogeneity of variances was tested. For variances with homogeneity, one-way analysis of variance (ANOVA) with the Bonferroni test was used for multiple comparisons. For variances with heterogeneity, a Kruskal-Wallis ANOVA on ranks with Dunnett’s test was used for multiple comparisons. Data were presented as mean ± standard deviation. The significance level was set at 0.05. A commercial software package (SigmaStat for Windows; SPSS Inc., Chicago, IL, USA) was used for data analysis.

3. Results

3.1. Effects of dexmedetomidine on pulmonary inflammation

Lung tissues harvested from rats in the CV group revealed mild lung inflammation (Figure 1A). In contrast, lung tissues harvested from rats in the HVT group revealed severe lung inflammation as evidenced by marked edematous changes of the alveolar wall, marked swelling of the alveolar epithelium, and massive polymorphonuclear leukocyte infiltration (Figure 1C). Similar results were observed in the HV_T -D(0.5), HV_T -D(2.5), HV_T -D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T -D(5.0)-Y groups. On the other hand, lung tissues harvested from rats in the HVT -D(5.0) group revealed moderate lung inflammation (Figure 1B).

Figure 1

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3.2. Effects of dexmedetomidine on arterial blood gas analysis

The values of pH, PaO₂, and base excess of the HV_T group were significantly lower than those of the CV group (p= 0.021, 0.027, and 0.005, respectively; Table 1). In contrast, the PaCO₂ value of the HV_T group was significantly higher than that of the CV group (p= 0.019; Table 1). The values of pH, PaO₂, PaCO₂, and base excess of the HV_T -D(0.5), HV_T -D(2.5), HV_T -D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T -D(5.0)-Y groups were not significantly different from those of the HV_T group (Table 1). In contrast, the values of pH, PaO₂ and base excess of the HV_T -D(5.0) group were significantly higher than those of the HV_T group (p= 0.035, 0.031, and 0.017, respectively; Table 1), whereas the PaCO₂ value of the HV_T -D(5.0) group was significantly lower than that of the HV_T group (p= 0.036; Table 1).

Table 1

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3.3. Effects of dexmedetomidine on pulmonary TNF-α, IL-1β, IL-6, and MIP-2 production

Our ELISA data revealed that the concentrations of TNF-α, IL-1β, IL-6, and MIP-2 of the CV group were low (Figure 2). The concentrations of TNF-α, IL-1β, IL-6, and MIP-2 of the HV_T, HV_T -D(0.5), HV_T-D(2.5), HV_T -D(5.0), HV_T-D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T -D(5.0)- Y groups were significantly higher than those of the CV group (all p< 0.001; Figure 2). TNF-α concentrations of the HV_T -D(0.5), HV_T -D(2.5), HV_T -D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T -D(5.0)-Y groups were not signif icantly different from that of the HVT group, whereas the TNF-α concentration of the HVT -D(5.0) group was significantly lower than that of the HVT group (p= 0.018; Figure 2). Moreover, the TNF-α concentration of the HV_T -D(5.0)-Y group was significantly higher than that of the HVT -D(5.0) group (p= 0.023; Figure 2). Similarly, IL-1β concentrations of the HV_T - D(0.5), HV_T -D(2.5), HV_T -D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T -D(5.0)-Y groups were not significantly different from that of the HV_T group, whereas the IL-1β concentration of the HV_T -D(5.0) group was significantly lower than that of the HV_T group (p= 0.027; Figure 2). The IL-1β concentration of the HV_T -D(5.0)-Y group was also significantly higher than that of the HV_T -D(5.0) group (p= 0.039; Figure 2). The IL-6 and MIP-2 concentrations of the HV_T -D(0.5), HV_T -D(2.5), HV_T -D(0.5)-Y, and HV_T -D(2.5)-Y groups were not significantly different from that of the HV_T group, whereas the IL-6 and MIP-2 concentrations of the HV_T -D(5.0) and HV_T - D(5.0)-Y groups were significantly lower than that of the HV_T group (IL-6: p= 0.016 and 0.029, respectively; MIP-2: p= 0.022 and 0.026, respectively; Figure 2). Moreover, IL-6 and MIP-2 concentrations of the HV_T -D(5.0)-Y group were significantly higher than those of the HV_T -D(5.0) group (p= 0.025 and 0.031, respectively; Figure 2).

Figure 2

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Figure 2 Assays of the concentrations of tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-6, and macrophage inflammatory protein-2 (MIP-2) in the lung tissues harvested from rats of the CV, HVT, HVT plus dexmedetomidine (0.5, 2.5 or 5.0 μg/kg/mL), or HVT plus dexmedetomidine (0.5, 2.5 or 5.0 μg/kg/mL) plus yohimbine groups. Data are mean ± standard deviation. D = dexmedetomidine; Y = yohimbine. *p < 0.05 vs. CV group; †p < 0.05 vs. HVT group; ‡p < 0.05, HVT -D(5.0)-Y group vs. HVT -D(5.0) group.

3.4. Effects of dexmedetomidine on pulmonary iNOS transcription and NO production

Data from RT-PCR and chemiluminescence revealed that the pulmonary concentrations of iNOS mRNA and NO of the CV groups were low (Figure 3). The concentrations of iNOS mRNA and NO of the HV_T, HV_T -D(0.5), HV_T -D(2.5), HV_T -D(5.0), HV_T -D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T -D(5.0)-Y groups were significantly higher than those of the CV group (all p< 0.001; Figure 3). The concentrations of iNOS mRNA and NO of the HV_T -D(0.5), HV_T -D(2.5), HV_T -D(0.5)-Y, and HV_T - D(2.5)-Y groups were not significantly different from those of the HV_T group (Figure 3). In contrast, iNOS mRNA concentration of the HV_T -D(5.0) group was significantly lower than that of the HV_T group (p= 0.015; Figure 3), whereas the iNOS mRNA concentration of the HV_T -D(5.0)-Y group was not significantly different from that of the HV_T group (Figure 3). The iNOS mRNA concentration of the HV_T -D(5.0)-Y group was also significantly higher than that of the HV_T -D(5.0) group (p= 0.019; Figure 3). The NO concentrations of the HV_T -D(5.0) and HV_T -D(5.0)-Y groups were significantly lower than that of the HV_T group (p= 0.017 and 0.031, respectively; Figure 3). Moreover, the NO concentration of the HV_T -D(5.0)-Y group was significantly higher than that of the HV_T -D(5.0) group (p= 0.026; Figure 3). 

Figure 3

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Figure 3 Assays of the concentrations of inducible nitric oxide synthase (iNOS) mRNA and nitric oxide (NO) in the lung tissues harvested from rats of the CV, HVT, HVT plus dexmedetomidine (0.5, 2.5 or 5.0 μg/kg/mL), or HVT plus dexmedetomidine (0.5, 2.5 or 5.0 μg/kg/mL) plus yohimbine groups. Data are mean ± standard deviation. D = dexmedetomidine; Y = yohimbine. *p < 0.05 vs. CV group; †p < 0.05 vs. HVT group; ‡p < 0.05, HVT -D(5.0)-Y group vs. HVT -D(5.0) group.

3.5. Effects of dexmedetomidine on pulmonary COX-2 transcription and PGE2 production

Data from RT-PCR and ELISA revealed that the concentrations of COX-2 mRNA and PGE₂ of the CV groups were also low (Figure 4). The concentrations of COX-2 mRNA and PGE₂ of the HV_T, HV_T -D(0.5), HV_T -D(2.5), HV_T -D(5.0), HV_T -D(0.5)-Y, HV_T -D(2.5)-Y, and HV_T - D(5.0)-Y groups were significantly higher than those of the CV group (all p< 0.001; Figure 4). The COX-2 mRNA concentrations of the HV_T -D(0.5), HV_T -D(0.5)-Y, HV_T -D(2.5), and HV_T -D(2.5)-Y groups were not significantly different from those of the HV_T group, whereas the HV_T -D(5.0) and HV_T -D(5.0)-Y groups were significantly lower than that of the HV_T group (p= 0.031 and 0.036, respectively; Figure 4). Moreover, the COX-2 mRNA concentration of the HV_T -D(5.0)-Y group was significantly higher than that of the HV_T -D(5.0) group (p= 0.028; Figure 4). The PGE2 concentrations of the HV_T -D(2.5), HV_T -D(5.0), and HV_T -D(5.0)-Y groups were significantly lower that of the HV _T group (p= 0.016, 0.005, and 0.019, respectively; Figure 4), whereas the PGE₂ concentration of the HV_T -D(2.5)-Y group was not significantly different from that of the HV_T group (Figure 4). The PGE₂ concentration of the HV_T -D(2.5)-Y group was significantly higher than that of the HV_T -D(2.5) group (p= 0.039; Figure 4). Similarly, the PGE₂ concentration of the HV_T -D(5.0)-Y group was significantly higher than that of the HV_T -D(5.0) group (p= 0.033; Figure 4).

Figure 4

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Figure 4 Assays of the concentrations of cyclooxygen- ase-2 (COX-2) mRNA and prostaglandin E2 (PGE2) in the lung tissues harvested from rats of the CV, HVT, HVT plus dexmedetomidine (0.5, 2.5 or 5.0 μg/kg/mL), or HVT plus dexmedetomidine (0.5, 2.5 or 5.0 μg/kg/mL) plus yohimbine groups. Data are mean ± standard deviation. D = dexmedetomidine; Y = yohimbine. *p < 0.05 vs. CV group; †p < 0.05 vs. HVT group; ‡p < 0.05, HVT -D(5.0)-Y group vs. HVT -D(5.0) group.

4. Discussion

The use of dexmedetomidine has been approved for sedation in critically ill adult patients. In mechanically ventilated ICU patients, dexmedetomidine has been shown to reduce the duration of delirium and coma while providing adequate sedation.⁹ How ever, findings from this study revealed that dexmedetomidine, in the clinical dose range, did not show significant effects on regulating the pulmonary inflammatory changes or the upregulation of pulmonary inflammatory molecules in a rat model of VILI. Recent data indicate that some patients, especially pediatric patients, might need higher doses of dexmedetomidine (approximately 2−5 times higher than the clinical dose) to achieve adequate sedation.^9,17 Our data also indicated that dexmedetomidine at such a dose range did not affect the lung injury and production of pulmonary inflammatory molecules induced by HV_T. These results seem to suggest that use of clinically relevant doses of dexmedetomidine in treatment of patients under mechanical ventilation should provide negligible therapeutic effects against VILI.

In contrast, findings from this study revealed that dexmedetomidine at a dose approximately 10 times higher than the clinical dose significantly attenuated the HVT -induced lung injury and upregulation of pulmonary inflammatory molecules. These findings, in concert with those results previously observed in septic rats,10 highlight the possible therapeutic potential of dexmedetomidine. Although the safety of higher doses of dexmedetomidine in clinical application remains to be elucidated, previous results have indicated that the most significant adverse effects associated with higher doses of dexmedetomidine are hypotension and bradycardia.^8,18 Therefore, it is unlikely that such a high dose of dexmedetomidine would be administered in clinical practice. From these findings, the use of dexmedetomidine against VILI is likely to be of minimal clinical significance.

Nevertheless, data from this study also revealed that the observed therapeutic effects of dexmedetomidine were mediated, at least in part, by α2- adrenergic receptors as the anti-inflammatory effects of dexmedetomidine were significantly attenuated by the α2-adrenergic receptor antagonist yohimbine. The data not only suggest a crucial role for α2-adrenergic receptors but also highlight the therapeutic potential of α2-adrenergic receptor activation. Judging from our data, future studies aimed at potentiating the anti-inflammatory capacity of the α2-adrenergic receptors may very likely prove to be fruitful and thus warrants further investigation.

Results from this study, in concert with previous results,¹² confirmed that mechanical ventilation with HVT induced significant lung injury with inflammatory changes and upregulation of pulmonary inflammatory molecules, including MIP-2, TNF-α, IL-1β, IL-6, iNOS/NO, and COX-2/PGE₂. The production of these inflammatory molecules is under the regulation of nuclear factor-κB (NF-κB) and mitogen-activated protein kinases (MAPKs), the two pathways that have also been shown to play crucial roles in mediating the development of VILI.¹⁹⁻²¹ Previous data indicated that activation of NF-κB and MAPKs required a transient increase in intracellular calcium concentration.²² Dexmedetomidine has been shown to increase intracellular calcium concentrations at clinical doses, whereas this effect was attenuated when the dose of dexmedetomidine was increased to approximately 10 times the clinical dose.²³ Judging from these data, we thus speculate that the observed therapeutic effects of dexmedetomidine on attenuating the upregulation of pulmonary inflammatory molecules induced by HVT may very likely be associated with its effect in regulating intracellular calcium concentrations.

Pulmonary inflammatory changes and upregulation of pulmonary inflammatory molecules have been proposed to play crucial roles in mediating the development of VILI.²⁻⁵ In addition to these mechanisms, previous reports also indicated that several other crucial mechanisms, such as vascular remodeling, activation of the renin-angiotensin system, stretch-activated ion channels, extracellular matrixintegrin-cytoskeleton complex, and growth factor receptors, may also contribute to the development of VILI.²⁴⁻²⁶ The effects of dexmedetomidine on these above-mentioned mechanisms remain to be elucidated. Judging from our data, we speculate that dexmedetomidine at clinically relevant doses may very likely have negligible effects on regulating these mechanisms. However, more studies are needed before further conclusions can be drawn.

In summary, dexmedetomidine at clinically relevant doses offered no significant effects on the regulation of HV_T-induced lung injury and upregulation of pulmonary inflammatory molecules in a rat model. In contrast, dexmedetomidine at a dose approximately 10 times higher than the clinical dose significantly attenuated VILI. Moreover, these effects of dexmedetomidine were mediated, at least in part, by α2-adrenergic receptors.

Acknowledgments

This work was performed mainly in Mackay Memorial Hospital and supported by a grant from Mackay Memorial Hospital (MMH9758) awarded to Dr C.J. Huang.