Mechanical ventilators are increasingly used for life-saving procedures. Although they are indispensable, they cause lung injury, characterized by pulmonary edema. Modulating ion-channel activities can improve the outcomes of acute lung injury. This present study, for the first time, investigated the role of GABA receptors in modulating ventilator-induced lung injury. GABA decreased pulmonary edema, as shown by the decreased wet-to-dry ratio. The reduced pulmonary edema was due to the improved AFC and a decreased apoptosis.
Ventilator-induced lung injury is characterized by pulmonary edema and inflammation . We observed increases in total BALF protein, wet-to-dry ratio, and vascular damage, as indicated by EB-dye extravasation. The conditions used to induce lung injury vary among investigators and laboratories. For example, tidal volumes range from 20 to 40 ml/kg BW, and ventilation times, from 40 minutes to a few hours. Sometimes, ventilation was done with 100% oxygen. Thus, a great variation is noted in the lung-injury parameters. Wet-to-dry ratio is the simplest method to use to evaluate pulmonary edema. However, it does not differentiate between interstitial and alveolar edema . The wet-to-dry ratios in our study were similar to those in Sprague-Dawley rats [35, 38, 48], but higher than those reported in Wistar-Kyoto rats . The increase in wet-to-dry ratio was evident as early as 30 minutes, but remained the same for up to 60 minutes. Thus, our lung-injury model conforms with those in earlier studies.
Histopathologic analysis indicates that alveolar epithelium was relatively less damaged after the instillation of GABA when compared with saline. Thus GABA might have protected alveolar epithelium from damage, leading to the alleviation of ventilator-induced lung injury. Indeed, GABA reduced apoptosis of distal lung epithelial cells. Increased apoptosis of distal lung epithelial cells occurs in acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) in human patients . Lung epithelial cell apoptosis is one of the critical parameters for evaluating lung injury . Previous studies have shown that apoptotic inhibitors reduce endotoxin-mediated lung injury . GABA attenuated ischemia-mediated neuronal apoptosis . Mechanical ventilation has been shown to activate mitogen-activated protein kinase, which in turn activates various apoptotic pathways [23, 24, 54]. We think that the GABA-mediated reduction in apoptosis enables the distal lung epithelial cells to function properly, including AFC.
Mechanical ventilation for 1 hour decreased AFC in adult rats. The reduction in AFC by mechanical ventilation in our studies was smaller than that in earlier studies in rats [30, 48, 55]. The differences in AFC rates could be due to the systems used: in situ and isolated perfused lungs versus in vivo in our case. AFC was measured in the absence of blood flow in the perfused-lung models. Our current data showed that GABA prevented the pulmonary edema. Our data further support that the effects of GABA on the ventilator-induced pulmonary edema are due to, at least in part, an increase in fluid clearance. High-volume ventilation causes vascular damage  and increases permeability to proteins and small solutes [30, 55, 57]. However, instillation of GABA did not prevent the vascular damage. Furthermore, GABA increased AFC, as directly measured by the FITC-albumin dye. Previous studies have reported that β-adrenergic agonists increase AFC in the injured lungs, and this effect is effectively blocked by amiloride [30, 55], suggesting a role of ENaC in the resolution of edema.
In the present study, GABA increased AFC in ventilator-induced lung injury. We think GABA-mediated effects are due to its effects on the increased Cl-
influx and not efflux. In normal adult rats, GABA receptors mediate Cl-
effluxes and decrease AFC [11
]. Two possible explanations exist:
It is possible that GABA mediates Cl- influxes in the lung under injurious conditions. GABA receptors have been shown to switch the Cl- conductance patterns in fetal and adult cells, depending on intracellular Cl- concentrations [14, 16, 58].
Our recent study showed that AEC I and AEC II operate Cl- fluxes in completely opposite directions, even when activated by the same ligand, UTP. AEC II mediates Cl- efflux, whereas AEC I mediates Cl- influx . Mechanical ventilation might injure AEC I, leading to cessation or inhibition of Cl- influxes. GABA prevents the death of AEC I and thus an increased of net influx. It is unknown whether AEC II reverses Cl- fluxes in injurious conditions.