Dead space ventilation promotes alveolar hypocapnia reducing surfactant secretion by altering mitochondrial function

Area of Science:

• Pulmonary physiology
• Cellular biology
• Respiratory medicine

Background:

• Acute respiratory distress syndrome (ARDS) is characterized by increased dead space ventilation (VD/VT) and decreased alveolar CO2 (low [CO2]iA).
• Alveolar hypocapnia in ARDS is linked to atelectasis and surfactant depletion, but the underlying mechanisms remain unclear.

Purpose of the Study:

• To elucidate the mechanism by which alveolar hypocapnia affects surfactant secretion and alveolar ventilation.
• To investigate the role of intracellular calcium and mitochondria in CO2 sensing in alveolar epithelial cells (AECs).

Main Methods:

• Experiments were conducted in isolated perfused rat lungs and ventilated rabbits.
• Measurements included cytosolic and mitochondrial calcium concentrations ([Ca2+]cyt, [Ca2+]mito) in AECs, surfactant secretion, and alveolar size using real-time imaging and Western blot.
• Pulmonary artery ligation and inspiratory CO2 administration were used to modulate ventilation and assess responses in vivo.

Main Results:

• Low [CO2]iA induced a shift of calcium from the cytosol to mitochondria in AECs, decreasing [Ca2+]cyt and impairing surfactant secretion.
• Mitochondrial inhibitors blocked these CO2-induced changes, highlighting mitochondria's role in CO2 sensing.
• In rabbits, pulmonary artery ligation reduced alveolar ventilation and surfactant secretion, which was counteracted by adding CO2 to the inhaled air.

Conclusions:

• Alveolar hypocapnia in ARDS causes a mitochondrial calcium shift, reducing surfactant secretion and regional ventilation.
• This mechanism explains the link between high VD/VT, hypocapnia, and impaired lung function in ARDS.
• Inspiratory CO2 application can mitigate the detrimental effects of alveolar hypocapnia and hypoventilation in ARDS.