CO2 relaxes parenchyma in the liquid-filled rat lung

CO2 regulation of lung compliance is currently explained by pH- and CO2-dependent changes in alveolar surface forcesand bronchomotor tone. We hypothesized that in addition to, but independently of, those mechanisms, the parenchyma tissue responds to hypercapniaand hypocapniaby relaxing and contracting, respectively, thereby improving local matching of ventilation (Va) to perfusion (Q). Twenty adult rats were slowly ventilated with modified Krebs solution (rate = 3 min−1, 37°C, open chest) to produce unperfused living lung preparations free of intra-airway surface forces. The solution was gassed with 21% O2, balance N2, and CO2 varied to produce alveolar hypocapnia( Pco2= 26.1 ± 2.4 mmHg, pH = 7.56 ± 0.04) or hypercapnia( Pco2= 55.0 ± 2.3 mmHg, pH = 7.23 ± 0.02). The results show that lung recoil, as indicated from airway pressure measured during a breathhold following a large volume inspiration, is reduced ∼30% when exposed to hypercapniavs. hypocapnia(P < 0.0001, paired t-test), but stress relax...