Segmental pulmonary vascular responses to changes in pH in rat lungs: Role of nitric oxide

Background: Respiratory or renal failure is associated with changes in blood pH. Changes in pH may have profound effects on vascular tone and reactivity. Site of action of acidosis in the pulmonary vasculature and the role of nitric oxide production remain unclear. Methods: We utilized isolated rat lung preparation perfused with autologous blood (Hct=20%, flow rate=33 ml/min), and investigated the effect of acidosis and alkalosis (induced by ventilation with high and low inspired CO₂) on vascular resistance and the role of nitric oxide during resting and elevated tone conditions. Changes in resistance were described in terms of small and large arteries and veins, using the vascular occlusion technique. Results: Acidosis (P(CO₂):=66.7±0.7 mmHg, pH=7.17±0.01, P(O₂)= 255±3 mmHg) caused vasoconstriction under resting and increased vascular tone conditions (U46619-induced). The changes in resistance occurred primarily in the small arteries. In contrast, alkalosis (P(CO₂)=20.1±0.3 mmHg, pH=7.61±0.01, P(O₂)=244±3 mmHg) caused vasodilation only at elevated tone conditions. Nitrow-L-arginine (LNA), an inhibitor of nitric oxide synthase, increased vascular resistance slightly but did not modulate the responses to pH, suggesting that such responses are not nitric oxide dependent. During KCl-induced contraction, the effects of pH were abolished. Conclusions: We conclude that in rat lung, acidosis causes an increase in pulmonary vascular resistance at normal and elevated tone conditions. Furthermore, the response is limited primarily to the small arteries, and is not mediated by nitric oxide. Alkalosis tends to cause the opposite effects. The effects of acidosis and alkalosis were abolished when vascular tone was elevated with a low dose of KCl, suggesting that vascular response to pH may involve changes in membrane potential.