Inhibition of nitric oxide synthesis results in a selective increase in arterial resistance in rabbit lungs

Endogenous nitric oxide (NO) opposes the vasoconstriction that occurs when lungs are ventilated with a hypoxic gas mixture. However, the contribution of NO to pulmonary vascular resistance when alveolar gas tension is not reduced remains to be defined. Here, we investigated the hypothesis that endogenous NO is a determinant of pulmonary vascular resistance in isolated perfused rabbit lungs ventilated with a normoxic gas mixture. Moreover, we wished to establish that, as flow rate increases, the contribution of NO to vascular resistance increases. In addition, we examined the contribution of NO to the longitudinal distribution of pulmonary vascular resistance. Pressure-flow curves were generated in isolated blood perfused rabbit lungs by varying flow rate from 50 ml/min to 300 ml/min in the presence and absence of the cyclooxygenase inhibitor, indomethacin (100 microM) and the inhibitor of NO synthesis, NG-nitro-L-arginine methyl ester (L-NAME, 100 microM). Indomethacin did not alter total pulmonary vascular resistance or the longitudinal distribution of resistance. In contrast, L-NAME administration resulted in significant, flow-related increases in total vascular resistance, i.e., after L-NAME, as flow rate increased, the increment in resistance increased. L-NAME-induced increases in total pulmonary vascular resistance were the result of flow-related increases in the arterial component of vascular resistance. These results provide support for the hypothesis that NO is an important determinant of pulmonary vascular resistance in the rabbit and that the major site of NO activity resides in the arterial side of that circulation.