Background—Chronic hypoxia results in the development of pulmonary hypertension and subsequent right heart failure. A role of the plasminogen system in the pathogenesis of pulmonary hypertension and pulmonary vascular remodeling has been suggested.

Methods and Results—Mice with targeted deficiency of the gene encoding tissue-type plasminogen activator (t-PA^−/−), urokinase-type plasminogen activator (u-PA^−/−), u-PA receptor (u-PAR^−/−), or plasminogen (plg^−/−) were subjected to hypoxic conditions. Hypoxia caused a significant 2.5-fold rise in right ventricular pressure in wild-type mice. Deficiency of u-PA or plasminogen prevented this increase in right ventricular pressure, t-PA^−/− mice showed changes that were fully comparable with wild-type mice, and u-PAR^−/− mice showed a partial response. Hypoxia induced an increase in smooth muscle cells within pulmonary arterial walls and a vascular rarefaction in the lungs of wild-type but not of u-PA^−/− or plg^−/− mice. Elastic lamina fragmentation, observed in hypoxic wild-type but not in u-PA or plasminogen-deficient mice, suggested that proliferation of vascular smooth muscle cells was dependent on u-PA–mediated elastic membrane degradation. Hypoxia-induced right ventricular remodeling in wild-type mice, characterized by cardiomyocyte hypertrophy and increased collagen contents, was not seen in u-PA^−/− and plg^−/− mice.

Conclusions—Loss of the u-PA or plasminogen gene protects against the development of hypoxia-induced pulmonary hypertension and pulmonary vascular remodeling. These observations point to an essential role of u-PA–mediated plasmin generation in the adaptive response to chronic hypoxia and the occurrence of hypoxic pulmonary vascular disease.