Angiotensin-converting enzyme activity in ovine bronchial vasculature

Angiotensin-converting enzyme (ACE) plays a major role in the metabolism of bradykinin, angiotensin, and neuropeptides, which are all implicated in inflammatory airway diseases. The activity of ACE, which is localized on the luminal surface of endothelial cells (EC), has been well documented in pulmonary EC; however, few data exist regarding the relative activity of ACE in the airway vasculature. Therefore, we measured ACE activity in cultured EC from the sheep bronchial artery and bronchial mucosa (microvascular) and compared it with pulmonary artery EC. The baseline level of total ACE activity (cellular plus secreted) was significantly greater in bronchial microvascular EC (1.24 ± 0.24 mU/10⁶ cells) compared with bronchial artery EC (0.59 ± 0.15 mU/10⁶ cells; P < 0.05) and comparable to pulmonary artery EC (1.12 ± 0.14 mU/10⁶ cells; P > 0.05). Measured ACE activity secreted into culture medium for each cell type was 64-74% of total activity and did not differ among the three EC types (P = 0.17). Hydrocortisone (10 μg/ml; 48-72 h) treatment resulted in a significant increase in ACE activity in bronchial EC. Likewise, TNF-α (0.1 ng/ml) treatment markedly increased ACE activity in all cell lysates (P < 0.05). We confirmed the importance of ACE activity in vivo since, at the highest dose of bradykinin studied (10^-8 M), bronchial artery pressure at constant flow showed a greater decrease after captopril treatment (36% before vs. 60% after; P = 0.05). These results demonstrate high ACE expression of the bronchial microvasculature and suggest an important regulatory role for ACE in the metabolism of kinin peptides known to contribute to airway pathology.