Normoxic lung ischemia/reperfusion accelerates shedding of angiotensin converting enzyme from the pulmonary endothelium

Normoxic lung ischemia/reperfusion (I/R) leads to oxidative injury of the pulmonary tissue. We analyzed angiotensin-converting enzyme (ACE) in perfused rat lungs upon I/R in order to assess the endothelial injury produced. I/R led to a time-dependent increase in ACE activity in the perfusate, from 145 +/- 14 mU to 252 +/- 1 mU, and to reduction of ACE activity in the lung tissue homogenate, from 29.7 +/- 2.3 U to 22.7 +/- 1.7 U. About 80% of ACE activity in control and I/R lungs was associated with an aqueous phase of extracted perfusates, thus indicating that I/R accelerates shedding of the hydrophilic form of ACE from the plasma membrane. To specifically assess ACE localized on the luminal surface of the pulmonary endothelium, we perfused rat lungs with a radiolabeled monoclonal antibody (mAb) to ACE (anti-ACE mAb 9B9). Pulmonary uptake of mAb 9B9 with I/R was reduced from 32.1 +/- 1.7% to 24.8 +/- 0.9%. In contrast, I/R led to a marked increase in the pulmonary uptake of nonspecific [I-125]IgG, from 0.17 +/- 0.02% to 0.67 +/- 0.04%. Lung wet weight was equal to 0.78 +/- 0.08% of body weight in the I/R group versus 0.57 +/- 0.02% at the control level. The observed increase in [I-125]IgG uptake and wet lung weight indicate that I/R causes an increase in lung vascular permeability. These results indicate that normoxic lung I/R induces injury to the pulmonary vascula endothelium.