The fate of circulating biologically active peptides in the lungs

The factors which regulate the disappearance of endogenous active substances during pulmonary transit are discussed. The presence of hydrolytic enzymes in the cytosol is not the only factor regulating pulmonary metabolism. An uptake-transport process is required to permit access to intracellular enzymes. No endothelial cell transport mechanism has yet been described for peptides. However, bradykinin and angiotensin I are metabolized by the pulmonary circulation because the enzyme is localized on the luminal surface of the endothelial cells. The pulmonary and extrapulmonary conversion of angiotensin in rat is still a matter of debate. Evidence is presented which supports the idea that when low doses of angiotensin I (greater than 10 ng) are given intravenously, most of the conversion is pulmonary. With higher doses, extrapulmonary conversion also contributes to the final pressor effect of angiotensin I.