Characterization of rat pulmonary vascular aminopeptidase P in vivo: role in the inactivation of bradykinin.

The nonapeptide bradykinin (BK) is hydrolyzed at multiple sites during a single passage through the rat pulmonary vascular bed. Hydrolysis of one bond, Arg1-Pro2, appears to be catalyzed by an aminoacylproline hydrolase called aminopeptidase P (AmP). To help clarify its role in BK degradation, we have characterized rat pulmonary AmP in vivo in terms of its ability to react with intravascular substrates, its saturability and its contributions to the inactivation of circulating BK. By using indicator dilution methodology, hydrolysis of tracer doses of the AmP substrate Arg-Pro-Pro-[3H]benzylamide ([3H]APPB) during a single transit through the pulmonary vascular bed was measured. Transpulmonary hydrolysis of [3H]APPB obeyed first-order enzyme kinetics and was inhibited by carrier substrate (APPB) and two alternative AmP substrates, BK and des-Arg9-BK. APPB, des-Arg9-BK and des-Arg1-BK, all capable of binding to AmP in vitro, potentiated hypotensive effects of BK injected i.v. A saturating dose of APPB, 2 mumol/kg, in coinjections with BK, potentiated effects of i.v. BK by about 4-fold when pulmonary angiotensin converting enzyme (ACE) was active or inhibited completely. Complete inhibition of ACE potentiated blood pressure effects of i.v. BK by 40- to 120-fold. When both AmP and ACE were inhibited, the effects of i.v. BK were potentiated by up to 800-fold, and the hypotensive effects of BK injected i.v. on systemic mean arterial blood pressure were equivalent to effects of BK injected into the ascending aorta (i.a.); the BK i.v. and i.a. log dose-response curves were virtually superimposable.(ABSTRACT TRUNCATED AT 250 WORDS)