ANGIOTENSIN-I IS CONVERTED TO ANGIOTENSIN-II BY A SERINE-PROTEASE IN HUMAN DETRUSOR SMOOTH-MUSCLE

The aim of the present study was to investigate whether a pathway for conversion of angiotensin I (ANG I) to angiotensin II (ANG II) other t han that via angiotensin-converting enzyme (ACE) is present in the smo oth muscle of the human detrusor. Isolated detrusor strips from 11 pat ients were contracted by ANG I (1 mu M) in the absence or presence of enalaprilat (10 mu M), soybean trypsin inhibitor (STI, 200 mu g/ml), o r both. The metabolic activity in detrusor membranes from four patient s was studied separately using Hip-Gly-Gly or ANG I as a substrate, wi th or without various protease inhibitors. The contractile response to ANG I (1 mu M) was depressed by enalaprilat from 66 +/- 22 (mean c SD ) to 39 +/- 13% of the K+ (124 mM)-induced response (P < 0.01, n = 11) , and the combination of enalaprilat and STI resulted in a further red uction in contractile amplitude to 25 +/- 14% (P < 0.01 vs. K+, and P < 0.05 vs. enalaprilat alone) and a significantly slower developing co ntraction with a time to peak of 3.7 +/- 1.7 vs. 1.1 +/- 0.3 min for A NG I alone (P < 0.01). In detrusor membranes, a low ACE activity, inhi bitable by captopril, was demonstrated by the formation of hippuric ac id (0.70 nmol.min(-1).mg protein(-1)) from the synthetic ACE substrate , Hip-Gly-Gly. However, the conversion of ANG I (166 nmol.min(-1).mg p rotein(-1)) to ANG II was not affected by ACE inhibition, while serine protease inhibitors, e.g., STI and chymostatin, completely prevented ANG II formation. These results indicate the presence of an alternativ e pathway for ANG II formation in the human detrusor, involving one or several serine protease(s).