Angiotensin I is converted to angiotensin II by a serine protease in human detrusor smooth muscle

Abstract

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 than that via angiotensin-converting enzyme (ACE) is present in the smooth muscle of the human detrusor. Isolated detrusor strips from 11 patients were contracted by ANG I (1 microM) in the absence or presence of enalaprilat (10 microM), soybean trypsin inhibitor (STI, 200 micrograms/ml), or both. The metabolic activity in detrusor membranes from four patients was studied separately using Hip-Gly-Gly or ANG I as a substrate, with or without various protease inhibitors. The contractile response to ANG I (1 microM) was depressed by enalaprilat from 66 +/- 22 (mean +/- 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 reduction in contractile amplitude to 25 +/- 14% (P < 0.01 vs. K+, and P < 0.05 vs. enalaprilat alone) and a significantly slower developing contraction with a time to peak of 3.7 +/- 1.7 vs. 1.1 +/- 0.3 min for ANG I alone (P < 0.01). In detrusor membranes, a low ACE activity, inhibitable by captopril, was demonstrated by the formation of hippuric acid (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 protein-1) to ANG II was not affected by ACE inhibition, while serine protease inhibitors, e.g., STI and chymostatin, completely prevented ANG II formation.(ABSTRACT TRUNCATED AT 250 WORDS)