Angiotensin-converting enzyme activity in tissues of the rainbow trout

Angiotensin-converting enzyme activity (ACE) was assayed in homogenized rainbow trout tissues and plasma. The physiological role of ACE was examined by injection of the ACE inhibitor captopril (SQ 14,225) into the dorsal aorta of chronically cannulated trout. Gills and corpuscles of Stannius exhibited greatest ACE activity and contained over 30 times more enzyme than plasma on a per-wet-weight basis. ACE was barely detectable in skeletal muscle, liver, and kidney tissues. Captopril lowered dorsal aortic pressure (DAP) within 1 min after injection. Pressure fell 5-7 mm Hg below control over the next 30 min and remained at this level for an additional hour. Because the gills receive the entire cardiac output they are, like the mammalian lungs, ideally situated to regulate plasma hormone levels. Activation of angiotensin II (AII) appears to be only one example of branchial metabolic capability.