Localization of angiotensin-converting enzyme in the trout gill

Angiotensin-converting enzyme (ACE) was localized in perfused trout gills by measuring gill extraction of two radiolabeled ACE inhibitors, 125I-351A (an iodinated derivative of lisinopril) and 3H-RAC-X-65, and by autoradiography of gills perfused with 125I-351A. A 125I-351A pulse was preferentially extracted by the arterio-arterial (AA) pathway (61.7% +/- 1.8% extraction; mean +/- SE, N = 4); the arteriovenous (AV) pathway extracted an additional 10%. Extraction by either pathway was reduced by simultaneous perfusion with 10(-5) M unlabeled lisinopril. AA extraction of RAC-X-65 during continuous perfusion was maximal (75% +/- 5%, N = 6) during the first few minutes of perfusion and decreased steadily to 38% +/- 9% by 20 min and to less than 10% by 40 min. AV extraction of RAC-X-65 was negligible. Autoradiography of gills continuously perfused with 125I-351A showed that the radiolabel was concentrated in the respiratory lamellae. The highest grain density was associated with the pillar cells nearest the medial (inner) lamellar margin. Afferent filamental arteries and afferent lamellar arterioles were labeled to a lesser extent. Relatively little label was found on the efferent lamellar arterioles or efferent filamental arteries. 125I-351A binding was not evident in AV vessels. These findings support the hypothesis that the gill is an important site for formation of plasma angiotensin II and they suggest that enzymes associated with mammalian endothelial cells are also common to gill pillar cells.