Immunohistochemistry of angiotensin I-converting enzyme in rat eye structures involved in aqueous humor regulation.

Angiotensin-converting enzyme (ACE) was localized by immunohistochemical methods in rat eye structures involved in aqueous humor secretion and reabsorption, i.e., the ciliary processes, the trabeculum, and Schlemm's canal. The three-layer peroxidase-antiperoxidase method was used with light and electron microscopy. In the ciliary processes, ACE was found on the basal plasma membrane of the internal epithelium facing the aqueous humor and on vesicles in its vicinity, whereas neither the external epithelium nor the blood vessels running through the ciliary processes were immunoreactive. A faint ACE immunoreactivity was found on some endothelial cell plasma membranes of both the internal collector channels and Schlemm's canal. These results suggest that the ciliary process internal epithelium could be an important site for the production of angiotensin II and/or the metabolism of bradykinin. In addition, the present investigation provides evidence for ACE involvement in aqueous humor secretion, whereas its involvement in aqueous humor reabsorption may be minor.