Na(+),K(+)-ATPase and the angiotensin I converting enzyme in the retina of streptozotocin-induced and Zucker diabetic rats: Early biochemical and ultrastructural change

Diabetic retinopathy (DR) is a long term microangiopathic complication of diabetes that evolves insidiously and continues to be a leading cause of blindness despite the advent of laser and vitreous surgery. The early stage of DR is characterized by hemodynamic alterations and increased blood-retinal barrier (BRB) permeability. Fluid leakage, from the retinal capillaries (RV) and retinal pigmented epithelium (RPE), two components of the BRB, ultimately results in retinal damage and macular edema.;In this dissertation I have investigated early biochemical and ultrastructural changes in streptozotocin (STZ)-induced and Zucker spontaneously diabetic rats. The biochemical change that I assayed was the activity of the Na,K-ATPase, an enzyme that plays a key role in ion homeostasis and regulation of cell volume, that is decreased in the retina of diabetic rats. The morphological changes that I studied were ultrastructural alterations in the regions of the BRB.;One focus of the work was to study the relation of the renin-angiotensin system (RAS) to biochemical and morphological changes due to diabetic retinopathy. The RAS, which modulates ionic balance and blood pressure has been proposed to influence the development of microvascular complications of diabetes such as diabetic retinopathy. Components of the tissue RAS such as renin, angiotensin converting enzyme (ACE) which converts Angiotensin I (ANG I) into the vasoconstrictor, ANG II, and inactivates bradykinin, are present in the retina and are increased in patients with DR. In the blood brain barrier, a structure embryologically similar to the BRB, ANG II increases permeability of microvessels and decreases Na,K-ATPase activity suggesting that RAS and Na,K-ATPase are associated with vascular hyperpermeability in the early stage of DR.;Captopril, an ACE inhibitor, is recommended by the American Diabetes Association for delaying and treating microangiopathic complications of diabetes. However, the mechanisms leading to this beneficial effect have not been determined. In the first set of experiments I asked whether a relationship exists between the activity of Na,K-ATPase and ACE levels in the blood and retina, and whether captopril restores retinal Na,K-ATPase activity. I found that ANG II applied to control retinas in vitro, decreased Na,K-ATPase activity. An AT1 antagonist restored the activity of retinal Na,K-ATPase activity indicating that the effect of ANG II was mediated by an AT1 receptor. Whereas serum ACE activity was increased in STZ-induced diabetic rats, retinal ACE activity was decreased. These results suggest that a local rather than systemic RAS is involved in the development of DR. Captopril further decreased retinal ACE activity and restored retinal Na,K-ATPase activity.;In a second set of experiments I asked whether Na,K-ATPase activity was altered predominantly in the structures that form the BRB, and whether anatomical changes accompany the early biochemical. I found that biochemical changes in the RV preceded anatomical changes in the BRB: Na,K-ATPase activity was already significantly decreased at two months of diabetes in retinal microvessels. Later, at five months, changes in the basal infoldings of the RPE were the only structural changes. Finally I found that the ultrastructural changes in the RPE were not reversed by captopril, but instead were increased. These findings suggest that the use of captopril for controlling DR should be examined more thoroughly.