Regulation of the early subnormal retinal oxygenation response in experimental diabetes by inducible nitric oxide synthase.

We aimed to test the hypothesis that the inducible form of nitric oxide synthase (iNOS) contributes to the development of an early subnormal retinal oxygenation response in preclinical models of diabetic retinopathy. In urethane anesthetized Sprague Dawley rats or C57BL/6 mice, functional magnetic resonance imaging was used to noninvasively measure the change in retinal oxygen tension (Delta PO(2)) during a carbogen-inhalation challenge. In the rat experiments, the retinal Delta PO(2) of the following groups were compared: control rats (n = 9), 3-month diabetic rats (n = 5), and 3-month diabetic rats treated orally with L-N(6)-(1-iminoethyl)lysine 5-tetrazole amide, a prodrug of an inhibitor of iNOS (n = 6). In addition, the retinal Delta PO(2) of the following mouse groups were compared: C57BL/6 mice (n = 20), C57BL/6-Nos2(tm1 Lau) mice (n = 10), 4-month diabetic mice (n = 13), and 4-month diabetic knockout mice (n = 6). Only the Delta PO(2) of the superior hemiretina of the diabetic rat and mice groups were significantly subnormal (P < 0.05). The superior Delta PO(2) of the diabetic rats treated with the prodrug was not significantly (P > 0.05) different from their respective normal controls. In the mice experiments, the superior retinal Delta PO(2) of the iNOS null mice was not statistically different (P > 0.05) from that of normal control mice. iNOS is required for the development of an early subnormal Delta PO(2) in experimental diabetic retinopathy.

Effect of PKCbeta on retinal oxygenation response in experimental diabetes.

PURPOSE: To test the hypotheses that, in mice, breathing carbogen (95% O(2)-5% CO(2)) oxygenates the retina better than breathing 100% oxygen, the superior hemiretinal oxygenation response to carbogen inhalation is subnormal early in diabetes, and diabetes-induced elevation of retinal protein kinase C (PKC)-beta contributes to this pathophysiology. METHODS: Retinal oxygenation response (DeltaPO(2)) was measured using functional magnetic resonance imaging (MRI) and either carbogen or 100% oxygen inhalation challenge in C57BL/6J control (C) mice. Retinal DeltaPO(2) during carbogen breathing was also measured in PKCbeta knockout (C57BL6-Prkcb1; [KO]), 4 month C57BL/6J diabetic (D), and 4-month diabetic PKCbeta KO (D+KO) mice. Retinal PKCbeta protein expression was assessed by Western analysis. RESULTS: In C mice, retinal DeltaPO(2) during carbogen breathing was significantly greater (P < 0.05) than during oxygen breathing. In D mice, DeltaPO(2) during carbogen breathing was significantly lower than normal in the superior, but not the inferior, hemiretina and was normal (P > 0.0 5) in the KO group. In the D+KO mice DeltaPO(2) was normal (P > 0.05) only at distances less than 1.5 mm from the optic nerve head. PKCbeta expression was elevated in the retina in diabetes (P < 0.05), but was significantly decreased (P < 0.05) in D+KO mice. CONCLUSIONS: The present study confirms that, in the mouse, retinal DeltaPO(2) patterns during different inhalation challenges or in the presence of diabetes are similar to what has been reported in rats. Diabetes-induced elevation of PKC appears to contribute only focally to subnormal retinal DeltaPO(2). This raises the possibility that PKC inhibition therapy may be only regionally effective in treating retinal pathophysiology associated with diabetic retinopathy.