Characterization and diabetes-induced impairment of nitric oxide synthase in rat choroid

Abstract

Purpose. To examine the nitric oxide (NO) system in the choroid of normal rats and rats with streptozotocin (STZ)-induced diabetes. Methods. We assayed NO synthase (NOS) activity by monitoring the conversion of L -[ 14 C] arginine to L -[ 14 C] citrulline, identified the NOS isoforms by immunoblotting, and examined the effects of STZ-induced diabetes on NOS in the rat choroid. Results. Calcium-independent NOS activity was insignificant in the choroid of normal and diabetic rats. Choroidal calcium-dependent NOS activity was high and comparable to that in the cerebellum. Neuronal (n) NOS protein in the choroid was found in both the membrane and cytosolic fractions and showed similar subcellular distribution as NOS activity, while endothelial (e) NOS protein in the choroid was present almost solely in the membrane fraction. Total NOS activity (nNOS + eNOS) and protein levels of nNOS and eNOS in the choroid were significantly reduced 6 weeks after the induction of diabetes. Conclusions. The high NOS activity in the choroid measured in vitro appears to come mostly from nNOS. Because choroidal nNOS exists in the parasympathetic perivascular nerve fibers, the decrease in choroidal nNOS in diabetic eyes suggests that the choroid undergoes a diabetes-induced neuronal disorder. Thus, diabetic choroidopathy encompasses diabetic neuropathy and microangiopathy.