Lieth E, LaNoue K F, Antonetti D A, Ratz M
Department of Neuroscience and Anatomy, Pennsylvania State University College of Medicine, Hershey, PA 17033, USA.
Exp Eye Res. 2000 Jun;70(6):723-30. doi: 10.1006/exer.2000.0840.
Retinas of diabetic individuals develop early functional changes measurable by electrophysiological and psychometric testing. Using a rat model of diabetes, we previously identified diabetes-induced alterations in metabolism of the neurotransmitter glutamate which may ultimately lead to accumulation of glutamate in the retina (Diabetes, 47: 815, 1998). We therefore investigated the function of enzymes that mediate the synthesis and breakdown of glutamate in retinas from rats made diabetic by injection of streptozotocin. De novo synthesis of nitrogen-containing amino acids including glutamate, glutamine and aspartate was assessed by measuring the rate of carbon fixation in freshly dissected retinas, and was unchanged by diabetes. In contrast, the oxidation of glutamate was significantly reduced in retinas from diabetic rats (62%, P < 0.05). Furthermore, diabetic retinas were less susceptible to inhibition of glutamate oxidation by the transaminase inhibitor aminoxyacetate (80%, N.S.), compared to the significant decrease seen in control rats (61%, P < 0.001). The activity and content of glutamine synthetase were also significantly reduced in retinas from rats diabetic for 2-6 months [range of 48% (P < 0.005) to 83% (P < 0.05) compared to control]. The activity of glutamine synthetase was normalized by acute injections of insulin, but not by reducing blood sugar levels with injections of phlorizin. These results indicate two enzymatic abnormalities in the glutamate metabolism pathway in the retina during diabetes: transamination to alpha-ketoglutarate and amination to glutamine. The reduced flux through these pathways may be associated with the accumulation of glutamate. These results are also consistent with the possibility that some of the glial changes in the retina during diabetes may be caused by hypoinsulinemia rather than hyperglycemia.
糖尿病患者的视网膜会出现早期功能变化，可通过电生理和心理测量测试来检测。利用糖尿病大鼠模型，我们之前发现糖尿病会导致神经递质谷氨酸代谢改变，这最终可能导致视网膜中谷氨酸积累（《糖尿病》，47: 815, 1998）。因此，我们研究了介导谷氨酸合成和分解的酶在注射链脲佐菌素致糖尿病大鼠视网膜中的功能。通过测量新鲜分离视网膜中的碳固定速率来评估包括谷氨酸、谷氨酰胺和天冬氨酸在内的含氮氨基酸的从头合成，糖尿病对此无影响。相比之下，糖尿病大鼠视网膜中谷氨酸的氧化显著降低（62%，P < 0.05）。此外，与正常大鼠中观察到的显著降低（61%，P < 0.001）相比，糖尿病视网膜对转氨酶抑制剂氨氧基乙酸抑制谷氨酸氧化的敏感性较低（80%，无显著差异）。糖尿病2至6个月大鼠视网膜中谷氨酰胺合成酶的活性和含量也显著降低[与对照组相比，范围为48%（P < 0.005）至83%（P < 0.05）]。谷氨酰胺合成酶的活性可通过急性注射胰岛素恢复正常，但注射根皮苷降低血糖水平则不能。这些结果表明糖尿病期间视网膜谷氨酸代谢途径存在两种酶异常：转氨生成α - 酮戊二酸和氨化生成谷氨酰胺。这些途径通量的降低可能与谷氨酸积累有关。这些结果也与糖尿病期间视网膜中一些神经胶质细胞变化可能由低胰岛素血症而非高血糖症引起的可能性一致。
