Zeevalk G D, Nicklas W J
Department of Neurology, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway 08854, USA.
J Neurochem. 2000 Sep;75(3):1027-34. doi: 10.1046/j.1471-4159.2000.0751027.x.
Under conditions of energy impairment, CNS tissue can utilize substrates other than glucose to maintain energy metabolism. Retinas produce large amounts of lactate, although it has not been shown that lactate can be utilized by retina to prevent the cell damage associated with hypoglycemia. To investigate this, intact, isolated retinas were subjected to aglycemic conditions in the presence or absence of 20 mM lactate. Retinas incubated in the absence of glucose for 60 min showed a threefold elevation in tissue aspartate and 60% decreases in tissue glutamate and glutamine, demonstrating a mobilization of carbon from glutamine and glutamate to the tricarboxylic acid cycle. Lactate prevented these changes in tissue amino acids, indicating metabolism of lactate with sparing of tissue glutamate and glutamine. Tissue ATP was 20 and 66% of control values with zero glucose or zero glucose plus lactate, respectively. Consistent with previous findings, incubation of retinas in the absence of glucose caused acute swelling of retinal neurons and release of GABA into the medium at 60 min. These acute toxic affects caused by the absence of glucose were completely prevented by the presence of lactate. At 24 h of recovery following 60 min of zero glucose, many pyknotic profiles were observed and lactate dehydrogenase (LDH) release into the medium was elevated sevenfold, indicating the extent of cell death. In contrast, no elevation in LDH was found and histology appeared normal in retinas exposed to zero glucose in the presence of lactate. alpha-Cyano-4-hydroxy cinnamate (4-CIN; 0.5 mM), an inhibitor of the monocarboxylic acid transporter and mitochondrial pyruvate carrier, blocked the ability of lactate to maintain ATP and protect retinas from aglycemia but had no effect on ATP or toxicity per se. Derangements in tissue aspartate, glutamate, and glutamine, which were prevented by lactate during zero glucose incubation, were again observed with lactate plus zero glucose in the presence of 4-CIN. However, 0.5 mM 4-CIN alone in the presence of glucose produced similar increases in aspartate and decreases in glutamate and glutamine as observed with zero glucose while having only modest inhibitory effects on [U-(14)C]lactate uptake, suggesting the mitochondrial pyruvate carrier as the main site of action. The above findings show that lactate is readily utilized by the chick retina during glucose deprivation to prevent derangements in tissue amino acids and ATP and retinal neuronal cell death.
在能量受损的情况下，中枢神经系统组织可以利用葡萄糖以外的底物来维持能量代谢。视网膜会产生大量乳酸，尽管尚未证实乳酸可被视网膜利用以预防与低血糖相关的细胞损伤。为了对此进行研究，将完整的离体视网膜置于无糖条件下，并分别添加或不添加20 mM乳酸。在无葡萄糖的情况下孵育60分钟的视网膜，其组织天冬氨酸含量升高了三倍，组织谷氨酸和谷氨酰胺含量降低了60%，这表明谷氨酰胺和谷氨酸中的碳被转运至三羧酸循环。乳酸可防止组织氨基酸发生这些变化，表明乳酸代谢可节省组织中的谷氨酸和谷氨酰胺。在葡萄糖为零或葡萄糖为零加乳酸的情况下，组织ATP分别为对照值的20%和66%。与先前的研究结果一致，在无葡萄糖的情况下孵育视网膜会导致视网膜神经元在60分钟时急性肿胀，并使GABA释放到培养基中。乳酸的存在可完全预防因无葡萄糖而导致的这些急性毒性作用。在零葡萄糖处理60分钟后的24小时恢复过程中，观察到许多固缩形态，并且乳酸脱氢酶（LDH）释放到培养基中的量升高了七倍，这表明细胞死亡的程度。相比之下，在有乳酸存在的情况下暴露于零葡萄糖的视网膜中，未发现LDH升高，并且组织学表现正常。单羧酸转运体和线粒体丙酮酸载体的抑制剂α-氰基-4-羟基肉桂酸（4-CIN；0.5 mM）可阻断乳酸维持ATP以及保护视网膜免受无糖血症影响的能力，但对ATP本身或毒性没有影响。在4-CIN存在的情况下，乳酸加零葡萄糖处理时，再次观察到组织天冬氨酸、谷氨酸和谷氨酰胺的紊乱，而在零葡萄糖孵育期间，乳酸可防止这种紊乱。然而，在有葡萄糖存在的情况下，单独使用0.5 mM 4-CIN会使天冬氨酸含量出现类似零葡萄糖处理时的升高，谷氨酸和谷氨酰胺含量降低，同时对[U-(14)C]乳酸摄取仅有适度的抑制作用，这表明线粒体丙酮酸载体是主要作用位点。上述研究结果表明，在葡萄糖缺乏期间，雏鸡视网膜可轻易利用乳酸来防止组织氨基酸和ATP紊乱以及视网膜神经元细胞死亡。
