Espat N J, Copeland E M, Souba W W
Department of Surgery, University of Florida College of Medicine, Gainesville.
JPEN J Parenter Enteral Nutr. 1993 Nov-Dec;17(6):493-500. doi: 10.1177/0148607193017006493.
During starvation, the liver switches from an organ of net glutamine uptake to an organ of net glutamine release to help maintain blood glutamine levels. We hypothesized that this shift in hepatic glutamine exchange was regulated at the level of the hepatocyte plasma membrane by adaptive changes in glutamine transport. To test this hypothesis, adult rats (200 g) were allowed to consume regular rat food ad libitum (fed, n = 8) or were fasted for 72 hours (fasted, n = 8, access to water allowed). Livers were excised and hepatocyte plasma membrane vesicles were prepared by differential and Percoll density gradient centrifugation. Vesicle purity and functionality were assessed by marker enzyme measurements, classic "overshoots," and time courses, which showed similar vesicle size. Uptake of 3H-glutamine by hepatocyte plasma membrane vesicles in the presence and absence of sodium was assayed by a rapid mixing/filtration method, which reflects actual transport across the hepatocyte cell membrane in vivo. Fasted rats lost 15 +/- 2% of body weight; fed rats gained weight. Na(+)-dependent glutamine transport (system "N," mediates uptake into the hepatocyte) fell by 22% in the starved group, indicating a diminished rate of glutamine transport into the hepatocyte. In contrast, carrier-mediated Na(+)-independent glutamine transport (system "n," mediates the release of glutamine out of the cell) doubled in the starved animals. Diffusion of glutamine across the vesicle membrane was unchanged by starvation.(ABSTRACT TRUNCATED AT 250 WORDS)
在饥饿期间，肝脏从一个净摄取谷氨酰胺的器官转变为一个净释放谷氨酰胺的器官，以帮助维持血液中的谷氨酰胺水平。我们假设肝脏谷氨酰胺交换的这种转变是由谷氨酰胺转运的适应性变化在肝细胞质膜水平上调节的。为了验证这一假设，成年大鼠（200克）被允许随意食用常规大鼠食物（喂食组，n = 8）或禁食72小时（禁食组，n = 8，允许饮水）。切除肝脏，通过差速离心和Percoll密度梯度离心制备肝细胞质膜囊泡。通过标记酶测量、经典的“过冲”现象和时间进程评估囊泡纯度和功能，结果显示囊泡大小相似。采用快速混合/过滤法测定在有无钠存在的情况下肝细胞质膜囊泡对3H-谷氨酰胺的摄取，该方法反映了体内谷氨酰胺跨肝细胞细胞膜的实际转运情况。禁食大鼠体重减轻了15±2%；喂食大鼠体重增加。饥饿组中依赖钠的谷氨酰胺转运（“N”系统，介导谷氨酰胺进入肝细胞）下降了22%，表明谷氨酰胺进入肝细胞的转运速率降低。相反，在饥饿动物中，载体介导的不依赖钠的谷氨酰胺转运（“n”系统，介导谷氨酰胺从细胞中释放）增加了一倍。饥饿对谷氨酰胺跨囊泡膜的扩散没有影响。（摘要截短至250字）
