Clow Kathy A, Ewart K Vanya, Driedzic William R
Ocean Sciences Centre, Memorial Univ. of Newfoundland, St. John's, Newfoundland, Canada A1C 5S7.
Am J Physiol Regul Integr Comp Physiol. 2008 Sep;295(3):R961-70. doi: 10.1152/ajpregu.90372.2008. Epub 2008 Jul 16.
Rainbow smelt (Osmerus mordax) accumulate high levels of glycerol in winter that serve as an antifreeze. Liver glycogen is a source of glycerol during the early stages of glycerol accumulation, whereas dietary glucose and amino acids are essential to maintain rates of glycerol synthesis. We presently report rates of glycerol and glucose production by isolated hepatocytes. Cells from fish held at 0.4 to -1.5 degrees C and incubated at 0.4 degrees C were metabolically quiescent with negligible rates of glycerol or glucose production. Hepatocytes isolated from fish maintained at 8 degrees C and incubated at 8 degrees C produced glucose but not glycerol. Glycerol production was activated in cells isolated from 8 degrees C fish and incubated at 0.4 degrees C without substrate or when glucose, aspartate, or pyruvate was available in the medium. Incubation at 0.4 degrees C without substrate resulted in similar molar rates of glucose and glycerol production in concert with glycogen mobilization. Glycogenolysis and glycerol production were associated with increases in total in vitro activities of glycogen phosphorylase and glycerol-3-phosphate dehydrogenase. Maximal in vitro activities of hexokinase and glucokinase were not influenced by temperature, but high activities of a low-K(m) hexokinase may serve to redirect glycogen-derived glucose to glycolysis as opposed to releasing it from the cells. Rates of glycerol production were not enhanced in cells from fish held at 8 degrees C and incubated at 0.4 degrees C with adrenergic or glucocorticoid stimulation. As such, low temperature alone is sufficient to activate the glycerol production mechanism and results in a shift from glucose to a mix of glucose and glycerol production.
彩虹香鱼（Osmerus mordax）在冬季会积累高水平的甘油作为抗冻剂。在甘油积累的早期阶段，肝糖原是甘油的来源，而膳食中的葡萄糖和氨基酸对于维持甘油合成速率至关重要。我们目前报告了分离的肝细胞产生甘油和葡萄糖的速率。来自饲养在0.4至 -1.5摄氏度并在0.4摄氏度孵育的鱼的细胞代谢静止，甘油或葡萄糖产生速率可忽略不计。从饲养在8摄氏度并在8摄氏度孵育的鱼中分离的肝细胞产生葡萄糖但不产生甘油。从8摄氏度的鱼中分离并在0.4摄氏度无底物孵育或培养基中有葡萄糖、天冬氨酸或丙酮酸时，甘油的产生被激活。在0.4摄氏度无底物孵育会导致葡萄糖和甘油产生的摩尔速率相似，同时伴有糖原动员。糖原分解和甘油产生与糖原磷酸化酶和甘油 -3-磷酸脱氢酶总的体外活性增加有关。己糖激酶和葡萄糖激酶的最大体外活性不受温度影响，但低K(m)己糖激酶的高活性可能有助于将糖原衍生的葡萄糖重定向到糖酵解，而不是从细胞中释放出来。在0.4摄氏度对来自饲养在8摄氏度的鱼的细胞进行肾上腺素能或糖皮质激素刺激时，甘油产生速率并未提高。因此，仅低温就足以激活甘油产生机制，并导致从产生葡萄糖转变为产生葡萄糖和甘油的混合物。
