低溫刺激對於虱目魚和草魚醣類代謝調控之探討

Abstract

虱目魚為本省傳統養殖魚類，但其為狹溫性魚種，每當寒流過境，虱目魚往往因不耐低溫而大量死亡。本篇論文之研究目的係探討虱目魚在急性低溫刺激下，醣類能量代謝之酵素活性和代謝物含量之變化情形，並與較耐寒之草魚比較能量代謝調節的差異。 將在25℃馴化下之虱目魚施予急速的降溫，分別降至15℃及12℃。在15℃的刺激下，虱目魚之腎臟均質液及血清中 norepinephrine含量有增加的現象，但腦中的含量在壓迫初期為升高，但隨後即呈下降。而草魚與虱目魚在低溫壓迫期間，血液滲透壓均較平常為低，然而虱目魚在低溫致死的階段甚至出現大幅滑落的現象。在血糖濃度方面，虱目魚與草魚在低溫壓迫階段，血液中葡萄糖含量有顯著增加，又以虱目魚所增加的幅度為大。顯示出虱目魚在低溫下產生葡萄糖的代謝機制上，受到相當大程度的激發。本研究中顯示，虱目魚血液中葡萄糖的來源主要是經由糖質新生作用產生的，而肝糖分解在低溫刺激下有增高的現象，但與血糖濃度之增加無必然結果。虱目魚在15℃低溫壓迫的後期，以及12℃的刺激下，血糖濃度呈現大幅下降的現象，顯示出在低溫壓迫之致死階段，葡萄糖的供應上有所阻礙。而在糖解作用以產生能量的代謝途徑上，經由測定虱目魚及草魚的肝臟與肌肉的lactate dehydrogenase, pyruvate kinase, citrate synthase和G6PDH之活性，發現草魚在低溫刺激下主要是增強有氧糖解作用以獲得較多的能量，以及五碳糖的代謝途徑以產生NADPH而有利於脂質的生合成。而虱目魚在低溫下這些代謝途徑的增強效果並不顯著，甚至有降低的趨勢，顯示出虱目魚對葡萄糖的利用率並不高，這可能是其無法在低溫下長期存活的原因。 另一方面探討離體培養之虱目魚肝組織，在溫度與壓迫激素-catechloamines的作用下，對於肝組織葡萄糖產生與釋放的影響。研究發現在12℃低溫培養下的肝組織，在葡萄糖的釋放量以及糖質新生和肝糖分解之酵素活性，均較25℃下培養為高。同時catecholamines對促進葡萄糖釋放及肝糖分解的效果極為顯著，其中又以norepinephrine較epinephrine效果為佳。然而catecholamine對糖質新生的增強效果並不顯著。這些結果顯示catecholamine在某種程度上參與了調節葡萄糖的產生與釋放。然而在無外界因數參與下，細胞本身單純受到低溫的刺激，也能激發特殊的調節機制，以增強葡萄糖的產生。

also studied to compare energy production and utilization of these two spicies at low temperature. Milkfish and grass carp were acclimated at 25°C for one month. Milkfish under 15°C cold shock treatment. the blood glucose content increased 5 folds, and both the activities of gluconeogenicc and glycogenolytic enzymes increased. The blood glucose content of grass carp had less increase than milkfish under cold shock. But in 12°C cold treated milkfish and the late period of 15°C cold shock. the blood glucose content decreased. It showed the glucose supply were shorted under extreme or prolong cold shock. To estimate the activitiees of glycolytic. pentose phosphate. anaerobic and aerobic pathway, the activities of PK. G6PDH. LDH. and CS in the liver and muscle of milkfish and grass carp under cold shock were measured. The results indicated that milkfish had enhanced aerobic glycolysis to produce more ATP under cold shock, grass carp favored in aerobic glycolysis and enhanced the pentose phosphate pathway to produce NAPDH for lipid biosynthesis. In addition. in vitro cultured milkfish liver was used to study how low temperature ane catecholamines influence glucose production. The results showed that 12℃-cultured liver tissue releasd more glucose and had higher enzyme activitties of glycogenolysis and gluconeogenesis than 25-cultured liver tissue. The effects of norepinephrine on the increae of hepatic glucose release were significant. Epinephrine reduced the glucose release, but it activated glycogenolysis. Norepinephrine affected the increased of gluconeogenesis more significantly than epinephrine. These suggested that except of stress hormones, liver cells themself. could regulate hepatic glucogenolysis and gluconeogenesis.