低溫刺激對淡水長臂大蝦（Macrobrachium roenbergii) 醣類代謝酵素活性變化的影響

Abstract

淡水長臂大蝦為台灣重要養殖蝦種是屬於熱帶蝦種。由於牠不耐寒，每在冬季養殖，往往有存活率不穩定的情況發生。本研究之主要目的除探討低溫刺激下高糖壓迫反應現象之葡萄糖代謝相關酵素活性之變化外，並以能量代謝酵素 Lactate dehydrogenase (LDH)、Citrate synthase (CS)及 Pyruvate kinase (PK)之活性比值，比較不同低溫刺激下生理調適變化。 一般認為甲殼類血糖調控主要與眼柄中 X-器官靜竇腺之 CHH 釋放有關。生物體在壓迫下初級反應產物兒茶酚胺，可間接刺激 CHH 的釋出外，並可直接作用於標的器官，造成血糖含量的增加。淡水長臂大蝦在低溫刺激下，主要促進肝醣分解及葡萄糖新生作用增加葡萄糖含量，且肌肉組織可利用血糖做為細胞內能量來源。在不同低溫刺激下，馴養在 23℃ 的淡水蝦在13℃水溫處理下，比馴養於 28℃ 之蝦體有較長時間的耐受性。於 28℃ 轉至18℃及13℃低溫刺激 4 小時後之回溫恢復過程中，血糖含量有明顯恢復至低血糖水準。由於血糖是許多生理調節之最後步驟，在低溫刺激中若持續維持高血糖累積，可能因生理調節機制喪失所造成，可做為生理衰竭、死亡的指標。在糖解作用途徑上經由測定肌肉、肝胰臟組織 LDH、CS及PK之活性變化，發現在13℃低溫刺激下肝胰臟組織能量代謝相關酵素CS/PK活性比值呈較劇烈的變動幅度，而與回溫過程中能量相關酵素活性比值均在某特定範圍內平穩變動相較，可能是生理機能受損、衰竭的特徵。 淡水蝦在正腎上腺素注射後，血糖於1小時時程點增加到 65.62士5.93 mg/dl之高值，隨後1小時內降至19.87 土4.06 mg/dl與對照組無異。葡萄糖來源主要是由於增強肝醣分解，葡萄糖新生作用並不顯著。在離體培養中正腎上腺素會促進肝胰臟釋放大量的葡萄糖，為血糖增加之主要器官。正腎上腺素亦有促進肌肉組織釋出葡萄糖，雖量上不如肝胰臟多，但由於肌肉是蝦體最大的組織，故對血糖的累積應有一定的貢獻。此外，於16℃低溫培養中葡萄糖釋出量較26℃培養下低，似乎溫度效應對正腎上腺素引發之一連串酵素反應有相當程度的影響。

Freshwater giant prawns, Macrobrachium rosenbergii, a tropical decapods. is a commercially important culture species in Taiwan. Mass mortality of this species has often occurred in the winter season, because of their intolerability to cold temperatures. The main objectives of this study is aimed to document hyperglycemic responses and activity changes of the enzymes relevant to glucose metabolism under cold shock, and to monitor the dominance of energy metabolic pathways determined by the shifts in the activity ratio of lactate dehydrogenase(LDH),citrate synthase(CS)and pyruvate kinase(PK). Crustacean hyperglycemic hormone (CHH) synthesized and released from X-organ sinus gland complex,induces hyperglycemia of the prawns.Catecholamines mediate the release of CHH under stress or acts directly on target tissues,and in consequence, blood glucose is eventually elevated. The increase in the blood glucose of the prawns under cold shock is primarily resulted from the activation of gluconeogenesis and glycogenolysis pathways, and glucose is utilized as the energy source in muscular tissues. The prawns acclimated under 23℃ was better tolerant to cold shock at 13℃ than those acclimated under 28℃. When the prawns was transferred directly from 28℃ to 18℃ or 13℃, the blood glucose increased notably but in the process of recovery after 4 hr, the glucose content showed to decline gradually down to the original level. The physiological compensation to the cold stress is reflected in the changes of hemolymph glucose over the cold exposure, and sustained elevation of glucose accumulation in the hemolymph would be indicative of possible physiological disorder, exhaustion leading towards the death. In glycolysis process, the enzyme activities of LDH, CS and PK in the muscular and hepatopancreatic tissues were monitored under 13℃ cold shock. Significant fluctuation of the CS/PK activity ratio was observed in the hepatopancreatic tissues under the cold shock, but the ratio maintained rather constant in the recovery period. It might suggest that great fluctuations in the enzyme activity ratio well indicate the exhaustion state of physiological acclimation to the cold stress. The hemolymph glucose peaked at 65.62±5.93 mg/dl in 1 hr after the prawns received an injection of norepinephrine at the dose of 10 nM/prawn, followed by glucose decline to 19.87±4.06 mg/dl within an hour, at which the hemolymph glucose content is insignificantly different from those in control. The hyperglycemic response of the prawns in this case is mainly resulted from the enhancement of glycogenolysis, rather than gluconeogenesis. Furthermore, norepinephrine was found effective to stimulate the release of glucose in vitro from the hepatopancrease , which is considered one of important source of glucose in this species. Release of glucose from the muscular tissue in vitro was also observed, but the quantity released was not as much as those from the hepatopancreatic tissues. However, importance of the muscles as the source of glucose can not be ignored. The glucose accumulation in the culture media under in vitro stimulation at 16℃ was much less than those at 26℃, and the effectiveness of the norepinephrine on the glucose release via a chain of glucose metabolism is certainly related to the temperatures employed.