重金屬鎘與鋅對白蝦之毒性影響研究

Abstract

白蝦(Litopenaeus vannamei)是目前全球性的重要蝦類養殖蝦種之一，也是一種相當適合用來進行重金屬影響相關研究的水生生物。在本實驗中，我們透過急速毒性試驗得到鎘(Cd)與鋅(Zn)對白蝦的24、48、72、96小時半致死濃度(LC50)分別為2.58、1.30、1.14、1.07 mg Cd/L 與3.98、2.14、1.75、1.35 mg Zn/L。當白蝦急性曝露於鎘或鋅之後，我們發現到其鰓部功能發生異常的現象。曝露於鎘與鋅會造成白蝦耗氧量分別下降為91.3%與75.9%，但會使得白蝦排氨量分別增加為153.7%與144.1%。同時，曝露過鎘或鋅的白蝦其血淋巴液滲透壓也出現下降的情形，我們認為此應跟排氨量的上升有關係。在實驗中，我們也檢測了白蝦鰓部組織鎘與鋅分佈的情形，發現到其存在量隨著曝露時間增加而有明顯上升的趨勢。此外，在組織學方面的研究上，我們發現到急性曝露鎘或鋅的白蝦其鰓部會有嚴重的組織病變發生。我們認為，當白蝦急性曝露於鎘或鋅之後所產生的這些鰓部組織改變，跟先前研究所得的鰓部功能異常結果應該有所關聯，因此在本文當中也進行探討。 在得到急速毒性試驗結果之後，我們決定出慢性毒性研究所使用的金屬濃度。在慢性毒性研究方面，本研究進行白蝦成長、攝食以及肝胰臟中生化物質組成(三酸甘油脂、膽固醇與總溶解性蛋白質)檢測等實驗，來觀察重金屬鎘與鋅對於白蝦營養狀況的影響。結果顯示，長期曝露於鎘與鋅會造成白蝦成長上的延遲，而這相當有可能跟攝食率的降低、以及其肝胰臟內營養生化物質組成的改變或不可獲得性有很大的關係。同時，我們也觀察到長期曝露於鎘或鋅的白蝦，其肝胰臟的確也會出現組織上的病變，因此也影響到其正常的生理代謝功能。 因為先前在許多研究上都觀察到金屬硫蛋白(metallothioneins，MT)在許多種生物體內的存在量會隨著外界金屬濃度增加而被誘發且提高，因此長久以來其一直被視為具有潛力可以被利用來作為檢測環境重金屬污染是否存在的生物性指標。然而，相關的研究在對蝦種類上卻相當少。在本研究中，我們首先利用膠體過濾層析法分離並證明了白蝦亦具有金屬硫蛋白或類似蛋白質的存在。接著，我們同時檢測長期曝露於鎘或鋅的白蝦體內各器官中的金屬硫蛋白濃度以及重金屬累積存在量。結果顯示，白蝦對於鎘與鋅的反應並不相同，而這點也是當實際在野外利用金屬硫蛋白來進行檢測工作時必須考慮到的重點之一。除此之外，本實驗也發現到鎘與鋅亦會促進穀胱苷月太-硫-轉化酶(glutathione S-transferase，GST)的誘發，以提供白蝦本身對外來重金屬的解毒能力。

White shrimp, Litopenaeus vannamei, a globally important cultured prawn species, is an ideal animal for studying the impairment caused by the effects of heavy metals that are often detected in coastal areas. In this present study, we detected the acute toxicity of cadmium (Cd) and zinc (Zn) to L. vannamei, and investigated their effects on gill functions, which have not been carried out in this species before, hoping to obtain some important significance. First of all, we examined the acute toxicity of cadmium (Cd) and zinc (Zn) to L. vannamei and obtained 24-, 48-, 72-, and 96-h medium lethal concentration (LC50) values of 2.58, 1.30, 1.14, and 1.07 mg Cd/L and 3.98, 2.14, 1.75, and 1.35 mg Zn/L, respectively. Furthermore, we also found that acute exposure of shrimp to Cd or Zn caused some alterations in gill functions, including inhibitions in oxygen consumption of 91.3% and 75.9% lower than that of the control, elevations in ammonium excretion, which were 153.7% and 144.1%, respectively, higher than the control, and decreases in osmotic pressure of shrimp blood. The distribution of Cd and Zn within gills was also determined. Furthermore, effects on gill structures of L. vannamei after acute exposure to Cd and Zn were studied. We demonstrated that acute exposure to high concentrations of Cd and Zn resulted in morphological alterations in gills of L. vannamei in this present study. Combining with these results, we further discussed aspects of the effects of heavy metals on respiratory function by examining the relations between gill structural changes and injuries, and gill dysfunction in the present article. Using the results from acute toxicity experiments as foundations, we further studied the chronic toxicities of Cd and Zn on L. vannamei. We investigated the effects of Cd and Zn on the growth, food consumption, and biochemical parameters such as triacylglycerol (TAG), cholesterol, and total soluble protein (TSP) levels within the hepatopancreas, to determine the effects of Cd and Zn on the nutritional condition of L. vannamei. Results showed that Cd and Zn caused growth retardation in L. vannamei, which might be due to the decrease in their food consumption rates as well as changes in nutritional conditions, since alterations in levels of or unavailability of biochemical and nutritional materials within the hepatopancreas were observed. Histological injuries in hepatopancreas were also observed in L. vannamei after long-term exposure to Cd and Zn. Metallothioneins (MTs) have been widely considered for their potential use as specific biomarkers to reflect the existence of heavy metal pollution, because their induction has been observed to be obviously elevated after heavy metal exposure in a large number of organisms studied. However, relatively fewer efforts have been made in MT-related studies of prawn species, such as L. vannamei. With the results from gel filtration chromatography, we demonstrated the existence of MTs or MT-like proteins in L. vannamei. We further studied the relationship between MT induction and metal accumulation after long-term exposure to the heavy metals Cd and Zn. From our results, it is very clear that the response of L. vannamei to Cd differs from that to Zn, and this should be considered when using MTs in field applications to monitor metal contamination. Also, induction of glutathione S-transferase (GST) in L. vannamei was observed after exposure to Cd and Zn.