萊克多巴胺在斑馬魚模型中的安全性探討

Abstract

萊克多巴胺 (Ractopamine; RAC) 是一種β-腎上腺素受體促進劑 (β-adrenergic receptor agonists)，通常用作飼料添加劑以調節牲畜新陳代謝，促進蛋白質合成進而提高牲畜的產肉率。過去研究指出，RAC的使用可能會導致家畜的行為和生理的異常變化，並且在水體中也發現多種類似的畜牧用藥汙染。由於脊椎動物斑馬魚具有體型小、生長週期短、繁殖力高、與人類基因組高度相似等優點，因此本研究利用斑馬魚的胚胎和成魚作為實驗模型，參考ADI (Acceptable daily intake)以及不確定性係數 (Uncertainty factor) 決定施用於斑馬魚的RAC濃度範圍，綜合進行RAC對於早期胚胎外型及心臟發育、幼魚運動行為能力與成魚生殖能力的安全性評估。

首先將2 hpf (hours post fertilization) 野生型斑馬魚胚胎以不同濃度RAC溶液暴露至72 hpf後取魚體組織進行直接競爭型酵素免疫分析法 (cdELISA)，得知 RAC在組織內的蓄積作用濃度約為水中暴露濃度的15-20 %。以最高濃度RAC (2000 μg/L) 處理野生型斑馬魚胚胎至144 hpf，並未觀察到對幼魚形態、生存率和孵化率造成顯著的影響；此外RAC亦不會顯著改變心臟螢光魚Tg (BMP4: EGFP) 胚胎在72 hpf時心房、心室的截面積大小和靜脈竇-動脈球 (SV-BA) 的距離；但從心臟組織切片則發現高濃度RAC會顯著增加心包腔壁層的平均厚度。RAC處理亦會造成胚胎的心率和血液流速上升，並且具有劑量效應關係。觀察在高濃度RAC組別中144 hpf幼魚的泳動行為參數，其中平均泳速和自主泳動的占比時間都有顯著性提升。若以食品相關濃度RAC處理48 hpf胚胎進行RNA定序，分析得知有關啟動子轉錄調控、細胞內信號轉導、跨膜運輸和離子傳輸等生物功能亦會受到影響。

為了模擬人類口服攝入RAC之途徑，我們對成年斑馬魚連續進行14天RAC (100 x ADI 濃度) 的管餵實驗，犧牲成年斑馬魚所得到之肝體指數 (HSI) 和性腺指數 (GSI) 和對照組相比並沒有統計差異；性腺組織切片的結果則顯示接受RAC管餵斑馬雄魚的睪丸中精子所占的面積和雌魚卵巢中成熟卵細胞的數量皆出現明顯增加。此外檢視成魚管餵進行第五天和第十天後交配所產生的F1胚胎，發現其心血管功能和運動能力都和對照組F1相似。綜合以上結果，高濃度RAC (100 x ADI) 的暴露對於斑馬魚胚胎心臟功能、運動行爲和生殖功能可能會有程度不一的影響，需要更多研究數據進一步確認，以維護大眾及敏感族群之健康。

Ractopamine (RAC) is a β-adrenergic receptor agonist commonly used as a feed additive to regulate the metabolism of livestock, promote protein synthesis, and then increase meat production efficiency. Previous studies have indicated that the use of RAC may lead to behavioral and physiological abnormalities in livestock, and various similar residues of veterinary drugs have also been found in aquatic environments. Due to the advantages of zebrafish, such as small size, short lifecycle, high reproductive capacity, and genomic similarity to humans, this study employs zebrafish embryos and adults as experimental models. The RAC concentrations applied to zebrafish were determined by referencing Acceptable Daily Intake (ADI) and Uncertainty factor values to comprehensively assess the safety of RAC on early embryo morphology, cardiac development, juvenile fish locomotor behavior, and adult reproductive capacity.

Wild-type zebrafish embryos at 2 hours post fertilization (hpf) were initially exposed to various doses of RAC solutions until 72 hpf, and the competitive direct enzyme-linked immunosorbent assay (cdELISA) was applied to examine the RAC levels in fish tissues. It was found that the accumulated effective concentration of RAC within tissues was approximately 15-20% of the exposure concentration in the water. Treatment of wild-type embryos with the highest concentration of RAC (2000 μg/L) until 144 hpf did not result in significant effects on juvenile morphology, survival rate, or hatching rate. Furthermore, RAC did not significantly alter the atria and ventricles' sizes or the distance between the Sinus venosus (SV) and Bulbus arteriosus (BA) in Tg (BMP4: EGFP) embryos at 72 hpf. However, tissue section analysis revealed a significant increase in the average thickness of the pericardial wall exposed to high RAC concentrations. RAC treatment also led to increased heart rate and blood flow velocity in embryos, exhibiting a dose-response relationship. Abnormal swimming behaviors were observed in 144 hpf larvae from the high RAC groups, with significantly increased average swimming speeds and percentages of active swimming time. Additionally, RNA sequencing of 48 hpf embryos treated with food-relevant concentrations of RAC revealed significant effects on promoter transcription regulation, intracellular signal transduction, transmembrane transport, and ion transport.

To simulate the ingestion of RAC in humans accurately, adult zebrafish were subjected to a continuous 14-day gavage experiment with RAC at 100 times the ADI concentration. Sacrificed adult zebrafish showed no statistically significant differences in Hepatosomatic index (HSI) and Gonadosomatic index (GSI) compared to the control group. However, analysis of gonadal tissue sections revealed that RAC gavage significant increased in the area occupied by sperm in the testes of male fish and in the number of mature egg cells in the ovaries of female fish. Moreover, F1 embryos produced by gavaged adult mating on the 5th and 10th days showed the cardiovascular function and locomotor ability similar to those of control F1 embryos.

In summary, exposure to high concentrations of RAC (100 times the ADI) may have varying degrees of effects on zebrafish embryo cardiac function, locomotor behavior, and reproductive function, necessitating further research to confirm and safeguard public and sensitive population health.