萊克多巴胺(Ractopamine, RAC)是β2-腎上腺素受體素。RAC廣泛的使用在畜牧業當作飼料添加劑，促進蛋白質合成和減少脂肪堆積。由於RAC的生理作用尚未明瞭，所以被禁用在台灣及許多國家。目前RAC當作牲畜的飼料添加劑也有所爭議，於是我們測試RAC在小鼠的作用及潛在的心毒性。
我們所建立小鼠動物模型：BALB/c公成鼠皮下注射RAC 300 mg/kg一劑，等待4天後犧牲。採其心臟利用蘇木紫與伊紅、梅森氏三色染色法進行心臟病理分析；接著利用TUNEL染色法偵測心肌細胞凋亡情形，更進一步利用免疫組織化學染色法觀察心肌細胞遭受氧化壓力的程度。針對RAC的作用機制，我們以心肌細胞H9c2做測試。處理RAC 1 mM不同時間點以西方墨點法分析活化態caspase-3和磷酸化-p38程度；以DAPI染色檢測凋亡細胞。
結果顯示，在RAC刺激下造成心臟細胞明顯流失且受傷區域被浸潤細胞所取代。在RAC處理組相較於沒有受傷的DMSO組別其左心室有2.5%的受傷區域。我們也證實RAC透過氧化壓力的增加及細胞凋亡造成心肌細胞受損。此外細胞方面，RAC處理之下活化態caspase-3 和p-p38蛋白含量隨時間增加而增加。與單獨處理RAC組別比較，前處理p-p38抑制劑(SB-202190)或蛋白激酶A (Protein kinase A; PKA)的抑制劑(H-89)都可降低RAC處理後造成的細胞凋亡。此結果代表RAC透過活化PKA和p-p38路徑造成心肌細胞凋亡。本研究結果更了解萊克多巴胺對於心臟所造成的影響及作用機制，建立的動物模式可用於未來臨床之應用。

Ractopamine (RAC) is a sympathomimetic beta 2 adrenergic (β2-AR) agonist. RAC is commonly used as a feed additive for livestock to promote protein synthesis and reduce fat accumulation. The physiological role of RAC remains unclear, which leads to the ban of using RAC in certain countries, including Taiwan. In light of the controversy of ractopamine being used as a feed additive in livestock, we had hypothesis which the effects of RAC in mice and found that RAC possesses potential cardiotoxicity.
To test the hypothesis, we created an animal model. Adult male BALB/c mice were subcutaneously injected with RAC 300 mg/kg once. The hearts of the mice were excised and analyzed 4 days after the RAC injection. H&E and Masson’s trichrome staining were performed to analyze the pathology of the heart. Other procedures including, cardiac cell TUNEL staining for apoptosis, immunohistochemistry staining for oxidative stress. The mechanism of RAC was investigated in cultured cardiomyoblast H9c2 cells. The activation of caspase-3 and p38 MAPK was examined by Western blotting. Apoptosis was determined by the nuclear condensation after DAPI staining.
RAC treatment resulted in cardiomyocyte loss and the injury area was replaced by infiltrating cells. 2.5% injury area was observed in RAC-treated left ventricles compared with no injury found in DMSO-control mice. In addition, we demonstrated that RAC induced myocardial injury through increasing oxidative stress and apoptosis. Moreover, cellular levels of cleaved caspase-3 and p-p38 proteins were increased after RAC treatments. p38 inhibitor (SB-202190) or protein kinase A inhibitor (H-89) effectively blocked the apoptosis induced by RAC. These results indicated that RAC induced apoptosis through protein kinase A and p38 activation.
Here we provided evidence to demonstrate the cardiac toxicity of ractopamine in mice.

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