糖尿病乃因胰島素作用的失常，引致高血糖的一種新陳代謝疾病。本研究欲探討活化μ型接受體對胰島素抗性的緩解效果;使用μ型接受體作用劑的loperamide來探討其對胰島素敏感性的影響，評估改善胰島素抗性的可行性，並進一步研究改善胰島素抗性之可能機轉。依照文獻的方法，將摻有60﹪果糖飼料給予小鼠來誘發成胰島素抗性，再以tolbutamide測試；原本會促進胰島素釋放來造成降血糖的作用消失了，就表示動物已產生胰島素阻抗作用。在嗎啡μ型接受體剔除的小鼠會比普通的小鼠更快出現胰島素抗性。由此可知，μ型接受體在胰島素抗性形成的重要性。
接著，在果糖誘發的胰島素抗性大白鼠進行葡萄糖耐受性試驗 (glucose tolerance test)。與控制組相較，發現loperamide會顯著地降低 (Glucose insulin index) 此指標；另外，在遺傳性的第二型糖尿病老鼠 (Zucker-diabetic fatty rats) 也會看到相同的反應。當以loperamide連續給藥3天，取其果糖老鼠的骨骼肌，結果發現，loperamide會提高骨骼肌中IRS-1 (Tyr)，PI-3 kinase (p85α) 及Akt (Ser473) 的活性。
進一步使用C2C12小鼠肌母細胞來解明loperamide改善胰島素抗性的可能機轉。發現不論在正常C2C12小鼠肌母細胞或是對TNF-α引致的胰島素抗阻作用，loperamide都會以劑量相關的方式增強細胞對葡萄糖的吸入 (Glucose uptake) 能力，但此項作用會被μ型接受體的拮抗劑naloxone及naloxonazine所阻斷。接著，前處理μ型接受體下游的訊息分子 phospholipase C (PLC) 的阻斷劑U73122，看到U73122會解消原先loperamide促進細胞對葡萄糖的吸入作用。同樣地，在protein kinase C (PKC) 的阻斷劑chelerythrine或GF109203X存在下，發現lopermaide促進C2C12細胞吸入的作用會呈現濃度相關性的減少。最後，在胰島素抗性的C2C12肌母細胞，也發現Loperamide不僅會提高細胞中胰島素訊息分子的活性，同時，更增強下游葡萄糖轉運蛋白 (GLUT4) 的蛋白表現量。
由此可知，在C2C12小鼠細胞，當TNF-α破壞胰島素訊息路徑的傳遞，降低了細胞對葡萄糖的吸入作用。此時，藉由loperamide活化嗎啡μ型接受體，啟動PLC-PKC的訊息路徑，可提升周邊組織對胰島素的反應來達到改善的作用。綜合以上的結果，無論在體內或是體外試驗，以loperamide活化嗎啡μ型接受體，皆可使胰島素抗性獲得有效地緩解。

Diabetes mellitus (DM) is the metabolic disorder caused by abnormalities of insulin action. The present study is performed to investigate whether activation of opioid-μ receptor can improve insulin resistance. Loperamide has been used as the selective agonist of opioid-μ receptor. Insulin resistance has been induced in animals by feeding the chow containing 60% fructose. Induction of insulin resistance characterized by loss of tolbutamide-induced plasma glucose lowering effect. Insulin resistance has been observed in the wild-type mice more rapidly than that in opioid-μ receptors knockout mice. The important role of opioid-μ receptor in insulin resistance can thus be considered.
Loperamide causes the lowering of glucose-insulin index in rats with insulin resistance more markedly than that from vehicle-treated group response to the glucose challenge test. In addition, the response is similar to Zucker-diabetic fatty rats. Correspondingly, insulin receptor substrate-1 (IRS-1) phosphorylation, IRS-1-associated phosphatidylinositol (PI) 3-kinase (PI 3-kinase) and protein kinase B (AKT) activity in soleus muscle are significantly increased by loperamide after repeated treatment for 3 days in rats with insulin resistance.
Effect of loperamide on insulin resistance is identified in mouse myoblast cell line C2C12 cells. Loperamide enhance the uptake of radioactive glucose into C2C12 cells with or without pretreated TNF-a in a concentration-related manner, which are abolished by naloxone and naloxonazine. The results suggest that activation of opioid-μ receptor may increase the insulin action to ameliorate insulin resistance. Pharmacological inhibition of phospholipase C (PLC) by U73122 decrease in loperamide -stimulated uptake of radioactive glucose into C2C12 cells pretreatment with TNF-a. Also, cheleythrine or GF109203X, protein kinase C (PKC) inhibitor, diminish the action of loperamide in a concentration- related manner. Then, TNF-a treatment causes a decrease in the protein levels of glucose transporter subtype 4 (GLUT4) and phosphorylation of several insulin signaling proteins, including IRS-1, PI 3-kinase, and AKT in C2C12 cells are also reversed by loperamide .
Thus, it shows that TNF-α impaired insulin signaling and insulin- stimulate glucose uptake in C2C12 cells can be restored by loperamide in a dose-related manner. The activation of opioid-μ receptor by loperamide may increase the glucose uptake via PLC-PKC pathway in C2C12 cells pretreated with TNF-a. In conclusion, activation of opioid-μ receptor by loperamide can improve insulin resistance both in vivo and in vitro.

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