為了解明metformin可能的降血糖機轉，使用胰島素缺乏的大白鼠來研究。不論在正常大白鼠或胰島素缺乏的大白鼠，metformin皆能以劑量相關的方式降低血糖。同時，在葡萄糖挑戰試驗，metformin會增加週邊組織對葡萄糖的利用率。文獻指出，在胰島素缺乏的大白鼠身上，藥物會刺激腎上腺分泌?-endorphin來活化周邊組織的嗎啡?型受體，進而產生降血糖作用。因此，使用enzyme-linked immunosorbent assay技術檢測IDDM大白鼠血中?-endorphin的濃度。發現metformin會以劑量相關的方式增加血中?-endorphin的含量。而且，metformin的降血糖作用會被嗎啡μ型受體阻斷劑的naloxone或naloxonazine依劑量相關的型式阻斷。另外，在嗎啡μ型受體剔除的糖尿病小鼠，靜脈注射給予能產生降血糖濃度的metformin，卻看不到原先的降血糖作用；顯示metformin的降血糖作用與嗎啡μ型受體的活化有關。以口服的方式給予metformin至兩側腎上腺已摘除(bilateral adrenalectomy)的糖尿病大白鼠身上，血中?-endorphin的含量並無改變，且仍維持在高血糖的狀態；在STZ糖尿病大白鼠離體腎上腺(adrenal medulla)，metformin卻不能促進?-endorphin的釋放。由此可知，metformin並非直接作用在腎上腺來促使?-endorphin釋放。目前已知，交感神經的活化可使?-endorphin釋放，於是，進一步探討metformin促進?-endorphin釋放之機制。用HPLC檢測Amine的實驗中，發現metformin可抑制COMT之活性，減少交感神經傳遞物質的代謝。而且，metformin促進?-endorphin釋放之作用及降血糖作用均可被交感神經阻斷劑，prazosin或guanethidine解消。由此可知，在胰島素缺乏的糖尿病大白鼠，metformin會產生降血糖的作用，主要是透過抑制COMT活性，間接使體內交感神經傳遞物質增加；使交感神經活化，在腎上腺增加?-endorphin釋放，活化週邊組織的嗎啡μ型受體，因而增加肌肉葡萄糖的利用及抑制肝臟的糖質新生，於是，產生降血糖的作用。

In the present study, the antihyperglycemic action and possible mechanism of metformin were investigated. A dose-dependent lowering of plasma glucose was observed both in the diabetic rats lacking insulin and Wistar rats, after oral treatment of metformin. Metformin at an effective dose significantly attenuated the increase of plasma glucose induced by intravenous glucose challenge test in Wistar rats that indicated an increase of glucose utilization by metformin. In our previous study, we found that ?-endorphin is responsible for production of hypoglycemic effect in STZ-diabetic rats. Increase of plasma ?-endorphin in STZ-diabetic rats seems important for compensation of hyperglycemia. Metformin at the effective dose increased the plasma ?-endorphin level in STZ-diabetic rats. The plasma glucose lowering effect of metformin was also abolished by naloxone or naloxonazine. Plasma glucose lowering action of metformin at the dose effective in diabetic rats was disappeared in opioid ?-receptors knockout mice. The obtained results suggest that release of ?-endorphin seems responsible for the lowering of plasma glucose in STZ-diabetic rats by metformin. Bilateral adrenalectomy made the loss of the plasma glucose lowering effect of metformin and no increase of plasma ?-endorphin was obtained in metformin treated STZ-diabetic rats. Metformin could not enhance the ?-endorphin secretion from isolated adrenal medulla of STZ-diabetic rats. Sympathetic activation could induce release of ?-endorphin. Metformin could reduce the metabolism of sympathetic neurotransmitters by decreasing COMT activity. Similar to the plasma glucose lowering effect, the increase of plasma ?-endorphin by metformin was also abolished by prazosin or guanethidine. These results suggest that release of ?-endorphin from the adrenal gland seems responsible for the lowering of plasma glucose in STZ-diabetic rats by metformin through an inhibition of COMT activity and activation of sympathetic nerve. Activation of opioid ?-receptors by the released ?-endorphin can increase the utilization of glucose and /or decrease hepatic gluconeogenesis to lower plasma glucose in diabetic rats lacking insulin.

Aimee, D., Kohn, F.T. and Richard, A.R. Akt, a pleckstrin homology domain containing kinase, is activated primarily by phosphorylation. Journal of Biological Chemistry. 271 : 21920-21926,1996

Alastair, J.J. Metformin. The New England Journal of Medicine. 334 : 574-579, 1996

Akil, H.W., Young, S.J., Lewis, M.E., Khachaturian, H. and Walker, J.M. Endogenous opioid: biology and function. Annual Review of Neuroscience. 7 : 223-255, 1984

Baque, S., Montell, E., Camps, M., Guinovart, J.J., Zorzano, A. and Gomez-Foix, A.M. Overexpression of glycogen phosphorylase increase GLUT4 expression and glucose transport in cultured skeletal human muscle. Diabetes. 47 : 1185-1192, 1998

Berger, M., Muhlhauser, I. and Jorgens, V. Evaluation of education of the diabetic. Quality control in diabetes therapy . Fortschritte der Medizin. 101 : 212-215, 1983
Bertram, G.K. Basic＆Clinical Pharmacology.7th Edition, Appleton & Lange, pp.80-81, 1998

Bringer, J., Raingeard, I., Renard, E., Grigorescu, F. and Lefebvre P. Insulin sensitivity and polycystic ovarian syndrome. Diabetes & Metabolism. 27: 239-245, 2001
Bunn, S.J., Marley, P.D. and Livett, B.G. The distribution of opioid binding subtypes in the bovine adrenal medulla. Neuroscience. 27 : 1081-1094, 1998

Catret, M., Anselmi, E., Ivorra, M.D., Elorriaga, M., Tur, R. and D'Ocon, M.P. Alpha-adrenoceptor interaction of tetrandrine and isotetrandrine in the rat: functional and binding assays. Journal of Pharmacy and Pharmacology. 50 : 1267-1273, 1998

Civantos, C.B.Alpha-adrenoceptor subtypes. Pharmacological Research. 44 : 195-208, 2001

Chang, S.L., Lin, J.G., Chi, T.C., Liu, I.M. and Cheng, J.T. An insulin-dependent hypoglycaemia induced by electroacupuncture at the Zhongwan (CV12) acupoint in diabetic rats. Diabetologia. 42 : 250-255, 1999

Cheng, J.T. and Liu, I.M. Stimulatory effect of caffeic acid on 1A-adrenoceptors to increase glucose uptake into cultured C2C12 cells. Naunyn-Schmiedeber's Archives of Pharmacology. 362 : 122-127, 2000

Chou, P., Chen, H.H. and Hsiao, K.J. Community-base epidemiological study on diabetes in Pu-Li, Taiwan. Diabetes Care. 15 : 81-89, 1992

Eizirik, D.L., Sandler, S. and Palmer, J.P. Repair of pancreatic beta-cells. A relevant phenomenon in early IDDM? Diabetes. 42 : 1383-1391, 1993

Filz, H.P. Insulin sensitizer. A new therapy option for type 2 diabetic patients. Fortschritte der Medizin. 142 : 31-33, 2000

Fukumoto, H., Kayano, T., Buse, J.B., Edwards, Y., Pilch, P.F., Bell, G.I. and Seino, S. Cloning and characterization of the major insulin-responsive glucose transporter expressed in human skeletal muscle and other insulin-responsive tissues. Journal of Biological Chemistry. 264 : 7776-7779, 1989

Giff, B.F., Drake, R.L., and Cardell, R.R. Hepatic lobular patterns of phosphoenolpyruvate carboxykinase, glycogen synthase, and glycogen phosphorylase in fasted and fed rats. Journal of Histochemistry & Cytochemistry. 41 : 1849-1862, 1993

Giguere, V., Cote, J. and Labrie, F. Characteriristics of the α-adrenergic stimulation of adrenocorticotropin secretion in rat anterior pituitary cells. Endocrinology. 109 : 757-762, 1981

Goda, T., Yamada, K., Sugiyama, M., Moriuchi, S. and Hosoya , N. Effect of sucrose and acarbose feeding on the development of streptozotocin -induced diabetes in the rat. Journal of Nutritional Science & Vitaminology. 28 : 41-56, 1982

Harris, M.I., Hadden, W.C., Knowler, W.C. and Bennett, P.H. Prevalence of diabetes and impaired glucose tolerance and plasma glucose levels in the U.S. population aged 20-74 yr. Diabetes. 36 : 523-534, 1987

Hofmann, C., Lorenz, K., Williams, D., Palazuk, B.J. and Colca, J.R. Insulin sensitization in diabetic rat liver by an antihyperglycemic agent. Metabolism. 44 : 384-389, 1995

Holstad, M. and Sandler, S. Prolactin protects against diabetes induced by multiple low doses of streptozotocin in mice. Journal of Endocrino1ogy. 163 : 229-234, 1999

Hundal, R.S., Krssak, M., Dufour, S., Laurent, D., Lebon, V., Chandramouli, V., Inzucchi, S.E., Schumann, W.C., Petersen, K.F., Landau, B.R. and Shulman, G.I. Mechanism by which metformin reduces glucose production in type 2 diabetes. Diabetes. 49 : 2063-2069, 2000

Iwase, M., Yamaguchi, M., Yoshinari, M., Okamura, C., Hirahashi, T., Tsuji, H. and Fujishima, M. A Japanese case of liver dysfunction after 19 months of troglitazone treatment. Diabetes Care. 22 : 1382-1384, 1999

James, D.E., Strube, M. and Mueckler, M. Molecular cloning and characterization of an insulin-regulatable glucose transporter. Nature. 338 : 83-87, 1989

Koenig, J.I., Meltzer, H.Y., Devane, G.D. and Gudelsky, G.A. The concentration of arginine vasopressin in pituitary stalk plasma of the rat after adrenalectomy or morphine. Endocrinology. 118 : 2534-2539, 1986

Kohn, A.D., Takeuchhi, F. and Roth, R.A. Akt, a pleckstrin homology domain containing kinase, is activated primarily by phosphorylation. Journal of Biological Chemistry. 271 : 21290-21926, 1996

Kukkonen, J.P., Jansson, C.C. and Akerman, K.E. Agonist trafficking of G(i/o)-mediated alpha(2A)-adrenoceptor responses in HEL 92.1.7 cells. British Journal of Pharmacology. 132 : 1477-1484, 2001

Laemmli, U.K. Cleavage of structure protein during the assembly of the head of bacteriophage T4. Nature. 227 : 680-685, 1970

Law, P.Y., Wong, Y.H. and Loh, H.H. Molecular mechanisms and regulation of opioid receptor signaling. Annual Review of Pharmacology and Toxicology. 40 : 389-430, 2000

Liu, I.M., Chi, T.C., Chen, Y.C., Lu, F.H. and Cheng, J.T. Activation of opioid mu-receptor by loperamide to lower plasma glucose in streptozotocin -induced diabetic rats. Neuroscience Letters. 265 : 183-186, 1999a

Liu, I.M., Niu, C.S., Chi, T.C., Kuo, D.H. and Cheng, J.T. Investigations of the mechanism of the reduction of plasma glucose by cold-stress in streptozotocin-induced diabetic rats. Neuroscience. 92:1137-1142, 1999b

Liu, I.M., Huang, L.W. and Cheng, J.T. Gene expression of α1A-adrenoceptor but not α1B-adrenoceptor in cultured C2C12 cells. Neuroscience Letters. 294 : 93-96, 2000a

Liu, I.M., Hsu, F.L., Chen, C.F. and Cheng, J.T. Antihyperglycemic action of isoferulic acid in streptozotocin-induced diabetic rats. British Journal of Pharmacology. 129 : 631-636, 2000b

Lowry, O.H., Rosebrough, N.J., Farr, A.L. and Randall, R.J. Protein measurement with the folin phenol reagent. Journal of Biological Chemistry. 193 : 265-275, 1951

Malmejac, J. Cardiovascular actions of epinephrine and norepinephrine. Anesthesie, Analgesie, Reanimation. 26 : 693-700, 1969

Murphy, E.J., Davern, T.J., Shakil, A.O., Shick, L., Masharani, U., Chow, H., Freise, C., Lee, W.M. and Bass, N.M. Troglitazone-induced fulminant hepatic failure. Acute Liver Failure Study Group. Digestive Diseases & Sciences. 45 : 549-553, 2000

Musi, N., Hirshman, M.F., Nygren, J., Svanfeldt, M., Bavenholm, P., Rooyackers, O., Zhou, G., Williamson, J.M., Ljunqvist, O., Efendic, S., Moller, D.E., Thorell, A. and Goodyear, L.J. Metformin increases AMP-activated protein kinase activity in skeletal muscle of subjects with type 2 diabetes. Diabetes. 51 : 2074-2081, 2002

Park, K.S., Ciaraldi, T.P., Abrams-Carter L., Mudaliar, S.N. and Henry, R.R. PPAR-gamma gene expression is elevated in skeletal muscle of obese and type II diabetic subjects. Diabetes. 46 : 1230-1234, 1997

Pessin, J.E. and Bell, G.I. Mammalian facilitative glucose transporter family: structure and molecular regulation. Annual Review of Physiology. 54 : 911-930, 1992

Porstmann, T and Kiessig, S.T. Enzyme immunoassay techniques. Journal of Immunological Methods. 150 : 5-21, 1992

Schultzberg, M., Lundberg, J.M., Hokfelt, T., Terenius, L., Brandt, J., Elde, R.P. and Goldstein, M. Enkephalin-like immunoreactivity in gland cells and nerve terminals of the adrenal medulla. Neuroscience. 3 : 1169-1186, 1978

Spraul, M., Anderson, E.A., Bogardus, C. and Ravussin, E. Muscle sympathetic nerve activity in response to glucose ingestion. Impact of plasma insulin and body fat. Diabetes. 43 : 191-196, 1994

Sugaya, A., Sugiyama, T., Yanase, S., Shen, X.X., Minoura, H. and Toyoda, N. Expression of glucose transporter 4 mRNA in adipose tissue and skeletal muscle of ovariectomized rats treated with sex steroid hormones. Life Science. 66 : 641-648, 1999

Toyota, T. and Ueno, Y. Clinical effect and side effect of troglitazone. Nippon Rinsho - Japanese Journal of Clinical Medicine. 58 : 376-382, 2000

Vargo, T., Rossier, J., Minick, S., Ling, N., Rivier, C., Vale, W. and Bloom, F. -endorphin and adrenal corticotropin are secreted concomitantly by the pituitary gland. Science. 197 : 1367-1369, 1977

Vigouroux, C., Magre, J., Desbois, M.C., Lascols, O., Cherqui, G., Caron, M. and Capeau, J. Major insulin resistance syndromes: clinical and physiopathological aspects. Journal de la Societe de Biologie. 195 : 249-257, 2001

Wang, J.P., Liu, I.M., Tzeng, T.F. and Cheng, J.T. Decrease in catechol-O- methyltransferase activity in the liver of streptozotocin-induced diabetic rats . Clinical & Experimental Pharmacology & Physiology. 29 : 419-422, 2002

Winder, W.W. and Hardie, D.G. AMP-activated protein kinase, a metabolic master switch : possible roles in type 2 diabetes. American Journal of Physiology. 277 : E1-E10, 1999

Winter, H., Herschel, M., Propping, P., Friedl, W. and Vogel, F. A twin study on three enzymes (DBH, COMT, MAO) of catecholamines metabolism. Correlations with MMPI. Psychopharmacology. 57 : 63-69, 1978

Yamada, K., Yamakawa, K. and Terada, Y. Expression of GLUT4 glucose transporter protein in adipose and skeletal muscle from streptozotocin-induced diabetic pregnant rats. Hormone and Metabolic Research. 31 : 508-513, 1999

Yoshinaga, T.A. Morphological study on the mechanism of insulin -release, using sulfonylurea, l-leucine, and alpha-ketocarboxylic acids. Nippon Naibunpi Gakkai Zasshi - Folia Endocrinologica Japonica. 44 : 741-749, 1968

劉怡旻 中藥升麻酸降血糖作用之研究，成功大學基礎醫學研究所博士論文。pp. 100-101, 2001

游步青 中藥穿心蓮素降血糖作用之研究，成功大學藥理學研究所碩士論文。pp. 54-65, 2002