The effect of tramadol on the plasma glucose level of streptozotocin (STZ)-induced diabetic rats was investigated. A dose-dependent lowering of plasma glucose was seen in the fasting STZ-induced diabetic rats 30 min after intravenous injection of tramadol. This effect of tramadol was abolished by pretreatment with naloxone or naloxonazine at doses sufficient to block opioid -receptors. However, response to tramadol was not changed in STZ-induced diabetic rats receiving p-chlorophenylalanine at a dose sufficient to deplete endogenous 5-hydroxytrptamine (5-HT). Therefore, mediation of 5-HT in this action of tramadol is ruled out. In isolated soleus muscle, tramadol enhanced the uptake of radioactive glucose in a concentration-dependent manner. The stimulatory effects of tramadol on glycogen synthesis were also seen in hepatocytes isolated from STZ-induced diabetic rats. The blockade of these actions by naloxone and naloxonazine indicated the mediation of opioid -receptors. The mRNA and protein levels of the subtype 4 form of glucose transporter in soleus muscle were increased after repeated treatments for 4 days with tramadol in STZ-induced diabetic rats. Moreover, similar repeated treatments with tramadol reversed the elevated mRNA and protein levels of phosphoenolpyruvate carboxykinase in the liver of STZinduced diabetic rats. These results suggest that activation of opioid -receptors by tramadol can increase the utilization of glucose and/or decrease hepatic gluconeogenesis to lower plasma glucose in diabetic rats lacking insulin. Diabetes 50: [2815][2816][2817][2818][2819][2820][2821] 2001 U nlike the analgesic effects of opioids, their effects on glucose metabolism in diabetes have received little attention. In diabetic patients, ␤-endorphin stimulates insulin secretion (1). Also, ␤-endorphin is known to be involved in plasma glucose homeostasis (2,3). Opioid receptors in the pancreas have been investigated for this regulation of plasma glucose (4,5). However, the effect of opioids on glucose homeostasis does not depend entirely on insulin. In our previous study (6), we found that ␤-endorphin is also responsible for the reduction of plasma glucose during cold exposure in streptozotocin (STZ)-induced diabetic rats, which were used as a type 1 diabetes model. Actually, injection of exogenous ␤-endorphin lowered plasma glucose in STZ-induced diabetic rats (6). Moreover, we demonstrated that loperamide, an agonist of opioid -receptors, could lower plasma glucose in STZ-induced diabetic rats (7). Thus, it has been shown that activation of opioid -receptors may produce a plasma glucose-lowering effect in diabetic rats lacking insulin. Clinically, tramadol has widely been used as an analgesic through activation of opioid -receptors (8 -11) and others (9). In the present study, we investigated the effect of tramadol on plasma glucose and characterized the role of opioid -receptors in the action of tramadol during the absence of insulin, both in vivo and in vitro. We also examined the influence of repeated treatmen...

To better understand the insulin-independent plasma glucose-lowering action of metformin, we used streptozotocin (STZ)-induced diabetic rats to investigate the possible mechanisms. Oral intake of metformin decreased the plasma glucose of STZ-induced diabetic rats with a parallel increase of plasma ␤-endorphin-like immunoreactivity (BER). Mediation of opioid -receptors in the action of metformin was identified by the blockade of receptors with antagonist in STZ-induced diabetic rats and the failure of action in opioid -receptor knockout diabetic mice. Release of BER from adrenal glands by metformin was characterized, using bilateral adrenalectomy and the release of BER from isolated adrenal medulla of STZ-induced diabetic rats. Repeated treatment with metformin in STZ-induced diabetic rats increased the mRNA and protein levels of GLUT-4 in soleus muscle that was blocked by naloxonazine. Reduction of the mRNA or protein levels of hepatic PEPCK was also impeded in the same group of STZ-induced diabetic rats. In conclusion, our results provide novel mechanisms for the plasma glucose-lowering action of metformin, via an increase of ␤-endorphin secretion from adrenal glands to stimulate opioid -receptor linkage, leading to an increase of GLUT-4 gene expression and an attenuation of hepatic PEPCK gene expression in STZinduced diabetic rats. Diabetes 55:819 -825, 2006

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Our results suggest that agmatine may activate I(2)-imidazoline receptors in the adrenal gland. This enhances secretion of beta-endorphin, which can activate opioid mu-receptors to increase GLUT4 gene expression and/or suppress hepatic PEPCK gene expression, resulting in a lowering of plasma glucose in diabetic rats lacking insulin. The results provide a potential new target for intervention in type 1 diabetes.

The effect of beta-endorphin on plasma glucose levels was investigated in streptozotocin-induced diabetic rats (STZ-diabetic rats). A dose-dependent lowering of plasma glucose was observed in the fasting STZ-diabetic rat fifteen minutes after intravenous injection of beta-endorphin. The plasma glucose-lowering effect of beta-endorphin was abolished by pretreatment with naloxone or naloxonazine at doses sufficient to block opioid mu-receptors. Also, unlike wild-type diabetic mice, beta-endorphin failed to induce its plasma glucose-lowering effect in the opioid mu-receptor knock-out diabetic mice. In isolated soleus muscle, beta-endorphin enhanced the uptake of radioactive glucose in a concentration-dependent manner. Stimulatory effects of beta-endorphin on glycogen synthesis were also seen in hepatocytes isolated from STZ-diabetic rats. The blockade of these actions by naloxone and naloxonazine indicated the mediation of opioid mu-receptors. In the presence of U73312, the specific inhibitor of phospholipase C (PLC), the uptake of radioactive glucose into isolated soleus muscle induced by beta-endorphin was reduced in a concentration-dependent manner, but it was not affected by U73343, the negative control of U73312. Moreover, chelerythrine and GF 109203X diminished the stimulatory action of beta-endorphin on the uptake of radioactive glucose at a concentration sufficient to inhibit protein kinase C (PKC). The data obtained suggest that activating opioid mu-receptors by beta-endorphin may increase glucose utilization in peripheral tissues via the PLC-PKC pathway to lower plasma glucose in diabetic rats lacking insulin.