Mediation of Endogenous beta-endorphin by Tetrandrine to Lower Plasma Glucose in Streptozotocin-induced Diabetic Rats

Abstract

The role of beta-endorphin in the plasma glucose-lowering action of tetrandrine in streptozotocin-induced diabetic rats (STZ-diabetic rats) was investigated. The plasma glucose concentration was assessed by the glucose oxidase method. The enzyme-linked immunosorbent assay was used to determine the plasma level of beta-endorphin-like immunoreactivity (BER). The mRNA levels of glucose transporter subtype 4 (GLUT4) in soleus muscle and phosphoenolpyruvate carboxykinase (PEPCK) in the liver of STZ-diabetic rats were detected by Northern blotting analysis. The expressed protein of GLUT4 or PEPCK was characterized by Western blotting analysis. Tetrandrine dose-dependently increased plasma BER in a manner parallel to the decrease of plasma glucose in STZ-diabetic rats. Moreover, the plasma glucose-lowering effect of tetrandrine was inhibited by naloxone and naloxonazine at doses sufficient to block opioid μ-receptors. Further, tetrandrine failed to produce plasma glucose-lowering action in opioid μ-receptor knockout diabetic mice. Bilateral adrenalectomy eliminated the plasma glucose-lowering effect and plasma BER-elevating effect of tetrandrine in STZ-diabetic rats. Both effects were abolished by treatment with hexamethonium or pentolinium at doses sufficient to block nicotinic receptors. Tetrandrine enhanced BER release directly from the isolated adrenal medulla of STZ-diabetic rats and this action was abolished by the blockade of nicotinic receptors. Repeated intravenous administration of tetrandrine (1.0 mg/kg) to STZ-diabetic rats for 3 days resulted in an increase in the mRNA and protein levels of the GLUT4 in soleus muscle, in addition to the lowering of plasma glucose. Similar treatment with tetrandrine reversed the elevated mRNA and protein levels of PEPCK in the liver of STZ-diabetic rats. The obtained results suggest that tetrandrine may induce the activation of nicotinic receptors in adrenal medulla to enhance the secretion of beta-endorphin, which could stimulate opioid μ-receptors to increase glucose utilization or/and reduce hepatic gluconeogenesis to lower plasma glucose levels in STZ-diabetic rats.

Figure 1

The effect of tetrandrine on plasma BER level in STZ-diabetic rats. Values (means ± SE) were obtained from each group of 8 animals receiving an intravenous (i.v.) injection of tetrandrine at the indicated dose. *P < 0.05 and **P < 0.01 vs data from animals treated with the vehicle (0) administered in the same volume.

Figure 2

Effects of tetrandrine (1.0 mg/kg, i.v.) on plasma glucose concentrations in opioid μ-receptor knockout mice and wild-type controls. Values (means ± SE) were obtained from each group of 7 animals. **P < 0.01 vs data obtained from animals before treatment in each group.

Figure 3

Effect of tetrandrine on BER secretion from isolated adrenal medulla of STZ-diabetic rats. Results expressed as pg/mg protein are the means ± SE of 7 determinations. *P < 0.05 and **P < 0.01 vs data from samples treated with MKS (0), respectively.

Figure 4

A: Representative response of mRNA level for GLUT4 or β-actin in soleus muscle isolated from normal or STZ-diabetic rats receiving repeated treatment with tetrandrine (1.0 mg/kg) or the same volume of vehicle three times a day for 3 days. B: Identification of protein level of GLUT 4 or β-tubulin using immunoblot analysis. Lanes show vehicle-treated normal rats (lane 1), vehicle-treated STZ-diabetic rats (lane 2) and tetrandrine-treated STZ-diabetic rats (lane 3).

Figure 5

A: Representative response of mRNA level for PEPCK or β-actin in liver isolated from normal or STZ-diabetic rats receiving repeated treatment with tetrandrine (1.0 mg/kg) or the same volume of vehicle three times daily for 3 days. B: Identification of protein level of PEPCK or β-tubulin using immunoblot analysis. Lanes show vehicle-treated normal rats (lane 1), vehicle-treated STZ-diabetic rats (lane 2) and tetrandrine-treated STZ-diabetic rats (lane 3).