Mechanism for blockade of angiotensin subtype 1 receptors to lower plasma glucose in streptozotocin-induced diabetic rats

Aims: We investigated the mechanism(s) by which valsartan, a selective antagonist of angiotensin subtype 1 (AT₁) receptor, decreased plasma glucose in streptozotocin (STZ)-induced diabetic rats. Methods: The plasma glucose concentration was assessed by the glucose oxidase method. The concentration of β-endorphin in plasma or medium incubating adrenal medulla was measured using an enzyme-linked immunosorbent assay. The mRNA levels of the subtype 4 form of glucose transporter (GLUT4) in soleus muscle and phosphoenolpyruvate carboxykinase (PEPCK) in the liver were detected by Northern blotting analysis, while the protein levels of GLUT4 in isolated soleus muscle and hepatic PEPCK were investigated using Western blotting analysis. Results: A single intravenous injection of valsartan dose-dependently increased plasma β-endorphin-like immunoreactivity (BER) in parallel with the lowering of plasma glucose concentration in STZ-induced diabetic rats. Naloxone and naloxonazine inhibited the plasma glucose-lowering action of valsartan at doses sufficient to block opioid μ-receptors. In contrast to its action in wild-type diabetic mice, valsartan failed to modify plasma glucose in opioid μ-receptor knockout diabetic mice. Bilateral adrenalectomy in STZ-induced diabetic rats eliminated both the plasma glucose-lowering action and the plasma BER-elevating action of valsartan. In the isolated adrenal medulla of STZ-induced diabetic rats, angiotensin II (Ang II) or valsartan did not affect spontaneous BER secretion. Activation of cholinergic receptors by 1.0 μmol/l acetylcholine (ACh) enhanced BER secretion from the isolated adrenal medulla of STZ-induced diabetic rats, but not in the presence of 1.0 nmol/l Ang II, while valsartan reversed this inhibition by Ang II in a concentration-dependent manner. Treatment of STZ-induced diabetic rats with valsartan (0.2 mg/kg) three times daily for 3 days resulted in an increase in gene expression of GLUT4 in soleus muscle and impeded the reduction of elevated mRNA or protein level of hepatic PEPCK. Both of these effects were blocked by opioid μ-receptor antagonist. Conclusions: The results suggest that blockade of AT₁ receptor by valsartan may enhance the adrenal β-endorphin secretion induced by ACh, activating the opioid μ-receptors to increase glucose utilization and/or to decrease hepatic gluconeogenesis, resulting in the reduction of plasma glucose in STZ-induced diabetic rats.