AT1-receptor blockade by telmisartan upregulates GTP-cyclohydrolase I and protects eNOS in diabetic rats

Area of Science:

• Cardiovascular Research
• Endocrinology
• Pharmacology

Background:

• Diabetes mellitus is associated with endothelial dysfunction, partly due to endothelial nitric oxide synthase (eNOS) uncoupling.
• Reactive oxygen species (ROS) contribute to eNOS uncoupling in diabetes through various enzymatic sources.
• The BH4-synthesizing enzyme GTP-cyclohydrolase I (GCH-I) plays a role in maintaining eNOS function.

Purpose of the Study:

• To investigate the effects of telmisartan, an AT1-receptor blocker, on GCH-I expression, eNOS uncoupling, and endothelial dysfunction in a type 1 diabetes model.
• To assess telmisartan's impact on ROS production from various sources in diabetic aortas.
• To determine if telmisartan can prevent STZ-induced changes in GCH-I and eNOS phosphorylation.

Main Methods:

• Streptozotocin (STZ)-induced diabetes in rats, treated with telmisartan (25 mg/kg/day for 6.5 weeks).
• Isometric tension studies to evaluate aortic vascular function.
• Measurement of vascular and cardiac ROS production.
• Assessment of NADPH oxidase subunit expression and GCH-I expression.
• Analysis of eNOS phosphorylation at Ser1177.

Main Results:

• STZ-induced diabetes caused vascular dysfunction, increased ROS production (NADPH oxidase, mitochondria, eNOS, xanthine oxidase), and decreased GCH-I expression and eNOS phosphorylation.
• Telmisartan treatment did not alter blood glucose or body weight but normalized vascular function and reduced ROS levels.
• Telmisartan therapy prevented the STZ-induced decrease in GCH-I expression and eNOS phosphorylation at Ser1177.

Conclusions:

• AT1-receptor blockade with telmisartan prevents GCH-I downregulation and subsequent eNOS uncoupling in experimental diabetes.
• Telmisartan inhibits the activation of key superoxide sources, including NADPH oxidase, mitochondria, and xanthine oxidase.
• These findings suggest telmisartan's beneficial effects on endothelial dysfunction in diabetes are mediated by preserving GCH-I/BH4 pathway and reducing oxidative stress.