Acute vascular and metabolic actions of the green tea polyphenol epigallocatechin 3-gallate in rat skeletal muscle.

Epidemiological studies show a dose-dependent relationship between green tea consumption and reduced risk for type 2 diabetes and cardiovascular disease. Bioactive compounds in green tea including the polyphenol epigallocatechin 3-gallate (EGCG) have insulin-mimetic actions on glucose metabolism and vascular function in isolated cell culture studies. The aim of this study is to explore acute vascular and metabolic actions of EGCG in skeletal muscle of Sprague-Dawley rats. Direct vascular and metabolic actions of EGCG were investigated using surgically isolated constant-flow perfused rat hindlimbs. EGCG infused at 0.1, 1, 10 and 100 µM in 15 min step-wise increments caused dose-dependent vasodilation in 5-hydroxytryptamine pre-constricted hindlimbs. This response was not impaired by the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin or the AMP-kinase inhibitor Compound C. The nitric oxide synthase (NOS) inhibitor NG-Nitro-l-Arginine Methyl Ester (L-NAME) completely blocked EGCG-mediated vasodilation at 0.1-10 µM, but not at 100 µM. EGCG at 10 µM did not alter muscle glucose uptake nor did it augment insulin-stimulated muscle glucose uptake. The acute metabolic and vascular actions of 10 µM EGCG in vivo were investigated in anaesthetised rats during a hyperinsulinemic-euglycemic clamp (10 mU min-1 kg-1 insulin). EGCG and insulin both stimulated comparable increases in muscle microvascular blood flow without an additive effect. EGCG-mediated microvascular action occurred without altering whole body or muscle glucose uptake. We concluded that EGCG has direct NOS-dependent vasodilator actions in skeletal muscle that do not acutely alter muscle glucose uptake or enhance the vascular and metabolic actions of insulin in healthy rats.

Vascular and metabolic actions of the green tea polyphenol epigallocatechin gallate.

Epidemiological studies demonstrate robust correlations between green tea consumption and reduced risk of type 2 diabetes and its cardiovascular complications. However, underlying molecular, cellular, and physiological mechanisms remain incompletely understood. Health promoting actions of green tea are often attributed to epigallocatechin gallate (EGCG), the most abundant polyphenol in green tea. Insulin resistance and endothelial dysfunction play key roles in the pathogenesis of type 2 diabetes and its cardiovascular complications. Metabolic insulin resistance results from impaired insulin-mediated glucose disposal in skeletal muscle and adipose tissue, and blunted insulin-mediated suppression of hepatic glucose output that is often associated with endothelial/ vascular dysfunction. This endothelial dysfunction is itself caused, in part, by impaired insulin signaling in vascular endothelium resulting in reduced insulin-stimulated production of NO in arteries, and arterioles that regulate nutritive capillaries. In this review, we discuss the considerable body of literature supporting insulin-mimetic actions of EGCG that oppose endothelial dysfunction and ameliorate metabolic insulin resistance in skeletal muscle and liver. We conclude that EGCG is a promising therapeutic to combat cardiovascular complications associated with the metabolic diseases characterized by reciprocal relationships between insulin resistance and endothelial dysfunction that include obesity, metabolic syndrome and type 2 diabetes. There is a strong rationale for well-powered randomized placebo controlled intervention trials to be carried out in insulin resistant and diabetic populations.

Muscle insulin resistance resulting from impaired microvascular insulin sensitivity in Sprague Dawley rats.

AIMS: Enhanced microvascular perfusion of skeletal muscle is important for nutrient exchange and contributes ∼40% insulin-mediated muscle glucose disposal. High fat-fed (36% fat wt./wt.) rats are a commonly used model of insulin-resistance that exhibit impairment of insulin-mediated microvascular recruitment and muscle glucose uptake, which is accompanied by myocyte insulin-resistance. Distinguishing the contribution of impaired microvascular recruitment and impaired insulin action in the myocyte to decreased muscle glucose uptake in these high-fat models is difficult. It is unclear whether microvascular and myocyte insulin-resistance develop simultaneously. To assess this, we used a rat diet model with a moderate increase (two-fold) in dietary fat. METHODS AND RESULTS: Sprague Dawley rats fed normal (4.8% fat wt./wt., 5FD) or high (9.0% fat wt./wt., 9FD) fat diets for 4 weeks were subject to euglycaemic hyperinsulinemic clamp (10 mU/min/kg insulin or saline) or isolated hindlimb perfusion (1.5 or 15 nM insulin or saline). Body weight, epididymal fat mass, and fasting plasma glucose were unaffected by diet. Fasting plasma insulin and non-esterified fatty acid concentrations were significantly elevated in 9FD. Glucose infusion rate and muscle glucose uptake were significantly impaired during insulin clamps in 9FD. Insulin-stimulated microvascular recruitment was significantly blunted in 9FD. Insulin-mediated muscle glucose uptake between 5FD and 9FD were not different during hindlimb perfusion. CONCLUSIONS: Impaired insulin-mediated muscle glucose uptake in vivo can be the direct result of reduced microvascular blood flow responses to insulin, and can result from small (two-fold) increases in dietary fat. Thus, microvascular insulin-resistance can occur independently to the development of myocyte insulin-resistance.