Metformin Improves Skeletal Muscle Microvascular Insulin Resistance in Metabolic Syndrome
Aims: Microvascular insulin resistance is present in metabolic syndrome and may contribute to increased cardiovascular disease risk and the impaired metabolic response to insulin observed. Metformin improves metabolic insulin resistance in humans. Its effects on macro and microvascular insulin resistance has not been defined. Methods: Eleven non-diabetic, metabolic syndrome subjects were studied four times (before and after 12 weeks treatment with placebo or metformin) using a crossover design, with an eight week washout interval between treatments. On each occasion, we measured three indices of large artery function (pulse wave velocity-PWV, radial pulse wave separation analysis (PWSA), brachial artery endothelial function (flow-mediated dilation-FMD) as well as muscle microvascular perfusion (contrast-enhanced ultrasound-CEU) before and 120 min into a 150 min, 1 mU/min/kg euglycemic insulin clamp. RESULTS: Metformin decreased body mass index (BMI), fat weight, and % body fat (P<0.05, each), placebo had no effect. Metformin (not placebo) improved metabolic insulin sensitivity, (clamp glucose infusion rate, P<0.01). PWV, and FMD after insulin were unaffected by metformin treatment. PWSA improved with insulin only after metformin P<0.01). Insulin decreased muscle microvascular blood volume measured by contrast ultrasound both before and after placebo and before metformin (P<0.02 for each) but not after metformin. CONCLUSIONS: Short-term metformin treatment improves both metabolic and muscle microvascular response to insulin. Metformin's effect on microvascular insulin responsiveness may contribute to its beneficial metabolic effects. Metformin did not improve aortic stiffness or brachial artery endothelial function, but enhaced radial pulse wave properties consistent with relaxation of smaller arterioles.