Endothelial function will be assessed with novel high-resolution ultrasound devices, which allows to investigate endothelial function in vivo. This method makes use of the property of the endothelium to release nitric oxide in response to shear stress and increased flow as previously described. Endothelium-dependent flow-mediated dilation (FMD) and endothelium-independent glycerol trinitrate (GTN) (0.4 mg sublingual, Nitrolingual Spray, Pohl-Boskamp, Germany)-induced vasodilation of the brachial artery will be assessed by a high-resolution ultrasound vessel wall tracking device with a 10 MHz linear array transducer . FMD of the brachial artery is induced by release of a wrist cuff inflated 50 mmHg above systolic pressure for 5 minutes. After sublingual GTN application, used as endothelium-independent stimulus, the diameter will be recorded every 30 seconds for 6 minutes.