HMOX1 and endothelial dysfunction: A proof of concept study aiming to demonstrate that individuals with Morbus Meulengracht (Gilbert's syndrome), displaying increased HO-1 activity and bilirubin levels, are devoid of tolerance, endothelial dysfunction and oxidative stress in response to nitroglycerin (GTN) therapy could provide a new therapeutic option to overcome these undesired side effects of GTN treatment.