This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
[{'content': 'All  of the following  actions  of histamine are mediated through H1 receptors EXCEPT: \nSelect and explain. \nA. Release of EDRF from vascular endothelium resulting in vasodilation, B. Direct  action  on  vascular  smooth  muscle  causing vasodilation, C. Bronchoconstriction, D. Release of catecholamines from adrenal medulla', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Both H1 and H2 receptors cause vasodilation. H1 receptors are present in the endothelium and cause vasodilation by releasing NO. However, H2 receptors are present on the smooth muscles and cause direct vasodilation.', 'role': 'assistant'}]