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: Physiology
Which of the following directly inhibits insulin secretion? 
Select and explain. 
A. Alpha2-adrenergic agonist, B. Beta2-adrenergic agonist, C. Cholecystokinin, D. Glucagon
A: Alpha2-adrenergic agonist
Answer: A: Alpha2-adrenergic agonist 
Explanation: Alpha 2-receptor agonists directly inhibit pancreatic insulin secretion. Beta2-adrenergic agonists stimulate insulin secretion. Cholecystokinin is a hormone that not only causes gallbladder contraction, but also causes insulin secretion from the pancreas. Pancreatic glucagon release acts as a paracrine stimulus for insulin secretion. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 25. Overview of Gastrointestinal Function & Regulation. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.