How is insulin secretion controlled?

Beta cells in the pancreas contain potassium and calcium ion channels - potassium ion channels are normally open and calcium channels closed. When glucose concentrations outside of the cell are high the glucose molecules will diffuse down the concentration gradient and into the cell. The glucose now inside the cell is quickly metabolised into ATP. An increase in the concentration of ATP inside the cell causes the potassium ion channels to close - as the positive ions are not able to leave the cell, the cell becomes less negative inside. This change in the potential across the membrane causes voltage gated calcium ion channels to open and calcium ions diffuse into the cell. Presence of calcium causes movement of vesicles containing insulin to move towards the cell membrane where they fuse and are released by exocytosis.