Describe how insulin is secreted from beta cells in the Islets of Langerhans

Insulin is released in response to high blood glucose concentrations. High blood glucose concentrations means glucose is taken up by the beta-cells in the Islet's of Langerhans and is respired to produce ATP. This ATP then blocks ATP-sensitive potassium ion channels resulting in their closure. Thus, K+ ions are unable to diffuse out of the cell and hence accumulate inside the cell. This means that a positive charge builds up inside the beta cell and therefore causes voltage-gated calcium ion channels to open. This allows Ca2+ ions to diffuse down their concentration gradient into the cell. The Ca2+ ions then bind to special receptors on the vesicles containing insulin resulting in the vesicle migrating towards the cell surface membrane for release via exocytosis.