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.