Fig. (2) Glucose triggers insulin synthesis and signalling. Insulin secretion in beta cells is triggered by rising blood glucose levels. Starting with the uptake of glucose by the GLUT2 transporter, the glycolytic phosphorylation of glucose causes an increase in the ATP:ADP ratio. This increase inactivates the K⁺ channel that depolarises the membrane, causing the Ca²⁺ channel to open up and allowing Ca²⁺ ions to flow inward. The ensuing increase in levels of Ca²⁺ ions leads to the exocytotic release of pre-synthesised insulin stored in vesicles. Finally, insulin is released from the storage vesicles.