Meta-Biol

• Pathways

Voltage-gated calcium channels respond to a change in voltage across the plasma membrane by opening and allowing free movement of calcium ions. In an unstimulated cell the concentration of calcium ions outside the cells is higher than inside due to calcium transporters so channel opening results in an influx of calcium into the cytosol. In the cytosol the calcium ions cause an immediate exocytosis of the readily releasable pool of docked insulin granules as well as a migration of reserve granules toward the plasma membrane where they will be released during the second, sustained phase of insulin secretion.
Mouse and human beta cells are known to contain L type channels Cav1.2 and Cav1.3, both of which have been shown to physically associate with docked insulin granules via Syntaxin1A. Cav1.2 and Cav1.3 predominate in the initial rapid release of insulin. Human beta cells also contain the P/Q type channel Cav2.1 and the R type channel Cav2.3. Cav2.3 is involved in regulating the second, sustained phase of insulin release but signaling and regulatory differences between the two phases of secretion are not fully characterized. Human cells also exhibit T-type (brief burst) calcium currents but the responsible channel has not been identified.