At the heart of secretion of protein hormones cage is the process of exocytosis. Secretory granules close to the cell surface, their membrane fuses with the plasma membrane, granule opens and its contents are transferred into the extracellular fluid.
As a result of this process, the membrane pellets incorporated into the plasma membrane and the entire contents of granules is outside the cell. Thus, together with the hormone endocrine glands can also secrete pro-segments and prohormones.
However, these substances tend to have very weak or no biological activity.
Molecular mechanisms of exocytosis are not understood fully.This process requires energy (ATP), calcium and intact cellular structures (microtubules and microfilaments). In all likelihood, this process is further regulated by the concentration of cytoplasmic calcium.
Therefore, a key part of stimulating the secretion of hormones closely associated with the increase of calcium in the cytosol. This increase is due to the transition of extracellular calcium in the cytoplasm or the release of calcium by mitochondria and endoplasmic reticulum (Fig.2-1). Increased calcium triggers exocytosis by binding of calcium to the protein mediator.
To identify this protein remains elusive. However, currently there are three possible candidates:
1) actin in microfilaments motile secretory granules to the plasma membrane;
2) calmodulin, known receptor calcium ions, which is a mediator of many intracellular effects of these ions and
3) sineksin, the protein is able to carry out the merger of the secretory granules. In the process of activating exocytosis may involve one, two or all three proteins.
