Sygnalizacja hormonalna

Endocrine signaling occurs when cells located in different organs need to communicate, such as when the pituitary gland communicates with the kidneys. When this happens, hormones, the signaling ligands, use the bloodstream to reach their target cells.

For example, the pituitary gland signals the kidneys to reabsorb water from urine, by releasing the hormone arginine vasopressin, or AVP, into the blood. When blood is filtered in the kidneys, AVP binds to its G protein-coupled receptor, AVPR2, on targeted renal cells.

Upon activation by the hormone, the G protein subunits decouple from the receptor, and activate adenylate cyclase, to make the second messenger, cyclic AMP. Cyclic AMP activates the intracellular signaling cascade involving protein kinase A, or PKA.

PKA has two functions. First, it phosphorylates the aquaporin channel, APQ2, held in reserve in cytoplasmic vesicles. This action brings the vesicle, and the channels, to the cell membrane, allowing the flow of water back into the renal cells. 

Secondly, PKA phosphorylates CREB in the nucleus, causing it to bind to the aquaporin 2 gene, and start its transcription and then translation for new aquaporin channels.

Thus, endocr