Endocrine cells produce hormones to communicate with remote target cells found in other organs. The hormone reaches these distant areas using the circulatory system. This exposes the whole organism to the hormone but only those cells expressing hormone receptors or target cells are affected. Thus, endocrine signaling induces slow responses from its target cells but these effects also last longer.
There are two types of endocrine receptors: cell surface receptors and intracellular receptors. Cell surface receptors work similarly to other membrane bound receptors. Hormones, the ligand, bind to a hormone specific G-protein coupled receptor. This initiates conformational changes in the receptor, releasing a subunit of the G-protein. The protein activates second messengers which internalize the message by triggering signaling cascades and transcription factors.
Many hormones work through cell surface receptors, including epinephrine, norepinephrine, insulin, prostaglandins, prolactin, and growth hormones.
Steroid hormones, like testosterone, estrogen, and progesterone, transmit signals using intracellular receptors. These hormones are small hydrophobic molecules so they move directly past the outer cell membrane. Once inside, and if that cell is a target cell, the hormone binds to its receptor. Binding creates a conformational change in the receptor which activates its potential as a transcription factor. Once activated, the receptor or hormone-receptor complex promote or suppress gene expression.
The intracellular hormone receptors are a large superfamily of receptors but they all have a similar single polypeptide chain with three distinct domains. The N-terminus is the active transcription factor domain. The middle contains a DNA binding domain specific for the gene of interest. And the hormone binds to a domain at the C-terminus.
