This is the largest class of hormones and includes physiologically important hormones such as insulin (from the pancreas), growth hormone (from the pituitary gland) and calcitonin (from the thyroid gland). As their name suggests, peptide hormones consist of chains of amino acids. These vary significantly in size from thyrotropin-releasing hormone which is a tripeptide (i.e. consists of 3 amino acids), up to growth hormone which is made up of 191 amino acids.
Like most peptides, peptide hormones are synthesised in the rough endoplasmic reticulum of the cells within the appropriate endocrine gland and passed on to the Golgi apparatus for further processing. The hormones are then packaged in membrane-bound secretory vesicles where they are stored until released. After being exposed to the appropriate trigger the hormone is secreted by exocytosis into adjacent capillaries.
In the blood stream, most peptide hormones circulate in a free state (i.e. not bound to carrier proteins), enter into the extracellular space and bind to specific receptors associated with their target cells. In the case of peptide hormones, these receptors are membrane-spanning proteins with the binding site for the peptide hormone on the extracellular side of the membrane. The binding of the hormone to the binding site triggers a conformational change (change in shape) of the receptor which trigger a cascade of intracellular events that produce the response in the target cell.
The schematic diagram opposite illustrates how some peptide hormones bind to a membrane-associated receptor (R) that (in this case) activates an intracellular G-protein (GP) which in turn activates the enzyme adenylate cyclase (AC). This leads to production of the second-messenger (in this case cAMP) which diffuses through the cytoplasm and mediates the intracellular effects of the hormone.
Given the large number of peptide hormones and the diversity of their target tissue effects it probably isn't surprising that there are a variety of mechanisms by which hormone-receptor binding produces a response in target cells. A detailed consideration of these intracellular events is really the domain of the biochemist. As we don't want to get involved in a demarcation dispute, we will leave consideration of this material to our biochemistry colleagues.