The vast majority of all non-steroidal hormones act on their target cells by forming weak bonds with specialized receptor sites on a cell's plasma membrane. For specific hormones to bond, cells must have the corresponding receptor sites. The presence or lack of receptor sites for each type of hormone determines whether the cell will be affected by the hormone. Cells can, of course, have more than one type of receptor site and, thus, can be influenced by more than one type of hormone. Specificity of receptor sites explains how so many hormones can have specific tissue targets and, yet, use the same intracellular response mechanism to effect change within cells. In the intracellular response system of non-steroidal hormones, often called the two-messenger model of hormonal control, the bond of the extra cellular messenger, the hormone, to its receptor site triggers production of an intracellular messenger, often cyclic adenosine monophosphate (cAMP). cAMP is synthesized from ATP via a reaction catalyzed by an enzyme, adenylate cyclase. Adenylate cyclase is located on the interior of the plasma membrane at the hormone receptor sites and is activated to synthesize cAMP when a hormone bonds with its receptor. cAMP, in turn, acts as an enzyme activating the protein kinase which also acts as an enzyme in triggering the ultimate response of the cell to the hormonal stimulation. The fact that a single enzyme can be reused over and over again to act on many substrate molecules explains why only low concentrations of hormones are needed to bring about major responses in the tissues of the body. One hormone molecule and the
