Explain how so many hormones can have specific tissue targets and still use the same intracellular response system to effect change within cells. What category of hormones depends on the two-messenger model of intracellular response? Describe the two-messenger model of intracellular response, explaining the phenomenon of amplification. How does a cell return to its non-hormone-stimulated state?
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
