Answer
Cyclic AMP (cAMP) is called the second messenger because it translates the presence of the first messenger, the water soluble hormone, into a response inside the cell.
Work Step by Step
Water soluble hormones include amines(epinephrine), proteins and peptides( growth hormone, insulin), and eicosanoids (prostaglandins). These hormones cannot diffuse through the lipid bi-layer of the plasma membrane. They act by first binding to membrane receptors with access to the interior of the cell. When the water soluble hormone binds to the membrane receptor, the reaction triggers a response within the cell. The first messenger (the water soluble hormone) thus activates a second messenger (cyclic AMP, cyclic GMP) which brings about the desired intracellular response.
