The above statement is true, that all the hormones from the anterior pituitary affect their target cells via cyclic AMP second messenger system except the growth hormone. Cyclic AMP is an important second messenger that is formed when the membrane enzyme adenylyl cyclase is activated by the alpha sub unit of a G protein. The Cyclic AMP in turn then goes on to activate specific proteins. The hormone which is the first messenger binds to receptor , the receptor then activates G protein, the alpha sub unit of G protein activates adenylate cyclase which converts ATP to cAMP which then activates protein kinases.
