Insulin is a peptide hormone produced in the islets of Langerhans in the pancreas. Insulin interacts with membrane bound receptors and does not enter target cell but generates second messenger. Insulin receptors for most cells are less than 100 . Insulin binds to receptors located on plasma membrane to form hormone receptor complex which induces release of GDP from G protein which activates adenyl cyclase that in turn catalysis the formation of cyclic adenosine monophosphate (cAMP) from ATP. The hormone is called first messenger and cAMP is termed as second messenger.