Peptide hormones influence their target cells without actually entering them. They bind to receptors on the target cell’s plasma membrane. This will trigger one of a variety of responses. For example, epinephrine from the adrenal medulla binds to a receptor on the membrane of a liver cell. Epinephrine’s effect on this cell will be the breakdown of glycogen to glucose. The binding of epinephrine to the receptor activates the enzyme adenylate cyclase, which converts ATP to cyclic AMP. Cyclic AMP acts as a second messenger and sets off a cascade of reactions that activate a set of enzymes. In this cell the enzymes catalyze the breakdown of glycogen to glucose and glucose can be released from the cell.