A G protein may activate a membrane-bound enzyme phospholipase C. When this occurs, a phospholipid in the membrane, phosphatidylinositol (PIP2), is hydrolyzed into two intracellular molecules, inositol triphosphate (IP3), which causes Ca++ release from intracellular stores, and diacylglycerol (DAG). The calcium binds to and activates a protein called calmodulin, which alters cellular activity in much the same way as cyclic AMP does. The diacylglycerol acti vates protein kinase C (PKC) that, in turn, activates proteins in the cell (see Fig. 5.4). Cyclic AMP, Ca++ inositol triphosphate, and diacylglycerol are viewed as second messengers in the events following the hormone's binding to a receptor on the cell membrane. These second messengers should not be viewed as being independent of one another because changes in one can affect the action of the others (107, 138 ).