PIP 2 functions as an intermediate in the [IP 3 /DAG pathway], which is initiated by ligands binding to G protein-coupled receptors activating the G q alpha subunit . PtdIns(4,5) P 2 is a substrate for hydrolysis by phospholipase C (PLC), a membrane-bound enzyme activated through protein receptors such as α1 adrenergic receptors . PtdIns(4,5)P 2 regulates the function of many membrane proteins and ion channels, such as the M-channel . The products of the PLC catalyzation of PtdIns(4,5)P 2 are inositol 1,4,5-trisphosphate (Ins P 3 ; IP 3 ) and diacylglycerol (DAG), both of which function as second messengers . In this cascade, DAG remains on the cell membrane and activates the signal cascade by activating protein kinase C (PKC). PKC in turn activates other cytosolic proteins by phosphorylating them. The effect of PKC could be reversed by phosphatases. IP 3 enters the cytoplasm and activates IP 3 receptors on the smooth endoplasmic reticulum (ER), which opens calcium channels on the smooth ER, allowing mobilization of calcium ions through specific Ca 2+ channels into the cytosol. Calcium participates in the cascade by activating other proteins.