IP3 / DAG: - Phospholipase catalyzes the breakdown of phospholipids in the cell membrane into two different second messenger products- IP3 & DAG. IP3 mobilizes the calcium ion from mitochondria & endoplasmic reticulum lumen to cytoplasm & then the Ca++ ions have their own IInd messenger effects, such as smooth muscle contraction and changes in cell secretion. DAG the other lipid second messenger, activates the enzyme protein kinase C Release of Ca2+ in smooth muscle contraction can be from: Influx from extracellular site through voltage or ligand gated Ca2+channels in the plasma membrane Efflux from intracellular stores through a Ryr channel Efflux from intracellular stores through a IP3R Ca2+ channel.