 Dear students, in this topic we shall discuss the inositol phospholipid and calcium signaling second messenger systems. Some water soluble hormones use inositol phospholipid pathway to produce their effect. These hormones include angiotensin 2, catecholamines, some hypothalamic hormones, for example GNRH, growth hormone-releasing hormone, oxytocin, thyrotropane-releasing hormone, and wezo-pressin or ADH. Now we shall discuss in detail the mechanism of inositol phospholipid signaling system. The hormone binds to the transmembrane receptor which is linked to the G-protein. The G-protein activates an enzyme which is called phospholipase C. It is here that the pathway of inositol phospholipid signaling system separates or differentiates from the CAMP pathway where the G-protein activates and adenylates cyclase. The phospholipase C enzyme catalyzes the breakdown of some phospholipids in the plasma membrane. For example, phosphatidyl inositol bisphosphate or PIP2. As a result, two different second messengers are formed. First is the inositol trisphosphate and second is diacylglasrol. The inositol trisphosphate mobilizes calcium ions from intracellular stores of calcium. These stores include mitocontria and endoplasmic reticulum. Endoplasmic reticulum and the calcium ions in mitocontria mobilizes the inositol trisphosphate and the calcium ions come out of it and come into cytoplasm. As a result, calcium ion concentration in the cell is increased and these calcium ions act as second messengers or the inositol trisphosphate activates the calcium ions. These calcium ions which have their own second messenger effects act on the smooth muscle contraction and cause changes in cell secretions. The other second messenger produced in this pathway is the diacylglasrol which activates an enzyme which is called protein kinase C. The protein kinase C phosphorylates a large number of proteins which lead to cells response. Diacylglasrol also acts as a precursor for the synthesis of certain prostaglandins and some other local hormones. These hormones have multiple effects in the tissues throughout the body. In this way, the chain of reaction is produced by a lot of different types of responses. Dear students, now we shall discuss the calcium signaling systems. Some hormones interact with the receptor and open calcium channels. When enters through these channels acts as second messenger. When calcium ions enter the cell through these channels, they bind to a protein which is called calmodulin. Calmodulin has four active sites which bind to calcium. When calcium ions bind to these sites, as a result, calmodulin activates. This activates an enzyme which is called calmodulin-dependent protein kinase enzyme. Calmodulin-dependent protein kinase phosphorylates certain proteins. Kinase enzymes make all these phosphorylations. This enzyme which is called calmodulin-dependent protein kinase also phosphorylates specific proteins which generate the specific hormone of the calcium ion signaling system. Dear students, let us take an example of calcium signaling system which is found or used in the contraction of smooth muscles. In the smooth muscles, this signaling system is used. It involves activation of myocin kinase enzyme which phosphorylates the protein myocin in the smooth muscles. When the phosphorylation of myocin is in the smooth muscles, it contracts the smooth muscles. In this way, calcium signaling pathway contracts the smooth muscles.