Which component is ultimately responsible for calcium release from the endoplasmic reticulum in the phosphatidylinositol pathway?

• A. DAG
• B. IP3
• C. GTP
• D. Phospholipase C

Answer: (B)

In the phosphatidylinositol pathway, inositol trisphosphate (IP3) plays a critical role in mediating the release of calcium from the endoplasmic reticulum (ER). When a receptor on the cell surface is activated, it typically triggers the activation of phospholipase C, an enzyme that catalyzes the breakdown of a membrane lipid called phosphatidylinositol 4,5-bisphosphate (PIP2) into two secondary messengers: diacylglycerol (DAG) and IP3.

Once formed, IP3 diffuses through the cytosol and binds to specific receptors on the membrane of the endoplasmic reticulum. This binding induces a conformational change in the IP3 receptor, leading to the opening of calcium channels. As a result, calcium ions stored in the ER are released into the cytosol, raising intracellular calcium levels and initiating various intracellular signaling pathways.

DAG, while important in activating protein kinase C and facilitating various cellular responses, does not directly mediate calcium release from the ER. GTP is involved in G protein signaling, but it is not responsible for calcium release specifically in this pathway. Lastly, although phospholipase C is crucial