Question

In the ER, calcium release channels can be opened following

A. The formation of cAMP by adenylyl cyclase
B. The formation of IP3 by PI 3-kinase
C. The formation of IP3 by phospholipase C
D. The formation of cAMP by phospholipase C

Answer 1

Calcium release channels in the endoplasmic reticulum are opened by the formation of IP3 via phospholipase C. IP3 then binds to ligand-gated calcium channels, releasing Ca2+ into the cytosol and initiating cell-specific responses.

Step-by-step explanation:

In the endoplasmic reticulum (ER), calcium release channels can be opened by several mechanisms, particularly through the action of second messengers in signal transduction pathways. Among the options provided, the correct one is C. The formation of IP3 by phospholipase C. This enzyme cleaves a particular phospholipid in the membrane, PIP2, to form diacylglycerol (DAG) and inositol triphosphate (IP3).

While DAG remains in the plasma membrane and activates protein kinase C (PKC), IP3 diffuses into the cytoplasm where it binds to ligand-gated calcium channels on the endoplasmic reticulum. Consequently, this binding causes the channels to open and allows the stored Ca2+ to flood into the cytosol, activating various enzymes and producing a cell-specific response. A notable example of this signaling pathway is its role in the action of hormones like angiotensin II and growth hormone-releasing hormone (GHRH).

Answer 2

Calcium release channels in the endoplasmic reticulum (ER) are opened by the formation of inositol triphosphate (IP3) by phospholipase C (PLC), leading to the release of Ca2+ ions into the cytosol, which activates enzymes and cellular responses.

Step-by-step explanation:

In the ER, calcium release channels can be opened by the formation of inositol triphosphate (IP3) by phospholipase C (PLC).

This process begins when a signaling molecule activates a G protein-coupled receptor (GPCR) that in turn activates PLC. PLC cleaves a membrane-bound phospholipid, PIP2, to produce diacylglycerol (DAG) and IP3.

While DAG remains in the membrane to activate protein kinase C (PKC), IP3 diffuses into the cytosol, binds to IP3-receptor channels on the endoplasmic reticulum, and induces the release of Ca2+ ions into the cytoplasm. This increase in cytosolic calcium ion concentration activates various enzymes, some by binding to calmodulin or similar proteins, initiating cellular responses.