41. Effect of production of during Signal transduction pathway is
(1) Closure of Ca²⁺ channel in ER
(2) Increase in intracellular Ca²⁺ level
(3) Increase of extracellular Ca²⁺ level
(4) Inactivation of calmodulin proteins

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Introduction

IP3 (inositol 1,4,5-trisphosphate) is a vital second messenger generated during receptor-mediated signal transduction, especially through G protein-coupled receptors activating phospholipase C. The key effect of IP3 is to induce the release of Ca2+2+ from the endoplasmic reticulum (ER) into the cytosol, regulating many physiological processes.

Mechanism of Action

• Upon extracellular ligand binding, phospholipase C cleaves PIP2 into two second messengers: IP3 and diacylglycerol (DAG).

• IP3 diffuses through the cytoplasm and binds to IP3 receptors (IP3Rs), which are ligand-gated Ca2+2+ channels located on the ER membrane.

• Binding of IP3 to these receptors opens the channels, allowing stored Ca2+2+ to flow from the ER lumen into the cytosol.

• The resulting increase in intracellular Ca2+2+ concentration triggers further downstream signaling pathways essential for various cellular functions.

Evaluating the Options

(1) Closure of Ca2+2+ channel in ER: False – IP3 causes opening, not closing, of Ca2+2+ channels.

(2) Increase in intracellular Ca2+2+ level: True – This is the direct effect of IP3 production.

(3) Increase of extracellular Ca2+2+ level: Incorrect – IP3 mediates release from intracellular stores.

(4) Inactivation of calmodulin proteins: Incorrect – Elevated Ca2+2+ generally activates calmodulin, not inactivates it.

Correct Choice

(2) Increase in intracellular Ca2+2+ level directly results from IP3 production during signal transduction.

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