Inositol Phosphate Pathway — MCQs

10 questions

Products of Phospholipase C are -

Mechanism of action of cholecystokinin?

What is the most appropriate investigation for a patient with bipolar disorder on lithium therapy for 6 months, who presents with seizures, coarse tremors, confusion, and weakness of the limbs after a day of fasting?

Which of the following statements best describes the mechanism of action of insulin on target cells?

Which one of the following substances increases the release of Ca2+ from the endoplasmic reticulum?

In the mitogen activated protein kinase pathway, the activation of RAS is counteracted by

A primigravida at 38 weeks of gestation has gone into labor. Oxytocin was started to augment labor. The second messenger system through which oxytocin acts is:

Which hormone activates the phospholipase C pathway?

Phospholipid associated with the mechanism of hormone action is

Q10

Select the correct sequence of events in the cAMP signaling pathway.

Inositol Phosphate Pathway Indian Medical PG Practice Questions and MCQs

Question 1: Products of Phospholipase C are -

A. Lysophospholipid and fatty acid
B. Phosphatidate and IP2
C. Inositol triphosphate and Diacylglycerol (Correct Answer)
D. Inositol and diacylglycerol

Explanation: ***Inositol triphosphate and Diacylglycerol*** - **Phospholipase C (PLC)** cleaves **phosphatidylinositol 4,5-bisphosphate (PIP2)** into two important second messengers: **inositol 1,4,5-trisphosphate (IP3)** and **diacylglycerol (DAG)**. - **IP3** mobilizes intracellular **calcium**, while **DAG** activates **protein kinase C (PKC)**, both crucial for signal transduction pathways. *Lysophospholipid and fatty acid* - These are typically products of **Phospholipase A2**, which hydrolyzes a **fatty acid** from the *sn-2* position of a glycerophospholipid, leaving a **lysophospholipid**. - **Phospholipase C** acts on a different bond within the **phospholipid structure** and produces different second messengers. *Phosphatidate and IP2* - **Phosphatidate** is a precursor in lipid synthesis and is not directly produced by **PLC's primary action** on **PIP2**. - **IP2 (Inositol bisphosphate)** is a dephosphorylated product of **IP3** and not the direct initial product of **PLC** cleavage from **PIP2**. *Inositol and diacylglycerol* - **Inositol** itself is a cyclic alcohol and is part of the **inositol phosphate** structures. It is not directly released as a standalone product by **PLC** cleavage of **PIP2**. - It is **inositol trisphosphate (IP3)**, not free inositol, that is the second messenger formed alongside **diacylglycerol**.

Question 2: Mechanism of action of cholecystokinin?

A. Activation of adenylyl cyclase
B. Opening of ion channels
C. Through IP3- DAG system (Correct Answer)
D. Transcription factors

Explanation: ***Through IP3- DAG system*** - Cholecystokinin (CCK) primarily acts via **Gq protein-coupled receptors**, leading to the activation of **phospholipase C**. - This activation results in the hydrolysis of **PIP2 into IP3 and DAG**, which then mediate intracellular signaling cascades, causing actions like gallbladder contraction and pancreatic enzyme secretion. *Activation of adenylyl cyclase* - This mechanism is typically associated with **Gs protein-coupled receptors**, leading to increased levels of **cyclic AMP (cAMP)**. - Hormones like **glucagon** and **epinephrine** often utilize this pathway, which is distinct from CCK's primary signaling. *Opening of ion channels* - While ion channels are crucial for many cellular processes, CCK's direct mechanism of action typically involves **intracellular second messengers** rather than direct gating of ion channels. - Neurotransmitters like **acetylcholine** can directly open ion channels, but this is not the main signaling pathway for CCK. *Transcription factors* - Transcription factors regulate **gene expression** by binding to DNA, which is a slower, more long-term cellular response. - While CCK can eventually influence gene expression, its direct and immediate effects (e.g., gallbladder contraction) are mediated by **rapid second messenger systems**, not primary transcription factor modulation.

Question 3: What is the most appropriate investigation for a patient with bipolar disorder on lithium therapy for 6 months, who presents with seizures, coarse tremors, confusion, and weakness of the limbs after a day of fasting?

A. Serum Electrolyte
B. ECG
C. Serum Lithium (Correct Answer)
D. Serum creatinine

Explanation: ***Serum Lithium*** - The patient's symptoms (seizures, coarse tremors, confusion, weakness) are classic signs of **lithium toxicity**. - Fasting can lead to **dehydration** and electrolyte imbalances, which can increase lithium levels and precipitate toxicity. *Serum Electrolyte* - While electrolyte imbalances can contribute to lithium toxicity and symptoms, measuring electrolytes alone will not directly confirm **lithium overdose**. - **Hyponatremia** and **dehydration** can exacerbate lithium toxicity by increasing renal reabsorption of lithium. *ECG* - **ECG changes** (e.g., T-wave abnormalities, QTc prolongation) can occur with severe lithium toxicity, but it is not the primary diagnostic test. - ECG is a supportive investigation to assess cardiac complications, not the direct cause of the neurological symptoms. *Serum creatinine* - **Serum creatinine** is important for monitoring **renal function**, as lithium is cleared renally and renal impairment can lead to higher lithium levels. - However, assessing creatinine levels will not directly diagnose acute lithium toxicity; it helps evaluate the **renal health** and potential for toxicity.

Question 4: Which of the following statements best describes the mechanism of action of insulin on target cells?

A. Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.
B. Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.
C. Insulin enters the cell and causes the release of calcium ions from intracellular stores.
D. Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor. (Correct Answer)

Explanation: ***Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor.*** - **Insulin** is a **peptide hormone** and cannot freely pass through the lipid bilayer, thus it binds to a **transmembrane receptor** on the cell surface. - This binding leads to the activation of the receptor's intrinsic **tyrosine kinase activity** in the intracellular domain, initiating a signaling cascade. *Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.* - This mechanism describes the action of **steroid hormones**, which are lipid-soluble and can cross the cell membrane, binding to **intracellular receptors**. - **Insulin** acts via a **cell surface receptor** and its downstream effects are mediated through signal transduction pathways, not direct nuclear translocation. *Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.* - This mechanism is characteristic of **G-protein coupled receptors (GPCRs)**, which activate or inhibit enzymes like adenylate cyclase via G-proteins to produce second messengers like cyclic AMP. - The **insulin receptor** is a **receptor tyrosine kinase**, not a GPCR, and does not directly activate adenylate cyclase via Gs protein. *Insulin enters the cell and causes the release of calcium ions from intracellular stores.* - While some hormones and neurotransmitters can trigger the release of intracellular **calcium ions**, this is typically mediated by specific pathways (e.g., GPCRs linked to phospholipase C). - **Insulin** does not directly enter target cells to cause calcium release; its actions are primarily mediated through receptor tyrosine kinase signaling pathways.

Question 5: Which one of the following substances increases the release of Ca2+ from the endoplasmic reticulum?

A. Inositol triphosphate (Correct Answer)
B. 1,25 - dihydroxy cholecalciferol
C. Diacylglycerol
D. Parathyroid hormone

Explanation: ***Inositol triphosphate*** - **Inositol triphosphate (IP3)** is a secondary messenger that binds to specific receptors on the **endoplasmic reticulum (ER)**, triggering the release of stored **Ca2+** into the cytoplasm. - This calcium release plays a crucial role in various cellular processes, including **muscle contraction**, **neurotransmission**, and **hormone secretion**. *1,25 - dihydroxycholecalciferol* - This is the active form of **vitamin D**, primarily involved in **calcium absorption** from the gut and **calcium reabsorption** in the kidneys. - It does not directly increase Ca2+ release from the endoplasmic reticulum. *Diacylglycerol* - **Diacylglycerol (DAG)** is another secondary messenger, also produced from the cleavage of **PIP2** along with IP3. - DAG primarily activates **protein kinase C (PKC)**, which is involved in signal transduction pathways, but it does not directly trigger Ca2+ release from the ER. *Parathyroid hormone* - **Parathyroid hormone (PTH)** is a hormone that primarily regulates **extracellular calcium levels** by promoting bone resorption, increasing renal calcium reabsorption, and stimulating the synthesis of 1,25-dihydroxycholecalciferol. - While it ultimately increases blood calcium, it does not directly cause Ca2+ release from the endoplasmic reticulum.

Question 6: In the mitogen activated protein kinase pathway, the activation of RAS is counteracted by

A. Inositol triphosphate
B. GTPase activating protein (Correct Answer)
C. Phosphatidyl inositol
D. Protein kinase C

Explanation: ***GTPase activating protein*** - **GTPase Activating Proteins (GAPs)** facilitate the hydrolysis of **GTP bound to RAS** to GDP, converting active RAS back to its inactive state. - This inactivation is crucial for turning off the downstream signaling of the **MAPK pathway** and preventing uncontrolled cell proliferation. *Inositol triphosphate* - **Inositol triphosphate (IP3)** is a secondary messenger that triggers the release of **intracellular calcium** from the endoplasmic reticulum. - It is involved in various signaling pathways, but its primary role is not to directly counteract RAS activation. *Phosphatidyl inositol* - **Phosphatidylinositol (PI)** is a component of cell membranes and can be phosphorylated to produce various **phosphatidylinositol phosphates (PIPs)**, like **PIP2** and **PIP3**. - These molecules act as docking sites for signaling proteins but do not directly inactivate RAS. *Protein kinase C* - **Protein kinase C (PKC)** is a family of enzymes involved in signal transduction, typically activated by **diacylglycerol (DAG)** and calcium. - It phosphorylates various proteins, mediating diverse cellular responses, but it does not directly counteract the activation of RAS.

Question 7: A primigravida at 38 weeks of gestation has gone into labor. Oxytocin was started to augment labor. The second messenger system through which oxytocin acts is:

A. Tyrosine kinase
B. Phospholipase C (IP3/DAG pathway) (Correct Answer)
C. cGMP
D. cAMP

Explanation: ***Phospholipase C (IP3/DAG pathway)*** - Oxytocin binds to its receptor, which is a **Gq protein-coupled receptor**. This activates **phospholipase C**. - **Phospholipase C** then hydrolyzes **phosphatidylinositol 4,5-bisphosphate (PIP2)** into **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**, which act as second messengers to increase intracellular calcium and mediate myometrial contraction. *Tyrosine kinase* - **Tyrosine kinase receptors** are typically activated by growth factors (e.g., insulin, epidermal growth factor) and lead to phosphorylation cascades. - This mechanism is not primarily associated with the downstream signaling of **oxytocin receptors**. *cGMP* - **Cyclic guanosine monophosphate (cGMP)** is a second messenger primarily involved in signaling pathways initiated by **nitric oxide** and some peptide hormones. - It often acts to cause smooth muscle relaxation, which is contrary to oxytocin's role in uterine contraction. *cAMP* - **Cyclic adenosine monophosphate (cAMP)** is a common second messenger for many hormones that bind to **Gs protein-coupled receptors**. - Hormones such as **epinephrine (beta-adrenergic receptors)** and **glucagon** utilize cAMP, typically leading to different cellular responses than those of oxytocin.

Question 8: Which hormone activates the phospholipase C pathway?

A. FSH
B. GnRH (Correct Answer)
C. LH
D. TSH

Explanation: ***GnRH*** - **Gonadotropin-releasing hormone (GnRH)** binds to its receptor on pituitary cells, activating a **Gq protein**. - **Gq protein** then activates **phospholipase C (PLC)**, leading to the production of **inositol trisphosphate (IP3)** and **diacylglycerol (DAG)**, which ultimately increases intracellular calcium and triggers the release of LH and FSH. *FSH* - **Follicle-stimulating hormone (FSH)** primarily signals through the **adenylyl cyclase pathway**, increasing **cAMP** levels. - While it plays a crucial role in reproduction, its direct signaling cascade does not typically involve phospholipase C activation. *LH* - **Luteinizing hormone (LH)** also primarily acts via the **adenylyl cyclase pathway**, similar to FSH. - This pathway leads to increased **cAMP** and subsequent protein kinase A activation in target cells. *TSH* - **Thyroid-stimulating hormone (TSH)** primarily activates the **adenylyl cyclase pathway** in thyroid follicular cells. - Activation of this pathway leads to increased **cAMP** and stimulates thyroid hormone synthesis and release.

Question 9: Phospholipid associated with the mechanism of hormone action is

A. Phosphatidylcholine
B. Phosphatidylethanolamine
C. Plasmalogen
D. Phosphatidylinositol (Correct Answer)

Explanation: ***Phosphatidylinositol*** (Correct) - **Phosphatidylinositol (PI)** and its phosphorylated derivatives, particularly **PIP2 (phosphatidylinositol 4,5-bisphosphate)**, are critical in signal transduction pathways activated by many hormones. - Hormones binding to **G protein-coupled receptors** can activate phospholipase C, which cleaves PIP2 into **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**, leading to increased intracellular calcium and protein kinase C activation, respectively. *Phosphatidylcholine* (Incorrect) - **Phosphatidylcholine** is a major component of cell membranes and is involved in membrane structure and fluidity. - While it can be a source of signaling molecules like **lysophosphatidic acid**, it is not primarily associated with the initial intracellular signaling events of hormone action in the same way as phosphatidylinositol. *Phosphatidylethanolamine* (Incorrect) - **Phosphatidylethanolamine** is another abundant membrane phospholipid primarily involved in membrane structure and stability. - It can be a precursor for other lipids, but it does not directly participate in the **second messenger systems** triggered by most hormones as a primary signaling molecule. *Plasmalogen* (Incorrect) - **Plasmalogens** are a unique class of phospholipids containing an ether bond at the sn-1 position. - They are abundant in certain tissues, particularly nervous and cardiovascular tissues, and are thought to have antioxidant properties, but they are not directly involved in the initiating events of **hormone signaling pathways**.

Question 10: Select the correct sequence of events in the cAMP signaling pathway.

A. Adenylyl cyclase converts ATP to cAMP, which activates PKA. (Correct Answer)
B. PKA is activated before cAMP is formed.
C. Adenylyl cyclase activates PKA before producing cAMP.
D. cAMP directly activates adenylyl cyclase to produce more cAMP.

Explanation: ***Adenylyl cyclase converts ATP to cAMP, which activates PKA.*** - **Adenylyl cyclase** is an enzyme that catalyzes the conversion of **ATP (adenosine triphosphate)** into **cyclic AMP (cAMP)**, a crucial second messenger. - Subsequently, **cAMP** binds to and activates **Protein Kinase A (PKA)**, which then phosphorylates various target proteins to mediate cellular responses. *PKA is activated before cAMP is formed.* - **cAMP formation** is a prerequisite for **PKA activation**; PKA cannot be activated independently before cAMP is produced. - The binding of **cAMP** to the regulatory subunits of **PKA** is what causes the dissociation and activation of its catalytic subunits. *Adenylyl cyclase activates PKA before producing cAMP.* - **Adenylyl cyclase's** sole function in this pathway is to synthesize **cAMP** from ATP; it does not directly activate PKA. - **PKA activation** is mediated by **cAMP**, not directly by adenylyl cyclase. *cAMP directly activates adenylyl cyclase to produce more cAMP.* - While **cAMP** is a critical messenger, it does not directly activate **adenylyl cyclase** to produce more of itself in a positive feedback loop. - **Adenylyl cyclase** is typically activated by **G-protein coupled receptors (GPCRs)** binding to their ligands, which then stimulate G proteins to activate adenylyl cyclase.

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