cAMP and cGMP Signaling — MCQs
Which one of the following acts as second messenger?
Nesiritide causes vasodilation through?
What is the second messenger responsible for smooth muscle relaxation mediated by nitric oxide (NO)?
If there is a Gs alpha subunit gain-of-function mutation, this results in
Sodium nitroprusside is metabolized to form an active metabolite, which acts to activate:
Which enzyme does Vibrio cholerae toxin activate to cause profuse watery diarrhea?
With which of the following receptors does theophylline have an antagonistic interaction?
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:
Select the correct sequence of events in the cAMP signaling pathway.
Phospholipid associated with the mechanism of hormone action is
cAMP and cGMP Signaling Indian Medical PG Practice Questions and MCQs
Question 1: Which one of the following acts as second messenger?
- A. Cl-
- B. Ca++ (Correct Answer)
- C. PO4³⁻
- D. Mg++
Explanation: ***Ca++*** - **Calcium ions** act as a ubiquitous **second messenger** in various cellular processes, including **muscle contraction**, neurotransmission, and gene expression. - Their concentration in the cytoplasm is tightly regulated, and transient increases trigger specific cellular responses. *Cl-* - **Chloride ions** primarily contribute to maintaining cell **membrane potential** and regulating cell volume. - While important for cell function, they generally do not act as an intracellular second messenger to signal downstream events. *PO4³⁻* - **Phosphate ions** are essential components of **ATP**, DNA, and RNA, and play roles in **phosphorylation events** that regulate protein activity. - However, they do not function as a direct second messenger in the same way that calcium or cyclic nucleotides do. *Mg++* - **Magnesium ions** are critical cofactors for many enzymes and play a role in **ATP hydrolysis** and nucleic acid stability. - They are involved in many cellular processes but are not typically classified as a second messenger that transmits signals from cell surface receptors to intracellular effectors.
Question 2: Nesiritide causes vasodilation through?
- A. ATP
- B. Cyclic adenosine monophosphate (cAMP)
- C. K+ ions
- D. Guanosine 3',5'-cyclic monophosphate (cGMP) (Correct Answer)
Explanation: ***Guanosine 3',5'-cyclic monophosphate (cGMP)*** - **Nesiritide** is a synthetic **B-type natriuretic peptide (BNP)** that acts as a potent vasodilator [2]. - It works by binding to **guanylyl cyclase receptors**, leading to an increase in intracellular **cGMP**, which promotes smooth muscle relaxation [1], [2]. *Cyclic adenosine monophosphate (cAMP)* - While **cAMP** is a crucial second messenger in various cellular processes and can mediate some forms of vasodilation, it is primarily associated with **beta-adrenergic receptor activation**, not the mechanism of action of nesiritide. - Nesiritide's pathway is distinct from those involving **cAMP-mediated** relaxation, which often involves different kinases and protein phosphorylation. *ATP* - **ATP** (adenosine triphosphate) is the primary **energy currency** of the cell and is involved in numerous cellular functions, including muscle contraction and relaxation, but it is not a direct mediator of nesiritide's vasodilatory effects. - Though ATP can be broken down to produce **adenosine**, which has vasodilatory properties, this is not the specific mechanism through which nesiritide causes vasodilation. *K+ ions* - Changes in **potassium ion (K+)** flux across cell membranes are essential for regulating vascular tone, as K+ channel activation can lead to hyperpolarization and relaxation of smooth muscle. - However, **nesiritide's primary mechanism** of action does not involve direct modulation of K+ channels; its vasodilatory effects are mediated by the **cGMP pathway** [2].
Question 3: What is the second messenger responsible for smooth muscle relaxation mediated by nitric oxide (NO)?
- A. Calcium
- B. Cyclic AMP
- C. Cyclic GMP (Correct Answer)
- D. Magnesium
Explanation: ***Cyclic GMP*** - **Nitric oxide (NO)** activates **guanylyl cyclase**, an enzyme that converts **GTP to cGMP**. - Elevated **cGMP** levels activate **protein kinase G (PKG)**, leading to smooth muscle relaxation through various mechanisms, including reduced intracellular calcium and altered sensitivity of contractile proteins. *Calcium* - **Calcium** is primarily a key second messenger for **smooth muscle contraction**, not relaxation. - An increase in intracellular **calcium** promotes the binding of **calcium to calmodulin**, activating myosin light chain kinase and leading to contraction. *Cyclic AMP* - While **cyclic AMP (cAMP)** can cause smooth muscle relaxation (e.g., via beta-2 adrenergic stimulation), it is not the direct second messenger for **nitric oxide (NO)**-mediated relaxation. - **cAMP** is produced by **adenylyl cyclase** and primarily activates **protein kinase A (PKA)**. *Magnesium* - **Magnesium** is an important cofactor for many enzymes and can influence muscle contraction and relaxation, but it does not serve as a primary second messenger for **nitric oxide (NO)**. - High concentrations of **magnesium** can directly induce muscle relaxation by competing with **calcium** and modulating various channels and enzymes.
Question 4: If there is a Gs alpha subunit gain-of-function mutation, this results in
- A. Increased cAMP (Correct Answer)
- B. Decreased cAMP
- C. Increased GTPase activity
- D. Decreased IP₃
Explanation: ***Increased cAMP*** - A **gain-of-function mutation** in the **Gs alpha subunit** means it remains in its active, GTP-bound state for longer. - The activated Gs alpha subunit stimulates **adenylyl cyclase**, leading to persistently high levels of **cAMP**. *Decreased cAMP* - This would result from a **loss-of-function** mutation in the Gs alpha subunit or a gain-of-function in an inhibitory G protein (Gi), not a Gs gain-of-function. - A decrease in cAMP would inhibit downstream signaling pathways, which is the opposite of what occurs with Gs activation. *Increased GTPase activity* - **GTPase activity** is responsible for hydrolyzing GTP to GDP, which inactivates the G alpha subunit. - A gain-of-function mutation often implies **reduced GTPase activity**, causing the G protein to stay active longer, not increased activity. *Decreased IP* - **IP3 (inositol trisphosphate)** is a secondary messenger produced via the activation of **phospholipase C**, typically by Gq proteins. - Gs alpha subunit mutations primarily affect the **adenylyl cyclase/cAMP pathway**, not the inositol phosphate pathway.
Question 5: Sodium nitroprusside is metabolized to form an active metabolite, which acts to activate:
- A. Protein kinase C
- B. Guanylate cyclase (Correct Answer)
- C. Phospholipase A
- D. Phospholipase C
Explanation: ***Guanylate cyclase*** - **Sodium nitroprusside** releases **nitric oxide (NO)**, which is its active metabolite. - **NO** then activates **guanylate cyclase**, leading to increased production of **cyclic GMP (cGMP)**, which ultimately causes smooth muscle relaxation and vasodilation. *Protein kinase C* - **Protein kinase C (PKC)** is typically activated via the **inositol triphosphate (IP3)** and **diacylglycerol (DAG)** pathway, often initiated by **G protein-coupled receptors**. - It is involved in cell growth, metabolism, and immune responses, but not directly activated by **sodium nitroprusside's** mechanism of action. *Phospholipase A* - **Phospholipase A (PLA)** enzymes, such as **PLA2**, are involved in the release of **arachidonic acid** from membrane phospholipids, which is a precursor for prostaglandins and leukotrienes. - This pathway is distinct from the nitric oxide-cGMP pathway. *Phospholipase C* - **Phospholipase C (PLC)** is an enzyme that cleaves **phosphatidylinositol 4,5-bisphosphate (PIP2)** into **IP3** and **DAG**, leading to increases in intracellular calcium and PKC activation. - This is a separate signaling pathway and is not directly activated by the active metabolites of **sodium nitroprusside**.
Question 6: Which enzyme does Vibrio cholerae toxin activate to cause profuse watery diarrhea?
- A. Sodium channels
- B. Adenylate cyclase (Correct Answer)
- C. Potassium channels
- D. Guanylate cyclase
Explanation: ***Adenylate cyclase*** - The **Vibrio cholerae toxin** (cholera toxin/choleragen) causes activation of **adenylate cyclase** through ADP-ribosylation of the Gs alpha subunit of G-proteins. - The toxin's A subunit **permanently activates the Gs protein** by preventing its GTPase activity, keeping it in an active GTP-bound state. - The constitutively active Gs protein continuously activates **adenylate cyclase**, leading to sustained elevation of intracellular **cyclic AMP (cAMP)**. - This increased cAMP causes **excessive secretion of chloride ions and water** into the intestinal lumen, resulting in profuse watery "rice-water" diarrhea characteristic of cholera. *Sodium channels* - Sodium channels are ion channels, not enzymes. - While sodium and chloride transport is affected in cholera, the toxin does not directly activate sodium channels. - The increased cAMP affects the **CFTR (cystic fibrosis transmembrane conductance regulator)** chloride channel, which indirectly affects sodium absorption. *Potassium channels* - Potassium channels are ion channels, not enzymes. - The cholera toxin does not activate potassium channels. - Potassium loss occurs as a consequence of the massive fluid secretion, not through direct channel activation. *Guanylate cyclase* - **Guanylate cyclase** is activated by different enterotoxins, notably **E. coli heat-stable enterotoxin (STa)**, leading to increased cyclic GMP (cGMP). - Increased cGMP also causes secretory diarrhea, but this is **not the mechanism of cholera toxin**. - Heat-labile E. coli toxin (LT) has a similar mechanism to cholera toxin (activating adenylate cyclase).
Question 7: With which of the following receptors does theophylline have an antagonistic interaction?
- A. Histamine receptors
- B. Adenosine receptors (Correct Answer)
- C. Imidazoline receptors
- D. Bradykinin receptors
Explanation: ***Adenosine receptors*** - **Theophylline** acts as a **non-selective competitive antagonist** at **adenosine receptors** (A1, A2A, and A2B). - This antagonism contributes to its **bronchodilator effects** by blocking adenosine-induced bronchoconstriction and to its **stimulant effects** by enhancing neurotransmitter release. *Histamine receptors* - Theophylline does not primarily interact with **histamine receptors**. Its effects are mediated through different mechanisms. - While histamine plays a role in allergic reactions and airway smooth muscle contraction, theophylline's direct action is not on these receptors. *Imidazoline receptors* - Theophylline does not have a significant antagonistic interaction with **imidazoline receptors**. - These receptors are primarily involved in blood pressure regulation and sympathetic outflow, and are not a key target for theophylline's therapeutic effects. *Bradykinin receptors* - Theophylline does not directly antagonize **bradykinin receptors**. - Bradykinin is a potent vasodilator and inflammatory mediator, but its receptors are not the primary site of action for theophylline.
Question 8: 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 9: 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.
Question 10: 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**.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free