Sodium-Potassium ATPase — MCQs
What is the primary mechanism by which digoxin improves symptoms in heart failure?
Which of the following would cause an immediate reduction in the amount of potassium leaking out of a cell?
What is the PRIMARY mechanism by which the Na+-Ca2+ exchanger functions in cardiac muscle cells?
The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?
What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?
A 60-year-old patient with atrial fibrillation is prescribed digoxin. Which of the following is the MOST common EARLY side effect of digoxin?
Which of the following is the primary mechanism that drives sodium reabsorption in the proximal tubule?
A patient has hyperaldosteronism. Which lab finding is expected?
Which of the following conditions does not enhance the toxicity of digoxin?
Which of the following is true regarding Na+ (sodium) ions?
Sodium-Potassium ATPase Indian Medical PG Practice Questions and MCQs
Question 1: What is the primary mechanism by which digoxin improves symptoms in heart failure?
- A. Inhibits Na+/K+ ATPase (Correct Answer)
- B. Increases intracellular calcium levels
- C. Increases heart rate
- D. Decreases heart rate
Explanation: Inhibits Na+/K+ ATPase - Digoxin's primary mechanism involves inhibiting the Na+/K+ ATPase pump in cardiac myocytes [1]. - This inhibition leads to an increase in intracellular sodium, which in turn reduces the efficiency of the Na+/Ca2+ exchanger, ultimately increasing intracellular calcium [1]. Increases intracellular calcium levels - While digoxin does increase intracellular calcium, this is a downstream effect of its initial action on the Na+/K+ ATPase pump, not its primary mechanism of action [2]. - The elevated calcium then leads to increased contractility of the cardiac muscle [2]. Increases heart rate - Digoxin actually tends to decrease heart rate by increasing vagal tone, which is beneficial in heart failure, especially in patients with atrial fibrillation [1], [3]. - An increased heart rate would worsen cardiac output in a failing heart. Decreases heart rate - While digoxin does decrease heart rate, this is an indirect effect through vagal stimulation, and not its primary cellular mechanism of action for improving contractility in heart failure [3]. - The direct and primary mechanism is the inhibition of the Na+/K+ ATPase [1].
Question 2: Which of the following would cause an immediate reduction in the amount of potassium leaking out of a cell?
- A. Hyperpolarizing the membrane potential (Correct Answer)
- B. Reducing the activity of the sodium-potassium pump
- C. Decreasing the extracellular potassium concentration
- D. Increasing the permeability of the membrane to potassium
Explanation: ***Increasing (hyperpolarizing) the membrane potential*** - **Hyperpolarizing** the membrane means making the inside of the cell more negative relative to the outside. - This increased negativity inside the cell will **electrically attract** the positively charged **potassium ions** (K+) preventing them from leaking out. *Reducing the activity of the sodium-potassium pump* - The **sodium-potassium pump** actively transports potassium into the cell, helping to maintain the concentration gradient. - Reducing its activity would lead to an accumulation of potassium outside the cell and subsequent **increase in potassium leakage**. *Decreasing the extracellular potassium concentration* - A **lower extracellular potassium concentration** would steepen the potassium concentration gradient, causing more potassium to leak out of the cell. - This effect is due to the **chemical driving force** for potassium efflux. *Increasing the permeability of the membrane to potassium* - Increasing the **permeability** to potassium, typically through opening more **potassium channels**, would facilitate the movement of potassium ions down their electrochemical gradient. - This would result in a **greater leakage** of potassium out of the cell.
Question 3: What is the PRIMARY mechanism by which the Na+-Ca2+ exchanger functions in cardiac muscle cells?
- A. Na+-Ca2+ exchanger requires ATP directly
- B. Na+-Ca2+ exchanger acts to remove Ca2+ from heart muscle cells (Correct Answer)
- C. The Na+-Ca2+ exchanger operates in reverse mode during normal cardiac contraction
- D. The Na+-Ca2+ exchanger primarily moves Ca2+ into cardiac muscle cells during systole
Explanation: ***Na+-Ca2+ exchanger acts to remove Ca2+ from heart muscle cells.*** - The primary function of the **Na+-Ca2+ exchanger (NCX)** in cardiac muscle is to **extrude calcium from the cell** into the extracellular space. - It uses the electrochemical gradient of **sodium (Na+)** which flows into the cell, to power the removal of **calcium (Ca2+)** from the cell, contributing to muscle relaxation during diastole. *The Na+-Ca2+ exchanger operates in reverse mode during normal cardiac contraction* - While it can theoretically operate in reverse, its **primary physiological role** during normal cardiac contraction is forward mode (Ca2+ extrusion). - Reverse mode operation (Ca2+ influx) is typically seen under specific conditions, such as **pathological states** or severely altered intracellular Na+ concentrations. *Na+-Ca2+ exchanger requires ATP directly* - The **Na+-Ca2+ exchanger** is a **secondary active transporter** and does not directly use ATP. - Its energy comes from the **electrochemical gradient of Na+**, which is maintained by the **Na+/K+-ATPase** (primary active transport, which *does* use ATP). *The Na+-Ca2+ exchanger primarily moves Ca2+ into cardiac muscle cells during systole.* - Moving **Ca2+ into the cell** during systole would primarily be the role of **L-type calcium channels** on the sarcolemma. - The NCX's main role is to **reduce intracellular Ca2+** after contraction, facilitating relaxation during diastole.
Question 4: The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?
- A. Cyanide (Correct Answer)
- B. Carbon dioxide
- C. Oligomycin
- D. Ouabain
Explanation: ***Cyanide*** - **Cyanide** is a potent inhibitor of **cytochrome c oxidase (Complex IV)** in the electron transport chain, binding to the ferric iron (Fe3+) in the heme group of the enzyme. - This binding prevents the transfer of electrons to **oxygen**, thereby halting cellular respiration and ATP production. *Carbon dioxide* - **Carbon dioxide** is a metabolic waste product and a component of the **bicarbonate buffer system**, but it does not directly inhibit cytochrome complex IV. - While high levels can affect physiological pH and enzyme function, its primary role is not as an electron transport chain inhibitor. *Oligomycin* - **Oligomycin** inhibits **ATP synthase (Complex V)** by binding to its Fo subunit, which blocks the flow of protons through the ATP synthase channel. - This prevents the synthesis of ATP but does not directly affect the electron transfer steps of cytochrome complex IV. *Ouabain* - **Ouabain** is a cardiac glycoside that inhibits the **Na+/K+-ATPase pump** in the cell membrane. - It does not have any direct inhibitory effect on the components of the electron transport chain, including cytochrome complex IV.
Question 5: What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?
- A. Depolarization
- B. Action Potential
- C. Resting Membrane Potential
- D. Hyperpolarization (Correct Answer)
Explanation: ***Hyperpolarization*** - During **hyperpolarization**, the membrane potential becomes more negative than the **resting membrane potential**, primarily due to the outflow of **potassium (K+)** ions or influx of **chloride (Cl-)** ions. - This increased K+ efflux or Cl- influx signifies a state where potassium permeability is maximal relative to sodium permeability, making the membrane less excitable. *Action Potential* - An **action potential** involves a rapid **depolarization** phase due to a massive influx of **sodium (Na+)** ions, causing the PNa/PK ratio to be high, followed by repolarization where K+ efflux restores the resting potential. - Therefore, during an action potential, the ratio of PK/PNa is at its lowest during the rising phase when sodium channels are open. *Depolarization* - **Depolarization** is characterized by a decrease in the absolute value of the membrane potential, making it less negative or even positive, primarily due to the influx of **sodium (Na+)** ions. - During depolarization, the permeability to sodium is significantly higher than to potassium, thus the PK/PNa ratio is low. *Resting Membrane Potential* - At **resting membrane potential**, potassium permeability is already much higher than sodium permeability due to **leak potassium channels**, but it is not maximized to the extent seen during hyperpolarization. - The resting potential is established by a balance of ion movements, primarily K+ efflux and limited Na+ influx, maintained by the **Na+/K+-ATPase pump**.
Question 6: A 60-year-old patient with atrial fibrillation is prescribed digoxin. Which of the following is the MOST common EARLY side effect of digoxin?
- A. Nausea and vomiting (Correct Answer)
- B. Hypertension
- C. Visual disturbances
- D. Hyperkalemia
Explanation: ***Nausea and vomiting*** - **Gastrointestinal symptoms** such as nausea, vomiting, and anorexia are the **most common early signs** of **digoxin toxicity** due to its effect on the **chemoreceptor trigger zone**. - These symptoms can occur even at therapeutic levels, especially in susceptible individuals or with slight increases in concentration. - GI symptoms typically appear **before** other manifestations of toxicity, making them important early warning signs. *Hypertension* - Digoxin primarily affects **cardiac contractility** and **heart rate**, and it is not typically associated with causing **hypertension**. - In fact, digoxin can somewhat lower blood pressure due to its effects on **cardiac output** and **vasodilation** in some circumstances, though this is not its primary mechanism or side effect. *Visual disturbances* - **Visual disturbances**, including blurred vision, halos around lights, and changes in color perception (e.g., **yellow-green halos**), are a classic and **common symptom of digoxin toxicity**. - However, these typically appear **later** than gastrointestinal symptoms and often occur after or concurrently with GI manifestations. - While significant indicators of toxicity, they are not usually the **earliest** warning sign. *Hyperkalemia* - Digoxin inhibits the **Na+/K+-ATPase pump**, which can lead to **intracellular sodium accumulation** and **extracellular potassium accumulation**. However, **hyperkalemia** is primarily seen in cases of **acute, severe digoxin toxicity** or in patients with **renal impairment**. - More commonly, **hypokalemia** can actually potentiate digoxin's effects and increase the risk of toxicity, rather than digoxin directly causing hyperkalemia at therapeutic or mildly toxic levels.
Question 7: Which of the following is the primary mechanism that drives sodium reabsorption in the proximal tubule?
- A. Sodium reabsorption through cotransport with amino acids at the luminal membrane.
- B. Sodium reabsorption through cotransport with glucose at the luminal membrane.
- C. Sodium reabsorption through countertransport with hydrogen ions at the luminal membrane.
- D. Active sodium transport via the Na+-K+-ATPase pump at the basolateral membrane. (Correct Answer)
Explanation: ***Active sodium transport via the Na+-K+-ATPase pump at the basolateral membrane.*** - This pump **actively transports sodium out of the cell** into the interstitial fluid, creating a low intracellular sodium concentration. - The **Na+-K+-ATPase** is the primary driver of sodium reabsorption throughout the nephron, creating the electrochemical gradient for other sodium transporters. *Sodium reabsorption through cotransport with amino acids at the luminal membrane.* - While **sodium-amino acid cotransport** does occur in the proximal tubule, it accounts for only a fraction of total sodium reabsorption. - The primary driving force for this cotransport is the **low intracellular sodium concentration** maintained by the Na+-K+-ATPase. *Sodium reabsorption through cotransport with glucose at the luminal membrane.* - **Sodium-glucose cotransporters (SGLTs)** are crucial for glucose reabsorption in the proximal tubule, moving glucose into the cell along with sodium. - However, glucose cotransport represents a specific mechanism for glucose handling, not the overarching mechanism for sodium reabsorption. *Sodium reabsorption through countertransport with hydrogen ions at the luminal membrane.* - The **Na+-H+ exchanger (NHE3)** is significant for exchanging sodium for hydrogen ions at the luminal membrane in the proximal tubule. - This mechanism is important for **acid-base balance** and some sodium reabsorption, but it is secondary to the Na+-K+-ATPase in driving the overall sodium gradient.
Question 8: A patient has hyperaldosteronism. Which lab finding is expected?
- A. Metabolic acidosis
- B. Hyperkalemia
- C. Hypokalemia (Correct Answer)
- D. Hyponatremia
Explanation: ***Hypokalemia*** - **Aldosterone** increases the excretion of **potassium** in the kidneys, leading to decreased serum potassium levels [1]. - This effect is mediated by aldosterone's action on the principal cells of the collecting duct, promoting potassium secretion into the urine [1]. *Metabolic acidosis* - **Hyperaldosteronism** typically causes **metabolic alkalosis** due to increased hydrogen ion excretion by the kidneys [1]. - Aldosterone promotes the reabsorption of sodium and water, and the excretion of potassium and hydrogen ions, leading to alkalosis [2]. *Hyperkalemia* - **Aldosterone's primary role** is to promote **potassium excretion** in the kidneys [1]. - Therefore, **excessive aldosterone** production would lead to **hypokalemia**, not hyperkalemia. *Hyponatremia* - **Aldosterone** promotes **sodium reabsorption** in the kidneys, which usually leads to normal or even slightly elevated serum sodium levels [1]. - **Hyponatremia** would be an unexpected finding in hyperaldosteronism [3].
Question 9: Which of the following conditions does not enhance the toxicity of digoxin?
- A. Hypercalcemia
- B. Hyperkalemia (Correct Answer)
- C. Renal failure
- D. Hypomagnesemia
Explanation: ***Hyperkalemia*** - **Hyperkalemia** actually reduces the binding of digoxin to the Na+/K+-ATPase, thereby antagonizing its effect and decreasing its toxicity. - While very high potassium levels can be dangerous, they tend to mitigate, rather than enhance, **digoxin toxicity**. *Hypercalcemia* - **Hypercalcemia** enhances the inotropic effects of digoxin, leading to increased risk of toxicity, particularly arrhythmias. - High calcium levels contribute to the **calcium overload** within cardiac myocytes, which is a mechanism of digoxin toxicity. *Renal failure* - Digoxin is primarily excreted renally, so **renal failure** leads to reduced clearance and accumulation of the drug, increasing its serum concentration and toxicity. - Patients with impaired kidney function require **lower doses** of digoxin to avoid toxic levels. *Hypomagnesemia* - **Hypomagnesemia** exacerbates digoxin toxicity by increasing the binding affinity of digoxin to the Na+/K+-ATPase and contributing to the development of arrhythmias. - Low magnesium levels can destabilize the cardiac muscle, making it more susceptible to the **proarrhythmic effects** of digoxin.
Question 10: Which of the following is true regarding Na+ (sodium) ions?
- A. Does not help other ions in transport
- B. Responsible for depolarization (Correct Answer)
- C. Responsible for the resting membrane potential
- D. Sodium ion is responsible for Donnan effect
Explanation: ***Responsible for depolarization*** - The rapid influx of **Na+ ions** into the cell through voltage-gated sodium channels is the primary event that causes **depolarization** during an action potential. - This influx makes the inside of the cell more positive, shifting the membrane potential from negative toward positive values. *Sodium ion is responsible for Donnan effect* - The **Donnan effect** describes the unequal distribution of permeable ions across a semi-permeable membrane due to the presence of impermeant charged molecules (e.g., proteins). - **Na+ ions are small, permeable ions** - they do not create the Donnan effect. The effect is caused by large, non-diffusible charged molecules like proteins, not by sodium ions. *Does not help other ions in transport* - The **sodium-potassium pump (Na+/K+-ATPase)** actively transports Na+ out of the cell and K+ into the cell, maintaining their concentration gradients. - These Na+ gradients are crucial for **secondary active transport**, where the energy from Na+ moving down its electrochemical gradient is used to move other ions (e.g., in Na+-glucose cotransport) or molecules against their gradients. *Responsible for the resting membrane potential* - The **resting membrane potential** is primarily established by the differential permeability of the membrane to K+ ions and the activity of the Na+/K+-ATPase. - While Na+ leaking into the cell contributes slightly, the dominant factor is the efflux of **K+ ions** through leak channels, as the membrane is much more permeable to K+ than to Na+ at rest.
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