- In kidney, Na+ is reabsorbed across the second half of proximal tubule due to positive transepithelial voltage (i.e., tubular fluid becomes positive relative to blood) and by other mechanisms. The following proposed statements could explain the development of this positive transepithelial voltage.
A. Cl– concentration gradient in the second half of the proximal tubule favours diffusion of Cl– from tubular lumen to intercellular space via a paracellular route, which generates the positive transepithelial voltage.
B. The Na+– H+ antiporters in the second half of proximal tubules create the positive transepithelial voltage.
C. The Na+ – glucose symporters operating in the proximal part of renal tubules are responsible for this positive transepithelial voltage
D. The positive transepithelial voltage is created by the operation of 1Na+ – 1K+ – 2Cl–
symporter in the proximal tubules
Select the option with correct statement(s).
(1) only A (2) B and C
(3) C and D (4) only D
In the kidney’s second half of the proximal tubule, sodium (Na+) reabsorption is driven by a positive transepithelial voltage where the tubular fluid is positive relative to blood. This electrochemical gradient encourages Na+ to move passively across the epithelium, complementing active transport mechanisms.
Explanation of Positive Transepithelial Voltage Development
Statement A: Chloride Gradient and Paracellular Diffusion
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Correct
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High chloride (Cl-) concentration in the second half of the proximal tubule lumen favors its movement through the paracellular pathways (spaces between cells) into the intercellular space.
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This movement of negatively charged Cl- ions generates a lumen-positive transepithelial voltage because negative charges leave the lumen, leaving it relatively positive.
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This electrical gradient facilitates Na+ reabsorption paracellularly.khanacademy+1
Statement B: Na+-H+ Antiporters Role
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Incorrect
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The Na+-H+ antiporter primarily functions in the early proximal tubule to reabsorb Na+ in exchange for H+, but it does not create the positive transepithelial voltage in the second half.
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Its activity precedes the generation of this voltage, which is more pronounced in the late proximal tubule due to Cl- dynamics.derangedphysiology+1
Statement C: Na+-Glucose Symporters
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Incorrect
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These operate in the early proximal tubule segment, coupling Na+ with glucose for reabsorption.
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They do not contribute to the positive transepithelial voltage in the second half of the proximal tubule.
Statement D: 1Na+-1K+-2Cl- Symporter’s Contribution
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Incorrect
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This symporter is primarily located in the thick ascending limb of the loop of Henle, not the proximal tubule.
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It does not contribute to the positive transepithelial voltage in the proximal tubule.openaccessjournals+1
Correct Statements Summary
Statement Explanation Correctness A Cl- diffusion via paracellular route creates positive voltage Correct B Na+-H+ antiporter creates voltage Incorrect C Na+-glucose symporters cause voltage Incorrect D Na+-K+-2Cl- symporter causes voltage Incorrect Conclusion
The generation of the positive transepithelial voltage in the second half of the proximal tubule is mainly due to Cl- concentration gradient and paracellular diffusion (statement A).
Correct answer: (1) only A
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