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Related Experiment Videos

Active NH4+ absorption by the thick ascending limb.

J L Garvin1, M B Burg, M A Knepper

  • 1Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung and Blood Institute, Bethesda, Maryland 20892.

The American Journal of Physiology
|July 1, 1988
PubMed
Summary
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Rabbit kidneys actively absorb ammonium ions (NH4+) against an electrochemical gradient in the thick ascending limb. This process is crucial for renal medullary function and involves the Na+-K+-2Cl- cotransporter.

Area of Science:

  • Nephrology
  • Renal Physiology
  • Ion Transport

Background:

  • The renal medulla plays a critical role in concentrating urine and maintaining acid-base balance.
  • Understanding ammonium (NH4+) transport is essential for comprehending renal function and disorders.

Purpose of the Study:

  • To elucidate the mechanism of active ammonium ion absorption in the rabbit medullary thick ascending limb.
  • To investigate the role of the Na+-K+-2Cl- cotransporter in NH4+ transport.

Main Methods:

  • Isolated perfused rabbit medullary thick ascending limbs were utilized.
  • Electrochemical gradients were manipulated using chemical voltage clamping and altered bath NaCl concentrations.
  • The effects of luminal furosemide and basolateral ouabain on NH4+ flux were assessed.

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Main Results:

  • Ammonium ions were absorbed against an electrochemical gradient, indicating active transport.
  • NH4+ absorption was primarily mediated by the apical Na+-K+-2Cl- cotransporter, substituting for K+.
  • Passive paracellular diffusion also contributed to net NH4+ flux.
  • NH4+ permeability was high, while NH3 permeability was low.

Conclusions:

  • Active NH4+ absorption in the medullary thick ascending limb is dependent on apical entry via the Na+-K+-2Cl- cotransporter and basolateral Na+-K+-ATPase.
  • This active transport mechanism contributes to the countercurrent multiplication of NH4+ in the renal medulla.