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5.2 - Distal tubule and collecting ducts


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What is the osmolarity of the filtrate that enters the DCT from the ascending loop of Henle?
It is hyposmolar to plasma
Name three of the four functions of the DCT and the collecting ducts.
1.ADH-dependent water reabsorption
2.aldo-dependent NaCl reabsorption
3.aldo-independent NaCl reabsorption
4.aldo-dependent K+ secretion
What channel is used to move Na+ from the lumen to the cell in the distal tubule? How is the channel activated?
How is Na+ moved from the cell to the interstitium in the distal tubule?
Na+/K+ ATPase
In the distal tubule, Na+ moves down its gradient from the lumen into the cell. How is the charge balanced?
Passive diffusion of K+ from the cell to the lumen.
Describe the structure of the ENaC. What binding sites does it feature?
*multi-subunit channel
*apical sites bind amiloride which blocks the channel
*gates on the cytoplasmic side are responsive to aldosterone
What are 2 mechanisms used by the alpha cells of the distal tubule to secrete H+?
1.H+/K+ ATPase
2.specific primary transporter (H+ ATPase)
When CO2 enters an epithelial cell of the distal tubule it combines with water and forms H+ and HCO3-. What happens to the bicarbonate ion?
It is moved to the interstitium via a HCO3-/Cl- ATPase antiporter.
How do alpha cells of the distal tubule help the body cope with respiratory acidosis?
The activity of K+/H+ ATPase and other H+ transporters is increased to increase the secretion of H+.
What is the minimal urinary pH that can be attained?
What are the beta cells of the distal tubule? What transporters do they feature? During what condition are these cells most active?
*HCO3- secreting cells
*HCO3-/Cl- ATPase moves HCO3- into the lumen
*H+ ATPase moves H+ into the interstitium
*respiratory alkalosis
The osmolarity of the filtrate in the medullary collecting duct can vary widely - what causes this? What is the range of osmolarity of this filtrate?
*the presence of ADH
*80mOsm/L (no ADH) to 300mOsm/L (max ADH)
What are two ADH-dependent functions of the medullary collecting duct?
1.ADH-dependent activation of AQP-2 for reabsorption of water
2.ADH-dependent expression of UT-1 for urea reabsorption
How does ADH affect the reabsorption of Na+ in the medullary collecting duct?
It doesn't - ADH mediates only the reabsorption of water, independent of Na+.

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