Nephron Physiology - 3: DCT and Collecting tubule Flashcards
What is the Na and Cl content of the filtrate entering the DCT?
Hypotonic because of powerful reabsorption of Na and Cl by NKCC2.
What is the H20 content of the filtrate entering the DCT?
Dilute, because thick ascending was impermeable to water.
What is the NH4+ and K+ of the filtrate entering the DCT?
Very little
What is the bicarb content of the filtrate entering the DCT?
None (unless VERY high plasma content)
What is the Ca2+ and Mg2+ content of the filtrate entering the DCT?
Moderate
How is Na absorbed in the DCT?
NCC cotransporter with Cl.
NCC contransporter driven by Na+ gradient established by basolateral Na/K+ATPase.
- Cl diffues through basolateral channel
- Na leaves via ATPase
How does the NCC contransporter compare with NKCC2 in terms of sodium reabsorption?
NKCC2 is much more powerful than NCC. NCC has lower load coming, and isn’t as much of a boss. Only 5-10% Na+ reabsorbed.
How does Mg2+ transport occur in the DCT?
Mg2+ comes in through TRPM6 transporter on apical membrane.
Lumen is weakly positive due to K+ recycling, Mg2+ movement driven into cell by this gradient.
Transporter relies on intact EGF receptor to work.
How does Ca2+ transport occur in the DCT?
Calcium enters via TRPV5 transporter on apical membrane.
Once in the cell, associates with protein and goes to Ca2+/Na+ exchanger on the basolateral surface to be reabsorbed. Vitamin D helps.
What is the effect of PTH on Ca2+ reabsorption?
Increases Ca2+/Na+ exchange, increases calcium absorption.
What is the collecting duct?
Joining together of various nephrons; purpose is to determine final urine concentration for Na+, K+, H+ and bicarb depending on the needs of the body.
How is sodium reabsorbed in the collecting duct?
Epithelial Na Channel on luminal membrane of principal cells = ENac. Sodium moves down its concentration gradient from lumen to cell. Na+K+/ATPase pumps it out.
What is the effect of sodium reabsorption on K+ in the collecting duct?
As Na+/K+ ATPase pumps Na+ out of cell, it brings K+ in.
K+ escapes into lumen via ROMK leak channel.
K+ moves in the opposite direction as Na+ in the collecting duct.
What regulates the degree of ENac activity?
Aldosterone. Aldosterone enters epithelial cell nucleus, acts on mineralocorticoid to increase mRNA/protein synthesis of ENac. This leads to increased Na+ reabsorption, increased Na+/K+ATPase work, increased K+ out of the blood and into the lumen.
Aldo also increases expression of Na+/K+ATPases and K+ leak channels, adding to this effect.
How is H+ excreted in the collecting duct?
The alpha intercalated cell has a H+/ATPase, and an H+/K+ ATPase that removes H+ from the cell (and brings K+ in) and sends it for excretion (it jumps onto phosphate or ammonia buffers)
- This leaves bicarb behind; bicarb is exchanged with Cl- and absorbed.
