Renal Transport

Question 1

different transport mechanisms by energy source

Answer 1

Passive: down EC gradient
- simply or facilitated (pores, channels, carrier proteins) diffusion

active transport: against EC gradient, requires E

• primary= using ATP hydrolysis
• secondary=coupling transport up EC gradient with transport down EC gradient

Question 2

unique properties of epithelial cells, how these influence transport along nephron

Answer 2

• polar (distinct apical and basolateral)
• tight junctions near apical surface segregate membrane surfaces, determine transport directionality
• electric potential diff across peritubular space (0mV) and lumen with can be -, +, or 0
• apical SA amplified by brush border

Question 3

major pathways of Na+ transport along nephron, how distribution of tranposrters (apical/basal) allow regulation of transport

Answer 3

BULK Na+ reabsorption in PT
Regulation of Na+ reabsorption in CD

transport depends on:

• available transporters
• regional permeability
• composition delivered tubular fluid
• transepithelial voltage gradient

Question 4

proximal tubule Na+ reabsorption

Answer 4

Na+ reabsorption coupled with HCO3- reclamation

Na/K ATPase establish gradient
NHE3-Na/H exchanger- initiates Na+ reabsorption, secretes H+ into tubular lumen
NBC1-Na/HCO3- completes Na+ reabsorption, reclaims intracellularly produced HCO3-

OR coupled wo other solutes (glucose, a.a.)

Question 5

proximal tubule H2O/Cl reabsorption

Answer 5

Na+ reabs. promotes this.

bulk H2O movement will pull Cl- across tight junctions (solvent drag)
transepithelial EC gradient promotes paracellular CL- transport
transcellular Cl- transport path: apical and basolateral transporter

Question 6

thick ascending limb Na+ reabsorption

Answer 6

ascending thin limb = some passive Na_ reabsorption

TAL: NCC2 and Na/K ATPase
- generates hypertonic interstitium, hypotonic tubular fluid that re-enters cortex in DCT
- K+ recycled to tubular lumen, creates lumen-positive
which promotes paracellular cation reabsorption

Question 7

distal tubule Na+ reabsorption

Answer 7

principal cells of DCT

first portion relies on NCC
second relies on NCC and ENaC

Question 8

CD Na+ reabsorption

Answer 8

principal cells of CD rely on ENaC
site of regulation. tuned by aldosterone
aldosterone enhances ENaC synthesis, and insertion of ENaC apical, and open probability

Question 9

K+ Reabsorption/secretion basics

Answer 9

BULK reabsorption occurs in PT (same for all K+ levels)

Regulation K+ reabsorption/secretion occurs in CD
normal/elevated levels = secretion predominates
low levels=reabsorption

Question 10

PT K+ reabsorption

Answer 10

two paths:

1. solvent drag k+ reabsorption (early PT)
   - active Na+ reabsorption drives H2O reabsorption, coupled with K+
2. paracellular diffusion (late PT)
   - positive lumen drives

Question 11

composition of tubular fluid along PT

Answer 11

changes markedly.

• glucose/a.a./HCO3- largely reabsorbed
• amount H2O reduced (know this bc amount inulin is constant, but [] increases)
• H2O follows Na+ movement

also, transepithelial potential is negative at beginning, then becomes positive

flow rate decreases progressively, due to loss H2O

Question 12

TAL K+ reabsorption

Answer 12

2 paths:

1. transcellular K+ reabsorption
   - Na/K ATPase provides gradient for NKCC2
   - basolateral KCC and K+ channels
2. paracellular diffusion
   - some K+ needs to be recycled to tubular lumen, creates lumen-positive voltage, drives diffusion

Question 13

DCT K+ Secretion

Answer 13

KCC located apically = major pathway
couples basolateral NA/K ATPase activity to apical K+ transport

DCT late secretion relies on apical KCC and ROMK

Question 14

CD K+ handling, normal/elevated VS low levels K+

Answer 14

normal/elevated: similar to DCT late, relies on apical KCC and ROMK in principal cells

low levels: K+ reabsorption in CD alpha-intercalated cells. apical H/K+ ATPase brings K+ into cell and reabsorption completed by basolateral channel.