Proximal Tubular Transport

Question 1

overall processes in PCT and loop

Answer 1

PCT= isosmotic reabsorption

loop= separate reabsorption of water and salt, creates hyperosmotic medullary gradient

Question 2

reabsorption in PCT

Answer 2

2/3 of water and NaCl, all glucose and amino acids, most bicarb, 2/3 K+

Question 3

Na gradient in PCT

Answer 3

low Na inside cell maintained by Na-K ATPase on basolateral membrane, allows for filtered Na to flow down gradient into cell

Question 4

5 main transporters on luminal surface of PCT cell

Answer 4

• Na/Cl co transporter
• Na/glucose co transport
• Na/ aminos co transport
• Na/ H+ exchanger
• aquaporin 1

Question 5

describe secondary active transport

Answer 5

active transport of Na out of cell into interstitium, allows for Na gradient to help pull other solutes out of tubule

Question 6

amino acid reabsorption

Answer 6

99% in PCT, none excreted

follows Na into cell via cotransporter

Question 7

glucose reabsorption, what happens w/ DM

Answer 7

freely filtered at glomerulus, 100% reabsorbed at PCT

sodium dependent glucose transporter on luminal membrane (SGLT) and glucose transporter (GLUT) on basolateral

finite number of SGLTs, can be overwhelmed by excess plasma and filtrate glucose (DM) and glucose appears in urine, also polyuria and thirst via osmotic diuresis

Question 8

bicarb reabsorption- 2 mechanisms

Answer 8

2 ways to increase bicarb: reclaim from filtrate and generate new forms from glutamine in cell

Question 9

reclaiming filtered bicarb

Answer 9

1. bicarb becomes carbonic acid in acidic tubule (via Na/H+ exchanger), converted to H2O and CO2 by carbonic anhydrase in brush border

these can diffuse into cell where another CA converts back to bicarb

bicarb transported out of cell via Na/HCO3- symporter on basolateral (3 HCO3 for every 1 Na)

Question 10

generation of new bicarb in PCT

Answer 10

gln transported into cell, breaks down into bicarb and NH4+, HCO3 moves back across basolateral to interstitium (facillitated diffusion)

NH4+ secreted into tubular fluid

Question 11

K+ reabsorption in PCT

Answer 11

80% by solvent drag. water in paracellular route out of tubule pulls K+

Question 12

how is urea reabsorbed in PCT

Answer 12

H2O leaves first, concentrating urea in tubule, can then diffuse out along gradient

1/2 of filtered load is reabsorbed, more than any other segment

urea is fully filtered, filtered urea=plasma urea

Question 13

what happens to urea reabsorption w/ lower GFR

Answer 13

low GFR means low urine flow and more time for urea reabsorption

high plasma urea, increased BUN concentration

Question 14

secretion into PCT

Answer 14

metabolic products, pollutants, drugs

note: some drugs compete for same transporter, wont be fully secreted and excreted

Question 15

fluid entering vs leaving loop

Answer 15

enters isosmotic, hyperosmotic around the bend of the loop, leaves hyposmotic

Question 16

contrast descending and ascending limb of loop

Answer 16

descending: leaky, permeable to water and slightly to NaCl and urea

ascending (thick): impermeable to water, actively reabsorbing Na, Cl, and K, diluting urine

Question 17

NaCl reabsorption in ascending loop

Answer 17

depends on Na/K/2Cl transporter (NKCC)

inhibited by loop diuretics

Question 18

K transport in ascending limb, 4 transporters

Answer 18

into cell by NKCC on luminal, Na/K ATPase on basolateral

out of cell into lumen via ROMK, into interstitium via basolateral K+ channel

Question 19

how does ADH contribute to osmotic medullary gradient

Answer 19

inserts UT1 transporter in medullary collecting duct, urea is reabsorbed and adds to osmotic gradient to pull water out

Question 20

single effect and multiplication effect

Answer 20

single effect- horizontal osmotic gradient of 200 at each level from NKCC Na reabsorption

multiplication- flow of isosmotic urine and continued equilibration and Na reabsorption creates vertical gradient

Question 21

3 factors affecting magnitude of vertical gradient from loop

Answer 21

1. urea in interstitium- increased by ADH, reduced by low protein diet
2. length of the loop- longer loops = larger gradient and more concentrated urine
3. rate of flow- high flow rates decrease equilibrium in descending limb, from excess water ingestion, osmotic diuresis

Question 22

blood flow in vasa recta

Answer 22

slow and low, equilibrates on descending limb and ascending limb to prevent destruction of osmotic gradient