Lecture 5: Renal Tubular Transport (Bolser)

Question 1

what are the 4 different methods of membrane transport systems

Answer 1

1. diffusion
2. osmosis
3. facilitated transport (aka carrier mediated transport)
4. active transport - primary Na, K, ATPase
   and secondary transport (co-transport) glucose and amino acids

Question 2

why is there more creatinine in the urine than what is filtered?

Answer 2

because more is added after the filtration apparatus. aka its secreted.

Question 3

what are the characteristics of Na in tubular transport? (3 key points)

Answer 3

1. Na is reabsorbed - mostly in the proximal tubule, some in the thin and thick ascending limbs and tiny bit in the collecting duct
2. Na is NEVER secreted by the nephron
3. Na reabsorption is active and is powered by Na-K-ATPase at the basolateral membrane

Question 4

what are the characteristics of water in tubular transport (4 key points)

Answer 4

1. water follows Na
2. solute coupled in proximal tubules - solute reabsorption creates a small difference in osmolarity between the luminal fluid and the interstitium.
3. water is mostly absorbed in the proximal tubule, a bit (10%) in the descending thin limb and a wide range (1 - 24%) in the collecting ducts.
4. amount of water absorbed in collecting ducts is determined by ADH levels. if ADH is increased then water reabsorption will be increased.

Question 5

what are the characteristics of K+ in tubular transport (3 key points)

Answer 5

1. most of the filtered K is reabsorbed in the proximal tubule (55%), 30% is cotransported with Na in the ascending limb of the loop of Henle. only 10% of filtered K enters the distal tubule where active reabsorption occurs.
2. secretion of K is regulated, primarily at the distal tubule and collecting duct to control overall K excretion.
3. K is actively transported from the peritubular fluid into the cell and reaches the lumen by diffusion and active transport out of the cell.

Question 6

at what location within the nephron are glucose, amino acids and proteins reabsorbed by the nephron?

Answer 6

in the early proximal tubule

Question 7

why does inulin appear concentrated in the late proximal tubule?

Answer 7

because water has already left making inulin concentrations 3x higher

Question 8

when is HCO3 reabsorbed by the nephron?

Answer 8

its favored to be reabsorbed in the early proximal tubule because it is reabsorbed in association with Na.

Question 9

what are the 8 important features of action of the proximal tubule on the tubular fluid?

Answer 9

1. reabsorption is isosmotic
2. overall Na reabsorption is active
3. overall Cl reabsorption is passive
4. water reabsorption is passive
5. 2/3 filtrate reabsorbed
6. K passively reabsorbed
7. glucose, proteins, acetoacetate, ions, vitamins, AA all actively reabsorbed
8. H+ are secreted by a counter-transport involving sodium against concentration gradient

Question 10

what are the two mechanisms by which sodium moves across the apical membrane in the first half of the proximal tubule

Answer 10

1. co-transported with several other freely filtered molecules
2. transpored by a Na-H antiporter

Question 11

what drives Na transport in the first half of the proximal tubule?

Answer 11

the concentration gradient created by the Na/K ATPase as well as the reabsorption of the co-transported molecules

Question 12

what is the role carbonic anhydrase plays in the first half of the proximal tubule? (2 things)

Answer 12

1. it facilitates the production of H ions to exchange with Na ions in the first half of the proximal tubule
2. bicarbonate is also reabsorbed in the first half of the proximal tubule which promotes the transport of Na in association with Cl mostly in the second half of the proximal tubule

Question 13

how are Na and Cl reabsorbed in the second half of the proximal tubule (2 ways)

Answer 13

1. paracellular diffusion

2. a complex co-transporter system involving two antiporters and weak acids

Question 14

anions are moved from the lumen in exchange for secreted organic acids including ___?

Answer 14

chloride, urate and hydroxyl ions.

Question 15

tertiary active transport

Answer 15

• occurs in organic anion secretion

- the transport of an organic acid is linked to Na transport by the Na/K/ATPase through common intermediaries

Question 16

how do organic cations enter the cell from the blood?

Answer 16

by carrier-mediated facilitated diffusion

Question 17

what are some prominent organic cations?

Answer 17

acetylcholine, epinephrine, dopamine, atropine and morphine

Question 18

what is characteristic of tubular transport in the thin descending limb? (2 things)

Answer 18

1. cells have minimal metabolic activity - aka little active transport
2. HIGH permeability to water, moderate permeability to urea, sodium and other ions

Question 19

what is characteristic of tubular transport in the thin ascending limb? (3 things)

Answer 19

1. little active transport in these cells
2. little to no permeability to water
3. high permeability to Na

Question 20

what is characteristic of tubular transport in the thick ascending limb (4 things)

Answer 20

1. impermeable to water
2. net reabsorption of Na, Cl and K. active Cl transport via Na, K, and 2Cl co-transporter
3. high ion concentration gradient between lumen and peritublar fluid
4. high osmolar concentration gradient between lumen and peritubular fluid

Question 21

what is the effect of diuretic lasix in the thick ascending limb?

Answer 21

diuretic lasix binds to Cl on the Na/Cl/K symporter suppressing its activity. the reduction in Na transport affects the ability of more distal segments of the nephron to reabsorb water

Question 22

are cells in the thick ascending limb permeable or impermeable to water?

Answer 22

impermeable - this portion of the nephron is known as the diluting segment

Question 23

what is function of the Na/K/ATPase symporter in the thick ascending limb?

Answer 23

it creates a concentration gradient for the function of the apical membrane transporters

Question 24

is the early distal tubule permeable or impermeable to water? what is the significance of this in relation to Na transport here?

Answer 24

impermeable

b/c this segment is impermeable to water but Na is transported out of the lumen in the early distal tubule, dilution of the tubular fluid occurs here

Question 25

what is the effect of reabsorption of ions in the early distal tubule (2 things)

Answer 25

1. it can establish high ionic concentration gradients between urine and plasma
2. can establish a high osmolar concentration gradient between urine and plasma

Question 26

what happens to transport of Na in the early distal tubule

Answer 26

the Na/Cl symporter is responsible for Na reabsorption

Na concentration can be reduced to zero in the tubular fluid

Cl reabsorption is mostly passive

Question 27

what happens to the transport of water in the distal tubule?

Answer 27

variable permeability to water (controlled by ADH) both here and in the collecting ducts

Question 28

what happens to the transport of K in the distal tubule

Answer 28

principal cells are responsible for K secretion into the tubular fluid

varies from no net K transport to net secretion and is controlled by cell K content which is regulated by active transport into the cell across the peritubular membrane.

Question 29

what happens to the transport of H in the distal tubule

Answer 29

actively secreted into peritubular lumen by intercalated cells via H/ATPase

pump can produce TF/p gradient (tubular fluid/plasma) 1000:1

Question 30

what is the function of principal cells

Answer 30

reabsorption of Na and secretion of K in the late distal tubule and collecting ducts.

they are also responsible for the action of ADH

Question 31

what is the function of type B intercalated cells

Answer 31

bicarbonate secretion

Question 32

what is the function of type a intercalated cells

Answer 32

acid secretion