5 - Renal Tubular Transport 2

Question 1

What is responsible for regulation of tubular reabsorption?

Answer 1

Neuronal, hormonal, and physical factors alter tubular fluid and solute reabsorption.

Question 2

What effect can a change in the filtration fraction in the glomerular capillaries of change in vascular resistance un the upstream segments of the renal arterial vasculature have?

Answer 2

These will alter the postglomerular plasma oncotic pressure and the peritubular capillary hydrostatic pressure, which will alter the rate of fluid reabsorption in the capillary bed.

Question 3

What is backleak?

Answer 3

Alterations in physical factors such as hydrostatic and oncotic pressure that alter the tubular reabsorption of fluid and solute.

Question 4

What effect does Aldosterone have on tubular reabsorption? Where does it act?

Answer 4

Acts in principal cells of late distal tubule and collecting duct.

Increases Na+ reabsorption and K+ secretion.

Question 5

What effect does angiotensin II have on tubular reabsorption? Where does it act?

Answer 5

Acts primarily in the PT.

Increases Na+ and water reabsorption, increases H+ secretion.

Question 6

What effect does antidiuretic hormone (vasopressin, ADH) have on tubular reabsorption? Where does it act?

Answer 6

Acts in principal cells of the late distal tubule and collecting duct, inner medullary collecting duct.

Increases water reabsorption.

Question 7

What is the effect of Atrial natriuretic peptide (ANP, ANF) on tubular reabsorption? Where does it act?

Answer 7

Acts in the distal tubule and collecting duct.

Decreases Na+ reabsorption.

Question 8

What is the effect of parathyroid hormone (PTH) on tubular reabsorption? Where does it act?

Answer 8

In PT: decreases PO43-
reabsorption.

In thick ascending loop of henle and distal tubule: increases Ca++ reabsorption.

Question 9

What is the pattern of calcium reabsorption in the kidneys?

Answer 9

The percentage of filtered calcium that’s reabsorbed follows the same pattern as sodium.

Question 10

How is calcium reabsorbed in the PT, thick ascending loop of henle, and the early distal tubule?

Answer 10

PT: mainly paracellular, not under hormonal control, largely parallels Na reabsorption.

Thick ascending: mainly paracellular, stimulated by PTH.

Early distal tubule: Distal tubular mechanism stimulated by PTH.

Question 11

What is the primary controller of calcium reabsorption?

Answer 11

PTH, regulated by plasma calcium, increases calcium reabsorption.

It acts mainly in the loop of henle and distal tubule to influence calcium.

Kidney plays important role in body’s compensation to decreased plasma calcium.

Question 12

What are the compensatory responses to decreased plasma calcium concentration?

Answer 12

Increased PTH, which increases vit D3 activation which increases intestinal Ca2+ reabsorption, increased renal Ca2+ reabsorption, and increased Ca2+ released from bones.

Question 13

What is the result of decreased calcium excretion?

Answer 13

Increased Parathyroid hormone, decreases extracellular fluid volume, decreased BP, increased plasma phosphate, metabolic alkalosis, and vitamin D3 activation.

Question 14

What is the result of increased calcium excretion?

Answer 14

Decreased PTH, increased extracellular fluid volume, increased BP, decreased plasma phosphate, and metabolic acidosis.

Question 15

How is phosphate handled by the kidneys?

Answer 15

It it not freely filtered b/c 10% of phosphate in plasma is protein bound.

80 % of filtered phosphate is reabsorbed in the PT via a Na/Pi co-transporter

20% reabsorbed in the distal tubule.

Question 16

What is the renal overflow mechanism for phosphate?

Answer 16

The kidney will completely reabsorb the filtered phosphate if plasma phosphate concentration is less than 0.8 mM/L.

Kidney will excrete any phosphate above this threshold.

Question 17

What causes increased urinary phosphate excretion?

Answer 17

Increased PTH, PO4 loading, ECV expansion, acidosis, and glucocorticoids.

Question 18

What causes decreased urinary phosphate excretion?

Answer 18

Decreased PTH, PO4 depletion, ECV contraction, alkalosis, growth hormone.

Question 19

What is the most important phosphate controlling hormone? How does it work?

Answer 19

PTH. It increases cAMP in the PT and inhibits reabsorption.

Question 20

How do the kidneys handle magnesium? What percentage is protein bound? Where it is reabsorbed?

Answer 20

It is not freely filtered b/c ~50% of plasma magnesium is bound to protein.

30% filtered Mg reabsorbed in the PT

65% thick ascending limb

Question 21

What is the mechanism by which Mg is handled by the kidneys? What increases Mg excretion?

Answer 21

Not well known.

Mg excretion increases in response to increased extracellular Mg, ECV expansion, and increased extracellular calcium.

Question 22

Where is water reabsorbed in the kidneys?

Answer 22

65% PT: isosmotic reabsorption with Na and electrolytes.
20% loop
5% distal tubule (ADH)
9.5% collecting duct (ADH)

Question 23

Why are the PT and thin descending loop of henle known as “leaky epithelia”?

Answer 23

They have a relatively low resistance across the tight junctions and these layers are highly permeable to water.

Here water is reabsorbed both paracellularly and transcellularly.

Question 24

What portions of the nephron are impermeable to water?

Answer 24

Thin ascending loop, thick ascending loop, and the early distal tubule.

Question 25

Why are the epithelial layers in the late distal tubule and collecting duct known as “tight epithelia”? Does water reabsorption occur in these segments?

Answer 25

Relatively high electrical resistance in the tight junctions.

Water reabsorption occurs via transcellular mechanisms in the presence of ADH

Question 26

What does ADH do?

Answer 26

Bind to vasopressin V2 receptors on the basolateral membrane of principal cells.

These stimulates an increase synthesis of aquaporin 2 proteins (AQP2), causing insertion of AQP2 into the apical membrane.

Question 27

When is the basolateral surface permeable to water? What about the apical membrane?

Answer 27

Basolateral surface is always highly permeable to water due to presence of AQP3 and ASQP4.

Apical membrane is only permeable to water when the AQP2 proteins are inserted in the membrane.

Question 28

How does water move through the apical membrane in the collecting duct?

Answer 28

Since these are “tight epithelia” with tight junctions that have high electrical resistance and low permeability, water can only go through transcellularly and ONLY when ADH is present.

Question 29

How is urea handed by the kidneys? How is it filtered and where is it reabsorbed?

Answer 29

It’s freely filtered.

It’s reabsorbed passively in PT

Secreted by carrier-mediated diffusion in the thin ascending or descending LOH.

Reabsorbed by carrier-mediated diffusion in the inner medullary collecting duct (with ADH)

Question 30

What is the function of the UTI transporter in the inner medullary collecting duct?

Answer 30

Located on the apical membrane, and stimulated by ADH, which causes urea reabsorption.