Tubular Reabsorption

Question 1

What are the two potential routes for reabsorption from the tubular lumen?

Answer 1

Paracellular - diffusion between cells

Transcellualr - diffusion across the cell (from apical to basolateral membrane)

Question 2

How is sodium reabsorbed in the cortical collecting duct?

Answer 2

Move down their concentration gradient into the cell from the lumen through sodium channels

Actively transported across the basolateral membrane via the Na-K ATPase

Question 3

How is glucose reabsorbed in the proximal tubule?

Answer 3

Cotransported with sodium across the luminal membrane (secondary active transport)

Exits the basolateral membrane via facilitated diffusion

*Overall process ultimately depends on the primary active Na-K ATPase pump in the basolateral membrane

Question 4

What is the transport maximum?

Answer 4

Limit to the amounts of material the active transport systems in the renal tubule can transport per unit of time

Due to saturation of carriers

Question 5

What is the glucose titration curve?

Answer 5

Describes the mass flows of filtration, reabsorption, and excretion of glucose over the range of plasma glucose concentrations

Question 6

What is splay?

Answer 6

The appearance of glucose in the urine before threshold is reached

Question 7

What is threshold?

Answer 7

plasma concentration at which glucose first appears in the urine

Question 8

Describe glucose clearance?

Answer 8

Normally, the clearance of glucose is 0

However, when the plasma glucose threshold is exceeded, plasma begins to be cleared and increases progressively with increasing plasma glucose concentrations

Question 9

What is renal glycosuria?

Answer 9

Glucose in the urine as a result of a defective or missing transport mechanism

Tg = 0 or very low

Question 10

What is Diabetes mellitus (as it relates to kidney function)?

Answer 10

Glucosuria due to lack of insulin

Tg = normal

Question 11

How does pregnancy affect glucose reabsorption?

Answer 11

Glucosuria due to glomerular hyperfiltration (large increase in GFR)

Tg = normal

Question 12

Generally, how do the kidneys handle amino acids?

Answer 12

Major role is conservation, zero clearance

Actively reabsorbed, negligible quantities excreted

Kidneys do not primarily regulate the plasma concentrations of amino acids

Question 13

Describe the carriers used to reabsorb amino acids?

Answer 13

Lysine, Arginine, Ornithine, Cystine, Histidine carrier

Glutamic and Aspartic acid carrier

Everything else by one or more other carriers

Question 14

What organic anions are actively reabsorbed?

Answer 14

Citrate

a-KG

B-hydroxybutyrate

Vit C

Question 15

How do the kidneys handle proteins?

Answer 15

Very small amount is filtered (0.6%)

Rebasorbed in the proximal tubule by endocytosis

Kidney is the major site of catabolism of many plasma proteins

Question 16

What factors cause the low filtration rate of proteins?

Answer 16

Steric hinderance

Viscous drag

Electrical Hinderance

Question 17

How are peptides reabsorbed?

Answer 17

Small linear peptides are completely filterable

They are catabolized to amino acids within the proximal tubule and then the amino acids are actively reabsorbed

Question 18

What are the renal effects of parathyroid hormone (PTH)?

Answer 18

Promotes calcitriol formation in the kidney

Increases Ca reabsorption

Increases phosphate excretion

Question 19

What are the three components of total plasma calcium?

Answer 19

Protein bound (40%) - does not filter

Ionized and biologically active (50%) - filters

Complexed with anions (10%) - filters

Question 20

What is the relationship between plasma H+ and ionized calcium?

Answer 20

As plasma H+ concentration, so does the concentration of ionized calcium

This reflects the availability of protein binding sites

In acidosis, more H+ gets buffered by protein, so bound calcium is displaced

Question 21

What is the effect of the ECF calcium concentration on kidney reabsorption of calcium?

Answer 21

Low [Ca] in the ECF causes the release of PTH from the parathyroid gland, resulting in an increase in calcium reabsorption in the kidney

The reverse is also true

Question 22

Describe calcium reabsorption in the proximal tubule

Answer 22

Paracellular pathway - driven by solvent drag

Transcellular pathway - uptake across the brush border via an ion channel and exits the basolateral membrane via Ca-ATPase

Question 23

Describe calcium reabsorption in the thick ascending limb of Henle

Answer 23

Paracellular pathway - driven by the transepithelial electrochemical gradient for Ca

Question 24

Describe calcium reabsorption in the distal tubule

Answer 24

Transcellular pathway - Ca enters the cell via ion channels

Inside the cell, Ca binds to calmodulin, and the complex diffuses across the cell to deliver Ca to the BL membrane

Ca is transported across the BL membrane by a 3Na - 1Ca antiporter

Question 25

How does the kidney handle phosphate?

Answer 25

90% of plasma phosphate is freely filtered

90% of the filtered load is actively reabsorbed

Linear relationship

Question 26

How is phosphate reabsorbed in the proximal tubule?

Answer 26

Apical transport - 3Na-1P symporter

Phosphate leaves the BL membrane via a P-anion antiporter

Question 27

How does PTH regulate phosphate homeostasis?

Answer 27

Inhibits reabsorption of phosphate by the proximal tubule

Stimulates the endocytic removal of NPT2 (phosphate symporter) from the apical membrane

Question 28

How does dietay phosphate intake regulate phosphate homeostasis?

Answer 28

Increased intake increases excretion and vice versa

Changes in intake alter the activity and number of NPT2 transporters