Renal Transport Mechanisms

Question 1

HCO3- Reabsorption

Answer 1

Na+ coupled in the early proximal tubule. Indirect absorption because it does not transverse the apical membrane. The PT secretes H+ which combines with filtered HCO3- to form carbonic acid (H2CO3) which dissociates into CO2 and H20 and is reabsorbed as such. This is catalyzed by carbonic anhydrase. The Na+/H+ transporter is driven by a low [Na] gradient by the Na/K ATPase in the basolateral surface of the cell. The Na/HCO3 brings it from the cell to the blood.

Question 2

Acetazolamide

Answer 2

Carbonic anhydrase inhibitor. Cannot reabsorb HCO3- and therefore less water escapes the tubules.

Question 3

Sodium Co-Transporters

Answer 3

Na-Glucose, Na-amino acids, Na-lactate, and Na-inorganic phosphate. Early PT. Increasing ratios of Na to co-transporters in series, increasing the avidity of nutrient reabsorption along the length of Pt.

Question 4

Cl- in the PT

Answer 4

Anions are reabsorbed along the PT, so the Cl- stays and eventually the concentration difference drives the Cl- out of the tubes through the paracellular pathway. This continued gradient drives out even more Na+ and other cation paracellular later in the tubules.

Question 5

NaCl Reabsorption in the PT

Answer 5

Second half of the tubule. Apical Na/H and a Cl/Organic Anion exchanger. Basolateral Na/K ATPase creates the gradient for the Na/H. The Cl/organic anion is driven by the higher concentration of the organic anions in the cell. H+ also neutralize the organic anion in the lumen while the resulting cellular alkalinization aids in the dissociation of the acid into anion and H+ in the cell. The neutralized organic acids are lipophilic. Basolateral Na/K-ATPase and K/Cl cotransporter. Nonionic diffusion.

Question 6

Starling forces in the PT

Answer 6

The rate limiting step is the uptake of fluid from the interstitium to the blood because the apical membrane PT is so leaky. Changes in interstitial hydrostatic pressure and hydrostatic and oncotic pressure in the capillary have a significant impact of rate on fluid reabsorption.

Question 7

Glomerulotubular Balance

Answer 7

Regulation of salt and water. 1. Increase in filtration increases cotransporters for Na which increases water reabsorption. 2. When GFR changes because of efferent arteriolar tone, changes occur in Starling. 3. Bending of microvilli in the PT sense flow and this stimulates Na transport.

Question 8

PT hormonal and neural

Answer 8

Angiotensin II- Stimulates the Na/H exchanger. Sympathetic acts directly on PT cells and in severe volume depletion, increased sympathetic increases salt and water retention. Alpha-adrenergic receptors increase the apical Na/H exchanger.

Question 9

Dopamine

Answer 9

Inhibitor of the PT produced locally by the PT from L-Dopa. This is stimulated by ECFV expansion.

Question 10

ANP

Answer 10

Stretch in the atria.

Question 11

Xenobiotics

Answer 11

First line of defense is binding to albumin. This limits their filtration. Taken up actively in the basolateral membrane by high affinity transporters. Cleared at a rate comparable to blood flow. Electrochemical gradients and ATP efflux pumps.

Question 12

Protein and Oligopeptide in PT

Answer 12

Albumin is endocytosed and degraded and the amino acid are recycled in the blood. Smaller proteins were filtered and degraded into amino acids in the brush border. Renal elimination = metabolic clearance.

Question 13

Loop of Henle

Answer 13

Indispensable for elaborating both dilute as well as concentrated urine. It reabsorbs 25% of Na but only 15% of filtered water.

Question 14

Descending Limb

Answer 14

Water reabsorption is restricted. Follows a gradient in the interstitium created by the TAL.

Question 15

Ascending Limb

Answer 15

First segment in the nephron impermeable to water. Water cannot follow the salt. Salt reabsorption is both transcellular and paracellular. Na/H somewhat. Luminal Na/K/2Cl cotransporter is the main source. Recycle K through an apical K channel. The K+ makes it lumen positive, allowing more sodium reabsorption through paracellular. Also drive Ca++ and Mg++ this way.

Question 16

Regulation of the LH

Answer 16

Rate of Na reabsorption increases with increased salt delivery. Ca++ receptor that senses extracellular Ca++. This inhibits K+ channels. NO and prostaglandins also contribute.

Question 17

Macula Densa

Answer 17

Na/K/2Cl transport. Signals the NaCl load at the distal tubule and contributes to tubuglomerular feedback and renin.

Question 18

Distal Tubule

Answer 18

6% filtered NaCl entirely through the transcellular pathway. Enters via luminal Na/Cl cotransporter and leaves via Na/K ATPase while Cl- leaves on its own in a Cl- channel. DT is also impermeable to water. A problem with the cotransporter is called Gitelman syndrome, salt wasting. Thiazide diuretics.

Question 19

Collecting Duct

Answer 19

Final Na and H2O. 4% NaCl and 15% H2O. Principal cells. Epithelial Na channel. Chemical gradient and negative potential. Can render the urine Na free. Exits via Na/K ATPase. K-sparing diuretics.

Question 20

Regulation of DT

Answer 20

Chronic. Upregulation of NaCl cotransporter. Aldosterone increases Na reabsorption in the DT by upregulation of NaCl cotransporter.

Question 21

Regulation of CD

Answer 21

Hormonal regulation. Aldosterone stimulates Na reabsorption. ANP and urodilatin inhibit reabsorption. Can be water permeable via aquaporin II. ADH stimulates them.

Question 22

Urea Transport

Answer 22

Freely filtered. Passive reabsorption and secretion. 50% filtered by the PCT by solvent drag and paracellular absorption. Thin limb of the LH has high urea permeability and it diffuses into the tube from the medullary interstitial. ADH increases its permeability in the inner meduallary CD, making it reabsorbed. High water 70% low water 15% secreted.

Question 23

Proximal Tubule

Answer 23

2/3 filtered water and Na, 80% bicarbonate and phosphate, and all nutrients. Secretion of xenobiotics. Permeability is large (expression of water channels) and water follows out the nutrients.