Bioc L12 Kidney

Question 1

What are the five main anatomical regions of the renal nephron?

Answer 1

Glomerulus: blood filtration

Proximal convoluted tubule: Reabsorption, Ammoniagenesis, Gluconeogenesis (RAG)

Loops of Henle (descending and ascending limbs): establish salt gradient

Distal convoluted tubule: fine tuning electrolyte reabsorption, buffer urine pH

Collecting duct: water balance, concentrate urine

Question 2

What are the three barriers that make up the filtration membrane?

Answer 2

fenestrated capillary endothelium

filtration slits

basement membrane

Question 3

What are the criteria for general glomerular filtration?

Answer 3

compounds that are both low in molecular weight and are water-soluble are filtered from the blood at the glomerulus

Question 4

What are the criteria for protein glomerular filtration?

Answer 4

In order of importance:

Molecular weight: compounds less than 5,000 D are easily filtered, where as proteins that are above 75,000 D are not filtered. Filtration rates decrease with larger weights

Net charge: the negatively charged GAG in glycocalyx and basal lamina of the glomerulus impede the passage of negatively charged proteins. Neutral proteins of a given molecular weight are more easily filtered than negatively charged proteins of a similar size. * albumin is negatively charged

Shape or configuration: flexible coils are more easily filtered than rigid spheres

Question 5

Explain the steps of resorption of Renal-Filtered Protein at Proximal Tubule.

Answer 5

1. Proteins that meet size, charge, and shape criteria are filtered at glomerulus.
2. Filtered proteins are reabsorbed in the proximal tubule cells via receptor-mediated endocytosis
3. Following endocytosis, phagosomes merge with lysosomes to form phagolysosomes where reabsorbed proteins are degraded to free amino acids which are released into circulation
4. Some intact protein may escape tubular reabsorption, this amount increases as more protein is filtered or tubular function is compromised (increased protein in urine)

Question 6

How are insulin and glucagon processed in the kidney?

Answer 6

In healthy humans, insulin (5,800 D) and glucagon (3,500 D) are filtered at the glomerulus but are never returned to the bloodstream and only insignificant quantities are lost in the urine

Question 7

What is Proteinuria?

Answer 7

Also called albuninuria, a hallmark of renal disease which can result from diabetes and causes inflammation in the kidneys

A condition in which urine contains an abnormal amount of protein

Normally a small amount of albumin does enter the filtrate but less than 1% of this filtered amount is excreted in the urine

Albumin is 69,000 D, just below the size selectivity for filtration and it is also highly negative, cutting down on its filtration

Question 8

What are three factors that contribute to Proteinuria?

Answer 8

Amount of protein presented to glomerulus (limited to amount of protein that can be filtered)

Defects in the slit diaphragm

Efficiency of proximal tubule reabsorption of filtered proteins is compromised

Question 9

What is Diabetic Microalbuminuria?

Answer 9

The heparan sulfate content of the glomerular basal lamina and the glycocalyx is diminished as a consequence of endothelial cell dysfucntion specifically due to hyperglycemia and the subsequent increase in the production ROS

The loss in charge selectivity (not as negative) of the filtration barrier results in an increased filtration rate for albumin

The increased filtration rate for albumin exceeds the kidney’s capacity to reabsorb the albumin resulting in albuminuria

Question 10

What is Haptoglobin (HP) and what is its main purpose?

Answer 10

It is a plasma alpha2-globulin which binds to free hemoglobin or myoglobin in a one to one ratio

Plasma haptoglobin prevents hemoglobin or myoglobin-mediated nephrotoxicity and iron loss following intravascular hemolysis or acute muscle injury, respectively

Question 11

What is the purpose of the HP-Hb complex and HP-Mb complexes?

Answer 11

They are too large to be filtered at the kidney, making sure there is no toxicity in the body

They are taken up by macrophages where the iron is stored

When the concentration of Hb and Mb exceed the binding capacity of serum haptoglobin, the circulating hemoglobin and/or myoglobin is filtered at the glomerulus

Filtered Hb and Mb are nehprotoxic

Toxicity may be due to the iron released during heme catabolism which can catalyze the formation of ROS

Question 12

Explain reabsorption in the proximal convoluted tubule.

Answer 12

PCT reabsorbs 99% of all filtered glucose and amino acids and 85% of NaCl and HCO3-. Water follows the solute reabsorption

The Na+ dependent resorption of glucose and amino acids will activate the Na+K+ ATPase in PCT

The resulting increase in ADP will activate ATP synthesis and Na+ is actively transported out

All reabsorption of glucose and amino acids is transcellular

Question 13

What pathway is missing in the proximal tubules?

Answer 13

GLYCOLYSIS!

The lack of glycolysis in the PCT is due to the near absense of the three irreversible enzymes: Hexokinase, PFK1, and pyruvate kinase

This means that the proximal tubule is unable to catabolize glucose during fed or fasting state

PCT therefore uses fatty acids, ketones, and glutamine in the fed state

Question 14

How does the proximal convuluted tubule cells increase their absoprtive capacity?

Answer 14

The apical surfaces of these cells have abundant microvilli

Question 15

What are the advantages of not having glycolysis of the proximal tubule?

Answer 15

HK activity would result in trapping glucose in the cell sof PCT, while the goal of resorption is just opposite to more glucose through the cell

Transcellular reabsorption of glucose proceeds without glucose utilization thereby conserving a limited resource

This is very beneficial during the post-absorptive period and while fasting when glucose is in short supply

If PCT used glucose, protein degradation must occur to supply glucose, which would be very costly

Overall, the lack of of glycolysis minimizes any regulatory issues that would arise because gluconeogenesis is located in teh same cells

Question 16

What are the disadvantages of not having glycolysis of the proximal tubule?

Answer 16

Unable to generate ATP when anaerobic conditions prevail

With infection, septic shock results in a decrease in renal perfusion. Renal failure is often a consequence of septic shock

Question 17

How does the kidney buffer urine?

Answer 17

Buffering urine facilitates acid excretion. The addition of ammonia to urine helps prevent urinary pH form dropping below 4.5

Normally the distal convoluted tubule is primarily responsible for correcting acid-base balance.

In starvation and acidosis, the PCT plays the largest role in buffering urinary acid, however, the distal convoluted tubule still plays a role in correcting the final urinary pH.

Question 18

What is ammoniagenesis?

What are the two main enzymes involved?

Answer 18

Ammoniagenesis pathway generates the ammonia used to buffer urinary acid.

Pathway degrades one glutamine to α-ketoglutarate and generates two ammonia ions.

The two enzymes in ammoniagenesis:

glutaminase: Strips amine group

glutamate dehydrogenase: Removes nitrogen from glutamate; generates 2nd nitrogen. alpha-ketoglutarate then enters the TCA cycle and provides an important percentage of the kidney’s energy needs: Fed (15%); Fasting (25%); Acidosis (40%)

During metabolic acidosis, kidney is the predominant site for glutamine uptake and catabolism.

Question 19

How is renal ammoniagenesis regulated?

Answer 19

Renal ammoniagenesis is increase by 50% within 24 hours of developing acidosis

Three enzymes in the PCT are regulated by increasing [H+] leading to an activation of ammoniagenesis.

Acidosis increases gene expression of of glutaminase and glutamate dehydrogenase by 3 to 20-fold.

Hydrogen ions act as a positive allosteric effector of α-ketoglutarate dehydrogenase

The activation α-ketoglutarate dehydrogenase ensures that the α-ketoglutarate generated from ammoniagenesis enters the TCA cycle.

Question 20

Why is fasting and/or starvation associated with an increase in ammoniagenesis?

Answer 20

Ketone bodies resulting from ketogenesis increases the acid load being excreted in the kidney

Question 21

What is the urea cycle?

What are the two mechanisms for ammonia removal?

Answer 21

The urea cycle is located at the vascular inflow region of the liver (periportal hepatocytes) while glutamine synthesis is located at the vascular outflow region of the liver (perivenous hepatocytes).

The two mechanisms for ammonia removal, urea cycle and glutamine synthesis, provides a fail-safe system in which a high-capacity, low-affinity system (urea cycle) is followed by a low-capacity, high-affinity system (glutamine synthesis).

Question 22

How is urea synthesis and glutamine balanced?

Answer 22

Normally, urea synthesis is more predominant than glutamine synthesis.

During fasting and/or an acidosis, there is a decrease in urea synthesis and a proportional increase in glutamine synthesis (“Mop-up” function).

CPS-I (carbamoylphosphate synthetase; rate-limiting step in urea cycle) is inhibited by acid (e.g., increase in H+) which results in less urea synthesis.

The decrease in urea synthesis also saves 4 high energy phosphates/urea, which is helpful when fasting.

Activity of glutamine synthetase (in perivenous hepatocytes) increases as ammonia levels increase.

Question 23

Site of gluconeogenesis is shifted from liver to kidney during fasting and/or acidosis

Answer 23

The extra glutamine made in the liver is used in the kidney for ammoniagenesis (buffer acidic urine) and gluconeogenesis

Renal ammoniagenesis results in ammonia being released directly into urine, thus reducing the need for urea synthesis

During acidosis, the rate of hepatic amino acid catabolism will remain constant.

However, the carbon skeletons are diverted from hepatic gluconeogenesis and are used to make more glutamine which is used for gluconeogenesis and ammoniagenesis in the kidney

Question 24

How does renal amino acid catabolism supplies much of energy needed for gluconeogenesis?

Answer 24

The carbon skeleton of glutamine enters the TCA cycle as α-ketoglutarate and moves on to oxaloacetate before entering gluconeogenesis.

In the TCA cycle, the α-ketoglutarate generates two NADH, one FADH2 and one GTP (equivalent to 9 high energy phosphates).

From oxaloacetate, only 4 high energy phosphates are needed to make glucose.

vs. from pyruvate, 6 ATP are needed

Hepatic gluconeogenesis is an ATP-consuming process when starting with lactate, alanine, pyruvate or glycerol

Question 25

How is renal gluconeogenesis regulated?

Answer 25

PEPCK (PEP carboxykinase) catalyzes the rate-limiting step in renal gluconeogenesis.

Acidosis and cortisol increase gene expression of PEPCK.

Lack of glycolysis decreases importance of fructose-1,6-bisphosphatase in the regulation of renal gluconeogenesis.

Recall no PFK1.

Overall total glucose production via gluconeogenesis does not change even though location of gluconeogensis does change.

Question 26

How is the kidney divided into regions?

Answer 26

Kidney is divided into two regions, cortex and medulla.

Medulla is further divided into the inner and outer medulla :

Inner medulla is fairly anaerobic

Outer medulla is aerobic

The further delineation of the medulla into outer and inner regions is based on differences in oxygen and mitochondria density which then limit pathways that can operate in these conditions

Question 27

How is mitochonidrial density determined by oxygen density in the kidney?

Answer 27

Renal cortical pO2 is in the range of 40 to 50 mm Hg.

Renal cortex and outer medulla is second only to the heart with respect to oxygen consumption and the number of mitochondria.

Inner medullary pO2 is in the range of 10 mm Hg due to limited blood flow.

Mitochodirial density of inner medulla is ≤20% of that of cortex or outer medulla

Percentages in diagram to the left represents % of cell volume occupied by mitochondria

Question 28

What is the preminant metabolic pathway in the thin loop of henle?

What does the decreased circulation in the inner medulla imply?

Answer 28

The thin loop of Henle, especially that portion located in the inner medulla, is primarily limited to anaerobic glycolysis in both the fed and fasted state.

The decreased circulation in the inner medulla implies that glucose might also be limiting. This is not the case as most of the glucose used by cells in the inner medulla is obtained from the luminal contents of the nephron (reabsorption of glucose is not 100% in the PCT)

Question 29

What is the fuel preference in the proximal tubule?

Answer 29

fatty acids and glutatime

Question 30

What is the fuel preference in the thin loop of henle/inner medulla?

Answer 30

Glucose, because of low O2 and not enough mitochondria

Question 31

What is the fuel preference in the thick ascending loop of henle?

Answer 31

Glucose in fed state;
fatty acids and ketones during fasted state