Renal IIIb

Question 1

About ___ of total body potassium is located within cells because of the Na-K-ATPase pump

Answer 1

98%

Question 2

The Na-K-ATPase pump actively transport potassium ___ cells.

Answer 2

into

Question 3

What can occur because the amount of potassium in the ECF is so small?

Answer 3

even very small changes in potassium into or out of cells can produce large changes in extracellular potassium concentration

Question 4

EC K+ concentration in humans is….

Answer 4

~3.5-5.7 mM

Question 5

Why is it important to maintain EC concentration of potassium relatively constant?

Answer 5

because of the role of potassium in nerve and muscle excitability

Question 6

Extracellular potassium is a function of 2 variables:

Answer 6

• total amount of potassium in the body → function of amount of potassium ingested and excreted
• distribution of potassium between extracellular and intracellular fluid compartments → regulated by renal and non-renal mechanisms

Question 7

What process regulated the amount of Na in the body?

Answer 7

reabsorptive process

Question 8

What process regulated the amount of K in the body?

Answer 8

net secretory process

⇡ K+ → ⇡ aldosterone → ⇡ K secretion

Question 9

Amount of potassium excreted in urine by a normal person eating a typical American diet is…

Answer 9

5-15% of filtered quantity

Question 10

Reabsorption of potassium by the kidney occurs in….

Answer 10

proximal tubule (65%)

thick ascending limb of the Loop of Henle (25%)

medullary collecting duct (5%)

Question 11

How is K+ reabsorbed in the proximal tubule?

Answer 11

mostly paracellularly

can be transcellular but the mechanisms hasn’t been established

Question 12

How is K+ reabsorbed in the thick ascending limb of the Loop of Henle?

Answer 12

via the Na-K-2Cl cotransporter

and paracellular mechanisms

Question 13

How is K+ reabsorbed in the collecting duct?

Answer 13

in the α-intercalated cells there is a H+-K+-ATPase countertransporter which exchanges H+ with K+

Question 14

What happens to potassium in the cortical collecting duct?

Answer 14

it is secreted under normal circumstances

Question 15

___ secrete potassium while ___ reabsorb potassium

Answer 15

Principal cells; α-intercalated

Question 16

What happens to principal cells in the cortical collecting duct during periods of potassium depletion?

Answer 16

they cease secretion of potassium

Question 17

What mainly regulates changes in potassium excretion?

Answer 17

the cortical collecting duct

Question 18

Regulation of K+ secretion by the Cortical Collecting Duct:

Answer 18

• Plasma K+ concentration
• Aldosterone
• Flow of Tubular fluid
• Diuretics
• Acid-base balance

Question 19

What is the critical event for potassium secretion in cortical collecting duct? What does it do?

Answer 19

active transport of potassium from interstitial fluid across the basolateral membrane into the cell, mediated by the basolateral Na-K-ATPase pumps

it creates a concentration gradient for potassium to diffuse into the lumen

Question 20

Regulation of K+ secretion by the Cortical Collecting Duct: Plasma K+ concentration

Answer 20

high potassium diet → ⇡ plasma potassium (⇡ amount of K+ in blood) → drives uptake of K+ via Na-K-ATPase → ⇡ K+ in cell → drives K+ into tubular lumen

Question 21

Regulation of K+ secretion by the Cortical Collecting Duct: Aldosterone

Answer 21

Aldosterone enhances tubular secretion of potassium

Question 22

How does aldosterone work to enhance tubular secretion of potassium?

Answer 22

Aldosterone secreting cells in the adrenal cortex are sensitive to EC K+ concentration → ⇡ in plasma potassium stimulates aldosterone secretion → stimulates potassium secretion by the principal cells of the cortical collecting duct

Question 23

Anything with a large impact on aldosterone will….

Answer 23

have an impact on K+ secretion and excretion

Question 24

Regulation of K+ secretion by the Cortical Collecting Duct: Flow of Tubular Fluid

Answer 24

⇡ flow rate to the cortical collecting duct → ⇡ K+ secretion in the cortical collecting tubule

B/C ⇡ flow rate → tends to dilute K+ concentration in the luminal fluid - enhances K+ gradient for secretion

⇡ Flow rate → ⇡ K+ secretion → ⇡ K+ excretion

⇡ Flow rate → ⇡ Na reabsorbed → ⇡ K+ secretion

Question 25

Regulation of K+ secretion by the Cortical Collecting Duct: Acid-Base Balance

Answer 25

Alkalosis stimulates Na-K-ATPase activity in the cortical collecting duct → K+ levels ⇡ within the principal cells → secretion via K+ channels into the tubule lumen increases

Question 26

What happens to K+ in patients suffering from alkalosis induced by vomiting?

Answer 26

there is ⇡ K+ secretion and hence excretion

Question 27

Regulation of K+ secretion by the Cortical Collecting Duct: Diuretics

Answer 27

diuretics → ⇡ Flow rate → always ⇡ K+ secretion and excretion

Question 28

Relationship between sodium excretion and potassium secretion mechanisms

Answer 28

changes in Na reabsorption mechanisms are not coupled with K+ secretion mechanisms

Question 29

Why are changes in Na reabsorption mechanisms are not coupled with K+ secretion mechanisms?

⇡⇣

Answer 29

high sodium diet → ⇡ ECF → ⇣ sympathetic activity and renal Ang II → ⇣ aldosterone secretion → ⇣ K+ secretion at the cortical collecting ducts

high sodium diet → ⇡ ECF → ⇡ GFR → ⇡ fluid delivery to cortical collecting ducts → ⇡ K+ secretion at the cortical collecting ducts

overall effect is that K+ secretion is unchanged

Question 30

Renal Calcium regulation

Answer 30

calcium is reabsorbed paracellularly down its electrochemical gradient in the thick ascending limb of the loop of henle

Ca is reabsorbed via calcium channels in the distal tubule

Question 31

What occurs in response to hypocalcemia (⇣ Ca in blood)?

Answer 31

Parathyroid hormone (PTH; released by parathyroid gland) is released and acts to increase calcium reabsorption at the level of the distal tubule

Question 32

Renal Phosphate Regulation

Answer 32

Phosphate that is not bound to plasma proteins is filtered across glomerular capillaries

85% of phosphate is reabsorbed in the proximal tubules

15% is excreted

Question 33

How does PTH regulate reabsorption of phosphate in the proximal tubule?

Answer 33

by inhibiting Na+-phosphate cotransport, thereby decreasing the phosphate reabsorption

Question 34

PTH

Answer 34

increases Ca reabsorption in the distal tubule

decreases Phosphate reabsorption in proximal tubule (inhibits phosphate reabsorption)

Question 35

Renal Mg reabsorption

Answer 35

95% reabsorption of Mg

30% in proximal tubules

60% in thick ascending limb of loop of henle

5% in distal tubule

reabsorbed paracellularly down its electrochemical gradient in the thick ascending limb of the loop of henle

Question 36

Proteins and Peptides

Answer 36

binding of proteins to specific receptors then endocytosis

Intracellular vesicles merge with lysosomes whose enzymes degrade protein to low molecular weight fragments (amino acids)

End products the exit the cells across the basolateral membrane into the interstitial fluid from which they enter peritubular capillaries

Question 37

Active Proximal Tubule Secretion of Organic Anions

Answer 37

Active secretory pathway for organic anions in proximal tubule has relatively low specificity → single kind of transporter is responsible for secretion of all organic anions

Question 38

Active Proximal Tubule Secretion of Organic Anions: PAH

Answer 38

PAH is actively transported into proximal tubular cell across the basolateral membrane → the resulting intracellular concentration then provides the gradient for facilitated diffusion of PAH across the luminal membrane into the tubular lumen

Question 39

Renal handling of Organic cations

Answer 39

transported across the basolateral membrane by facilitated diffusion mechanism → drive by negative potential difference across the basolateral membrane

transport across the luminal membrane is mediated by OC+-H+ antiporter → antiporter is nonspecific and other organic cations can compete with it