Exam #4: Potassium Regulation

Question 1

Define secretion in terms of net movement of a solute.

Answer 1

Renal secretion= movement of solute from blood to the lumen of the nephron

Question 2

Explain why all organic acids competitively inhibit secretion of other organic acids.

Answer 2

Organic acids use the same carrier mediated proteins for transport; the presence of one organic acid inhibits the secretion of the others

Question 3

Why do organic bases competitively inhibit secretion of all other organic bases.

Answer 3

Organic bases use the same carrier mediated proteins for transport; the presence of one organic base inhibits the secretion of the others

Question 4

Explain the effect of low urine pH on the net secretion.

Answer 4

Low urine pH decreases net secretion

Question 5

Explain the effect of low urine pH on the net excretion.

Answer 5

Low urine pH decreases net excretion

Question 6

Explain the effect of low urine pH on plasma concentration of organic acids.

Answer 6

• Low urine pH= high H+ concentration (e.g. acidosis)
• Organic acids e.g. ASA, and anions & bind free H+ in acidic urine
• Organic acids bound to H+ are more permeable
• These bound acids LEAK BACK INTO THE CIRCULATION

Thus, low urine pH increases plasma concentration of organic acids

Question 7

Explain why alkalinizing the urine (by ingesting bicarbonate) would be helpful in treating an aspirin overdose.

Answer 7

• Aspirin is an organic acid i.e. organic anion
• Low urine pH causes association of ASA w/ H+, making ASA-H more permeable–>reabsorption into the blood

Conversely, high urine pH (alkaline) causes dissociation of ASA & H, making ASA more likely to stay in the urine and be excreted

Question 8

Explain why an excretion rate that exceeds the filtration rate for a compound means that the compound was secreted.

Answer 8

Excretion= filtration + secretion

Thus, if a compound’s excretion rate is greater than its filtration rate alone, then it must have also been secreted

Question 9

Explain why the clearance of PAH is a measure of the effective renal plasma flow (ERPF).

Answer 9

• PAH is removed from renal circulation (via filtration & secretion) with nearly 0 PAH appearing in the renal vein.
• Clearance= volume/time from which all of substance has been removed
• Thus, clearance of PAH= volume of plasma that entered the kidney per unit time i.e. effective renal plasma flow, or ERPF

Question 10

How do you calculate ERPF when given the plasma and urinary concentrations of PAH and the rate of urine production? Write the necessary equations.

Answer 10

N/A

Question 11

How do you calculate the filtration fraction when given GFR and ERPF?

Answer 11

This is the fraction of plasma delivered to the kidneys that is filtered

FF= GFR/ERPF

Question 12

Explain why the nephron segments upstream of the distal tubule do not contribute to the regulation of potassium balance.

Answer 12

• The majority of K+ is reabsorbed in the proximal tubule regardless of ECF K+ concentration
• Similarly, what hasn’t been reabsorbed in the proximal tubule is reabsorbed in the thick ascending limb–Loop of Henle (~90% of filtered load)
• Distal tubule is where changes in reabsorption & secretion occur based on ECF K+

Question 13

Describe the effect of hyperkalemia on aldosterone secretion.

Answer 13

High plasma K+ increases secretion of Aldosterone from the adrenal cortex.

Question 14

Describe the effect of increased plasma potassium on the Na+/K+ATPase pump on the basolateral membrane of the principal cells in the late distal tubule (second half of distal tubule) and collecting duct/tubule.

Answer 14

• High plasma K+ increases the Na+/K+ ATPase on the basolateral surface of the principal cells in the late distal tubule and collecting duct
• Increasing the Na+/K+ ATPase increases K+ into the cell from the blood
• K+ flows down its electrochemical gradient and is secreted into the lumen

Question 15

What is the probable effect of aldosterone on the Na+/K+ ATPase pump?

Answer 15

Aldosterone increases the effect of the Na+/K+ ATPase, causing increased secretion of K+

Question 16

What effects does aldosterone have on the collecting ducts?

Answer 16

Aldosterone increases the number of epithelial sodium channels (ENaCs) in the collecting ducts, which increases the reabsorption of Na+ & secretion of K+

Question 17

Describe the effect of increased aldosterone and increased potassium on the reabsorption or secretion of potassium in the late distal tubule (second half of distal tubule) and the collecting duct.

Answer 17

• Hyperkalemia & increased aldosterone increase the Na+/K+ ATPase on the basolateral side of the principal cells in the late distal tubule & collecting duct.
• K+ concentration in the cell is elevated
• K+ flows down its concentration and into the tubular lumen
• K+ is secreted

Question 18

Explain why diuretics which inhibit sodium and water reabsorption can cause the depletion of body potassium.

Answer 18

Diuretics increase excretion of K+

• Loop block the Na+/2Cl-/K+ pumps in the thick ascending limb of the loop of henle
• Thiazide diuretics inhibit NaCl reabsorption in the early distal tubule, which increases the rate of K+ secretion

Question 19

State the effect of alkalosis on potassium.

Answer 19

Like hyperkalemia & aldosterone, metabolic alkalosis stimulates the Na+/K+ ATPase on the basolateral membrane of the principal cells in the late distal tubule/ collecting ducts.
–> HYPOKALEMIA

Question 20

Does alkalosis cause hypokalemia or hyperkalemia?

Answer 20

Hypokalemia