Renal IX: Renal Regulation of Electrolytes

Question 1

Which cells secrete K+ and which reabsorb K+?

Answer 1

principal cells secrete it and alpha-intercalated cells reabsorb it

Question 2

Why can K+ excretion vary from 1%-110% of the filtered load?

Answer 2

This is because it depends on many factors, like dietary intake, aldosterone levels, and acid-base status. K+ excretion is able to exceed 100% because of secretion at the distal tubule and collecting duct (secretion adds to the filtered component).

Question 3

What are the normal percentages for K+ reabsorption and K+ secretion?

Answer 3

85% reabsorbed, 15% excreted

Question 4

What happens under conditions of potassium depletion in the body?

Answer 4

no K+ is secreted into the tubules– it is all reabsorbed and only 1% is excreted

Question 5

What are physiological substances that contribute to maintaining plasma [K+] constant?

Answer 5

epinephrine (catecholamines), insulin, aldosterone

Question 6

What are examples of drugs that induce hyperkalemia?

Answer 6

dietary K+ supplements, ACE inhibitors, K+ sparing diuretics

Question 7

Why does plasma [Ca2+] decrease when plasma phosphate increases?

Answer 7

Phosphate binds up free Ca2+ in the plasma, thus decreasing its concentration in the plasma. This can be damaging to our bones, which is why the body must compensate by increasing [Ca2+].

Question 8

Is there more phosphate in the ICF or ECF and why?

Answer 8

There is more in the ICF, as that is where our nucleotides and metabolic pathways are (and phosphate is an important component of DNA, RNA, nucleotides, and metabolic pathway intermediates).

Question 9

What is the most important hormone controlling phosphate excretion?

Answer 9

PTH! It inhibits phosphate reabsorption by the proximal tubule, thereby increasing phosphate excretion by the kidneys.

Question 10

How is elevated plasma phosphate related to calcitriol production?

Answer 10

It suppresses calcitriol production, resulting in a decrease in intestinal phosphate reabsorption into the blood stream (and, therefore, decreased plasma phosphate).

Question 11

What is the effect of FGF-23?

Answer 11

It inhibits phosphate reabsorption into the blood and also inhibits calcitriol production by the PT. Thus, FGF-23 is released in response to hyperphosphatemia and works to decrease the plasma [phosphate].

Question 12

98% of our body’s K+ is located where?

Answer 12

within cells (ICF) – only 2% is located in the ECF

Question 13

What happens under conditions of increased dietary K+, and how can this affect the body?

Answer 13

Increased dietary potassium will enhance K+ excretion to 80% (remember: normal excretion is only 15%). This can also lead to hyperkalemia (too much K+ in ECF), which may cause cardiac arrhythmias.

Question 14

What is the most important hormone that shifts K+ into cells?

Answer 14

insulin (especially postprandial in the liver)

Question 15

What is the range for potassium excretion (% of the filtered load)?

Answer 15

1-110%

Question 16

What happens to calcium in the body in acidosis vs. alkalosis?

Answer 16

• acidosis: increase in ionized [Ca2+]

- alkalosis: decrease in ionized [Ca2+]

Question 17

What are two substances we talked about that protect against low [Ca2+]?

Answer 17

calcitriol and parathyroid hormone (PTH)

Question 18

How do calcitriol and PTH affect the kidneys?

Answer 18

They increase reabsorption of Ca2+, thus decreasing excretion into the urine and overall increasing [Ca2+] in the blood.

Question 19

How does phosphate affect acid-base balance in the body?

Answer 19

Urinary phosphate is an important buffer (ie, it is one of the many titratable acids).

Question 20

How do calcitriol and PTH affect phosphate vs. calcium?

Answer 20

• phosphate: calcitriol increases [Pi] by increasing reabsorption into the blood stream in the intestines and kidneys, while PTH decreases [Pi] by inhibiting Pi reabsorption by the PT and thus increasing Pi excretion
• calcium: both calcitriol and PTH increase [Ca2+] in the blood