April 27, 2016 - Potassium Handling

Question 1

Serum Potassium and Action Potentials

Answer 1

Serum potassium has big influences on the resting membrane potential of the cells. Low levels of potassium will make the cell more negative, and higher levels of potassium will make the cell more positive.

This creates challenges with starting an action potential.

Sodium plays a role too, but only 1/100 of a role as the potassium levels.

Question 2

Regulators of Intracellular / Extracellular [K⁺ ]

Answer 2

There is a sodium-potassium ATPase which brings two potassium ions into the cell and brings three sodium ions out. This is stimulated by beta-2, inhibited by alpha-1, stimulated by aldosterone (more Na/K to the surface)

Question 3

Effect of Beta-2 on Potassium

Answer 3

Stimulates the sodium-potassium ATPase to work harder by phosphorylating it.

This results in more potassium being uptaken from the serum which will decrease the serum potassium levels.

Question 4

Effect of Alpha-1 on Potassium

Answer 4

De-phosphorylates the sodium-potassium ATPase which results in it working less effectively.

Less potassium enters the cell and it builds up in the serum, thereby increasing the level of potassium in the blood.

Question 5

Effect of Aldosterone on Potassium

Answer 5

Aldosterone increases the number of sodium-potassium ATPases on the surface of the cell. This results in more potassium being brought into the cell which will decrease the serum potassium.

Question 6

Effect of Digoxin on Potassium

Answer 6

Digoxin is a competitive potassium inhibitor for the sodium-potassium ATPase.

When digoxin binds to potassium, less can enter the cell and therefore potassium levels build up in the serum.

Question 7

Effect of Insulin on Potassium

Answer 7

Insulin stimulates the sodium-hydrogen exchanger on the cells which will result in more sodium entering the cell. This increase in sodium allows the sodium-potassium ATPase to work more and bring more potassium into the cell and out of the serum, causing the serum levels of potassium to drop.

Question 8

Effect on Potassium Levels on Potassium

Answer 8

Low serum potassium will cause potassium to passively leave the cells and increase the serum potassium level.

Question 9

Major Site of Potassium Regulation

Answer 9

In the cortical collecting duct.

Potassium is continually absorbed and reabsorbed, but ultimately we make the decision at the end as to how much we will keep.

Question 10

Principal Cell Physiology

Answer 10

To secrete calcium through the principal cell, we need 1) aldosterone, 2) sodium to bring into the cell, and 3) a negative charge (from chloride ions)

Basically, sodium enters the cell from the tubular lumen, leaving the negative charge behind. Potassium can then leave the parietal cell into the tubular lumen where it will be excreted. Potassium is brought into this cell from the capillaries through the 6 mechanisms talked about earlier, and then shunted into excretion.

Question 11

Trans-Tubular Potassium Gradient (TTKG)

Answer 11

TTKG = ( Urine_[K] x Serum_osm ) / ( Serum_[K] x Urine_osm )

If the patient is hyperkalemic and the TTKG is <7, this is abnormal and the principal cell is to blame.

If the patient is hypokalemic and the TTKG is >4, this is abnormal and hte principal cells it to blame.