Potassium

Introduction

Potassium is carefully regulated in the body - the consequences of altered Potassium levels are significant, including:

reduced concentration of potassium in the ECF leads to plasma membranes hyperpolarization resulting in decreased firing of action potentials. This causes skeletal muscle weakness and cardiac abnormalities.
increased concentration of potassium in the ECF leads to membrane depolarisation which is inappropriately triggered by action potentials. This can make the membrane insensitive to further stimulation causing cardiac abnormalities.

Sources of Potassium

Potassium is absorbed via passive diffusion from the small intestine and via active transport from the colon. It is regulated efficiently by aldosterone levels and recovery from cellular breakdown during haemolysis or tissue damage.

Methods of Control

The K⁺ in the ECF only represents a very small amount of the total K⁺ in the body; however its concentration is maintained within very strict parameters. The homeostasis of K⁺ is managed by three routes:

Cellular translocation - this is the main method of control; it is an acute response that triggers Potassium movement either into or out of the cells.
Renal excretion - this method makes up 90% of the chronic response (takes 4-6 hours to respond). It allows fine control and is regulated by aldosterone
GI excretion - this route makes up the other 10% of the chronic response and becomes significant in cases of renal failure. This response is also influenced by aldosterone

Cellular Translocation

Vital for rapid control of potassium loads
Helps control plasma concentration
Moves potassium into the cell
Stores potassium in skeletal muscle and liver
Balances ECF and ICF
Controlled by insulin and beta2 adrenoreceptors
- Increases the activity of Na⁺ / K⁺ ATPases causing sodium efflux and potassium influx

Renal Control

Potassium ions are reabsorbed and secreted at different points along the nephron
Active reabsorption of potassium occurs along the proximal tubule (70%) and along the ascending limb of the Loop of Henle (10-20%)
This results in there only being 10% of the original amount left in the distal tubule
However net reabsorption / secretion of potassium occurs in the distal tubule and first part of collecting duct
- Depends on bodies need
Under the influence of aldosterone
This is where the amount of potassium excreted is determined
Reabsorption occurs in the final part of the collecting duct

Potassium and Aldosterone

Aldosterone is the most important regulator of potassium
It causes increased secretion of potassium
Increased potassium directly stimulates Aldosterone secretion
Increases the activity and number of Na⁺ / K⁺ ATPase in basolateral membranes of the principal cells in the collecting duct and distal tubule
Potassium moves into the cells and is then excreted down an electro-chemical gradient

Factors Influencing Potassium Excretion

1.Sodium: High sodium = increased potassium excretion and:

More sodium into cells
Increased Na⁺ / K⁺ ATPase
Pumps sodium into peritubular renal interstitium
The resulting increased cellular uptake of potassium results in it moving down the electrochemical gradient into the nephron

2.Potassium: High potassium = increased potassium excretion which triggers the release of aldosterone.