Potassium

 

Potassium is the major intracellular cation, playing a critical role in the membrane potential of excitable cells. The body contains about 3500 mEq of potassium.


Intracellular [K+] is about 150 mM, while normal plasma [K+] ranges from 3.5-5 mM/L. Accordingly, only 2% of total body potassium is outside cells.

 

Plasma [K] is a poor indicator of total body stores.

 

 

Potassium Intake

 

Bananas are the best known source of potassium. Others include:

One choice= 3 mmol

Between 40-120 mmol of K+ is taken up through the gut daily.

 

 

Potassium Functions in the Body

Potassium is a key excitatory signal.

The K+ gradient - ratio of intracellular to excellular potassium - ie is the major determinant of cell membrane resting potential.

Potassium enters cells using a pump-leak mechanism, whereby they are pumped in through the Na/K ATPase and leak out through various channels. Potassium concentration can be controlled by changing pump activity, pump number, or channel permeability.

 

Nervous System

 

Cardiovascular System

Potassium is responsible for repolarizing cardiac pacemaker and muscle cells. Following influx of sodium and calcium during depolarizing, potassium flow down its concentration gradient and out of the cell restores intracellular negativity.

 

 

 

 

Regulation of Potassium Levels

 

Cellular Homeostasis

In response to dietary potassium, rapid removal from the extracellular space is necessary to prevent life-threatening hyperkalemia.

Potassium can easily move across membranes, and there are several mechanisms that regulate its transport:

Increases K+ Entry

Decreases K+ Entry

  • insulin
  • aldosterone
  • beta-adrenergic stimulation
  • alkalosis
  • glucagon
  • alpha-adrenergic stimulation
  • acidosis

 

Renal filtration and secretion

Average daily intake is 40-120 mmol. As plasma levels are 3.5-5 mmol, and the kidneys filter 180L daily, alomst 800 mmol of K+ is filtered. To maintain its balance, the kidney only excretes 5-15% of filtered K+.

Potassium, similar to other electrolytes, is freely filtered through the glomerulus. Almost all potassium is reabsorbed by the time filtrate reaches the collecting tubules. Potassium destined to be excreted is secreted into the collecting duct.

All cell types have potassium channels, which vary according to cell type. Each is a tetramer. Tubular cells habe different pumps and channels on their apical and basolateral aspects. The major driving force is the Na/K ATPase.

The ROMK channel is present along the kidney, save the proximal tubule. They tend to be open and allow potassium to flow out of the cell, allowing for secretion in the principal cells of the cortical collecting ducts.

Site

Mechanism

%

Inhibitor

Stimulator

proximal

unknown channels; may follow Na and water paracellularly

 

65

  

 

thick ascending loop

Na/K/Cl2 triporter

significant paracellular movement driven by +ve potential

30

furosemide

 

distal

sodium-dependent

5

 

  

collecting duct

principal cell secretion

variable

 

 

 

90% eliminated through the kidney and 10% in the stool.

 

The Na/K ATPase drives potassium secretion in principal cells by pumping basolaterally, allowing apical secretion through K+ channels or chloride co-transporters. Sodium reabsorption results in negative lumenal potential, promoting potassium secretion.

Potassium secretion into the lumen will only continue if filtrate concentration is kept low. Accordingly, the higher the flow rate, the higher the level of K+ secretion down its concentration gradient into the lumen.

Excretion is also increased by:

Excretion is decreased by