Potassium

Potassium is generally in the range of 2 to 8 mmol/l. Hypokalaemia in reptiles will occur from inadequate intake or excessive loss (diarrhoea). In mammals hyperkalaemia with excessive potassium intake, decresed secretion or shift from intracellular to extracellular fluid (e.g.severe acidosis).

Potassium

Importance of Regulation

Decreased Extracellular Potassium

If the concentration of potassium in the ECF is reduced then the plasma membranes hyperpolarize resulting in decreased firing of action potentials. This causes skeletal muscle weakness and cardiac abnormalities.

Increased Extracellular Potassium

In this state the membrane is depolarised and is inappropriately triggered by action potentials. This can make the membrane insensitive to further stimulation causing cardiac abnormalities.

Sources

• Potassium is absorbed via passive diffusion from the small intestine
• Also via active transport from the colon
  • Affected by Aldosterone
• Highly efficient
• It's recovered from cellular breakdown
  • Haemolysis
  • 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

• Potassium is moved either into or out of the cells
• Acute response
• Main method of control

Renal excretion

• Makes up 90% of the chronic response
• Takes 4-6 hours to respond
• Allows fine control
• Influenced by Aldosterone

GI excretion

• Makes up the other 10% of the chronic response
• Also influenced by Aldosterone
• Most important in renal failure

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

Sodium

• High sodium = increased potassium excretion
  • 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

Potassium

• High potassium = increased potassium excretion
• Triggers aldosterone

Potassium - Reptiles