Electrolytes

Master answer

Potassium

• Predominany intracellular cation
• Total Stores: Approx 3200mmol (50mmol/kg)
• Key Functions:
  • Main determinant of ICF osmolality and tonicity
  • Responsible for RMP of excitable cells via Goldmann-Hodgkin-Katz due to ↑↑gK relative to other species
  • Role in action potential → repolarisation phase
  • Secretion of insulin and multiple other KATP dependant processes
  • Regulation of IC processes (protein/glycogen synthesis)
  • Involved in Na+/K+ ATPase in cell membranes

Potassium flux is dictated by

1. Fick’s law of diffusion
   • D proportional to Δ [K] → ↑/↓ ECF K → ↑/↓ diffusion 

2. Nernst Equation

• Temperature via the nernst equation → ↑ temperature → more negative V → increase potassium permeability → ↑ intracellular potassium

3. Gibbs-donnan

• In the presence of non-diffusable species the relative osmolarity drives flow of diffusable species.
  • ↑ intracellular osmolarity or ↓ extracellular osmolarity → potassium efflux

4. Homeostatic processess

• Na/K ATPase
  • ↑ intracellular K
    • β adrenergic stimulation → ↑ Na/K ATPase activity
    • Insulin release → ↑ Na/K ATPase activity
• Acidosis:
  • ↑ H/K antiporter activity →
    • ↑ extracellular potassium
    • Utilisation of intracellular protein buffer
• Hormonal factors
  • Insulin → ↑ Na/K ATPase activity → ↑ intracellular K
  • Aldosterone → ↑ Na/K ATPase activity → ↑ intracellular K
• Activity of ATP sensitive potassium channels
  • Glucose (eg in pancreatic cells) → Diffuses into cells → ↑ ATP/ADP ratio → closure of ATP sensitive K channels → ↓ ECF K
  • Gluagon → direct increase of cAMP production → ↓ ATP → ↑ gK → ↓ intracellular K
• Burns and UMN lesions
  • upregulation of foetal/extrajunctional form of nAChRs
  • Prolonged opening relative to foetal form → ↑ potassium efflux on depolarisation

Gladwin 2016

Answer by Gladwin