3. Potassium And Magnesium Homeostasis Flashcards
What happens if extracellular K+ rises to the resting membrane potential?
Resting membrane potential is decreased (depolarised)
What happens if extracellular K+ falls to the resting membrane potential?
Resting membrane potential is increased (hyperpolarised)
Where is K+ reabsorbed?
PCT reabsorbs 65% - passive, through tight junctions, via conc gradients/solvent drag
TAL reabsorbs 20% - transcellular, paracellular
What can cause hypokalemia?
Excess insulin Alkalosis Certain catecholamines Insufficient intake - anorexia, prolonged fasting Too much aldosterone - primary aldosteronism, compensated heart failure, cirrhosis Diuretics Vomiting Diarrhoea Sweat
What are the symptoms of hypokalemia?
Asymptotic until conc is below 2-2.5mmol/L
Decreased resting potential
Less sensitive to depolarising stimuli
Less action potentials generated and paralysis ensues
What are the clinical effects of hypokalaemia?
Muscle weakness, cramps, tetany
Impaired liver conversion of glucose to glycogen
Vasoconstriction and cardiac arrhythmias
Impaired ADH action causing thirst, polyuria and no conc of urine
Metabolic alkalosis due to increase in intracellular H+ conc
What is the treatment of hypokalaemia?
Treating underlying cause
Oral or IV K+ may be required
What can hyperkalaemia result from?
Reduced renal excretion due to AKI or CKD, mineralocorticoid deficiency, K+ sparing diuretics or renal tubular defects Increased plasma load Insulin deficiency Transcellular shift of K+ out of cells Pseudohyperkalaemia Certain catecholamines Hypoaldosteronism
What are the clinical features of hyperkalaemia?
Can be asymptomatic
Muscle weakness, cardiac arrhythmias
What is the emergency treatment of hyperkalaemia?
Calcium gluconate
Insulin
Calcium resonium
How does calcium gluconate treat hyperkalaemia?
Ca2+ stabilises the myocardium, preventing arrhythmias
How does insulin treat hyperkalaemia?
Drives K+ into cells to lower plasma concentrations, given with glucose
How does calcium resonium treat hyperkalaemia?
Removes K+ by increasing excretion from the bowels
What are some other treatments for hyperkalaemia?
Salbutamol
Sodium bicarbonate
Renal replacement therapy
How does salbutamol treat hyperkalaemia?
Drives K+ into cells when given nebuliser or IV
How does sodium bicarbonate treat hyperkalaemia?
Correction of acidosis, would also drive K+ into cells
What is the role of magnesium?
Controls mitochondrial oxidative metabolism and so regulates energy production
Is vital for protein synthesis
Regulates K+ and Ca2+ channels in cell membranes
What is hypomagnesaemia caused by?
Decreased intake Diarrhoea Absorption disorder including fat absorption defects Renal wasting Uncontrolled diabetes Excessive alcohol consumption
What is hypomagnesaemia commonly associated with?
Hypokalaemia
Hypocalcaemia - Mg2+ needed to make PTH
What are the signs and symptoms of hypomagnesaemia?
Uncontrolled stimulation of nerves and tetany
What is the treatment for hypomagnesaemia?
Oral supplementation or IV/IM magnesium sulphate
Treat underlying cause
What is hypermagnesaemia caused by?
Renal failure
Ingestion of Mg2+
What are the signs and symptoms of hypermagnesaemia?
Reduced muscle contraction
High Mg2+ inhibits PTH release - hypocalcaemia
Very high levels Mg2+ alter the electrical potential across cardiac cell membrane, lead to cardiac arrhythmias
What is the treatment for hypermagnesaemia?
Reduce intake
Calcium gluconate injection
Furosemide to increase excretion
Haemodialysis in severe cases
