Disorders of Potassium Balance Flashcards
What is the intracellular concentration of potassium in the body?
150 mEq/L
What maintains the large potassium gradient across the cell membrane?
Na+-K+-ATPase
What are the cutoffs for hyperkalemia and hypokalemia?
[K+] > 5 mEq/L is hyperkalemia
[K+] < 3.5 mEq/L is hypokalemia
What is the major effect of potassium imbalance?
hyperkalemia causes the membrane potential to become less negative
hypokalemia causes the membrane potential to become more negative
What is the normal range of potassium in the plasma?
3.5-4.9 nM
What hormones promote the uptake of K+ into cells?
epinephrine and insulin
increased uptake by stimulating the Na+-K+-ATPase pump
act within a few minutes
insulin is the most important
causes of hypokalemia
excess insulin - stimulates Na+/K+ ATPase
beta adrenergic agonists - stimulates Na+/K+ ATPase
acute illness (MI, head injury) - catecholamine stimulation of Na+/K+ ATPase
hypokalemic periodic paralysis - mutations in cation channels
alkalemia - stimulates Na+/K+ ATPase
major pathophysiologic factors influencing the distribution of K+ between the ICF and ECF
acid-base balance
plasma osmolality
cell lysis
exercise
How does acid-baes balance affect potassium distribution?
metabolic acidosis increases plasma K+
metabolic alkalosis decreases plasma K+
How does plasma osmolality affect potassium distribution?
increased osmolality increases K+ release by cells
decreased osmolality ddecreases K+ release by cells
How does exercise affect potassium distribution?
K+ is released from skeletal muscle
causes of hyperkalemia
beta adrenergic blockade - inhibits Na+/K+ ATPase
digitalis toxicity - inhibits Na+/K+ ATPase
intense exercise - activates K+ ATP channel
acidosis - inhibition of Na+/K+ ATPase and activation of K+ channels
hyperosmolality (hyperglycemia) - water rich in K+ exits the cells because of osmolar effect
What is the distribution of K+ reabsorption in the nephron?
67% reabsorbed in the PT
about 20% absorbed by the loop of Henle
reabsorption is a constant fraction of the amount filtered
10% to 50% in the DCT
5% to 30% in the CCD
What is the process of K+ transport in the distal tubule and collecting duct?
1) K+ uptake across the basolateral membrane by Na+-K+ ATPase
2) diffusion of K+ from the cell into the tubular fluid, operation of Na+-K+ ATPase pump creates a high tracellular [K+], which provides the chemical driving force for K+ exit across the apical membrane through K+ channels
What are the major factors that control the rate of K+ secretion by the distal renal tubule and the collecting duct?
the activity of the Na+-K+ ATPase in the collecting tubule
the driving force for K+ in the apical membrane, influenced by urine flow rate
the ability of K+ to cross the apical membrane via a K+ channel called RMK
the ability of Na+ channels to function, creating a favorable electrical gradient for potassium secretion
aldosterone which increases the activity of the Na+-K+ ATPase pump and opens the sodium channel
major physiologic regulators of K+ secretion
plasma [K+}
aldosterone
distal Na+ delivery
Describe the mechanisms in a principle cell for potassium excretion
ROMK K+ channels and ENaC Na+ channels on the apical membrane
Na+-K+ ATPase and K+ channels on the basolateral membranes
factors that perturb K+ secretion
levels of circulating aldosterone in pathological states such as excess (increased K+ secretion) or deficiency (decreased K+ secretion)
flow of tubular fluid and distal sodium delivery and urine flow
acid-base balance - alkaloses increases K+ secretion an dacidosis decreases K+ secretion
amiloride
a diuretic that blocks sodium channels in the CCD
has the same effect as decreasing nluminal Na+
indirectly decreases K+ secretion
ECG findings of hyperkalemia
P wave flattened
PR interval prolonged
QRS complex widens
ventricular fibrillation or aystole leading to cardiac arrest may occur at high levels
extrarenal causes of hyperkalemia
tissue damage and factors that modify transcellular distribution of potassium and result in a shift of potassium from the intracellular to the extracellular fluid compartment
renal causes of hyperkalemia
decreased GFR (reduced to less than 10 mL/minute) when the mechanisms fail
aldosterone deficiency - most commonly caused by diabetes and interstitial renal disease, drugs blocking RAAS and NSAIDs can also lead to this, as well as chronic heparin therapy
decreased distal Na delivery or blockade of Na channels in the cortical collecting tubule
distal Na delivery or blockade of Na channels in the cortical collecting tube - best characterized in patients with chronic obstructive nephropathy with damage to collecting duct cells interfering with both H+ and K+ secretion
potassium-sparing diuretics such as amiloride or triamterene also interfere with sodium transport and directly inhibit potassium secretion, spironolactone can cause it as well
factors affecting adrenal aldosterone release
angiotensin II +++
K+ ++
ACTH +
ANP -
Dopamine -
causes of selective aldosterone deficiency (normal cortisol)
low plasma renin activity - hyporeninemic hypoaldosteronism
normal plasma renin activity - normoreninemic hypoaldosteronism, cyclosporin A
