Tuesday, 21 August 2012

Potassium Homeostasis

Potassium, the most abundant intracellular cation, is essential for the life of the organism. Potassium is obtained through the diet, and common potassium-rich foods include meats, beans, fruits, and potatoes.
Gastrointestinal absorption is complete, resulting in daily excess intake of approximately 1 mEq/kg/d (60-100 mEq). Ninety percent of this excess is excreted through the kidneys, and 10% is excreted through the gut. Potassium homeostasis is maintained predominantly through the regulation of renal excretion. The most important site of regulation is the collecting duct, where aldosterone receptors are present.
Excretion is increased by (1) aldosterone, (2) high sodium delivery to the collecting duct (eg, diuretics), (3) high urine flow (eg, osmotic diuresis), (4) high serum potassium level, and (5) delivery of negatively charged ions to the collecting duct (eg, bicarbonate).
Excretion is decreased by (1) absence or relative deficiency of aldosterone, (2) low sodium delivery to the collecting duct, (3) low urine flow, (4) low serum potassium level, and (5) renal failure
Kidneys adapt to acute and chronic alterations in potassium intake. When potassium intake is chronically high, potassium excretion likewise is increased. In the absence of potassium intake, obligatory renal losses are 10-15 mEq/d. Thus, chronic losses occur in the absence of any ingested potassium. The kidney maintains a central role in the maintenance of potassium homeostasis, even in the setting of chronic renal failure. Renal adaptive mechanisms allow the kidneys to maintain potassium homeostasis until the glomerular filtration rate drops to less than 15-20 mL/min. Additionally, in the presence of renal failure, the proportion of potassium excreted through the gut increases. The colon is the major site of gut regulation of potassium excretion. Therefore, potassium levels can remain relatively normal under stable conditions, even with advanced renal insufficiency. However, as renal function worsens, the kidneys may not be capable of handling an acute potassium load.

Serum potassium level

Potassium is predominantly an intracellular cation; therefore, serum potassium levels can be a very poor indicator of total body stores. Because potassium moves easily across cell membranes, serum potassium levels reflect movement of potassium between intracellular and extracellular fluid compartments, as well as total body potassium homeostasis.
Mechanisms for sensing extracellular potassium concentration are not well understood. Evidence suggests that adrenal glomerulosa cells and pancreatic beta cells may play a role in potassium sensing, resulting in alterations in aldosterone and insulin secretion.[1, 2] As both of these hormonal systems play important roles in potassium homeostasis, these new findings are no surprise; however, the molecular mechanisms by which these potassium channels signal changes in hormone secretion and activity have still not been determined.
Muscle contains the bulk of body potassium, and the notion that muscle could play a prominent role in the regulation of serum potassium concentration through alterations in sodium pump activity has been promoted for a number of years. Insulin stimulated by potassium ingestion increases the activity of the sodium pump in muscle cells, resulting in an increased uptake of potassium. Studies in a model of potassium deprivation demonstrate that acutely, skeletal muscle develops resistance to insulin-stimulated potassium uptake even in the absence of changes in muscle cell sodium pump expression. However, long term potassium deprivation results in a decrease in muscle cell sodium-pump expression, resulting in decreased muscle uptake of potassium.[3, 4, 5]
Thus, there appears to be a well-developed system for sensing potassium by the pancreas and adrenal glands, resulting in rapid adjustments in immediate potassium disposal and for long-term potassium homeostasis. High potassium states stimulate cellular uptake via insulin-mediated stimulation of sodium-pump activity in muscle and stimulate potassium secretion by the kidney via aldosterone-mediated enhancement of distal renal expression of secretory potassium channels (ROMK). Low potassium states result in insulin resistance, impairing potassium uptake into muscle cells, and cause decreased aldosterone release, lessening renal potassium excretion.
Several factors regulate the distribution of potassium between the intracellular and extracellular space, as follows:
  • Glycoregulatory hormones: (1) Insulin enhances potassium entry into cells, and (2) glucagon impairs potassium entry into cells.
  • Adrenergic stimuli: (1) Beta-adrenergic stimuli enhance potassium entry into cells, and (2) alpha-adrenergic stimuli impair potassium entry into cells.
  • pH: (1) Alkalosis enhances potassium entry into cells, and (2) acidosis impairs potassium entry into cells.
An acute increase in osmolality causes potassium to exit from cells. An acute cell/tissue breakdown releases potassium into extracellular space.

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