Potassium in biology

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Potassium is an essential mineral micronutrient and is the main intracellular ion for all types of cells. It is important in maintaining fluid and electrolyte balance in the bodies of humans and animals.[1][2] Potassium is necessary for the function of all living cells, and is thus present in all plant and animal tissues. It is found in especially high concentrations within plant cells, and in a mixed diet, it is most highly concentrated in fruits. The high concentration of potassium in plants, associated with comparatively very low amounts of sodium there, historically resulted in potassium first being isolated from the ashes of plants (potash), which in turn gave the element its modern name. The high concentration of potassium in plants means that heavy crop production rapidly depletes soils of potassium, and agricultural fertilizers consume 93% of the potassium chemical production of the modern world economy.

The functions of potassium and sodium in living organisms are quite different. Animals, in particular, employ sodium and potassium differentially to generate electrical potentials in animal cells, especially in nervous tissue. Potassium depletion in animals, including humans, results in various neurological dysfunctions.

Function in plants[edit]

Function in animals[edit]

Potassium is the major cation (positive ion) inside animal cells, while sodium is the major cation outside animal cells. The difference between the concentrations of these charged particles causes a difference in electric potential between the inside and outside of cells, known as the membrane potential. The balance between potassium and sodium is maintained by ion transporters in the cell membrane. All potassium ion channels are tetramers with several conserved secondary structural elements. A number of potassium channel structures have been solved including voltage gated,[3][4][5] ligand gated,[6][7][8][9][10] tandem-pore,[11][12][13] and inwardly rectifying channels,[14][15][16][17][18] from prokaryotes and eukaryotes. The cell membrane potential created by potassium and sodium ions allows the cell to generate an action potential—a "spike" of electrical discharge. The ability of cells to produce electrical discharge is critical for body functions such as neurotransmission, muscle contraction, and heart function.[19]

Deficiency[edit]

High blood pressure/Hypertension[edit]

Diets low in potassium lead to hypertension.[20]

Hypokalemia[edit]

A severe shortage of potassium in body fluids may cause a potentially fatal condition known as hypokalemia. Hypokalemia typically results from loss of potassium through diarrhea, diuresis, or vomiting. Symptoms are related to alterations in membrane potential and cellular metabolism. Symptoms include muscle weakness and cramps, paralytic ileus, ECG abnormalities, intestinal paralysis, decreased reflex response and (in severe cases) respiratory paralysis, alkalosis and arrhythmia.

In rare cases, habitual consumption of large amounts of black licorice has resulted in hypokalemia. Licorice contains a compound (Glycyrrhizin) that increases urinary excretion of potassium.

Insufficient intake[edit]

Although low dietary intake of potassium does not lead to hypokalemia in healthy individuals, many long-term health risks are related to insufficient dietary potassium.

The 2004 guidelines of the Institute of Medicine specify an RDA of 4700 mg of potassium for adults,[21] based on intake levels that have been found to lower blood pressure, reduce salt sensitivity, and minimize the risk of kidney stones. However, most Americans consume only half that amount per day.[22] Similarly, in the European Union, particularly in Germany and Italy, insufficient potassium intake is widespread.[23]

Diseases that may be prevented by adequate potassium intake include stroke, osteoporosis, kidney stones, and hypertension.

Food sources[edit]

Eating a variety of foods that contain potassium is the best way to get an adequate amount. Foods with high sources of potassium include kiwifruit, orange juice, potatoes, bananas, coconut, avocados, apricots, parsnips and turnips, although many other fruits, vegetables, legumes, and meats contain potassium.

Common foods very high in potassium:[24]

  • beans (white beans and others), dark leafy greens (spinach, Swiss chard, and others), baked potatoes, dried fruit (apricots, peaches, prunes, raisins; figs and dates), baked squash, yogurt, fish (salmon), avocado, and banana;
  • nuts (pistachios, almonds, walnuts, etc.) and seeds (squash, pumpkin, sunflower)

The most concentrated foods (per 100 grams) are:[24]

  • dried herbs, sun dried tomatoes, dark chocolate, whey powder, paprika, yeast extract, rice bran, molasses, and dry roasted soybeans

Side effects and toxicity[edit]

Gastrointestinal symptoms are the most common side effects of potassium supplements, including nausea, vomiting, abdominal discomfort, and diarrhea. Taking potassium with meals or taking a microencapsulated form of potassium may reduce gastrointestinal side effects.

Hyperkalemia is the most serious adverse reaction to potassium. Hyperkalemia occurs when potassium builds up faster than the kidneys can remove it. It is most common in individuals with renal failure. Symptoms of hyperkalemia may include tingling of the hands and feet, muscular weakness, and temporary paralysis. The most serious complication of hyperkalemia is the development of an abnormal heart rhythm (arrhythmia), which can lead to cardiac arrest.

Although hyperkalemia is rare in healthy individuals, oral doses greater than 18 grams taken at one time in individuals not accustomed to high intakes can lead to hyperkalemia. All supplements sold in the U.S. contain no more than 99 mg of potassium; a healthy individual would need to consume more than 180 such pills to experience severe health risks.

See also[edit]

References[edit]

  1. ^ Pohl, Hanna R.; Wheeler, John S.; Murray, H. Edward (2013). "Chapter 2. Sodium and Potassium in Health and Disease". In Astrid Sigel, Helmut Sigel and Roland K. O. Sigel. Interrelations between Essential Metal Ions and Human Diseases. Metal Ions in Life Sciences 13. Springer. pp. 29–47. doi:10.1007/978-94-007-7500-8_2. 
  2. ^ *Clausen, Michael Jakob Voldsgaard; Poulsen, Hanne (2013). "Chapter 3 Sodium/Potassium Homeostasis in the Cell". In Banci, Lucia (Ed.). Metallomics and the Cell. Metal Ions in Life Sciences 12. Springer. doi:10.1007/978-94-007-5561-1_3. ISBN 978-94-007-5560-4.  electronic-book ISBN 978-94-007-5561-1 ISSN 1559-0836 electronic-ISSN 1868-0402
  3. ^ "Crystal structure of a voltage-gated K+ channel pore module in a closed state in lipid membranes.". J Biol Chem 287: 43063–70. Dec 2012. doi:10.1074/jbc.M112.415091. PMID 23095758. 
  4. ^ Long SB, Campbell EB, Mackinnon R (August 2005). "Crystal structure of a mammalian voltage-dependent Shaker family K+ channel". Science 309: 897–903. doi:10.1126/science.1116269. PMID 16002581. 
  5. ^ Jiang Y, Lee A, Chen J; et al. (May 2003). "X-ray structure of a voltage-dependent K+ channel". Nature 423: 33–41. doi:10.1038/nature01580. PMID 12721618. 
  6. ^ Jiang Y, Lee A, Chen J, Cadene M, Chait BT, MacKinnon R (May 2002). "Crystal structure and mechanism of a calcium-gated potassium channel". Nature 417: 515–22. doi:10.1038/417515a. PMID 12037559. 
  7. ^ Yuan P, Leonetti MD, Pico AR, Hsiung Y, MacKinnon R (July 2010). "Structure of the human BK channel Ca2+-activation apparatus at 3.0 A resolution". Science 329: 182–6. doi:10.1126/science.1190414. PMC 3022345. PMID 20508092. 
  8. ^ Wu Y, Yang Y, Ye S, Jiang Y (July 2010). "Structure of the gating ring from the human large-conductance Ca(2+)-gated K(+) channel". Nature 466: 393–7. doi:10.1038/nature09252. PMC 2910425. PMID 20574420. 
  9. ^ "Functional and structural analysis of the human SLO3 pH- and voltage-gated K+ channel.". Proc Natl Acad Sci U S A 109: 19274–9. Nov 2012. doi:10.1073/pnas.1215078109. PMID 23129643. 
  10. ^ "Distinct gating mechanisms revealed by the structures of a multi-ligand gated K(+) channel.". Elife 1: e00184. 2012. doi:10.7554/eLife.00184. PMC 3510474. PMID 23240087. 
  11. ^ Brohawn SG, del Mármol J, MacKinnon R (January 2012). "Crystal structure of the human K2P TRAAK, a lipid- and mechano-sensitive K+ ion channel". Science 335: 436–41. doi:10.1126/science.1213808. PMC 3329120. PMID 22282805. 
  12. ^ Miller AN, Long SB (January 2012). "Crystal structure of the human two-pore domain potassium channel K2P1". Science 335: 432–6. doi:10.1126/science.1213274. PMID 22282804. 
  13. ^ "K2P channel gating mechanisms revealed by structures of TREK-2 and a complex with Prozac.". Science 347: 1256–9. Mar 2015. doi:10.1126/science.1261512. PMID 25766236. 
  14. ^ "Domain reorientation and rotation of an intracellular assembly regulate conduction in Kir potassium channels.". Cell 141: 1018–29. Jun 2010. doi:10.1016/j.cell.2010.05.003. PMID 20564790. 
  15. ^ "Crystal structure of the potassium channel KirBac1.1 in the closed state.". Science 300: 1922–6. Jun 2003. doi:10.1126/science.1085028. PMID 12738871. 
  16. ^ "Crystal structure of the mammalian GIRK2 K+ channel and gating regulation by G proteins, PIP2, and sodium.". Cell 147: 199–208. Sep 2011. doi:10.1016/j.cell.2011.07.046. PMID 21962516. 
  17. ^ "Structural basis of inward rectification: cytoplasmic pore of the G protein-gated inward rectifier GIRK1 at 1.8 A resolution.". Cell 111: 957–65. Dec 2002. doi:10.1016/S0092-8674(02)01227-8. PMID 12507423. 
  18. ^ "Crystal structure of the eukaryotic strong inward-rectifier K+ channel Kir2.2 at 3.1 A resolution.". Science 326: 1668–74. Dec 2009. doi:10.1126/science.1180310. PMID 20019282. 
  19. ^ Mikko Hellgren, Lars Sandberg, Olle Edholm (2006). "A comparison between two prokaryotic potassium channels (KirBac1.1 and KcsA) in a molecular dynamics (MD) simulation study". Biophys. Chem. 120 (1): 1–9. doi:10.1016/j.bpc.2005.10.002. PMID 16253415. 
  20. ^ Whelton PK, He J, Cutler JA, Brancati FL, Appel LJ, Follmann D, Klag MJ (May 1997). "Effects of Oral Potassium on Blood Pressure". JAMA 277: 1624–32. doi:10.1001/jama.1997.03540440058033. PMID 9168293. 
  21. ^ http://www.iom.edu/Reports/2004/Dietary-Reference-Intakes-Water-Potassium-Sodium-Chloride-and-Sulfate.aspx Dietary Reference Intakes: Water, Potassium, Sodium, Chloride, and Sulfate
  22. ^ http://www.mayoclinic.com/health/potassium/AN00884 Mayo Clinic
  23. ^ http://content.karger.com/ProdukteDB/produkte.asp?Aktion=ShowPDF&ProduktNr=223977&Ausgabe=230671&ArtikelNr=83312&filename=83312.pdf Energy and Nutrient Intake in the European Union
  24. ^ a b [1]

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