Editing Potassium (section)

====Control mechanisms====
Potassium content in the plasma is tightly controlled by four basic mechanisms, which have various names and classifications. These are:
# a reactive negative-feedback system,
# a reactive feed-forward system,
# a predictive or [[circadian]] system, and
# an internal or cell membrane transport system.
Collectively, the first three are sometimes termed the "external potassium homeostasis system";<ref>{{cite journal |last1=Gumz |first1=Michelle L. |last2=Rabinowitz |first2=Lawrence |last3=Wingo |first3=Charles S. |date=2015-07-02 |title=An Integrated View of Potassium Homeostasis |journal=The New England Journal of Medicine |volume=373 |issue=1 |pages=60–72 |doi=10.1056/NEJMra1313341 |issn=0028-4793 |pmc=5675534 |pmid=26132942}}</ref> and the first two, the "reactive potassium homeostasis system".
* The reactive negative-feedback system refers to the system that induces renal secretion of potassium in response to a rise in the plasma potassium (potassium ingestion, shift out of cells, or intravenous infusion.)
* The reactive feed-forward system refers to an incompletely understood system that induces renal potassium secretion in response to potassium ingestion prior to any rise in the plasma potassium. This is probably initiated by gut cell potassium receptors that detect ingested potassium and trigger [[vagal]] [[afferent nerve fiber|afferent]] signals to the pituitary gland.
* The predictive or circadian system increases renal secretion of potassium during mealtime hours (e.g. daytime for humans, nighttime for rodents) independent of the presence, amount, or absence of potassium ingestion. It is mediated by a [[circadian oscillator]] in the [[suprachiasmatic nucleus]] of the brain (central clock), which causes the kidney (peripheral clock) to secrete potassium in this rhythmic circadian fashion.[[File:Scheme sodium-potassium pump-en.svg|thumb|right|upright=1.8|The action of the [[sodium-potassium pump]] is an example of primary [[active transport]]. The two carrier proteins embedded in the cell membrane on the left are using [[Adenosine triphosphate|ATP]] to move sodium out of the cell against the concentration gradient; The two proteins on the right are using secondary active transport to move potassium into the cell. This process results in reconstitution of ATP.]]
* The ion transport system moves potassium across the cell membrane using two mechanisms. One is active and pumps sodium out of, and potassium into, the cell. The other is passive and allows potassium to leak out of the cell. Potassium and sodium cations influence fluid distribution between intracellular and extracellular compartments by [[osmotic]] forces. The movement of potassium and sodium through the cell membrane is mediated by the [[Na⁺/K⁺-ATPase]] pump.<ref>{{cite book|last=Campbell|first=Neil|title=Biology|date=1987|isbn=978-0-8053-1840-1|page=795|publisher=Benjamin/Cummings Pub. Co.|location=Menlo Park, California}}</ref> This [[Ion transporter|ion pump]] uses [[Adenosine triphosphate|ATP]] to pump three sodium ions out of the cell and two potassium ions into the cell, creating an electrochemical gradient and electromotive force across the cell membrane. The highly selective [[potassium ion channels]] (which are [[tetramer]]s) are crucial for [[Hyperpolarization (biology)|hyperpolarization]] inside [[neuron]]s after an action potential is triggered, to cite one example. The most recently discovered potassium ion channel is KirBac3.1, which makes a total of five potassium ion channels (KcsA, KirBac1.1, KirBac3.1, KvAP, and MthK) with a determined structure. All five are from [[prokaryotic]] species.<ref name="pmid16253415">{{cite journal|first1=Mikko |last1 = Hellgren| first2= Lars |last2= Sandberg|first3= Olle |last3= Edholm|title=A comparison between two prokaryotic potassium channels (K<sub>ir</sub>Bac1.1 and KcsA) in a molecular dynamics (MD) simulation study|journal=Biophysical Chemistry| volume=120|issue=1|pages=1–9|year=2006|pmid=16253415|doi=10.1016/j.bpc.2005.10.002}}</ref>