A channel that is "inward-rectifying" is one that passes current (positive charge) more easily in the inward direction (into the cell). By convention, this inward current is considered a negative current, while an outward current (positive charge moving out of the cell) is considered a positive current.
At membrane potentials below the channel's resting potential, inwardly rectifying K+ channels support the flow of positive charge into the cell, pushing the membrane potential back to the resting potential. This can be seen in figure 1: when the membrane potential is clamped below the channel's resting potential (e.g. -60 mV), negative current flows (i.e. positive charge flows into the cell). However, when the membrane potential is set higher than the channel's resting potential (e.g. +60 mV), these channels pass very little charge out of the cell.
Simply put, this channel passes much more current in the inward direction than the outward one.
This current is caused by the interaction between electrical forces and diffusion (due to a difference in K+ ion concentrations on the inside and outside of the cell), as encapsulated by the Nernst equation.
Note that these channels are not perfect rectifiers, as they can pass some outward current in the voltage range up to about 30 mV above resting potential. It is thought that this current may play an important role in regulating the resting level of neuronal activity.
The phenomenon of inward rectification of Kir channels is the result of high-affinity block by endogenous polyamines, namely spermine, and magnesium ions that plug the channel pore at positive potentials, resulting in a decrease in outward currents. This voltage-dependent block by polyamines causes currents to be conducted well in the inward direction. While the principal idea of polyamine block is understood, the specific mechanisms are still controversial.
|cardiac myocytes||Kir channels close upon depolarization, slowing membrane repolarization and helping maintain a more prolonged action potential. This type of inward-rectifier channel is distinct from delayed rectifier K+ channels, which help re-polarize nerve and muscle cells after action potentials; and potassium leak channels, which provide much of the basis for the resting membrane potential.|
|endothelial cells||Kir channels are involved in regulation of nitric oxide synthase.|
|kidneys||Kir export surplus potassium into collecting tubules for removal in the urine, or alternatively may be involved in the reuptake of potassium back into the body.|
|neurons and in heart cells||G-protein activated IRKs (Kir3) are important regulators. A mutation in the GIRK2 channel leads to the weaver mouse mutation. "Weaver" mutant mice are ataxic and display a neuroinflammation-mediated degeneration of their dopaminergic neurons. Weaver mice have been examined in labs interested in neural development and disease for over 30 years.|
|pancreatic beta cells||KATP channels (comprised of Kir6.2 and SUR1 subunits) control insulin release.|
Kir channels are formed from as homotetrameric membrane proteins. Each of the four identical protein subunits is composed of two membrane-spanning alpha helices (M1 and M2). Heterotetramers can form between members of the same subfamily (ie Kir2.1 and Kir2.3) when the channels are overexpressed.
|Kir2.1||IRK1||Kir2.2, Kir4.1, PSD-95, SAP97, AKAP79|
|Kir2.2||IRK2||Kir2.1 and Kir2.3 to form heteromeric channel, auxiliary subunit: SAP97, Veli-1, Veli-3, PSD-95|
|Kir2.3||IRK3||Kir2.1 and Kir2.3 to form heteromeric channel, PSD-95, Chapsyn-110/PSD-93|
|Kir2.4||IRK4||Kir2.1 to form heteromeric channel|
|Kir3.1||GIRK1, KGA||Kir3.2, Kir3.4, Kir3.5, Kir3.1 is not functional by itself|
|Kir3.2||GIRK2||Kir3.1, Kir3.3, Kir3.4 to form heteromeric channel|
|Kir3.3||GIRK3||Kir3.1, Kir3.2 to form heteromeric channel|
|Kir3.4||GIRK4||Kir3.1, Kir3.2, Kir3.3|
|Kir4.1||Kir1.2||Kir4.2, Kir5.1, and Kir2.1 to form heteromeric channels|
|Kir6.2||KATP||SUR1, SUR2A, and SUR2B|
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