Cyclic adenosine monophosphate

Cyclic guanosine monophosphate

Mechanism

The channels are gated by a chemical ligand (the cyclic nucleotide), but they are more similar in structure to the family of voltage-gated ion channels than to the ligand gated ones. In fact, the cyclic nucleotide-gated channels often have positively- or negatively-charged areas that may respond to changes in membrane potential. The purpose and function of these charged areas are not yet fully understood.

Functions

Cyclic nucleotide-gated channels are particularly important in several systems:

• The mammalian olfactory system.^[1]

• In the visual system, a cGMP- (cyclic guanosine monophosphate-) gated channel is found in the outer membrane of retinal photoreceptor cells. In response to high levels of cGMP, the channels are open and allow positively-charged ions to flow into the cell, causing depolarization. This is the state of the cell in the dark (called the dark current), but a photon striking a photoreceptor in the cell causes a chain reaction that results in lower levels of cGMP and therefore hyperpolarization. Thus, these cells are actually more active in dark than in light.

• Funny currents have been associated with the HCN channels.^[2]

• cGMP-gated cation channels are present in the inner medullary collecting ducts of the kidney. They promote Na⁺ reabsorption. A part of the atrial natriuretic peptide mechanism of action is inhibition of these channels^[3], just as the diuretic drug amiloride^[3]. Aldosterone has an opposite effect.^[3]

Examples

Alpha

"Cyclic nucleotide gated channel alpha" subunits include

• Cyclic nucleotide-gated channel alpha 1 ()
• Cyclic nucleotide-gated channel alpha 2 ()
• Cyclic nucleotide-gated channel alpha 3 ()
• Cyclic nucleotide-gated channel alpha 4 ()

Beta-subunits include:

• Cyclic nucleotide-gated channel beta 1 ()
• Cyclic nucleotide-gated channel beta 3 ()

Hyperpolarization-activated

Other examples include the following "hyperpolarization-activated cyclic nucleotide-gated potassium channels" (HCN channels): HCN1, HCN2, HCN3, HCN4

References

1. ↑
2. ↑
3. ↑ ^{a b c}

External links

• Overview at wustl.edu

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