Membrane cation channels

Mechanism for stimulus-response coupling of photon absorption to the closure of plasma membrane cation channels. I, Inhibitory subunit of phosphodiesterase. 

In addition to activating protein kinases, cyclic AMP and cyclic GMP also bind to and directly regulate the activity of plasma membrane cation channels referred to as cyclic nucleotide—gated (CNG) channels. CNG ion channels have been found in kidney, testis, heart, and the CNS. These channels open in response to direct binding of intracellular cyclic nucleotides and contribute to cellular control of the membrane potential and intracellular Ca levels. The CNG ion channels are multisubunit pore-forming channels that share structural similarity with the voltage-gated K channels. 

This review addresses the issues of the chemical and physical processes whereby inorganic anions and cations are selectively retained by or passed through cell membranes. The channel and carrier mechanisms of membranes permeation are treated by means of model systems. The models are the planar lipid bilayer for the cell membrane, Gramicidin for the channel mechanism, and Valinomycin for the carrier mechanism. 

Cyclic Adenosine Monophosphate Table Appendix Membrane Transport Proteins Cyclic Guanosine Monophosphate Non-Selective Cation Channels... 

Inositol 1,4,5-trisphosphate (IP3) receptors are intracellular cation channels. They are expressed in most cells and predominantly within the membranes of the endoplasmic reticulum. They mediate release of Ca2+ from intracellular stores by the many receptors that stimulate IP3 formation. 

Non-selective cation channels are macromolecular pores in the cell membrane that form an aqueous pathway. These enable cations such as Na+, K+ or Ca++ to flow rapidly, as determined by their electrochemical driving force, at roughly equal rates (>107 cations per channel pore and per second). 

Non-selective Cation Channels. Figure 1 The nicotinic acetylcholine receptor (nAChR) is localized within the cell membrane above the cell membrane is the synaptic cleft, below the cytoplasm. Drawing of the closed (left) and open (right) nAChR showing acetylcholine (ACh) binding and cation movement. Dimensions of the receptor were taken from references [2, 3]. 

PKD2, also called polycystin 2, is a TRP-related protein defective in human autosomal polycystic kidney disease, the most common life-threatening genetic disease. PKD2 appears to be a cation channel in the plasma membrane, although there is evidence that it is an intracellular Ca2+release channel. Mammalian homologs include polycystin-like (PCL). 

The actions of proteins isolated from sea anemones, or other coelenterates, involve mechanisms different from those described for saponins. Thus, hemolysins from sea anemone R macrodactylus are capable of forming ion channels directly in membranes (98). The basic protein from S. helianthus also forms channels in black-lipid membranes. These channels are permeable to cations and show rectification (99). This ability of S. helianthus toxin III to form channels depends upon the nature of the host lipid membrane (100). Cytolysin S. helianthus binds to sphingomyelin and this substance may well serve as the binding site in cell membranes (101-106). 

The molecules that transduce noxious heat or cold are members of the transient receptor potential (TRP) receptor family. TRP proteins (Table 57-2) form tetra-meric nonselective cation channels within the plasma membrane, allowing sodium and calcium ion influx [4]. The TRPV3 channel is activated at temperatures between 31 and39°C, TRPV1 at43°C, and TRPV2 at 52-55°C. The heat pain threshold in humans is 43°C, suggesting that... 

Allen MC, Gale PA, Hunter AC, Lloyd A, Hardy SP (2000) Membrane impermeant antioestrogens discriminate between ligand- and voltage-gated cation channels in NG108-15 cells. Biochim Biophys Acta 1509(1—2) 229—236... 

The methods of solute transfer across the serosal/basolateral membrane can include ion channels and antiporters similar to those described earlier. In the case of serosally located cation channels, these primarily work because the intracellular electrolyte concentration is high enough to overcome the electrical gradient (e.g. some K+ channels). For anion channels, the negative charge inside the cell compared with the blood will help drive (repel) anions from the cell (e.g. CL efflux on voltage-sensitive channels in the intestine [58]). In the case of antiporters, the operation is fundamentally the same as that used in the mucosal membrane, except that the driving force is derived from an ion... 

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