Channel mechanisms

II The Channel Mechanism of Ion Transport The Gramicidin Channel Model 181... 

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. 

Seeburg PH, Higuchi M, Sprengel R (1998) RNA editing of glutamate receptor channels mechanism and physiology. Brain Res Brain Res Rev 26 217-219... 

Receptors Linked to Ion Channels Mechanisms of Activation and Block... 

Membrane depolarization typically results from an increase in Na+ conductance. In addition, mobilization of intracellular Ca2+ from the endoplasmic or sarcoplasmic reticulum and the influx of extracellular Ca2+ appear to be elicited by ACh acting on muscarinic receptors (see Ch. 22). The resulting increase in intracellular free Ca2+ is involved in activation of contractile, metabolic and secretory events. Stimulation of muscarinic receptors has been linked to changes in cyclic nucleotide concentrations. Reductions in cAMP concentrations and increases in cGMP concentrations are typical responses (see Ch. 21). These cyclic nucleotides may facilitate contraction or relaxation, depending on the particular tissue. Inhibitory responses also are associated with membrane hyperpolarization, and this is a consequence of an increased K+ conductance. Increases in K+ conductance may be mediated by a direct receptor linkage to a K+ channel or by increases in intracellular Ca2+, which in turn activate K+ channels. Mechanisms by which muscarinic receptors couple to multiple cellular responses are considered later. 

Figure 7.6 A cartoon showing a strict channelling mechanism transfer of Fem from Fet3p to Ftrlp is non-dissociative and the Fem does not equilibrate with the bulk phase. (Reprinted with permission from Kwok et al. 2006. Copyright (2006) American Chemical Society.)...

Carmeliet E and Mubagwa K. Antiarrhythmic drugs and cardiac ion channels mechanisms of action. Prog Biophys Molec Biol 1998 70 1-72. 

In spite of the overwhelming importance of the channel mechanism for the transport of alkali and alkaline earth metal ions in biological systems, only carrier transport has been studied extensively by chemists. Studies on ion channel mimics of simple structures have long been limited to antibiotic families of gramicidin, amphotericin B, and others. Several pioneers have reported successful preparation of non-peptide artificial channels. However, their claims have been based on kinetic characteristics observed for the release of metal ions through liposomal membrane and lacked the very critical proofs of channel formation. Such a situation was... 

Before concluding this review, it is appropriate to comment on estimation methods of artificial ion channels. As described in each experimental method, both planar and liposomal membranes could be used for detecting the ion transport via the channel mechanism. For estimating the ion transport rate across the liposomal membrane, a variety of methodologies have been employed. These may be summarized as follows ... 

Discrimination of the channel mechanism from the carrier is very critical and a variety of criteria are proposed from research groups. The following criteria have been employed ... 

Comparison of rates in the gel and liquid state of the membrane. In the case of channel mechanism, ions can pass through a structured pathway which is more or less similar to the bulk aqueous phase, while carriers encapsulate ions and transfer in the membrane. Therefore, the former is insensitive to gel-liquid phase transition of the membrane, while the gel state inhibits the carrier transport. 

The answer is in the question itself. There is no proof at all for demonstrating whether or not carrier or channel mechanisms exist in permeation phenomena among biological systems. 

Figure 2.4 (a) Carrier, (b) channel and (c) gated channel mechanisms of ion transport across a bio-... 

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