Activators, slow channel transport

In sum, the natural tendency will be for sodium, calcium, and chloride ions to flow into the neuron and for potassium ions to flow out, and in so doing to reduce the membrane potential to zero. In reality, this is not so easy. The plasma membrane of the neuron is not very permeable to these ions. If it were, it would be impossible to sustain concentration gradients across it. The rate of passive diffusion of these ions across this membrane is very slow, though not zero, and different for each ion. So how do ions get across the neuronal plasma membrane rapidly There are two ways gated channels and active transport by pumps. 

Figure 1. The channels which can be available for proton release in different cells. These may be activated by ligands attached to receptors or signals generated by the electron transport system. The electron transport across the membrane can also be accompanied by proton movement, depending on the orientation of electron transport, but th is movement would be I imited because of the slow rate of electron transport compared to the rapid rate which can be elicited through channels. Any possible relation of oxidase control to the H+-ATPase or the H -K+-ATPase has not been tested by inhibitors such as bafilomycin or omeprazole, respectively (Swallow et al., 1990).

NBD probes are often used to assay flip-flop. Flip-flop refers to the reversible transversal diffusion of lipids from one leaflet to the other leaflet of a lipid bilayer membrane. In intact membranes, this transversal diffusion is very slow (fi/2 on the order of hours to days). However, it can be accelerated by biological or synthetic flippases, which are a special class of membrane transporters related to ion carriers. Alternatively, micellar pores are synthetic ion channels and pores with flippase activity and can thus be identified with flip-flop assay (Figure 2 interfacial location of the transporter, as second distinctive characteristic of micellar pores, can be identified by fluorescence depth quenching experiments with DOXYL probes). 

The dependence of the transport activity on the mole fraction of a binary mixture of fast and slow ions is a classical test for a cooperative multi-ion transport mechanism. Commonly, a negative deviation from a simple linear additivity is considered an anomalous mole fraction effect (AMFE) and confirms the existence of multiple ion binding sites along the conduction pathway (Figure I4a). i The conventional explanation refers to the presumption of single-file ion channels, in which occupation of more than one binding site is necessary for the ions to move really fast (Figure 14c and d). However, this... 

---