Ionic permeabilities

Two questions arise in connection with the ionic permeability present in membranes How can the ionic concentration gradients between the two sides of the membrane be preserved despite leakage of ions, and how is the membrane potential related to these gradients These questions have been explored in extended studies stiU continuing. 

In all three frames of Fig. 3.4, there is evidence of ionic-species transport, labeled as log Pi in Fig. 3.4. The pH at the bend in the curves corresponding to the onset of ionic permeability is labeled pK and corresponds to the pH where 50% of the transport is by the neutral species and 50% by the ionic species. This is a conditional constant, but unlike and pJ< it is dependent mainly on the con-... 

The simple pore was originally considered in the context of osmosis as an explanation of how water might move across a biological structure (e.g. an epithelium) in the absence of solute movement. This notion introduced by Brucke in the mid 19th century, (see Hille, 1984) was subsequently extended by Boyle and Conway (1941) to consider the selective ionic permeability of the resting cell membrane. Here the explanation for the high membrane permeability to potassium and to chloride, as compared to sodium, was simple. The hydrated ionic radius of sodium was greater than that of either the hydrated potassium or chloride ion, hence the pores postulated to be present in the membrane would act as a molecular sieve and permit the movement of potassium and of chloride but not of sodium. 

Levy, D., Seigneuret, M., Bluzat, A., Rigaud, J.-L. (1990). Evidence for proton countertransport by the sarcoplasmic reticulum Ca2+-ATPase during calcium transport in reconstituted proteo-liposomes with low ionic permeability. J. Biol. Chem. 265,19524-19534. 

The ionic permeabilities for the resting membrane are as follows V W ci-1 0.12 1.44. Calculate the resting membrane potential. (Contractor)... 

Schafer JA, Troutman SL, Andreoli TE (1974) Volume reabsorption, transepithelial potential differences, and ionic permeability properties in mammalian superficial proximal straight tubules. J Gen Physiol 64 582-607 Schlatter E, Greger R, Weidtke C (1983) Effect of high ceiling diuretics on active salt transport in the cortical thick ascending limb of Henle s loop of rabbit kidney. Correlation of chemical structure and inhibitory potency. Pfltigers Arch 396 210-217... 

Sugiura, M., Shinbo, T., Kikkawa, M., and Toyoda, H., Membrane potential and ionic permeability of formaldehyde-tanned collagen membranes, Nippon Nogei Kagaku Kaishi, 48, 493. 1974 Cherru Absir., 82, 100090, 1975. 

Meissner G. Ionic permeability of isolated muscle sarcoplasmic reticulum and liver endoplasmic reticulum vesicles. Methods En-zymol. 1988 157 417-437. 

Nerve impulses are communicated across most synapses by small, diffusible molecules called neurotransmitters, of which one is acetylcholine, referred to as a cholinergic neurotransmitter because it is derived from choline (Section 12.3.1). The presynaptic membrane of a synapse is separated from the postsynaptic membrane by a gap of about 50 nm, called the synaptic cleft. The end of the presynaptic axon is filled with synaptic vesicles, each containing about 10 acetylcholine molecules (Figure 13.14). The arrival of a nerve impulse leads to the synchronous export of the contents of some 300 vesicles, which raises the acetylcholine concentration in the cleft from 10 nM to 500 iM in less than a millisecond. The binding of acetylcholine to the postsynaptic membrane markedly changes its ionic permeabilities... 

Baffin F, Duru C, Jacob M, et al. Physico-chemical characterization of the ionic permeability of an enteric coating polymer. Int J Pharm 1995 120(2) 205-214. 

Mechanism of action of cholera toxin. The steps consist of cholera toxin (CT) binding to GMi receptors anchored by its ceramide moiety internalization of CT in endosomes the release of A1-subunit of CT from the trans-Golgi network and endoplasmic reticulum (ER) ADP ribosylation of the a-subunit of stimulatory G-protein by At activation of adenylyl cyclase and production of cAMP activation of protein kinase A phosphorylation of the regulatory (R) subunit of CFTR and finally opening of the chloride channel. Vibrio cholerae also produces a zona occludens toxin (ZOT) which increases the ionic permeability of the zona occludens. 

Whereas the mechanism of action of insecticides has been studied for many years since the development of synthetic insecticides such as DDT, lindane and parathion during and after World War II, it was not until around the mid-1960 s that their actions on the nervous system were understood at the cellular and membrane level. Since these neuroactive insecticides are known to alter membrane excitation which takes place with a time course of milliseconds, the study of their mechanisms of action can best be performed with the aid of advanced electrophysiological techniques such as voltage clamp which allows us to measure the ionic permeabilities of excitable membranes. Studies along this line unveiled a variety of important features concerning the... 

Understanding of how complex ligands (e.g., messengers, drugs) affect selectivity and sensitivity of ionic permeabilities of protein membranes, films, or lamina... 

Better understanding of deterioration of ionic permeability, usually associated with unwanted protein adsorption, in order to to design synthetic systems that retain for practical periods their desired capabilities... 

The molecular basis of the ion selectivity sequences of membrane ion channels (cf Ref. 7). It is not clear what molecular structures can preferentially select Na over K+ by a twelvefold ratio. Current theories attribute the selectivity to negative charge groups at the channel entrance (cf Fig. 2). These can account for ionic permeability sequences but not for the magnitudes of the selectivity. 

Zeaxanthin accumulation in the thylakoid membrane correlated with the increased resistance of lipids against peroxidation (Havaux et al. 1991, Sarry et al., 1994) and increased the thylakoid membrane thermostability of the ionic permeability (Havaux et al., 1996). 

Fixed Charge Hypothesis. According to the computation described above, the movement of an ion across the red cell membrane is governed by its concentration gradient and the transmembrane potential. It has been suggested (33) that the membrane potential is one factor that, in addition to its direct effect on flux, is a determinant of ionic permeability. For cation movements over wide ranges of membrane potential, a relatively small dependence of permeability on potential has been demonstrated (34). 

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