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Membrane Transport
Published in Lelio G. Colombetti, Biological Transport of Radiotracers, 2020
In red blood cells, the Donnan ratios of H+ and Cl- were found to be equal,20 and in lysosomes, Donnan ratios for H+, K+, and Cl- were equal.21 In both cases the membrane potential was considered to be determined by a Donnan potential.
The Importance of Temperature Control When Investigating High Threshold Calcium Currents in Mammalian Neurones
Published in Avital Schurr, Benjamin M. Rigor, BRAIN SLICES in BASIC and CLINICAL RESEARCH, 2020
R. Hamish McAllister-Williams, John S. Kelly
The change in the voltage sensitivity of current activation with time is of a similar nature to that seen by Belles et al.54 in cardiac cells, where a parallel shift of current inactivation was also seen. The leftward shift in the voltage dependence could be explained by two possible mechanisms that involve the redistribution of ionic charges. Firstly, while small ions tend to equilibrate rapidly between the cell and the pipette, large macromolecules (with a net negative charge) tend to remain inside the cell, with the small ions following a Donnan equilibrium, leading to the inside of the cell initially being more negative than the pipette.60 However, as the macromolecules slowly diffuse out of the cell, there is a decline in this Donnan potential. Since the direction and time course of the shift in the voltage dependence would correspond to that which would be expected for the change in Donnan potential, this has been proposed as a possible mechanism.60,61 The second possibility is that the shift could result from a change in the electric double layer at the inner surface of the membrane, with a consequent change in the effective field experienced by the gating charge. To account for the leftward shift observed in these experiments, the charge at the inner surface would have to become more positive, and this could result from the build-up of barium ions that entered the cell each time it was depolarized. This is the explanation favored by Belles et al.54
Overcoming problems of poor drug penetration into bacteria: challenges and strategies for medicinal chemists
Published in Expert Opinion on Drug Discovery, 2018
Davide Benedetto Tiz, Danijel Kikelj, Nace Zidar
Two other well-known classes of antibiotics, FQs and TCs, possess functional groups that can be protonated or deprotonated at physiological pH. Charged groups greatly affect both the compound’s diffusion rate and its equilibrium distribution across the membrane [74]. Under physiological conditions, a large fraction of FQs and TCs are charged (cationic, anionic, and zwitterionic), and are thus expected to cross the OM through porin channels. However, the diffusion through the OM may be a more complicated process. A possible explanation of FQs and TCs crossing the OM is their ability to chelate Mg2+ ions to form Mg2+ complexes that diffuse preferentially via porins, which usually prefer cations [75]. Accumulation in the periplasm, on account of the interior-negative Donnan potential across the OM, is followed by their dissociation from Mg2+ [58]. Moreover, passive diffusion through the OM is not necessarily negligible for TCs and FQs. For the more hydrophobic TCs it has been shown that they cross the OM rapidly, most probably through the bilayer region [76]. FQs and lipophilic TCs, such as minocycline cross the cytoplasmic membrane freely [77].