K+ diffusion potential

The same group also demonstrated that doxorubicin and vinblastine can be rapidly accumulated into egg-PC LUV in response to a valino-mycin-dependent K" " diffusion potential (Mayer et al., 1985b). 

Maintenance of unequal concentrations of ions across membranes is a fundamental property of living cells. In most cells, the concentration of K+ inside the cells is about 30 times that in the extracellular fluids, while sodium ions are present in much higher concentration outside the cells than inside. These concentration gradients are maintained by the Na+-K+-ATPase by means of the expenditure of cellular energy. Since the plasma membrane is more permeable to K+ than to other ions, a K+ diffusion potential maintains membrane potentials which are usually in the range of -30 to -90 mV. H+ ions do not behave in a manner different from that of other ions. If passively distributed across the plasma membrane, then the equilibrium intracellular H+ concentration can be calculated from the Nernst equation via... 

The electrochromic shift of the carotenoids is usually calibrated with K-diffusion potential in the presence of valinomycin. One problem is that the shifts observed in respiring chromatophores (where the proton electrochemical potential is predominantly in the form of a membrane potential) are much larger than those induced by the calibrating diffusion potential, so that an extensive extrapolation is required. Thus, the carotenoids in illuminated chromatophores may indicate a membrane potential in excess of 300 mV, whereas the distribution of CNS, an electrically permeant anion, in the same system only indicates 140 mV [32]. The extent of this discrepancy, and the uncertainty as to whether the carotenoids see the bulk-phase potential, or only the local electrical field within the membrane, limits the confidence with which carotenoids may be used for quantitative as opposed to qualitative potential measurements. 

FIG. 2 (RIGHT). Calculated dependence of the K+ -diffusion potential on the internal diameter of the proteoliposomes. The calculations were based on an external K+ -concentration of 50 mM. The initial internal K+ -concentration (in mM) is indicated in the Figure. 

Fig. 1 shows that the curves obtained with 0.5 and 5 mM K in the internal volume were displaced by only about 0.35 pH-units, or approx. 21 mV from one another. This is due to the effect of the membrane electrical capacitance on the distribution of at equilibrium [4,7]. We used the method of Apell and Bersch [ ] to calculate equilibrium values of the -diffusion potential after addition of proteoliposomes with a known internal -concentration to a medium with 50 mM K, in the presence of valinomycin. Fig. 2 shows the dependence of these diffusion potentials on the internal diameter of the proteoliposomes. The dashed line in Fig. 2 shows that with proteoliposomes of 27 nm internal diameter, the -diffusion potential obtained with an initial internal K -concentration of 0.5 mM is only 21 mV higher than the one obtained with an initial internal -concentration of 5 mM. The diffusion potential obtained in the latter case is 52 mV. These diffusion potentials correspond with ApH-values of 0.36 and 0.88 units, respectively. This is in good agreement with the results shown in Fig. 1, and the required internal diameter of 27 nm is in good agreement with electron-microscopic and other evidence on the size of the proteoliposomes [2]. Furthermore, Fig. 2 shows that vesicles of this diameter generate a K -diffusion potential of only 77 mV even if the initial internal -concentration is zero. Since ATP-synthesis was observed only above a threshold Apjj+ of 90 mV (Fig. 1), this explains why... 

This is consistent with the fact that in Necturus the proximal tubule does not modify the filtered potassium load. There is no significant net transport, reabsorptive or secretory, in the Necturus proximal tubule. The observed electrical asymmetry of this epithelium may be due to the difference in magnitude of two K" " diffusion potentials in series. Thus the simultaneously observed activity distribution ratios can account for all the electrical asjmimetry without the need to invoke a significant effect of passive leak of Na" " on either membrane boundary. 

Fig. 3 shows a single sodium pump in the peritubular cell membrane since the Na extrusion mechanism is not necessarily linked with K uptake into the cell. The intracellular [K ] can all be accounted for on the basis of a passive electro-chemical equilibrium distribution of potassium. The peritubular membrane is known (Giebisch, 1961) to be highly permeable to K" ion and to exhibit a high K selectivity. The observation that the peritubular membrane of the Necturus proximal tubule is characterized by = E may be explained in one of two ways. Either that the PD across the peritubular cell membrane is mainly due to a K" " diffusion potential, or that the peritubular cationic pump is an electrogenic Na pump which by its operation generates E. Then a passive influx occurs to the point where Ej = E. ... 

A series of experiments was first carried out to determine the response of the carotenoid band shift to K+-diffusion potentials. The procedure was similar to that employed on earlier occasions (Jackson, Crofts, 1969 Clark, Jackson, 1981) except that sodium ferri- and ferrocyanide were present as a redox buffer. The treatment of the chromatophores with valinomycin was followed by a period of 10 min to allow ionic equilibration across the membrane. The extent of the absorbance change corresponding to the carotenoid band shift, resulting from a subsequent KCl addition was plotted as a function of the final KCl concentration as shown in Fig.3. 

Fig.4. The size of the K -diffusion potential does not affect the extent of the carotenoid absorbance change elicited by a single flash. Data taken from a series of experiments similar to those described in the (inset to Fig.2. The extent of the flash-induced carotenoid absorbance change is plotted against the value of the diffusion potential reached during the preceding KCl pulse.

According to the nine assumptions and approach a) for the diffusion potential inside the membrane the selectivity coefficient Kg , can be expressed by other parameters. Table 3 shows the results for the different kinds of membranes 66). In some cases the expressions for K J i contain ion-mobilities inside the membrane... 

Because of solubility changes, the saturated calomel RE has a large temperature coefficient (0.65 mV/K). Its main advantages are ease of preparation (an excess of KCl is added to the solution) and low values of diffusion potential at interfaces with other solutions (see Section 5.2). The potentials of calomel REs can be reproduced to 0.1 mV. These electrodes are very convenient for measurements in neutral solutions (particularly chloride solutions). 

Consider a test solution containing both determinand and interferent K, neither of which fonns an ion-pair with ion-exchanger ion A". The mobilities of and K are identical, so that no diffusion potential is fonned in the membrane. The effect of the interferent is based on the fact that it may replace the determinand in the membrane phase as a result of the exchange reaction... 

Consider a system in which the analyte contains both determinand J and interferent K, and where a diffusion potential is formed in the membrane as a result of their different mobilities. A simplification that provides the basic characteristics of the membrane potential employs the Henderson equation for calculation of the diffusion potential in the membrane. According to (2.1.9) the membrane potential is separated into three parts, two potential differences between the membrane and the solutions A 0 and Aq with which it is ip contact, and the diffusion potential inside the membrane... 

The concentrations of ions J and K at point p, which are needed to calculate the diffusion potential, are given by (3.2.16), while the concentration... 

This group of membranes is also frequently called membranes with immobile ion-exchange sites. First systems in which a diffusion potential is formedin the membrane will be considered [10, 12, 18, 19, 25, 41, 54, 70]. It will be assumed that all sites are equivalent and that each has a single negative charge, while the membrane is permeable only for cations. It will be assumed also that two types of univalent cations are present in solution 1, J and K ... 

Equation (4.19) can be used only when (4.20) is valid. In simple systems (rapid processes at the membrane/electrolyte interface and a simple diffusion potential in the membrane) the apparent selectivity coefficient is a function of theflj/flK ratio alone, whereas in more complicated systems it also depends on the activities of J and K. 

K+ ions in the presence of valinomycin do not distribute passively at electrochemical equilibrium rather, this represents a nonequilibrium state in which creates a diffusion potential following which protons move. 

Table I presents six basic equations in a general way. Those on the left apply to transfer within a phase A, and those on the right to transfer across a phase boundary AB. The top row expresses the mutual definition of force F, proportionality constant K, and potential . The second row expresses the phenomenological proportionality between flux J and force F. The bottom row states the conservation constraints. The left equation says merely that in a given volume the difference between the accumulation rate and the emanation rate must be attributed to a source S. As stated, these equations apply to any conserved quantity which is diffusing, either within a phase under the influence of a potential gradient or across a phase under the influence of a potential difference.

We note that fik is sometimes called the diffusion potential of component k in the metal physics literature. 

K+ diffuses down its concentration gradient accompanied by an equivalent amount of CP. Equilibrium is established when the potentials... 

Taking for the diffusion potential the Henderson diffusion potential and adding the two Donnan potentials one gets the well-known formula of K. H. Meyer (97, 98), J. F. Sievers (97, 98) and T. Teorell (168). 

The interfacial pd selectivity coefficient, the factor multiplying a is determined by the ratio K /K, by the activity coefficient ratio, and by the mobility ratio, when the internal diffusion potential contribution is added. Clearly interferences should correlate with the ratio K /1C, which can be determined from salt extraction coefficients K KX/K K for a series of positive drugs, using common anion salts. This result is well documented in the literature (7,8). A curious correlation for N-based drugs studied by us and by Freiser ( ) is a trend in selectivity... 

The addition of valinomycin, a K+ ionophore, leads to generation of a diffusion potential through efflux of K+ down its concentration gradient. The F protein permits uptake of H+ by the vesicles (which are impermeable to H+ in the absence of F ) in response to this diffusion-generated gradient. 

Example 10.5 Diffusion cell and transference numbers The diffusion cell shown in Figure 10.2 has NaCl mixtures in the two chambers with concentrations c1A = lOOmmol/L and c1B = lOmmol/L. The mobilities of Na+ and Cl- ions are different and their ratio yields their transference numbers b+lb = t+/t = 0.39/0.61 (NaCl). The transference number t for an ion is the fraction of the total electric current carried by the ion when the mixture is subjected to an electric potential gradient. For monovalent ions, we have t+lt = 1. Estimate the diffusion potential of the cell at steady-state conditions at 298 K. Assume that activity coefficients are equal in the two reservoirs (Garby and Larsen, 1995). 

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