Quantitation membrane potential

Free Ions Versus Complexed Ions In discussing the ion-selective electrode, we noted that the membrane potential is influenced by the concentration of F , but not the concentration of HF. An analysis for fluoride, therefore, is pH-dependent. Below a pH of approximately 4, fluoride is present predominantly as HF, and a quantitative analysis for total fluoride is impossible. If the pH is increased to greater than 4, however, the equilibrium... 

The conclusion above is valid for ideally selective membranes. Real membranes in most cases have limited selectivity. A quantitative criterion of membrane selectivity for an ion to be measured, relative to another ion M +, is the selectivity coefficient The lower this coefficient, the higher the sefectivity wifi be for ions relative to ions An electrolyte system with an imperfectly selective membrane can be described by the scheme (5.16). We assume, for the sake of simplicity, that ions and have the same charge. Then the membrane potential is determined by Eq. (5.17), and the equation for the full cell s OCV becomes... 

The membrane potential and the concentration gradient can reinforce each other or they can be in opposition to each other. The total force tending to move a molecule or ion through a membrane is called the electrochemical potential. When the concentration gradient and the electrical potential work to oppose each other, the stronger effect wins. If someone forces you to get quantitative (this may be a physiologist rather than a biochemist), see the section on the Nernst equation at the end of the chapter. 

Reers M, Smith TW, Chen LB (1991) J-aggregate formation of a carbocyanine as a quantitative fluorescent indicator of membrane potential. Biochemistry 30 4480-4486... 

The extent of ion permselectivity displayed by a membrane can be expressed quantitatively by the transference numbers [88] for cations (t+) and anions (t ) within the membrane. Transference numbers can be determined potentiometrically by using a concentration cell [88], in which the membrane to be evaluated separates two electrolyte solutions that contain the same salt but at different concentrations. For a 1 1 salt, the membrane potential (E ,) is given by... 

The behavior of an ion type is described quantitatively by the Nernst equation (3). A /g is the membrane potential (in volts, V) at which there is no net transport of the ion concerned across the membrane (equilibrium potential). The factor RT/Fn has a value of 0.026 V for monovalent ions at 25 °C. Thus, for K, the table (2) gives an equilibrium potential of ca. -0.09 V—i. e., a value more or less the same as that of the resting potential. By contrast, for Na ions, A /g is much higher than the resting potential, at +0.07 V. Na" ions therefore immediately flow into the cell when Na channels open (see p. 350). The disequilibrium between Na" and IC ions is... 

It is at present still difficult to correlate the absolute intensity of the SHG with the number of cationic complexes at the membrane surface. Therefore, a quantitative discussion, showing how the permselective uptake of primary cations forming SHG active complexes into the membrane side of the phase boundary corresponds to the increase in the membrane potential, is not possible yet. Lipophilic derivatives of photoswitchable azobis(benzo-15-crown-5) were recently shown as a molecular probe to determine photoinduced changes in the amount of the primary cation uptake into the membrane phase boundary in relation to the photoinduced EMF changes under otherwise identical conditions. 

It should be acknowledged that understanding of the effects of dopamine on whole cell behavior, in terms of modulatory effects on ion channels, is at a more preliminary and somewhat speculative stage. Data on the modulation of individual channels by dopamine now has to be put in the perspective of the membrane potential fluctuations of the whole cell. A detailed and quantitative analysis is crucial to understanding the modulatory actions of dopamine on membrane currents, because exactly which currents are available depends on the recent history of the cell, for this is what determines which of the many currents are turned on and thus available for modulation by dopamine. 

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. 

Quantitatively, the usual resting membrane potential of — 70 mV Is close to but lower In magnitude than that of the potassium equilibrium potential calculated from the Nernst equation because of the presence of a few open Na channels. These open Na channels allow the net Inward flow of Na" ions, making the cytosolic face of the plasma membrane more positive, that is, less negative, than predicted by the... 

---