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J Sandblom

Publications and source records attributed to J Sandblom.

32 records · Page 2Linked to original sources
PubMed1982

Displacement currents in lipid bilayer membranes.

The processes of charge redistribution in phospholipid bilayer membranes have been examined by voltage clamping. Different types of phospholipids dissolved in n-decane were used as membranes and the membranes were made conducting by adding ionophores. The relaxation processes were found to be composed of at least two exponential decays of which the fast process can be explained in terms of a charge redistribution between the surface layers and the interior of the membrane, following the step in potential. The effect on this of varying the lipid composition, the external solution conditions and the applied potential are described in terms of changes in the capacitance CP and the resistance RP of the surface layers. A model in which a structural rearrangement of the lipid polar groups is responsible for the observed changes in CP and RP is also discussed.

Anti-Bacterial Agents↗
PubMed1980

The current-voltage behavior of ion channels: important features of the energy profile of the gramicidin channel deduced from the conductance-voltage characteristic in the limit of low ion concentration.

The conductance-voltage (G-V) characteristic of a single-filing, multi-barrier, multi-occupancy channel depends in the limit of low ion concentration upon only two parameters: the voltage dependence of the entry step and the ratio of the rate constant for leaving the channel to that for crossing its middle (14,17,20). We show that the G-V shape in this low concentration limit can be measured accurately using a triangular wave, many-channel technique and demonstrate that the observed shape is incompatible with that expected if the only important rate limiting barrier at low concentration were at the channel mouth. Instead the central barrier turns out, surprisingly, in view of the markedly sublinear I-V shape at low concentration, to be even slightly larger than the exit barrier. Additionally, we find that it is not possible to fit both the G-V shape and the concentration dependence of the zero-current conductance simultaneously with a 3-barrier 2-site model. However, by adding additional sites to yield a 3-barrier 4-site model either of the type 3B4S" where the extra site in each channel half is external to the mouth of the channel or of the type 3B4S' where the extra site is internal to the mouth of the channel, we obtain good agreement. Additionally, using the flux ratio data of Procopio and Andersen (19) to discriminate between 3B4S and 3B4S" models, we find the 3B4S" model to be the only satisfactory one.

Electrolytes↗
PubMed1980

Impedance of the frog gastric mucosa, the Teorell-Wersäll effect.

The small signal impedance of frog gastric mucosa has been measured from 1 Hz to 500 kHz. The results obtained in Ringer's solution have been compared with the effect of replacing sodium with potassium on the nutrient side of the mucosa. The substitution to potassium reduces the membrane resistance and changes the impedance locus from a flat to a semicircular shape. We have examined this effect, the Teorell-Wersäll effect, in relation to changes in membrane permeabilities. The experimental data are consistent with a model based on the electrical behaviour of transmission cables (the geometric requirement for this may be found in the gastric duct) and the observed electrical properties can be predicted from an increase in conductance of the secretory membrane when replacing sodium with potassium.

Animals↗
PubMed1978

Ionic selectivity, saturation, and block in gramicidin A channels. II. Saturation behavior of single channel conductances and evidence for the existence of multiple binding sites in the channel.

A theory, recently developed by Sandblom, Eisenman and Neher (1977) for the conductance of single gramicidin A cha-nelspred icts three limiting behaviors of the relation between conductance and salt concentration. These are: (i) a saturating behavior resembling a simple adsorption isotherm at medium and high concentrations, (ii) a decrease in conductance at the highest obtainable concentrations and (iii) deviations from the isotherm at very low concentrations. Features i and ii have been described before. Experimental evidence for point iii is given here. The new feature points towards interactions among ions in the channel at ionic concentrations as low as 1--10 mM. Particular emphasis is given to the behavior at very low salt concentrations and the experimental problems encountered in this situation. In addition, mutual blocking effects among monovalent ions in symmetrical salt mixtures are characterized and found to be in satisfactory agreement with theoretical expectations, based upon the single salt conductance data presented here and zero-current potentials in salt mixtures to be described in a subsequent paper.

Binding Sites↗
PubMed1978

Interactions in cation permeation through the gramicidin channel. Cs, Rb, K, Na, Li, Tl, H, and effects of anion binding.

As a prototype for binding and interaction in biological Na and K channels, the single channel conductances for Li, Na, K, Rb, Cs, H, and Tl and the membrane potentials for Tl-K mixtures are characterized for gramicidin A over wider concentration rangers than previously and analyzed using an "equilibrium domain" model that assumes a central rate-determining barrier. Peculiarities in the conductance-concentration relationship for TlF, TlNO3, and TlAc suggest that anions bind to Tl-loaded channels, and the theory is extended to allow for this. For concreteness, the selectivity of cation permeation is characterized in terms of individual binding and rate constants of this model, with the conclusions that the strongest site binds Cs greater than Rb greater than K greater than Na greater than Li, while the next strongest binds Na greater than K greater than Li greater than Rb greater than Cs. However, because Schagina, Grinfeldt, and Lev's recent finding of single filing (personal communication) indicates that the channel sites in gramicidin cannot be at equilibrium with the solution, and work in progress with Hägglund and Enos (Biophys. J. 21:26a. [Abstr.]) indicates that the simplest model adequate to account for the observed concentration-dependences of flux-ratio, conductance, I--V characteristic, and permeability has three barriers and four sites, some implications of additional rate-determining barriers at the mouth of the channel are discussed. The results are summarized using phenomenological "experimental" parameters that provide a model-independent way to represent that data concisely and which can be interpreted physically in terms of any desired model.

Anions↗
PubMed1977

Ionic selectivity, saturation and block in gramicidin A channels: I. Theory for the electrical properties of ion selective channels having two pairs of binding sites and multiple conductance states.

A model for the gramicidin A channel is proposed which extends existing models by adding a specific cationic binding site at each entrance to the channel. The binding of ions to these outer channel sites is assumed to shift the energy levels of the inner sites and barriers and thereby alter the channel conductance. The resulting properties are analyzed theoretically for the simplest case of two inner sites and a single energy barrier. This for-site model (two outer and two inner) predicts that the membrane potential at zero current (Uo) should be a Goldman-Hodgkin-Katz equation with concentration-dependent permeability ratios. The coefficients of the concentration-dependent terms are shown to be related to the peak energy shifts of the barrier and to the binding constants of the outer sites. The thory also predicts the channel conductance in symmetrical solutions to exhibit three limiting behaviors, from which the properties of the outer and inner sites can be characterized. In two-cation symmetrical mixtures the conductance as a function of mole fraction is shown to have a minimum, and the related phenomenon of inhibition and block exerted by one ion on the other is explained explicitly by the theory. These various phenomena, having ion interactions in a multiply occupied channel as a common physical basis, are all related (by the theory) through a set of measurable parameters describing the properties of the system.

Electric Conductivity↗
PubMed1975

Electrical relaxation processes in black lipid membranes in the presence of a cation-selective ionophore.

The time course of relaxation of the electric current following steps in the applied potential across lipid bilayer membranes has been measured. The membranes were made cation-selective by the addition of nonactin. To permit the measurement of very short time constants a voltage clamp device was developed in order to reduce the charging period to less than 1 musec, regardless of the magnitude of the series resistances in the external solutions. It was possible by this method to establish the presence of two electric processes, which were found to behave differently with respect to temperature, applied potential and external solution conditions. The rapid process (tau approximately 10 musec) was interpreted in terms of the electric parameters of the polar part of the membrane according to the theory developed by Hägglund and Sandblom (T.I.T.J. Life Sic. 2: 107, 1972). The second process (tau approximately 100 musec) showed a behavior consistent with the model of Stark, Ketterer, Benz and Läuger (Biophys. J. 11:981, 1971), which considers the different rate constants involved in the net transfer of carriermediated ion transport across bilayer membranes.

Anti-Bacterial Agents↗
PubMed1972

The transient response and impedance locus of a mobile site membrane.

The kinetic properties of a mobile site membrane are analyzed theoretically and the results are compared with experimental data obtained from a simple model system. In the model system the measured response to a step change in electric current is a single relaxation process, in excellent agreement with the theoretically predicted behavior. To this relaxation process corresponds an anomalous capacitance whose properties are obtained by performing a frequency analysis of the system. By plotting the impedance locus of this mobile site membrane, the locus is shown to exhibit a 45 degrees phase angle at high frequencies, a phenomenon which is generally associated with depletion layers.

Binding Sites↗
PubMed1972

Anomalous reactances in electrodiffusion systems.

A frequency response analysis of a constrained diffusion boundary has been made by linearizing the Nernst-Planck equations for a small applied AC current. The number of time constants and their dependence on ionic concentrations and electric field as well as membrane parameters such as dielectric constant, thickness, etc. have been evaluated by this method. Numerical solutions have been carried out for cases when the Planck charging time can be neglected and the results are presented in the form of impedance loci. These impedance loci show that if the membrane separates two univalent electrolytes with a common anion it will exhibit a combined capacitative inductive response with a 90 degrees phase angle. The dependence of these anomalous reactances on ionic concentrations and the electric field is consistent with the behavior of the Hodgkin-Huxley axon suggesting that a homogeneous electrodiffusion regime could be adequate as a basic model for the kinetic behavior of biological membranes.

Diffusion↗