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P Mueller

Publications and source records attributed to P Mueller.

At least 55 records · Page 3Linked to original sources
PubMed1984

Protons suppress the dark current of frog retinal rods.

Outer segments of rod photoreceptors with the attached ellipsoid region of the inner segment were isolated from Rana pipiens retinae, and their membrane photocurrents measured with the suction electrode technique in the 'ellipsoid-in' configuration. Under dark adapted conditions in standard Ringer solution, isolated rod outer segments with ellipsoids exhibited maximal photocurrents of 10-30 pA, and light sensitivities of 0.2-1.0 pA/isomerization. A local perfusion technique was employed to change rapidly the solution bathing the outer segment. Rods were tested for their sensitivity to protons by perfusion with Ringer solution of altered pH. The dark current was reversibly suppressed by low pH: in Ringer solution with Calcium activity aCa = 10(-3)M dark current suppression obeyed a hyperbolic saturation law with apparent dissociation constant, pKa = 4.8. The decay of dark current of rods following poisoning with ouabain was retarded by low pH perfusion, as it was by light. Protons thus act to suppress the outer segment Na+ conductance. Three experiments support the hypothesis that protons act interior to the plasma membrane in suppressing the dark current. (1) Perfusion of rods at constant pH with Ringer solution having increased CO2 suppressed the dark current. (2) Removal of perfusate containing 50 mM-NH4Cl causes transient dark current suppression. (3) Acetate, which acts as a neutral proton carrier, when added to Ringer solution, shifts the apparent pKa of dark current suppression to a higher pH. Dark current suppression by protons and recovery occurred with a time constant of ca. 1 s. Low pH perfusion retarded the recovery of the dark current from a saturating flash, slowed the light response in its linear range, and increased light sensitivity. Perfusion at pH = 10.5 caused a slight increase in dark current, sped up the recovery of the rod from a saturating flash, accelerated the linear response and decreased the light sensitivity. Lowering aCa of the Ringer solution caused the proton sensitivity of the dark current to drop. At aCa = 5 X 10(-6) M the apparent pKa of dark current suppression was shifted about 0.8 pH units to pH = 4.0. Cells at aCa = 10(-9) M were insensitive to pH = 3.5, which completely suppressed the dark current at normal aCa. Lowered aCa decreased light sensitivity. Both proton sensitivity and light sensitivity of dark current suppression were estimated for each member of a population of rods in various aCaS:proton sensitivity and light sensitivity were found to be linearly correlated over a range of 3 log units.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1983

Formation and properties of cell-size lipid bilayer vesicles.

Hydration of single or mixed phospholipids or lipid protein mixtures at low ionic strength results in the formation of a population of large, solvent free, single bilayer vesicles with included volumes of up to 300 microliters/mumol lipid. Their size ranges from 0.1 to 300 microns and they can be sorted out according to size by centrifugation. When formed in distilled water their internal solution has a conductivity of 20-50 microseconds/cm-1, an osmolarity of 0.5-5 mOsM, and a density of 1.0005-1.001. The osmotic pressure produced by the internal solutes cause a surface stress of 25 dyn/cm for a 20-microns vesicle. Their elastic constant ranges from 75-150 dyn/cm. During formation they can internalize particles such as latex beads or cell nuclei. They can be impaled with microelectrodes, or patch clamped. They can also be sealed to a small Vaseline-treated hole in a thin partition between two aqueous compartments. Sealing occurs in two stages. In the first stage sealing resistance is similar to that seen with patch-clamp pipettes. In the second stage, a much tighter seal is obtained. After sealing, the smaller portion of the sealed vesicle can be selectively broken by an electric shock leaving a single membrane across the hole. The capacitance and resistance of such membranes, in the presence of 10 mM NaCl, are approximately 0.7 microF/cm2 and 10(8) omega cm2 for pure lipid vesicles. Gramicidin increases the membrane conductance and monazomycin induces voltage-dependent gating thus providing further evidence that the vesicles are bounded by a single bilayer.

Animals↗
PubMed1983

Protons block the dark current of isolated retinal rods.

Membrane currents of isolated frog rods were recorded with the suction pipette technique and tested by perfusion techniques for their sensitivity to H+. The following facts have been established. (i) Increased [H+] suppresses the Na+ conductance of the outer segment rapidly and reversibly. (ii) H+ acts in the rod interior. (iii) The [H+] necessary to cause a 50% decrement in Na+ conductance is inversely related to the [Ca2+] over 5 orders of magnitude. (iv) The sensitivity to H+ and the sensitivity to light, as a function of [Ca2+], have the same slope. Thus, H+ act like light in effecting membrane current suppression but behave as if their effect is mediated through Ca2+. Based on these results and properties of rod disk membrane phosphodiesterase, we propose that protons produced in the light-activated hydrolysis of cGMP liberate Ca2+ from the disks by ion exchange.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1982

Dipole moment of alamethicin as related to voltage-dependent conductance in lipid bilayers.

The dipole moment of alamethicin, which produces voltage-dependent conductance in lipid-bilayer membranes, was measured in mixed solvents of ethanol and dioxane. The value of the dipole moment was found to increase from 40 to 75 DU (Debye units), as the concentration of ethanol increased from 0 (pure dioxane) to 40%. The relaxation frequency of alamethicin also changes from 10 to 40 MHz, depending upon the concentration of ethanol in mixed solvents. The length of alamethicin was calculated by using the relaxation time and was found to range from approximately 40 to 20 A. The dipole moment was independently calculated from voltage-dependent conductance and compared with the measured value. The calculated value was found to be larger than the value of direct measurements, indicating that several alamethicin molecules are required to form a conducting pore and that their dipole moments are oriented parallel to each other.

Alamethicin↗
PubMed1982

Photoelectric currents across planar bilayer membranes containing bacterial reaction centers. Response under conditions of single electron turnover.

Light-induced electric current and potential responses have been measured across planar phospholipid membranes containing reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides. Under conditions in which the reaction centers are restricted to a single electron turnover, the responses can be correlated with the light-induced electron transfer reactions associated with the reaction center. The results indicate that electron transfer from the bacteriochlorophyll dimer to the primary ubiquinone molecule, and from ferrocytochrome c to the oxidized dimer occur in series across the planar membrane. Electron transfer from the primary to secondary ubiquinone molecule is not electrogenic.

Bacteriochlorophylls↗
PubMed1980

Construction of a flash-activated cyclic electron transport system by using bacterial reaction centers and the ubiquinone-cytochrome b-c1/c segment of mitochondria.

Single-turnover electron transfer within the mitochondrial complex III has been studied by combining, in solution, the isolated complex from bovine heart with detergent-solubilized reaction centers of Rhodopseudomonas sphaeroides. Initiation of electron transfer by short flash activation resulted in the prompt oxidation of cytochrome c and reduction of cytochrome b. The subsequent reduction of ferricytochrome c was observed to be concomitant with the oxidation of the ferrocytochrome b, both reactions being inhibited by the addition of actimycin A. The rate of electron transfer through complex III is dependent upon the ambient redox potential poise in a way that is consistent with the presence of a redox component, presumably analogous to the photosynthetic ubiquinone Qz, which is an obligatory intermediate in electron transfer between cytochromes b and c. These results demonstrate cyclic electron transfer in a constructed assembly of mitochondrial complex III, cytochrome c, and photochemical reaction centers.

Animals↗
PubMed1976

Molecular aspects of electrical excitation in lipid bilayers and cell membranes.

Several compounds of fungal or bacterial origin (EIM, alamethicin, monazomycin, DJ400B) can be incorporated into planar lipid bilayers where they form molecular channels and generate voltage-dependent ion conductances. When studied by voltage clamp, the kinetic and steady-state characteristics of these conductance changes are in every respect identical to those found in excitable cell membranes, and their major aspects can be quantitatively described by the Hodgkin-Huxley equations. Thus, the steady-state conductance is an expotential function of the membrane potential, the conductance rises with a sigmoid time course and decays exponentially, and the time constants of the conductance changes go through a maximum as a function of the potential. The conductances also show inactivation as seen in the sodium channels of nerve and the potassium channels of muscle. In addition, there appear for particular pulsing sequences certain kinetic transients that cannot be accounted for by the Hodgkin-Huxley equations but are also seen in identical form in nerve. Because the kinetics are identical in all excitable cell membranes and in these bilayers, it is likely that, in spite of the diverse chemical nature of the channel-forming molecules in the bilayers and the widely differing ion selectivities in the cellular systems, the mechanism by which the membrane opens and closes for the flow of ions is essentially the same in all cases. The kinetic data imply that a cooperative process is involved in the gating action. In principle, two different concepts could account for the kinetics--one involving an intramolecular configurational change within a complex permanent channel, the other, the assembly of a channel through the voltage-dependent aggregation of monomeric channel precursors. In the bilayers the high-order dependence of the steady-state conductance and of the gating time constants on the concentration of the channel formers suggests an aggregation mechanism in which the gating involves the voltage-induced insertion of all or part of the channel-forming molecules from the membrane surface into the hydrocarbon region and their subsequent aggregation into open channels by lateral diffusion. The mathematical description of this two-step insertion-aggregation mechanism accounts quantitatively for the entire conductancb-voltage kinetics including inactivation and other kinetic features which deviate from the Hodgkin-Huxley kinetics in the sense that the rate constants of the changes are dependent not only on the membrane potential but also on the value of the conductance and on time. The proposed mechanism is also in agreement with single-channel data for alamethicin which suggest that both the insertion and the aggregation rate constants are voltage-dependent...

Alamethicin↗
PubMed1975

Membrane excitation through voltage-induced aggregation of channel precursors.

Electrically excitable lipid bilayers show the same voltage-dependent kinetics as nerve and other excitable cells. In the bilayers the gating process involves the voltage-dependent insertion of channel-forming molecules into the hydrocarbon region and their subsequent aggregation by lateral diffusion into an open "barrel stave channel." This process can account quantitatively for the classical Hodgkin-Huxley kinetics including inactivation as well as for certain kinetic features that lie outside the Hodgkin-Huxley domain. The multi- and single-channel kinetics suggest that both the insertion and the aggregation reate constants are voltage-dependent, and it is argued that a voltage-induced lateral phase separation between the lipids and the channel-forming molecules increases the local concentration of channel precursors and their aggregation rates. Because the observed aggregation rates are faster than those calculated from an upper limit of the diffusion constants and the known average concentration in the lipid phase, it is likely that the channel-formers preaggregate at the membrane surface. The structural characteristics of the channel-formers and the evidence supporting a similar excitation mechanism in nerve are discussed.

Alamethicin↗
PubMed1972

Formation of bimolecular membranes from lipid monolayers and a study of their electrical properties.

Bimolecular membranes are formed from two lipid monolayers at an air-water interface by the apposition of their hydrocarbon chains when an aperture in a Teflon partition separating two aqueous phases is lowered through the interface. Formation of the membrane is monitored by an increase of the electrical capacity, as measured with a voltage clamp. Electrical resistance of the unmodified membrane is analogous to that of conventional planar bilayers (black lipid membranes) prepared in the presence of a hydrocarbon solvent, i.e., 10(6)-10(8) ohm cm(2); the resistance can be lowered to values of 10(3) ohm cm(2) by gramicidin, an antibiotic that modifies the conductance only when the membranes are of biomolecular thickness. In contrast to the resistance, there is a significant difference between the capacity of bilayers made from mono-layers and that of hydrocarbon-containing bilayers made by phase transition; the average values are 0.9 and 0.45 muF cm(-2), respectively. The value of 0.9 muF cm(-2) approximates that of biological membranes. Assuming a dielectric constant of 2.1 for the hydrocarbon region, the dielectric thickness, as calculated from a capacity of 0.9 muF cm(-2), is 22 A. This value is 6-10 A smaller than the actual thickness of the hydrocarbon region of bilayers and cell membranes, as determined by x-ray diffraction. The difference may be due to a limited penetration of water into the hydrocarbon region near the ester groups that would lower the electrical resistance of this region and reduce the dielectric thickness. Asymmetric membranes have been formed by adjoining two lipid monolayers of different chemical composition.

Electric Conductivity↗