Transport mechanisms in chloride channels.
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Amphotericin B↗
Biomedical subjects
F Franciolini
Publications and source records attributed to F Franciolini.
Newly identified steady-state potassium channels in rat hippocampal neurons.
We report two new types of potassium channels in cultured hippocampal neurons of rat. Both channels occurred in the soma membrane of these cells at very low density. They were active in steady-state conditions, within a wide voltage range that included the resting membrane potential. Their open probability was enhanced by membrane depolarization, but not influenced by Ca ions. In symmetrical 150 mM KCl the channels showed a slope conductance of ca. 40 and 80 pS, respectively. Current-voltage relations of both K channels show a negative slope at high positive voltages.
Animals↗
Chloride channels of biological membranes.
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Animals↗
Single-channel currents activated by low intracellular pH in cultured hippocampal neurons of rat.
Patch clamp technique was applied to the plasma membrane of cultured hippocampal neurons of rat. Elementary currents of a cation-selective channel were elicited by low intracellular pH (pHi 3.5-4.5). Channel activity starts with 1-2 min delay from the application of low pHi, and persists upon restoration of physiological pH conditions. The channel has a conductance of approx. 110 pS in symmetrical 300 mM NaCl, and is strongly selective for cations over anions. The channel is active over the whole voltage range tested (from +75 mV to -75 mV). Mean open time is function of voltage, increasing with depolarization. Low pH applied extracellularly did not activate the channel.
Animals↗
Determination of dependence of spin-lattice relaxation rate in serum upon concentration of added iron by magnetic resonance imaging.
Dependence of spin-lattice relaxation rate (1/T1) in serum upon concentration of added iron was studied in the concentration range 0.0179-0.179 mmol l-1 for each of ferrous and ferric iron. In conjunction with the serum study, 1/T1 in solutions of transferrin and a mixture of albumin and gamma globulin was also studied as a function of added iron concentration. At low concentrations 1/T1 in serum increases linearly with increasing amounts of iron for each ion, and then reaches saturation for ferrous iron, whereas it shows an inflection for ferric iron. To explain the partition of added iron between various serum components, the effect of iron on 1/T1 in serum was compared with those of transferrin and the mixture. This effect can be defined as relaxivity or the incremental increase in relaxation rate per millimolar of added iron. At low concentrations the relaxivities of iron in serum, about 0.91 mmol-1 l s-1 for ferric and 0.95 mmol-1 l s-1 for ferrous ion, approximate well to the relaxivity of iron in transferrin solutions, which was measured to be about 0.92 mmol-1 l s-1. Furthermore, at high concentrations the relaxivity of ferric iron in serum, 0.44 mmol-1 l s-1, becomes similar to that of the mixture which is about 0.39 mmol-1 l s-1. These findings imply that iron added to serum first satisfies the binding requirements of transferrin, and the binding of iron to the other serum proteins occurs at high concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)
Blood↗
Spin-lattice relaxation rates in Fe(III)-doped human serum measured by magnetic resonance imaging.
The proton spin-lattice relaxation time T1 in iron-doped serum was measured with a magnetic resonance imager operating at 0.5 T. The T1 in aqueous solutions of iron and iron-doped solutions of albumin and gamma globulin was also measured in order to analyse the paramagnetic contribution in iron-doped serum. The enhancement in serum is not linearly dependent on concentration of iron added. It is known that Fe(III) added to serum is mainly complexed with transferrin, albumin, gamma globulin and water. With serum pH (7.9) Fe(III) in the protein solution results in enhancement. Fe(III) in water does not cause any enhancement. As a result, the 1/T1 enhancement in serum should be caused solely by iron-binding serum proteins.
Blood↗
Evolution of ionic channels of biological membranes.
This paper presents a view of the evolution and phylogenetic distribution of ionic channels of biological membranes. The view is based on the assumptions that ionic channels (1) appeared very early in the history of life, (2) have evolved from a common ancestor, and (3) have been subjected to evolutionary pressure to reach precision and high speed of signaling. We propose that Ca2+ was the intracellular messenger and modulator of the most primitive biological systems, which implies that the first channel to appear may have been a calcium channel. Then, very soon the entire group of potassium channels evolved from the calcium channel to improve the shape of signals and to restore initial conditions. Sodium channels probably appeared relatively late, diversifying from calcium channels in the early metazoan groups. Mainly because Na+ ions do not interfere with cellular metabolism (thus allowing the inward current--and, consequently, the speed of conduction--to be greatly increased), sodium channels probably proved advantageous in the generation of the action potential, and selection replaced calcium channels with sodium channels in this function. Finally, with the acquisition of multicellularity, channels responsible for synaptic transmission appeared. The case of the acetylcholine receptor channel is briefly discussed.
Animals↗
Calcium and voltage dependence of single Ca2+-activated K+ channels from cultured hippocampal neurons of rat.
Calcium and voltage dependence of the Ca2+-activated K+ channel, K(Ca), was studied at the single-channel level in cultured hippocampal neurons from rat. The K(Ca) channel has approx. 220 pS conductance in symmetrical 150 mM K+, and is gated both by voltage and by Ca2+ ions. For a fixed Ca2+ concentration at the inner membrane surface, [Ca]i, channel open probability, Po, increases e-fold for 14 mV positive change in membrane potential. At a fixed membrane potential (0 mV), channel activity is first observed at [Ca]i = 10(-6) M, and increases with Ca2+ concentration approximating an absorption isotherm with power 1.4. The [Ca]i required to half activate (Po = 0.5) the channel is 4.10(-6) M. When compared to other preparations, the K(Ca) channel from hippocampal neurons reported here shows the lowest Ca2+ sensitivity and the highest voltage sensitivity. These findings are interpreted in evolutionary terms.
Animals↗
Single channel recording and gating function of ionic channels.
This review discusses several aspects of the kinetic analysis of the gating function of membrane channels, as carried out on single channel data obtained by the patch clamp method. In particular, the following three aspects of channel behavior are reviewed in some detail: 1) estimate of the number of states or conformations the channel can enter; 2) the lifetime of each state as a function of transmembrane voltage (voltage-gated channels), or of ligand concentration (ligand-gated channels); 3) estimates of the rates at which the transitions between the various states occur, and assessment of reaction mechanisms. An introductory description of conformational transitions of channel proteins is also provided.
Animals↗
Single chloride channels in cultured rat neurones.
Single-channel currents through chloride channels were recorded in cultured hippocampal neurones from rats using the patch-clamp method. The channel is active at voltages between -80 and +80 mV, and the time spent in the open state increases with depolarization (almost fourfold for 120 mV). The channel conductance is 62 pS in symmetrical 150 mM NaCl saline. In salt gradient conditions the channel was measurably permeable to Na+. Substitution of NO3- and Br- for Cl- gave higher single-channel currents, meaning a higher permeability of the channel toward the two anions than to Cl-. SO4(2-) ions were poorer charge carriers, yet contributed measurable inward current at negative voltages. Channel activity appeared independent of intracellular Ca2+ concentration. Taken together, these features would suggest for this channel a role in stabilizing resting membrane potential and in maintaining normal cell excitability.
Animals↗
Spontaneous firing and myelination of very small axons.
In this article the question of what evolutionary factors guided acquisition of myelin in the nervous system is addressed. The conclusion that conduction velocity of action potentials along the axon has been the only motive force needs reformulation, as other factors may have played a central role as well. In particular, protection against firing of spontaneous action potentials which may result from the simultaneous opening of only few (less than 10) sodium channels at the nodes of small (less than 1 micron diameter) myelinated axons, may have greatly contributed to discouraging myelination of axons smaller than 1 micron.
Action Potentials↗
Blocking of the squid axon K+ channel by noxiustoxin: a toxin from the venom of the scorpion Centruroides noxius.
We have studied the selective effects of noxiustoxin (NTX), a fraction of the venom of the scorpion Centruroides noxius, on the K currents of perfused squid giant axons using the voltage-clamp technique. At concentrations below 1.5 microM, NTX blocked K currents in a voltage-independent manner, with little effect on their turning-on and turning-off kinetics. Above 1.5 microM, the block by NTX became voltage-dependent and could be partially removed by repetitive pulsing and strong depolarizations. Long repolarizations and more negative holding potentials favoured the slow restoration of channel block. Reduction of K currents by internally perfusing the fibers with solutions of low K+ concentration (200 mM), affected very little the removal of NTX-block during repetitive pulsing, suggesting that block removal depended on membrane potential and not on outward movements of K+ ions through open channels. In high extracellular K+ (300 mM) the blocking action of NTX was reduced and the instantaneous I-V characteristics showed a marked outward rectification. At 20 microM NTX, inward tail currents measured on step repolarizations to -70 mV were fully blocked, suggesting a direct interaction of the toxin with the open channel. The effects of the total venom Centruroides noxius Hoffmann was also studied. External application of 0.25 mg/ml of the venom caused a marked reduction of both Na and K currents, an effect similar to that of other scorpion venoms.
Animals↗
Anion and cation permeability of a chloride channel in rat hippocampal neurons.
The ionic permeability of a voltage-dependent Cl channel of rat hippocampal neurons was studied with the patch-clamp method. The unitary conductance of this channel was approximately 30 pS in symmetrical 150 mM NaCl saline. Reversal potentials interpreted in terms of the Goldman-Hodgkin-Katz voltage equation indicate a Cl:Na permeability ratio of approximately 5:1 for conditions where there is a salt gradient. Many anions are permeant; permeability generally follows a lyotropic sequence. Permeant cations include Li, Na, K, and Cs. The unitary conductance does not saturate for NaCl concentrations up to 1 M. No Na current is observed when the anion Cl is replaced by the impermeant anion SO4. Unitary conductance depends on the cation species present. The channel is reversibly blocked by extracellular Zn or 9-anthracene carboxylic acid. Physiological concentrations of Ca or Mg do not affect the Na:Cl permeability ratio. The permeability properties of the channel are consistent with a permeation mechanism that involves an activated complex of an anionic site, an extrinsic cation, and an extrinsic anion.
Animals↗
Patch clamp technique and biophysical study of membrane channels.
The present work describes the patch clamp technique, which first allowed the recording of single channel currents in biological membranes. In particular, it describes procedures for preparation and applications of the four different patch clamp configurations. Briefly, the cell-attached configuration is widely used for investigating channel modulation by transmitters acting via second messengers. The cell-free configurations (inside-out and outside-out), complementary to one another with respect to the orientation of the membrane surface, are particularly indicated for the study of the biophysics (kinetics, conductivity, selectivity, mechanism of permeation and block) of ionic channels. Finally, the whole-cell configuration which, because of the remarkable feature that it allows voltage clamp of very small cells, has given access to a number of physiologically important preparations never studied before.
Animals↗
The sarcoplasmic reticulum: a comparative study.
The diversity of cellular membrane structures associated with regulation of intracellular calcium level is described in several different groups of organisms and cells. All the instances reported refer to cellular processes related to movement, in which calcium ion acts as trigger and/or modulator. In addition, a simplified five-stage picture of the underlying view of evolution of these structures is presented. In short: the choice made by nature in using calcium as intracellular messenger was very early in the history of life; all cellular structures devoted to intracellular calcium regulation, from the simplest form of amoeba to the highly sophisticated apparatus of mammalian skeletal muscle, can be linked together in the chain of evolution. Because the evidence is still sparse, any conclusion more positive would be speculative and of little value. Hopefully, in the coming years, with a better understanding of membrane architecture as a whole and its protein components (i.e. calcium channels, calcium-binding proteins), we will be able to test the first segments of this evolutionary hypothesis.
Amoeba↗
Circadian activity of rat kidney enzymes.
Alkaline phosphatase, LAP, beta-glucuronidase and cathepsin D activities and protein content of the kidney homogenate did not show any circadian rhythm in animals sacrificed at different hours of the day. The fluctuations of maltase appear modest and not dependent on a ligh/dark cycle.
Animals↗
Early effects in kidney enzyme activities after irradiation.
Brush border enzymes of proximal tubules, lysosomal activities and protein content of rat kidney were analysed after whole-body irradiation using two different experimental schedules. Maltase, alkaline phosphatase and beta-glucuronidase activities increased moderately during the first days after irradiation, whereas LAP, cathespsin D activities and protein content were not modified. No evident morphologic alterations were observed.
Alkaline Phosphatase↗
Effect of carbon dioxide on calcium transient and tension responses from fibres of the barnacle Balanus nubilus [proceedings].
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Animals↗