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Query: UMLS:C0027960 (mole)
 21,279 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The mechanism of Cl ion permeation through single cystic fibrosis transmembrane conductance regulator (CFTR) channels was studied using the channel-blocking ion gluconate. High concentrations of intracellular gluconate ions cause a rapid, voltage-dependent block of CFTR Cl channels by binding to a site approximately 40% of the way through the transmembrane electric field. The affinity of gluconate block was influenced by both intracellular and extracellular Cl concentration. Increasing extracellular Cl concentration reduced intracellular gluconate affinity, suggesting that a repulsive interaction occurs between Cl and gluconate ions within the channel pore, an effect that would require the pore to be capable of holding more than one ion simultaneously. This effect of extracellular Cl is not shared by extracellular gluconate ions, suggesting that gluconate is unable to enter the pore from the outside. Increasing the intracellular Cl concentration also reduced the affinity of intracellular gluconate block, consistent with competition between intracellular Cl and gluconate ions for a common binding site in the pore. Based on this evidence that CFTR is a multi-ion pore, we have analyzed Cl permeation and gluconate block using discrete-state models with multiple occupancy. Both two- and three-site models were able to reproduce all of the experimental data with similar accuracy, including the dependence of blocker affinity on external Cl (but not gluconate) ions and the dependence of channel conductance on Cl concentration. The three-site model was also able to predict block by internal and external thiocyanate (SCN) ions and anomalous mole fraction behavior seen in Cl/SCN mixtures.
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PMID:Multi-Ion mechanism for ion permeation and block in the cystic fibrosis transmembrane conductance regulator chloride channel. 937 69

A distinctive feature of the voltage-dependent chloride channels ClC-0 (the Torpedo electroplaque chloride channel) and ClC-1 (the major skeletal muscle chloride channel) is that chloride acts as a ligand to its own channel, regulating channel opening and so controlling the permeation of its own species. We have now studied the permeation of a number of foreign anions through ClC-1 using voltage-clamp techniques on Xenopus oocytes and Sf9 cells expressing human (hClC-1) or rat (rClC-1) isoforms, respectively. From their effect on channel gating, the anions presented in this paper can be divided into three groups: impermeant or poorly permeant anions that can not replace Cl- as a channel opener and do not block the channel appreciably (glutamate, gluconate, HCO3-, BrO3-); impermeant anions that can open the channel and show significant block (methanesulfonate, cyclamate); and permeant anions that replace Cl- at the regulatory binding site but impair Cl- passage through the channel pore (Br-, NO3-, ClO3-, I-, ClO4-, SCN-). The permeability sequence for rClC-1, SCN- approximately ClO4- > Cl- > Br- > NO3- approximately ClO3- > I- >> BrO3- > HCO3- >> methanesulfonate approximately cyclamate approximately glutamate, was different from the sequence determined for blocking potency and ability to shift the Popen curve, SCN- approximately ClO4- > I- > NO3- approximately ClO3- approximately methanesulfonate > Br- > cyclamate > BrO3- > HCO3- > glutamate, implying that the regulatory binding site that opens the channel is different from the selectivity center and situated closer to the external side. Channel block by foreign anions is voltage dependent and can be entirely accounted for by reduction in single channel conductance. Minimum pore diameter was estimated to be approximately 4.5 A. Anomalous mole-fraction effects found for permeability ratios and conductance in mixtures of Cl- and SCN- or ClO4- suggest a multi-ion pore. Hydrophobic interactions with the wall of the channel pore may explain discrepancies between the measured permeabilities of some anions and their size.
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PMID:Permeation and block of the skeletal muscle chloride channel, ClC-1, by foreign anions. 956 3

1. The effects of external anions on the decay kinetics of Ca2+-activated Cl- currents (ICl(Ca)) were studied in smooth muscle cells isolated from rabbit portal vein using the perforated patch whole-cell voltage clamp technique. 2. In normal NaCl-containing external solution the decay of spontaneous Ca2+-activated Cl- currents (STICs) and Ca2+-activated Cl- 'tail' currents (Itail) was described by a single exponential with a time constant (tau) that was prolonged by external anions which are more permeable than Cl- (Br-, I- and SCN-) and accelerated by less permeant anions. However, intracellular I- did not affect the tau of STICs and Itail. 3. There was a positive correlation between the ability of an external anion to affect the decay tau of ICl(Ca) and its permeability relative to Cl-. 4. The voltage dependence of STIC and Itail decay was not affected by external or internal anions. 5. External permeating anions were not obligatory for activation of ICl(Ca) and STIC tau was not altered in Cl--free external solution. 6. Modulation of tau by mole fractions of SCN- and Cl- ions was fitted by a logistic curve, suggesting competition between SCN- and Cl- ions for a binding site. 7. In conclusion, external anions affect the decay of ICl(Ca) by a mechanism compatible with an interaction with a binding site which modulates Cl- channel kinetics.
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PMID:Modulation of the decay of Ca2+-activated Cl- currents in rabbit portal vein smooth muscle cells by external anions. 1008 37

Ca(2+)-activated Cl channels (Cl(Ca)Cs) are an important class of anion channels that are opened by increases in cytosolic [Ca(2+)]. Here, we examine the mechanisms of anion permeation through Cl(Ca)Cs from Xenopus oocytes in excised inside-out and outside-out patches. Cl(Ca)Cs exhibited moderate selectivity for Cl over Na: P(Na)/P(Cl) = 0.1. The apparent affinity of Cl(Ca)Cs for Cl was low: K(d) = 73 mM. The channel had an estimated pore diameter >0.6 nm. The relative permeabilities measured under bi-ionic conditions by changes in E(rev) were as follows: C(CN)(3) > SCN > N(CN)(2) > ClO(4) > I > N(3) > Br > Cl > formate > HCO(3) > acetate = F > gluconate. The conductance sequence was as follows: N(3) > Br > Cl > N(CN)(2) > I > SCN > COOH > ClO(4) > acetate > HCO(3) = C(CN)(3) > gluconate. Permeant anions block in a voltage-dependent manner with the following affinities: C(CN)(3) > SCN = ClO(4) > N(CN)(2) > I > N(3) > Br > HCO(3) > Cl > gluconate > formate > acetate. Although these data suggest that anionic selectivity is determined by ionic hydration energy, other factors contribute, because the energy barrier for permeation is exponentially related to anion hydration energy. Cl(Ca)Cs exhibit weak anomalous mole fraction behavior, implying that the channel may be a multi-ion pore, but that ions interact weakly in the pore. The affinity of the channel for Ca(2+) depended on the permeant anion at low [Ca(2+)] (100-500 nM). Apparently, occupancy of the pore by a permeant anion increased the affinity of the channel for Ca(2+). The current was strongly dependent on pH. Increasing pH on the cytoplasmic side decreased the inward current, whereas increasing pH on the external side decreased the outward current. In both cases, the apparent pKa was voltage-dependent with apparent pKa at 0 mV = approximately 9.2. The channel may be blocked by OH(-) ions, or protons may titrate a site in the pore necessary for ion permeation. These data demonstrate that the permeation properties of Cl(Ca)Cs are different from those of CFTR or ClC-1, and provide insights into the nature of the Cl(Ca)C pore.
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PMID:Anion permeation in Ca(2+)-activated Cl(-) channels. 1109 50

1. Anion binding within the pores of wild-type and mutant cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels, expressed in two different mammalian cell lines, was assayed using patch clamp recording. Specifically, experiments measured both the conductance of different anions and the ability of other permeant anions to block Cl- permeation through the pore. 2. Under symmetrical ionic conditions, wild-type CFTR channels showed the conductance sequence Cl- > NO3- > Br- > or = formate > F- > SCN- congruent to ClO4-. 3. High SCN- conductance was not observed, nor was there an anomalous mole fraction effect of SCN- on conductance under the conditions used. Iodide currents could not be measured under symmetrical ionic conditions, but under bi-ionic conditions I- conductance appeared low. 4. Chloride currents through CFTR channels were blocked by low concentrations (10 mM) of SCN-, I- and ClO4-, implying relatively tight binding of these anions within the pore. 5. Two mutations in CFTR which alter the anion permeability sequence, F337S and T338A, also altered the anion conductance sequence. Furthermore, block by SCN-, I- and ClO4- were weakened in both mutants. Both these effects are consistent with altered anion binding within the pore. 6. The effects of mutations on anion permeability and relative anion conductance suggested that, for most anions, increased permeability was associated with increased conductance. This indicates that the CFTR channel pore does not achieve its anion selectivity by selective anion binding within the mutated region. Instead, it is suggested that entry of anions into the region around F337 and T338 facilitates their passage through the pore. In wild-type CFTR channels, anion entry into this crucial pore region is probably dominated by anion hydration energies.
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PMID:Relationship between anion binding and anion permeability revealed by mutagenesis within the cystic fibrosis transmembrane conductance regulator chloride channel pore. 1117 91

Immediately following exposure to thiocyanate (SCN-)-containing solutions, the cystic fibrosis conductance regulator Cl- channel exhibits high unitary SCN conductance and anomalous mole fraction behaviour, suggesting the presence of multiple anion binding sites within the channel pore. However, under steady-state conditions SCN-conductance is very low. Here I show, using patch clamp recording from CFTR-transfected mammalian cell lines, that under steady-state conditions neither SCN- conductance nor SCN- permeability show anomalous mole fraction behaviour. Instead, SCN conductance, permeability, and block of Cl- permeation can all be reproduced by a rate theory model that assumes only a single intrapore anion binding site. These results suggest that under steady-state conditions the interaction between SCN- and the CFTR channel pore can be understood by a simple model whereby SCN- ions enter the pore more easily than Cl-, and bind within the pore more tightly than Cl-. The implications of these findings for investigating and understanding the mechanism of anion permeation are discussed.
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PMID:Thiocyanate as a probe of the cystic fibrosis transmembrane conductance regulator chloride channel pore. 1147 90

The mole rat, Cryptomys hottentotus (Bathyergidae) is a gregarious subterranean rodent, which shows no entrainment to ambient light-dark cycles. The locomotor activity of individuals or of a whole colony, which shows no circadian rhythmicity. Since the lack of both synchronization to light-dark cycle and an endogenous rhythm of locomotor activity could be related to the organization of the circadian system, we have investigated the neuropeptidergic features of the SCN and IGL, and have used pseudorabies viral tracing methods to identify the visual and circadian pathways in this species. The precise topographic distribution of certain neuropeptide populations in the SCN differs from typical rodent pattern of organization and may be correlated with the apparent absence of light entrainment of activity and lack of endogenous rhythmicity. The IGL is highly reduced in size. This structure can nevertheless be identified by the presence of NPY and CALB positive neurons, as well as by a dense network of SP fibers. Viral tracing using intraocular injection of the PRV-Becker and PRV-Bartha strains, leads to differential infection of neurons in circadian and visual structures. With the Bartha strain, infected neurons are principally observed in the SCN, whereas the Becker strain leads primarily to infection of the dLGN and shows an anatomical regression of visual structures. Transsynaptic retrograde infection of the retina contralateral to the injected eye reveals a morphologically homogeneous population, which resemble to retinohypothalamic ganglion cells described in other mammals.
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PMID:Organization of the circadian system in the subterranean mole rat, Cryptomys hottentotus (Bathyergidae). 1265 Sep 65

The interaction of thiocyanate with human native and cross-linked oxyhemoglobin (oxyHb), and methemoglobin (metHb) has been investigated by optical spectroscopy, circular dichroism (CD) and nuclear spin lattice relaxation rate measurements. The interaction of thiocyanate anion with human hemoglobin has been investigated by NMR measurements of the nuclear spin lattice relaxation rate of N(15) labeled thiocyanate in the presence of cyanomethemoglobin and cross-linked cyanomethemoglobin. Results show that thiocyanate is located approximately 8.9 and 6.2 A away from the heme group in cyanomethemoglobin and cross-linked cyanomethemoblobin, respectively. These results are consistent with the binding of SCN(-) at the lys-alpha-99 in the unmodified hemoglobin. Since this site is blocked in the cross-linked hemoglobin, the binding site is different. Results show that one mole of SCN(-) is binding to one mole of oxyhemoglobin suggesting that binding at the lys-alpha-99 is linked to dissociation of the hemoglobin tetramer into dimers due to its location at the alpha(1)beta(2) interface. Circular dichroism studies show that the interaction of thiocyanate with oxyHb decreases the optical rotation at 240 nm indicating a conformational change of the protein, which influences the electronic transitions of a number of peptide bonds or (and) a few aromatic side chains.
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PMID:An NMR and circular dichroism study of the interaction of thiocyanate with human and cross-linked hemoglobin: identification of Lys-alpha-99 as a possible dissociation linked binding site. 1455 95

Mole-rats are strictly subterranean and hardly, if ever, come into contact with external light. As a result, their classical visual system is severely regressed and the circadian system proportionally expanded. The family Bathyergidae presents a unique opportunity to study the circadian system in the absence of the classical visual system in a range of species. Daily patterns of activity were studied in the laboratory under constant temperature but variable lighting regimes in individually housed animals from 3 species of mole-rat exhibiting markedly different degrees of sociality. All 3 species possessed individuals that exhibited endogenous circadian rhythms under constant darkness that entrained to a light-dark cycle. In the solitary species, Georychus capensis, 9 animals exhibited greater activity during the dark phase of the light cycle, while 2 individuals expressed more activity in the light phase of the light cycle. In the social, Cryptomys hottentotus pretoriae, 5 animals displayed the majority of their activity during the dark phase of the light cycle and the remaining 2 exhibited more activity during the light phase of the light cycle. Finally in the eusocial Cryptomys damarensis, 6 animals displayed more activity during the light phase of the light cycle, and the other 2 animals displayed more activity during the dark phase of the light cycle. Since all three mole-rat species are able to entrain their locomotor activity to an external light source, light must reach the SCN, suggesting a functional circadian clock. In comparison to the solitary species, the 2 social species display a markedly poorer response to light in all aspects. Thus, in parallel with the sociality continuum, there exists a continuum of sensitivity of the circadian clock to light.
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PMID:Circadian rhythms of locomotor activity in solitary and social species of African mole-rats (family: Bathyergidae). 1466 49

The blind subterranean mole rat superspecies Spalax ehrenbergi is an extreme example of mammalian adaptation to life underground. Though this rodent is totally visually blind, harboring a drastically degenerated subcutaneous rudimentary eye, its daily activity rhythm is entrainable to LD cycles. This indicates that it confers light information to the clock, as has been previously shown by the authors in behavioral studies as well as by molecular analyses of its Clock/MOP3 and its three Per genes. The Cryptochrome (Cry) genes found in animals and plants act both as photoreceptors and as essential components of the negative feedback mechanism of the biological clock. To further understand the circadian system of this unique mammal, the authors cloned and characterized the open reading frame of Spalax Cry1 and Cry2. The Spalax CRY1 protein is significantly closer to the human homolog than to the mice one, in contrast to the evolutionary expectations. They have found two isoforms of Cry2 in Spalax, which differ in their 5' end of the open reading frame and defined their expression in Spalax populations. They found a large and significant excess of heterozygotes of sCry2 (sCry2L/S genotype). Both sCry1 and sCry2 mRNAs were found in the SCN, the eye, the harderian gland, as well as in a wide range of peripheral tissues. Their expression pattern under different LD conditions has also been analyzed. As was already shown for other circadian genes, despite being blind and living in darkness, the Cry genes of Spalax behave in a similar, though not identical, pattern as in sighted animals. Once again, the results indicate that the uniquely hypertrophied harderian gland of Spalax plays a key role in its circadian system.
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PMID:Circadian genes in a blind subterranean mammal III: molecular cloning and circadian regulation of cryptochrome genes in the blind subterranean mole rat, Spalax ehrenbergi superspecies. 1496 1


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