scholarly journals Ion selectivity and current saturation in inward-rectifier K+ channels

2012 ◽  
Vol 139 (2) ◽  
pp. 145-157 ◽  
Author(s):  
Lei Yang ◽  
Johan Edvinsson ◽  
Henry Sackin ◽  
Lawrence G. Palmer
Keyword(s):  
Divalent Cations ◽  
Ion Selectivity ◽  
Monovalent Cation ◽  
Inward Rectifier ◽  
Ion Concentration ◽  
K Channel ◽  
Cation Selectivity ◽  
Current Saturation ◽  
Lower Permeability ◽  
A Site

We investigated the features of the inward-rectifier K channel Kir1.1 (ROMK) that underlie the saturation of currents through these channels as a function of permeant ion concentration. We compared values of maximal currents and apparent Km for three permeant ions: K+, Rb+, and NH4+. Compared with K+ (imax = 4.6 pA and Km = 10 mM at −100 mV), Rb+ had a lower permeability, a lower imax (1.8 pA), and a higher Km (26 mM). For NH4+, the permeability was reduced more with smaller changes in imax (3.7 pA) and Km (16 mM). We assessed the role of a site near the outer mouth of channel in the saturation process. This site could be occupied by either permeant ions or low-affinity blocking ions such as Na+, Li+, Mg2+, and Ca2+ with similar voltage dependence (apparent valence, 0.15–0.20). It prefers Mg2+ over Ca2+ and has a monovalent cation selectivity, based on the ability to displace Mg2+, of K+ > Li+ ∼ Na+ > Rb+ ∼ NH4+. Conversely, in the presence of Mg2+, the Km for K+ conductance was substantially increased. The ability of Mg2+ to block the channels was reduced when four negatively charged amino acids in the extracellular domain of the channel were mutated to neutral residues. The apparent Km for K+ conduction was unchanged by these mutations under control conditions but became sensitive to the presence of external negative charges when residual divalent cations were chelated with EDTA. The results suggest that a binding site in the outer mouth of the pore controls current saturation. Permeability is more affected by interactions with other sites within the selectivity filter. Most features of permeation (and block) could be simulated by a five-state kinetic model of ion movement through the channel.

scholarly journals Divalent Cation Selectivity Is a Function of Gating in Native and Recombinant Cyclic Nucleotide–gated Ion Channels from Retinal Photoreceptors

1999 ◽  
Vol 113 (6) ◽  
pp. 799-818 ◽  
Author(s):  
David H. Hackos ◽  
Juan I. Korenbrot
Keyword(s):  
Divalent Cation ◽  
Cyclic Nucleotide ◽  
Divalent Cations ◽  
Ion Selectivity ◽  
Ion Concentration ◽  
Cone Photoreceptors ◽  
Cation Selectivity ◽  
Channel Opening ◽  
Α And Β Subunits ◽  
Α Subunits

The selectivity of Ca2+ over Na+ is ∼3.3-fold larger in cGMP-gated channels of cone photoreceptors than in those of rods when measured under saturating cGMP concentrations, where the probability of channel opening is 85–90%. Under physiological conditions, however, the probability of opening of the cGMP-gated channels ranges from its largest value in darkness of 1–5% to essentially zero under continuous, bright illumination. We investigated the ion selectivity of cGMP-gated channels as a function of cyclic nucleotide concentration in membrane patches detached from the outer segments of rod and cone photoreceptors and have found that ion selectivity is linked to gating. We determined ion selectivity relative to Na+ (PX/PNa) from the value of reversal potentials measured under ion concentration gradients. The selectivity for Ca2+ over Na+ increases continuously as the probability of channel opening rises. The dependence of PCa/PNa on cGMP concentration, in both rods and cones, is well described by the same Hill function that describes the cGMP dependence of current amplitude. At the cytoplasmic cGMP concentrations expected in dark-adapted intact photoreceptors, PCa/PNa in cone channels is ∼7.4-fold greater than that in rods. The linkage between selectivity and gating is specific for divalent cations. The selectivity of Ca2+ and Sr2+ changes with cGMP concentration, but the selectivity of inorganic monovalent cations, Cs+ and NH4+, and organic cations, methylammonium+ and dimethylammonium+, is invariant with cGMP. Cyclic nucleotide–gated channels in rod photoreceptors are heteromeric assemblies of α and β subunits. The maximal PCa/PNa of channels formed from α subunits of bovine rod channels is less than that of heteromeric channels formed from α and β subunits. In addition, Ca2+ is a more effective blocker of channels formed by α subunits than of channels formed by α and β subunits. The cGMP-dependent shift in divalent cation selectivity is a property of αβ channels and not of channels formed from α subunits alone.

scholarly journals A Model for Predicting Cation Selectivity and Permeability in AMPA and NMDA Receptors Based on Receptor Subunit Composition

2021 ◽  
Vol 13 ◽  
Author(s):  
Sampath Kumar ◽  
Sanjay S. Kumar
Keyword(s):  
Experimental Data ◽  
Divalent Cations ◽  
Ion Selectivity ◽  
Gene Families ◽  
Subunit Composition ◽  
Receptor Subunit ◽  
Cation Selectivity ◽  
Subunit Stoichiometry ◽  
Ampa And Nmda Receptors

Glutamatergic AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and NMDA (N-methyl-D-aspartate) receptors are implicated in diverse functions ranging from synaptic plasticity to cell death. They are heterotetrameric proteins whose subunits are derived from multiple distinct gene families. The subunit composition of these receptors determines their permeability to monovalent and/or divalent cations, but it is not entirely clear how this selectivity arises in native and recombinantly-expressed receptor populations. By analyzing the sequence of amino acids lining the selectivity filters within the pore forming membrane helices (M2) of these subunits and by correlating subunit stoichiometry of these receptors with their ability to permeate Na+ and/or Ca2+, we propose here a mathematical model for predicting cation selectivity and permeability in these receptors. The model proposed is based on principles of charge attractivity and charge neutralization within the pore forming region of these receptors; it accurately predicts and reconciles experimental data across various platforms including Ca2+ permeability of GluA2-lacking AMPARs and ion selectivity within GluN3-containing di- and tri-heteromeric NMDARs. Additionally, the model provides insights into biophysical mechanisms regulating cation selectivity and permeability of these receptors and the role of various subunits in these processes.

scholarly journals Tuning the Voltage Dependence of Tetraethylammonium Block with Permeant Ions in an Inward-Rectifier K+ Channel

1999 ◽  
Vol 114 (3) ◽  
pp. 415-426 ◽  
Author(s):  
Maria Spassova ◽  
Zhe Lu
Keyword(s):  
Alkali Metal ◽  
Metal Ions ◽  
Ion Selectivity ◽  
Inward Rectifier ◽  
K Channel ◽  
Alkali Metal Ions ◽  
Internal Site ◽  
External Site ◽  
Ion Binding Sites

To understand the role of permeating ions in determining blocking ion–induced rectification, we examined block of the ROMK1 inward-rectifier K+ channel by intracellular tetraethylammonium in the presence of various alkali metal ions in both the extra- and intracellular solutions. We found that the channel exhibits different degrees of rectification when different alkali metal ions (all at 100 mM) are present in the extra- and intracellular solution. A quantitative analysis shows that an external ion site in the ROMK1 pore binds various alkali metal ions (Na+, K+, Rb+, and Cs+) with different affinities, which can in turn be altered by the binding of different permeating ions at an internal site through a nonelectrostatic mechanism. Consequently, the external site is saturated to a different level under the various ionic conditions. Since rectification is determined by the movement of all energetically coupled ions in the transmembrane electrical field along the pore, different degrees of rectification are observed in various combinations of extra- and intracellular permeant ions. Furthermore, the external and internal ion-binding sites in the ROMK1 pore appear to have different ion selectivity: the external site selects strongly against the smaller Na+, but only modestly among the three larger ions, whereas the internal site interacts quite differently with the larger K+ and Rb+ ions.

scholarly journals A conductance maximum observed in an inward-rectifier potassium channel.

1994 ◽  
Vol 104 (3) ◽  
pp. 477-486 ◽  
Author(s):  
Z Lu ◽  
R MacKinnon
Keyword(s):  
Single Channel ◽  
Outward Current ◽  
Inward Rectifier ◽  
Ion Concentration ◽  
K Channel ◽  
Single Channels ◽  
Ligand Interaction ◽  
Membrane Biology ◽  
Maximum Value ◽  
Conduction Pathway

One prediction of a multi-ion pore is that its conductance should reach a maximum and then begin to decrease as the concentration of permeant ion is raised equally on both sides of the membrane. A conductance maximum has been observed at the single-channel level in gramicidin and in a Ca(2+)-activated K+ channel at extremely high ion concentration (> 1,000 mM) (Hladky, S. B., and D. A. Haydon. 1972. Biochimica et Biophysica Acta. 274:294-312; Eisenmam, G., J. Sandblom, and E. Neher. 1977. In Metal Ligand Interaction in Organic Chemistry and Biochemistry. 1-36; Finkelstein, P., and O. S. Andersen. 1981. Journal of Membrane Biology. 59:155-171; Villarroel, A., O. Alvarez, and G. Eisenman. 1988. Biophysical Journal. 53:259a. [Abstr.]). In the present study we examine the conductance-concentration relationship in an inward-rectifier K+ channel, ROMK1. Single channels, expressed in Xenopus oocytes, were studied using inside-out patch recording in the absence of internal Mg2+ to eliminate blockade of outward current. Potassium, at equal concentrations on both sides of the membrane, was varied from 10 to 1,000 mM. As K+ was raised from 10 mM, the conductance increased steeply and reached a maximum value (39 pS) at 300 mM. The single-channel conductance then became progressively smaller as K+ was raised beyond 300 mM. At 1000 mM K+, the conductance was reduced to approximately 75% of its maximum value. The shape of the conductance-concentration curve observed in the ROMK1 channel implies that it has multiple K(+)-occupied binding sites in its conduction pathway.

scholarly journals Mechanism of the Voltage Sensitivity of IRK1 Inward-rectifier K+ Channel Block by the Polyamine Spermine

2005 ◽  
Vol 125 (4) ◽  
pp. 413-426 ◽  
Author(s):  
Hyeon-Gyu Shin ◽  
Zhe Lu
Keyword(s):  
Steady State ◽  
Voltage Dependence ◽  
Ion Selectivity ◽  
Inward Rectifier ◽  
Selectivity Filter ◽  
K Channel ◽  
Voltage Sensitivity ◽  
Channel Block ◽  
Head Groups ◽  
Voltage Dependent

IRK1 (Kir2.1) inward-rectifier K+ channels exhibit exceedingly steep rectification, which reflects strong voltage dependence of channel block by intracellular cations such as the polyamine spermine. On the basis of studies of IRK1 block by various amine blockers, it was proposed that the observed voltage dependence (valence ∼5) of IRK1 block by spermine results primarily from K+ ions, not spermine itself, traversing the transmembrane electrical field that drops mostly across the narrow ion selectivity filter, as spermine and K+ ions displace one another during channel block and unblock. If indeed spermine itself only rarely penetrates deep into the ion selectivity filter, then a long blocker with head groups much wider than the selectivity filter should exhibit comparably strong voltage dependence. We confirm here that channel block by two molecules of comparable length, decane-bis-trimethylammonium (bis-QAC10) and spermine, exhibit practically identical overall voltage dependence even though the head groups of the former are much wider (∼6 Å) than the ion selectivity filter (∼3 Å). For both blockers, the overall equilibrium dissociation constant differs from the ratio of apparent rate constants of channel unblock and block. Also, although steady-state IRK1 block by both cations is strongly voltage dependent, their apparent channel-blocking rate constant exhibits minimal voltage dependence, which suggests that the pore becomes blocked as soon as the blocker encounters the innermost K+ ion. These findings strongly suggest the existence of at least two (potentially identifiable) sequentially related blocked states with increasing numbers of K+ ions displaced. Consequently, the steady-state voltage dependence of IRK1 block by spermine or bis-QAC10 should increase with membrane depolarization, a prediction indeed observed. Further kinetic analysis identifies two blocked states, and shows that most of the observed steady-state voltage dependence is associated with the transition between blocked states, consistent with the view that the mutual displacement of blocker and K+ ions must occur mainly as the blocker travels along the long inner pore.

scholarly journals Ni2+ Block of CaV3.1 (α1G) T-type Calcium Channels

2008 ◽  
Vol 132 (2) ◽  
pp. 239-250 ◽  
Author(s):  
Carlos A. Obejero-Paz ◽  
I. Patrick Gray ◽  
Stephen W. Jones
Keyword(s):  
Calcium Channels ◽  
Voltage Dependence ◽  
Divalent Cations ◽  
Ion Concentration ◽  
Diffusion Limit ◽  
Exit Rate ◽  
Channel Activity ◽  
Inward Currents ◽  
A Site ◽  
Lock In

Ni2+ inhibits current through calcium channels, in part by blocking the pore, but Ni2+ may also allosterically affect channel activity via sites outside the permeation pathway. As a test for pore blockade, we examined whether the effect of Ni2+ on CaV3.1 is affected by permeant ions. We find two components to block by Ni2+, a rapid block with little voltage dependence, and a slow block most visible as accelerated tail currents. Rapid block is weaker for outward vs. inward currents (apparent Kd = 3 vs. 1 mM Ni2+, with 2 mM Ca2+ or Ba2+) and is reduced at high permeant ion concentration (110 vs. 2 mM Ca2+ or Ba2+). Slow block depends both on the concentration and on the identity of the permeant ion (Ca2+ vs. Ba2+ vs. Na+). Slow block is 2–3× faster in Ba2+ than in Ca2+ (2 or 110 mM), and is ∼10× faster with 2 vs. 110 mM Ca2+ or Ba2+. Slow block is orders of magnitude slower than the diffusion limit, except in the nominal absence of divalent cations (∼3 μM Ca2+). We conclude that both fast and slow block of CaV3.1 by Ni2+ are most consistent with occlusion of the pore. The exit rate of Ni2+ for slow block is reduced at high Ni2+ concentrations, suggesting that the site responsible for fast block can “lock in” slow block by Ni2+, at a site located deeper within the pore. In contrast to the complex pore block observed for CaV3.1, inhibition of CaV3.2 by Ni2+ was essentially independent of voltage, and was similar in 2 mM Ca2+ vs. Ba2+, consistent with inhibition by a different mechanism, at a site outside the pore.

scholarly journals Molecular Basis of Ion Selectivity, Block, and Rectification of the Inward Rectifier Kir3.1/Kir3.4 K+Channel

2003 ◽  
Vol 278 (49) ◽  
pp. 49537-49548 ◽  
Author(s):  
Katherine M. Dibb ◽  
Thierry Rose ◽  
Samy Y. Makary ◽  
Thomas W. Claydon ◽  
Decha Enkvetchakul ◽  
...  
Keyword(s):  
Molecular Basis ◽  
Ion Selectivity ◽  
Inward Rectifier ◽  
K Channel

scholarly journals Asymmetrical Contributions of Subunit Pore Regions to Ion Selectivity in an Inward Rectifier K+ Channel

1998 ◽  
Vol 75 (3) ◽  
pp. 1330-1339 ◽  
Author(s):  
Scott K. Silverman ◽  
Henry A. Lester ◽  
Dennis A. Dougherty
Keyword(s):  
Ion Selectivity ◽  
Inward Rectifier ◽  
K Channel

scholarly journals Divalent ion trapping inside potassium channels of human T lymphocytes.

1989 ◽  
Vol 93 (4) ◽  
pp. 609-630 ◽  
Author(s):  
S Grissmer ◽  
M D Cahalan
Keyword(s):  
Divalent Cations ◽  
Ion Trapping ◽  
K Channel ◽  
K Channels ◽  
Current Decay ◽  
Channel Inactivation ◽  
Cell Recording ◽  
Human T Lymphocyte ◽  
K Current ◽  
A Site

Using the patch-clamp whole-cell recording technique, we investigated the influence of external Ca2+, Ba2+, K+, Rb+, and internal Ca2+ on the rate of K+ channel inactivation in the human T lymphocyte-derived cell line, Jurkat E6-1. Raising external Ca2+ or Ba2+, or reducing external K+, accelerated the rate of the K+ current decay during a depolarizing voltage pulse. External Ba2+ also produced a use-dependent block of the K+ channels by entering the open channel and becoming trapped inside. Raising internal Ca2+ accelerated inactivation at lower concentrations than external Ca2+, but increasing the Ca2+ buffering with BAPTA did not affect inactivation. Raising [K+]o or adding Rb+ slowed inactivation by competing with divalent ions. External Rb+ also produced a use-dependent removal of block of K+ channels loaded with Ba2+ or Ca2+. From the removal of this block we found that under normal conditions approximately 25% of the channels were loaded with Ca2+, whereas under conditions with 10 microM internal Ca2+ the proportion of channels loaded with Ca2+ increased to approximately 50%. Removing all the divalent cations from the external and internal solution resulted in the induction of a non-selective, voltage-independent conductance. We conclude that Ca2+ ions from the outside or the inside can bind to a site at the K+ channel and thereby block the channel or accelerate inactivation.

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