Correspondence to: Zhe Lu, University of Pennsylvania, Department of Physiology, D302A Richards Building, 3700 Hamilton Walk, Philadelphia, PA 19104-6085. Fax:Fax: 215-573-5851; E-mail:zhelu{at}mail.med.upenn.edu.
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
To understand the role of permeating ions in determining blocking ioninduced 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.
Key Words: inward-rectifier K+ channel, rectification, ionic blocker, ion selectivity, tetraethylammonium
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Inward-rectifier K+ channels act as K+-selective diodes in the cell membrane (
It was first discovered that some inward-rectifier K+ channels are blocked by intracellular Mg2+ and that the extent of block depends on membrane voltage (
However, as early as a half century ago, it was observed that inward rectification is also sensitive to the concentration of extracellular K+ (
Recently, not only the extent but also the voltage dependence of blockade of the ROMK1 inward-rectifier K+ channel by both TEA and Mg2+ were shown to depend on the concentration of extracellular K+ ( 10 mM), and thus the degree of rectification. In other words, for a given intracellular ionic condition, the degree of rectification reflects the level of ion saturation at the external site.
Thus far, it is unclear how much of the voltage dependence is due to the movement of permeant ions versus the movement of blocking ions in the electrical field. However, it is clear that a significant fraction of the voltage dependence results from the movement of permeant ions (
![]() |
Methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Molecular Biology and Oocyte Preparation
ROMK1 cDNA was cloned into the p-SPORT1 plasmid (GIBCO BRL) (
Patch Recording
ROMK1 currents were recorded in the inside-out configuration from Xenopus oocytes (injected with ROMK1 cRNA) with an Axopatch 200B amplifier (Axon Instruments, Inc.). The recorded signal was filtered at 1 kHz and sampled at 5 kHz using an analogue-to-digital converter (DigiData 1200; Axon Instruments, Inc.) interfaced with a personal computer. pClamp6 software (Axon Instruments, Inc.) was used to control the amplifier and acquire the data. Macroscopic currentvoltage curves were recorded as membrane voltage was linearly ramped (50 mV/s). Background leak current correction was carried out as previously described (
Recording Solutions
Pipette solutions contained specified concentrations of alkali metal ions with (mM): 0.3 CaCl2, 1.0 MgCl2, and 10 HEPES, pH 7.6. Na+ and N-methyl-D-glucamine (NMG+) had nearly identical effects on all examined properties (see RESULTS). Thus, when K+, Rb+, or Cs+ concentrations were reduced, Na+ was used to maintain the total concentration of alkali metal ions at 100 mM. The bath solutions contained the specified concentrations of TEA with (mM): 90 KCl, 5 K2EDTA, and 10 HEPES, pH 7.6. In some experiments, K+ in the bath solution was replaced by Rb+.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Current-Voltage Relation of the ROMK1 Channel under Either Symmetric K+ or Biionic Conditions
In the presence of 100 mM K+ on both sides of the membrane and the absence of intracellular blocking ions, the IV curve of the ROMK1 channel was remarkably linear (Figure 1 A). However, when extracellular K+ was replaced by an equal concentration of Rb+, the channel conducted larger outward than inward current with a reversal potential of -5.3 ± 0.3 mV (mean ± SEM, n = 5) (Figure 1 B). As shown in Figure 1CE, no inward currents can be seen when extracellular K+ was replaced by either Cs+, Na+, or NMG+. The reversal potentials were less than -60 mV in extracellular Cs+ and less than -120 mV in extracellular Na+ or NMG+. From these, we estimated the permeability ratios (PK/PX) using the Goldman-Hodgkin-Katz equation as PK/PRb = 1.2, PK/PCS > 10, and PK/PNa and PK/PNMG > 100. Based on this empirical measure of ion selectivity, the channel has the same ion-selective sequence as other K+ channels, K+ Rb+ > Cs+ >> Na+, NMG+.
|
The Degree of Rectification Induced by Intracellular TEA in the Presence of Various Extracellular Cations
Figure 1 A shows the IV curves recorded in symmetric 100 mM K+ without or with intracellular TEA at the concentrations indicated. As shown previously, intracellular TEA causes the ROMK1 channel to conduct in an inwardly rectifying manner (
The Affinity of TEA and the Voltage Dependence of Channel Blockade by TEA
In Figure 2, the fractions of unblocked current in the presence of 100 mM extracellular K+, Rb+, Cs+, or Na+ were plotted against TEA concentration for several representative membrane voltages. Although the increment in membrane voltage between the adjacent curves is the same (20 mV) in all four plots, the distance between adjacent curves is not the same in the four cases. It decreases in the order of K+, Rb+, Cs+, and Na+, illustrating how voltage dependence of channel blockade by TEA varies with various extracellular alkali metal ions. The curves superimposed on the data are least-squares fits of an equation that assumes a 1:1 stoichiometry between the channel and TEA. From the fits, we determined the observed equilibrium dissociation constants for TEA (TEAKobs) in the presence of each of the four alkali metal ions.
|
In Figure 3, we plotted TEAKobs determined in the presence of 100 mM of each of the four species of extracellular ions as a function of membrane voltage. The lines superimposed on the data represent least-squares fits of the Woodhull equation ()obs (an empirical measure of the voltage dependence).
|
The averages of TEAKobs(0 mV) and TEA(z)obs determined in the presence of 100 mM extracellular K+, Rb+, Cs+, and Na+ are presented in Figure 4. TEAKobs(0 mV) determined in K+ was slightly smaller than that in Rb+, but larger than those in Cs+ and Na+. Judged from TEA(z
)obs, the voltage dependence of channel blockade by TEA is similar in both K+ and Rb+, although it may be slightly smaller in Rb+. However, the voltage dependence is significantly attenuated in Cs+ and even more in Na+. The value of TEA(z
)obs in Na+ is only about half what it is in K+.
|
Dependence of TEA Affinity on the Concentration of Extracellular Alkali Metal Ions
We next examined how channel blockade by TEA varies with the concentration of each ion species. When the concentrations of K+, Rb+, or Cs+ were reduced, we used Na+ as a substituting ion instead of the more commonly used NMG+, because in the present study extracellular Na+ and NMG+ behaved similarly and membrane patches tolerated Na+ better. Both the IV curves of the channel and its blockade by TEA were very similar in the presence of extracellular Na+ or NMG+ (Figure 1). Furthermore, TEA binds to the channel with nearly identical affinities in the presence of extracellular Na+ or NMG+ (Figure 5 D). Assuming NMG+ does not bind at the external ion-binding site, we estimated the equilibrium dissociation constant of the site for Na+ is in the molar range (see DISCUSSION).
|
Generally, the values of both TEAKobs(0 mV) and TEA(z)obs increase with the concentration of extracellular alkali metal ions. In Figure 5, we plotted TEAKobs(0 mV) as a function of the concentration of all four alkali metal ions. In the presence of K+, Rb+, and Cs+, TEAKobs(0 mV) increases linearly with ion concentration. The plots for K+ and Rb+ are very similar, but the slope of the plot for Cs+ is much smaller. In the case of Na+, the TEAKobs(0 mV) value is nearly the same in either 0 mM (i.e., 100 mM NMG+) or 100 mM extracellular Na+. Furthermore, the value of TEA(z
)obs also increases with the concentration of alkali metal ions. For presentation purposes, the TEA(z
)obs data are presented in Figure 7 (below).
|
|
Channel Blockade by TEA in the Presence of Intracellular Rb+
To gain insight into how intracellular permeating ions affect channel blockade by intracellular TEA, we examined how TEA-blocking behaviors would change if we replaced intracellular K+ by Rb+. Figure 6 A shows the IV curves obtained in the presence of 100 mM intracellular Rb+ and 100 mM extracellular K+. The inward current carried by K+ is larger than the outward current carried by Rb+, and the currents reverse at approximately +10 mV. Addition of TEA to the intracellular solution significantly reduced the outward current. However, the curvature of the outward current induced by intracellular TEA is significantly smaller than that observed when 100 mM K+ is present in both the intracellular and extracellular solutions (compare Figure 1 A with 6 A). The IV curves in Figure 6 B were recorded in the presence of 100 mM Rb+ on both sides of the membrane. Unlike the remarkably linear IV curve in symmetric K+, the IV curve in symmetric Rb+ is nonlinear. The outward current is slightly smaller than the inward current, which reveals the asymmetric property of the channel. Addition of intracellular TEA also significantly reduced the outward current. Again, the TEA-induced curvature of the IV curves obtained in the presence of intracellular Rb+ is much less than that obtained in intracellular K+ (compare Figure 1 B with 6 B).
Figure 6C and Figure D, shows the effects of reducing the concentration of extracellular permeant ions on channel blockade by intracellular TEA. All IV curves in Figure 6C and Figure D, were recorded in the presence of 100 mM intracellular Rb+. The extracellular permeant ions were 20 mM K+ and Rb+ for C and D, respectively. The concentrations of intracellular TEA were as indicated. In the presence of 20 mM extracellular K+ or Rb+, both the extent and the voltage dependence of channel blockade by TEA were very similar to those obtained in 100 mM corresponding extracellular ions (Figure 6A vs. C, and B vs. D).
Figure 7A and Figure B, plots TEAKobs(0 mV) determined in the presence of 100 mM intracellular K+ (open symbols) or 100 mM intracellular Rb+ (closed symbols) as a function of the concentration of extracellular K+ and Rb+, respectively. Replacing intracellular K+ by Rb+ dramatically reduced the dependence of TEAKobs(0 mV) on the concentration of either extracellular ion.
To further illustrate how different intracellular ions alter the voltage dependence of channel blockade, in Figure 7C and Figure D, we plotted the TEA(z)obs values determined under various intra- and extracellular conditions. Figure 7 C plots TEA(z
)obs values against the concentration of extracellular K+, while Figure 7 D plots the values against the concentration of extracellular Rb+. In both plots, the data represented by open and closed symbols were obtained in intracellular K+ and Rb+, respectively. The values of TEA(z
)obs increased with increasing concentrations of extracellular K+ or Rb+, compatible with a scenario where the observed changes in the voltage dependence result from titrating an ion-binding site in the external part of the pore. When K+ in the intracellular solution was replaced by Rb+, the value of TEA(z
)obs was significantly reduced at all tested concentrations of extracellular K+ or Rb+. However, regardless of whether the intracellular ion was K+ or Rb+, the TEA(z
)obs values converged to the same minimum at vanishing extracellular K+ or Rb+ concentrations. These observations show that replacing intracellular K+ by Rb+ dramatically reduces the ability of extracellular ions to alter the voltage dependence of channel blockade.
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In the present study, we found that both the extent and the voltage dependence of channel blockade by TEA depend on the type of alkali metal ions, including K+, Rb+, Cs+, and Na+, in the extracellular solution (Figure 1 and Figure 4). Since the voltage dependence is determined by the movement of all energetically coupled ions in the transmembrane electrical field (
As shown in Figure 5, TEAKobs(0 mV) increases linearly with not only the concentration of K+, as observed previously (
|
The diagram in Figure 8 is a simplified version of the kinetic models that we previously used to interpret the dependence of TEAKobs(0 mV) on the concentration of extracellular K+ (
![]() |
(1) |
and
![]() |
(2) |
The fraction of unblocked channel is
![]() |
(3) |
where [XextCh], [ChTEAint], and [Ch] are concentrations of the channels bound with an additional extracellular ion X, intracellular TEA, or neither of them. Combining Equation 1Equation 2Equation 3 yields:
![]() |
(4) |
where
![]() |
(5) |
Undoubtedly, the state diagram in Figure 8 is an over simplification for the interaction between extracellular permeant ions and intracellular TEA. Nevertheless, it provides a simple and instructive way for considering the interaction between the extracellular permeant ions and intracellular TEA. In fact, Equation 5 derived based on this simple state diagram is the same as one we previously derived based on a more complex model (
According to Equation 5, we determined the equilibrium dissociation constant for TEA at 0 mV and in zero extracellular K+ [TEAK(0 mV)] from the y intercept of the plots in Figure 5 as 1.4 ± 0.3 mM. We also determined the equilibrium dissociation constants of the channel for the four ions from the ratio of the y intercept and slope of the corresponding plots (mM): KK = 13.2 ± 2.4 (mean ± SEM), RbK = 15.6 ± 3.1, CsK = 34.0 ± 3.5, and NaK = 1.4 M. Since the slope of plot D is minimal, a NaK value of 1.4 M may not be very precise. Nevertheless, it does indicate that the affinity of the site for Na+ is much lower. The results of the analysis indicate that the external ion-binding site strongly selects K+ over smaller Na+, but only minimally or modestly selects K+ over larger Rb+ or Cs+.
The ion affinity sequence of the site (K+ Rb+ > Cs+ > Na+) correlates with the degree of voltage dependence of channel blockade by TEA in the presence of the corresponding alkali metal ions (Figure 4 B). To account for the dependence of TEAKobs on membrane voltage, we combined Equation 5 and the Woodhull equation:
![]() |
(6) |
which gives
![]() |
(7) |
Rearranging Equation 7, we obtain
![]() |
(8) |
Quantity X(z) in Equation 8 is related only to the binding and unbinding of extracellular permeant ions to the external site. However, TEA(z
) is related to the movement of TEA and possibly other permeant ions (excluding the one bound to the external site) in the electrical field along the pore (for more details, see
Although we exploited the apparent competition between extracellular alkali metal ions and intracellular TEA to determine Kd values for the alkali metal ions, these Kd values should characterize the interaction of alkali metal ions with the channel in the absence of TEA (see the diagram in Figure 8). Even so, the absolute Kd values determined here do not necessarily reflect the intrinsic affinities of the site for the various alkali metal ions for the reason discussed below.
To learn how intracellular permeant ions modify blocking ioninduced rectification, we compared TEA block in 100 mM intracellular K+ versus Rb+. Regardless of whether K+ or Rb+ was in the extracellular solution, the effect on TEAKobs(0 mV) of changes in the concentration of extracellular permeating ions was much larger when the intracellular ion was K+ than when it was Rb+. For example, when the concentration of extracellular K+ was decreased from 100 mM to near zero, TEAKobs(0 mV) decreased by eightfold in intracellular K+, whereas it decreased by less than twofold in intracellular Rb+ (Figure 7 A). A similar phenomenon was observed when the concentration of extracellular Rb+ was reduced (Figure 7 B).
Analyzing the data acquired in intracellular Rb+ using Equation 5, we obtained KK = 155 ± 21 mM, RbK = 159 ± 18 mM, and TEAK = 6.5 ± 0.4 mM (all at 0 mV). Thus, replacing intracellular K+ with Rb+ lowers channel affinity for extracellular K+ and Rb+ by ~10-fold (KK = 13 vs. 155 mM, RbK = 16 vs. 159 mM), and channel affinity for intracellular TEA by ~5-fold (TEAK = 1.4 vs. 6.5 mM). The increase in TEAK accounts for the higher y intercepts of the plots corresponding to intracellular Rb+ in Figure 7A and Figure B, while the larger changes in both KK and RbK than in TEAK account for the shallower slopes of the plots corresponding to intracellular Rb+. These findings suggest that K+ and Rb+ interact quite differently with an internal ion-binding site, despite the fact that the external site has similar affinities for K+ and Rb+ (KK = 13 mM vs. RbK = 16 mM).
Since the binding of intracellular Rb+ reduces the affinity of the external site for both K+ and Rb+, Equation 8 predicts that in the presence of subsaturating concentrations of extracellular permeating ions, the voltage dependence of channel blockade should be less in intracellular Rb+ than in K+. Also, regardless of the types of permeating ions present in the intracellular solution, the voltage dependence should be the same when the concentration of extracellular permeating ions is zero, because in this case only the second term in Equation 8 applies. These predictions are consistent with what was observed (Figure 7C and Figure D). Therefore, the reduction in the voltage dependence due to replacing intracellular K+ by Rb+ can be explained by the resulting reduction in the affinity, and thus in the level of ion saturation, of the external ion-binding site.
It is unclear thus far why the affinity of the channel for blocking ions, such as TEA, and the voltage dependence of channel blockade are much less sensitive to the concentration of extracellular K+ in the voltage-activated K+ channels than in the inward-rectifier K+ channels (e.g.,
We showed here that the external ion site in the ROMK1 channel interacts selectively with alkali metal ions with an affinity sequence K+ Rb+ > Cs+ >> Na+. This sequence is similar to that previously determined in a Ca2+-activated K+ channel by examining the interaction between K+ and Ba2+ ions (
A multi-ion theory, often used to explain ion selectivity in K+ channels (e.g.,
We found here that the external site in the ROMK1 channel has very different affinities for extracellular permeating ions depending on whether the internal site is exposed to K+ or Rb+, which provides experimental evidence for the hypothesized permeant ion interactions in the pore. This finding also argues that the interactions between permeating ions in the pore are not merely electrostatic. Conceivably, binding of ions of different sizes at one site (e.g., the internal site in this case) in the pore can induce different degrees of structural "deformation" at a second site (e.g., the external site) elsewhere in the pore by propagating the binding energy along the pore-lining protein elements. Since the interactions between ions and the narrow part of the pore almost certainly are iondipole interactions, they should be highly sensitive to a change in the distance between permeating ions and the dipole-generating atoms, such as carbonyl oxygen, in the channel protein (
In summary, the 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 various permeating ions at the internal site through a nonelectrostatic mechanism. Consequently, the saturation level of the external ion site depends on the ion species on both sides of the membrane. Since rectification is determined by the movement of all energetically coupled ions in the transmembrane electrical field along the pore, various degrees of rectification are observed with various combinations of extra- and intracellular ions. Although both the external and internal ion sites in the ROMK1 pore appear to be ion selective, they likely have different ion selectivity: the external site selects strongly against smaller Na+ but only modestly among the three larger ions, whereas the internal site interacts quite differently with the larger ions K+ and Rb+.
![]() |
Acknowledgements |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
We thank K. Ho and S. Hebert for ROMK1 cDNA, P. De Weer for critical reading of our manuscript, and A. Klem for assistance on manuscript preparation.
This study was supported by a National Institutes of Health (NIH) grant (GM55560). Z. Lu was a recipient of an Independent Scientist Award from the NIH (HL03814), and M. Spassova was a recipient of an Individual National Research Service Award from the NIH (GM19215).
Submitted: May 20, 1999; Revised: July 19, 1999; Accepted: July 21, 1999.
1used in this paper: IV, currentvoltage; NMG+, N-methyl-D-glucamine; TEA, tetraethylammonium
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Aleksandrov, A., Velimirovic, B., Clampham, D.E. (1996) Inward rectification of the IRK1 K+ channel reconstituted in lipid bilayers. Biophys. J. 70:2680-2687[Abstract].
Almers, W., McCleskey, E.W. (1984) Non-selective conductance in calcium channels of frog muscle: calcium selectivity in a single-file pore. J. Physiol. 353:585-608[Abstract].
Armstrong, C.M., Binstock, L. (1965) Anomalous rectification in the squid giant axon injected with tetraethylammonium. J. Gen. Physiol. 48:859-872
Baukrowitz, T., Yellen, G. (1996) Use-dependent blockers and exit rate of the last ion from the multi-ion pore of a K+ channel. Science. 271:653-656[Abstract].
Choi, K.L., Mossman, C., Aube, J., Yellen, G. (1993) The internal quaternary ammonium receptor site of Shaker potassium channels. Neuron. 10:533-541[Medline].
Doyle, D.A., Morais, J., Pfuetzner, R.A., Kuo, A., Gulbis, J.M., Cohen, S.L., Chait, B.T., MacKinnon, R. (1998) The structure of the potassium channel: molecular basis of K+ conduction and selectivity. Science. 280:69-77
Fakler, B., Branle, U., Glowatzki, E., Weidemann, S., Zenner, H.P., Ruppersburg, J.P. (1995) Strong voltage-dependent inward-rectification of inward-rectifier K+ channels is caused by intracellular spermine. Cell. 80:149-154[Medline].
Ficker, E., Taglialatela, M., Wible, B.A., Henley, C.M., Brown, A.M. (1994) Spermine and spermidine as gating molecules for inward rectifier K+ channels. Science. 266:1068-1072[Medline].
French, B.J., Shoukimas, J.J. (1981) Blockage of squid axon potassium conductance by internal tetra-n-alkylammonium ions of various sizes. J. Gen. Physiol. 34:271-291.
Hagiwara, S., Takahashi, K. (1974) The anomalous rectification and cation selectivity of the membrane of a starfish egg cell. J. Membr. Biol. 18:61-80[Medline].
Harris, R.E., Larson, H.P., Isacoff, E.Y. (1998) A permeant ion binding site located between two gates of the Shaker K+ channel. Biophys. J. 74:1808-1820
Heginbotham, L., Lu, Z., Abramson, T., MacKinnon, R. (1994) Mutations in the K+ channel signature sequence. Biophys. J. 66:1061-1067[Abstract].
Heginbotham, L., MacKinnon, R. (1993) Conduction properties of the cloned Shaker K+ channel. Biophys. J. 65:2089-2096[Abstract].
Hess, P., Tsien, R.W. (1984) Mechanism of ion permeation through calcium channels. Nature. 309:453-456[Medline].
Hille, B. (1992) Ionic Channels of Excitable Membranes. 2nd ed Sunderland, MA, Sinauer Association Inc.
Hille, B., Schwarz, W. (1978) Potassium channels as multi-ion single-file pores. J. Gen. Physiol. 72:409-442[Abstract].
Ho, K., Nichols, C.G., Lederer, W.J., Lytton, J., Vassilev, P.M., Kanazirska, M.V., Hebert, S.C. (1993) Cloning and expression of an inwardly rectifying ATP-regulated potassium channel. Nature. 263:31-38.
Hodgkin, A.L., Horowicz, P. (1959) The influence of potassium and chloride ions on the membrane potential of single muscle fibers. J. Physiol. 148:127-160[Medline].
Horie, M., Irisawa, H., Noma, H. (1987) Voltage-dependent magnesium block of adenosine-triphosphate-sensitive potassium channel in guinea-pig ventricular cells. J. Physiol. 387:251-272[Abstract].
Horowicz, P., Gage, P.W., Eisenberg, R.S. (1968) The role of the electrochemical gradient in determining potassium fluxes in frog striated muscle. J. Gen. Physiol. 51:193s-203s[Medline].
Katz, B. (1949) Les constantes electriques de la membrane du muscle. Arch. Sci. Physiol 2:285-299.
Lee, J.-K., Scott, J.A., Weiss, J.N. (1999) Novel gating mechanism of polyamine block in the strong inward rectifier K channel Kir2.1. J. Gen. Physiol. 113:555-563
Lopatin, A.N., Makhina, E.N., Nichols, C.G. (1994) Potassium channel block by cytoplasmic polyamines as the mechanisms of intrinsic rectification. Nature. 372:366-369[Medline].
Lopatin, A.N., Nichols, C.G. (1996) [K+] dependence of polyamine-induced rectification in inward rectifier potassium channels (IRK, Kir2.1). J. Gen. Physiol. 108:105-113[Abstract].
Lu, Z., MacKinnon, R. (1994a) Electrostatic tuning of Mg2+ affinity in an inward-rectifier K+ channel. Nature. 371:243-246[Medline].
Lu, Z., MacKinnon, R. (1994b) A conductance maximum observed in an inward-rectifier potassium channel. J. Gen. Physiol. 104:477-486[Abstract].
Matsuda, H., Saigusa, A., Irisawa, H. (1987) Ohmic conductance through the inward-rectifier K+ channel and blocking by internal Mg2+. Nature. 325:156-159[Medline].
Neyton, J., Miller, C. (1988a) Potassium block of barium permeation through a calcium-activated K+ channel. J. Gen. Physiol. 92:549-567[Abstract].
Neyton, J., Miller, C. (1988b) Discrete Ba2+ block as a probe of ion occupancy and pore structure in the high-conductance Ca2+-activated K+ channel. J. Gen. Physiol. 92:569-586[Abstract].
Noble, D., Tsien, R.W. (1968) The kinetics and rectifier properties of the slow potassium current in calf Purkinje fibers. J. Physiol. 195:185-214[Medline].
Oliver, D., Hahn, H., Antz, C., Ruppersberg, J.P., Fakler, B. (1998) Interaction of permeant and blocking ions in cloned inward-rectifier K+ channels. Biophys. J. 74:2318-2326
Spassova, M., Lu, Z. (1998) Coupled ion movement underlies rectification in an inward-rectifier K+ channel. J. Gen. Physiol. 112:211-221
Taglialatela, M., Drewe, J.A., Kirsch, G.E., De Biasi, M., Hartmann, H.A., Brown, A.M. (1993) Regulation of K+/Rb+ selectivity and internal TEA blockade by mutations at a single site in K+ pores. Pflügers Arch. 423:104-112.
Taglialatela, M., Ficker, E., Wible, B.A., Brown, A.M. (1995) C-terminus determinants for Mg2+ and polyamine block of the inward rectifier K+ channel IRK1. EMBO (Eur. Mol. Biol. Organ.) J. 14:5532-5541[Abstract].
Vandenberg, C.A. (1987) Inward rectification of a potassium channel in cardiac ventricular cells depends on internal magnesium ions. Proc. Acad. Sci. USA. 84:2560-2564[Abstract].
Woodhull, A.M. (1973) Ionic blockage of sodium channels in nerve. J. Gen. Physiol. 61:687-708
Yang, J., Jan, Y.N., Jan, L.Y. (1995) Control of rectification and permeation by residues in two distinct domains in an inward rectifier K+ channel. Neuron. 14:1047-1054[Medline].
Yellen, G., Jurman, M.E., Abramson, T., MacKinnon, R. (1991) Mutations affecting internal blockade identify the probable pore-forming region of a K+ channel. Science. 251:939-942[Medline].