Limits...
Kinetics of inward-rectifier K+ channel block by quaternary alkylammonium ions. dimension and properties of the inner pore.

Guo D, Lu Z - J. Gen. Physiol. (2001)

Bottom Line: This remarkable difference in QA selectivity between the two channels results primarily from differing QA unbinding kinetics.The apparent rate constant for binding (kon) of all examined QAs is significantly smaller than expected for a diffusion-limited process.These observations argue that between the intracellular solution and the QA-binding locus, there exists a constricted pathway, whose dimension ( approximately 9 A) is comparable to that of a K+ ion with a single H2O shell.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.

ABSTRACT
We examined block of two inward-rectifier K+ channels, IRK1 and ROMK1, by a series of intracellular symmetric quaternary alkylammonium ions (QAs) whose side chains contain one to five methylene groups. As shown previously, the ROMK1 channels bind larger QAs with higher affinity. In contrast, the IRK1 channels strongly select TEA over smaller or larger QAs. This remarkable difference in QA selectivity between the two channels results primarily from differing QA unbinding kinetics. The apparent rate constant for binding (kon) of all examined QAs is significantly smaller than expected for a diffusion-limited process. Furthermore, a large ( approximately 30-fold) drop in kon occurs when the number of methylene groups in QAs increases from three to four. These observations argue that between the intracellular solution and the QA-binding locus, there exists a constricted pathway, whose dimension ( approximately 9 A) is comparable to that of a K+ ion with a single H2O shell.

Show MeSH

Related in: MedlinePlus

Effects of QAs on the I-V relationship of ROMK1 channels. (A) Steady-state I-V curves without and with various concentrations of one of five QAs, obtained from the data as shown in Fig. 5. (B) Ratios of the I-V curves with and without the QAs shown in A. The curves superimposed on the data are fits of the equation I/Io = Kd / (Kd + [QA]), where Kd = Kd(0 mV)e−ZFVm/RT. The Kd(0 mV) and Z values obtained from the fits are summarized in Fig. 9.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2233664&req=5

Figure 6: Effects of QAs on the I-V relationship of ROMK1 channels. (A) Steady-state I-V curves without and with various concentrations of one of five QAs, obtained from the data as shown in Fig. 5. (B) Ratios of the I-V curves with and without the QAs shown in A. The curves superimposed on the data are fits of the equation I/Io = Kd / (Kd + [QA]), where Kd = Kd(0 mV)e−ZFVm/RT. The Kd(0 mV) and Z values obtained from the fits are summarized in Fig. 9.

Mentions: Fig. 5 shows the current traces of another inward-rectifier (ROMK1) in the absence or presence of one of five QAs, each at 0.3 mM, at membrane voltages from −100 to +100 mV in 10-mV increments. Contrary to our findings with IRK1, but consistent with previous reports, the extent of ROMK1 inhibition increases with increasing QA size (Oliver et al. 1998; Spassova and Lu 1998). Fig. 6 A shows I-V curves of ROMK1 in the absence or presence of one of the five QAs at three concentrations. As in the case of IRK1, the extent of ROMK1 inhibition increased with both QA concentration and membrane depolarization. The strongly voltage-dependent QA block rendered ROMK1 inwardly rectifying. Fig. 6 B plots, for each QA, the fraction of unblocked ROMK1 current against membrane voltage at three concentrations. From the fits of the Woodhull equation we determined, for each QA, the Kd at 0 mV and the corresponding valence factor (Z).


Kinetics of inward-rectifier K+ channel block by quaternary alkylammonium ions. dimension and properties of the inner pore.

Guo D, Lu Z - J. Gen. Physiol. (2001)

Effects of QAs on the I-V relationship of ROMK1 channels. (A) Steady-state I-V curves without and with various concentrations of one of five QAs, obtained from the data as shown in Fig. 5. (B) Ratios of the I-V curves with and without the QAs shown in A. The curves superimposed on the data are fits of the equation I/Io = Kd / (Kd + [QA]), where Kd = Kd(0 mV)e−ZFVm/RT. The Kd(0 mV) and Z values obtained from the fits are summarized in Fig. 9.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2233664&req=5

Figure 6: Effects of QAs on the I-V relationship of ROMK1 channels. (A) Steady-state I-V curves without and with various concentrations of one of five QAs, obtained from the data as shown in Fig. 5. (B) Ratios of the I-V curves with and without the QAs shown in A. The curves superimposed on the data are fits of the equation I/Io = Kd / (Kd + [QA]), where Kd = Kd(0 mV)e−ZFVm/RT. The Kd(0 mV) and Z values obtained from the fits are summarized in Fig. 9.
Mentions: Fig. 5 shows the current traces of another inward-rectifier (ROMK1) in the absence or presence of one of five QAs, each at 0.3 mM, at membrane voltages from −100 to +100 mV in 10-mV increments. Contrary to our findings with IRK1, but consistent with previous reports, the extent of ROMK1 inhibition increases with increasing QA size (Oliver et al. 1998; Spassova and Lu 1998). Fig. 6 A shows I-V curves of ROMK1 in the absence or presence of one of the five QAs at three concentrations. As in the case of IRK1, the extent of ROMK1 inhibition increased with both QA concentration and membrane depolarization. The strongly voltage-dependent QA block rendered ROMK1 inwardly rectifying. Fig. 6 B plots, for each QA, the fraction of unblocked ROMK1 current against membrane voltage at three concentrations. From the fits of the Woodhull equation we determined, for each QA, the Kd at 0 mV and the corresponding valence factor (Z).

Bottom Line: This remarkable difference in QA selectivity between the two channels results primarily from differing QA unbinding kinetics.The apparent rate constant for binding (kon) of all examined QAs is significantly smaller than expected for a diffusion-limited process.These observations argue that between the intracellular solution and the QA-binding locus, there exists a constricted pathway, whose dimension ( approximately 9 A) is comparable to that of a K+ ion with a single H2O shell.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.

ABSTRACT
We examined block of two inward-rectifier K+ channels, IRK1 and ROMK1, by a series of intracellular symmetric quaternary alkylammonium ions (QAs) whose side chains contain one to five methylene groups. As shown previously, the ROMK1 channels bind larger QAs with higher affinity. In contrast, the IRK1 channels strongly select TEA over smaller or larger QAs. This remarkable difference in QA selectivity between the two channels results primarily from differing QA unbinding kinetics. The apparent rate constant for binding (kon) of all examined QAs is significantly smaller than expected for a diffusion-limited process. Furthermore, a large ( approximately 30-fold) drop in kon occurs when the number of methylene groups in QAs increases from three to four. These observations argue that between the intracellular solution and the QA-binding locus, there exists a constricted pathway, whose dimension ( approximately 9 A) is comparable to that of a K+ ion with a single H2O shell.

Show MeSH
Related in: MedlinePlus