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Mechanism of IRK1 channel block by intracellular polyamines.

Guo D, Lu Z - J. Gen. Physiol. (2000)

Bottom Line: As in other K(+) channels, in the presence of intracellular TEA, the IRK1 channel current decreases with increasing membrane voltage and eventually approaches zero.However, in the presence of intracellular polyamines, the channel current varies with membrane voltage in a complex manner: when membrane voltage is increased, the current decreases in two phases separated by a hump.Furthermore, contrary to the expectation for a nonpermeant ionic pore blocker, a significant residual IRK1 current persists at very positive membrane voltages; the amplitude of the residual current decreases with increasing polyamine concentration.

View Article: PubMed Central - PubMed

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

ABSTRACT
Intracellular polyamines inhibit the strongly rectifying IRK1 potassium channel by a mechanism different from that of a typical ionic pore blocker such as tetraethylammonium. As in other K(+) channels, in the presence of intracellular TEA, the IRK1 channel current decreases with increasing membrane voltage and eventually approaches zero. However, in the presence of intracellular polyamines, the channel current varies with membrane voltage in a complex manner: when membrane voltage is increased, the current decreases in two phases separated by a hump. Furthermore, contrary to the expectation for a nonpermeant ionic pore blocker, a significant residual IRK1 current persists at very positive membrane voltages; the amplitude of the residual current decreases with increasing polyamine concentration. This complex blocking behavior of polyamines can be accounted for by a minimal model whereby intracellular polyamines inhibit the IRK1 channel by inducing two blocked channel states. In each of the blocked states, a polyamine is bound with characteristic affinity and probability of traversing the pore. The proposal that polyamines traverse the pore at finite rates is supported by the observation that philanthotoxin-343 (spermine with a bulky chemical group attached to one end) acts as a nonpermeant ionic blocker in the IRK1 channel.

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Channel block by intracellular PhTx. (A) Current traces recorded in the absence or presence of 10 μM PhTx. The voltage protocol was as for Fig. 3. (B) I-V curves in the absence or presence of 10 μM PhTx. (C) The fraction of current not blocked by 10 μM PhTx is plotted against membrane voltage. The curve is a fit of the Woodhull equation, I/Io = 1/(1 + [PhTx]/Kde−ZF V/RT). During the fit, the current at a given voltage was normalized to the value at −100 mV. The values of Kd and Z determined from the fits are 21.3 ± 0.4 μM and 2.8 ± 0.1 (mean ± SEM, n = 3), respectively.
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Figure 10: Channel block by intracellular PhTx. (A) Current traces recorded in the absence or presence of 10 μM PhTx. The voltage protocol was as for Fig. 3. (B) I-V curves in the absence or presence of 10 μM PhTx. (C) The fraction of current not blocked by 10 μM PhTx is plotted against membrane voltage. The curve is a fit of the Woodhull equation, I/Io = 1/(1 + [PhTx]/Kde−ZF V/RT). During the fit, the current at a given voltage was normalized to the value at −100 mV. The values of Kd and Z determined from the fits are 21.3 ± 0.4 μM and 2.8 ± 0.1 (mean ± SEM, n = 3), respectively.

Mentions: To test whether the residual current at positive voltages is due to spermine being a permeant blocker, we examined block of the IRK1 channel by PhTx, essentially spermine with a bulky chemical group attached to one end. Fig. 10 A shows the current traces recorded at membrane voltages between −100 and +100 mV in the absence or presence of 10 μM PhTx. The corresponding I-V curves are shown in Fig. 10 B. In Fig. 10 C, we plotted the fractions of unblocked current as a function of membrane voltage. As in the case of TEA, the extent of channel block by PhTx increased monotonically with membrane voltage. Both the hump and the trailing “plateau” phase seen in the spermine-blocking curve are absent. The curve superimposed on the data points in Fig. 10 C is a fit of the Woodhull equation. Thus, PhTx blocks the IRK1 channel very much like a typical (nonpermeant) ionic pore blocker.


Mechanism of IRK1 channel block by intracellular polyamines.

Guo D, Lu Z - J. Gen. Physiol. (2000)

Channel block by intracellular PhTx. (A) Current traces recorded in the absence or presence of 10 μM PhTx. The voltage protocol was as for Fig. 3. (B) I-V curves in the absence or presence of 10 μM PhTx. (C) The fraction of current not blocked by 10 μM PhTx is plotted against membrane voltage. The curve is a fit of the Woodhull equation, I/Io = 1/(1 + [PhTx]/Kde−ZF V/RT). During the fit, the current at a given voltage was normalized to the value at −100 mV. The values of Kd and Z determined from the fits are 21.3 ± 0.4 μM and 2.8 ± 0.1 (mean ± SEM, n = 3), respectively.
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Figure 10: Channel block by intracellular PhTx. (A) Current traces recorded in the absence or presence of 10 μM PhTx. The voltage protocol was as for Fig. 3. (B) I-V curves in the absence or presence of 10 μM PhTx. (C) The fraction of current not blocked by 10 μM PhTx is plotted against membrane voltage. The curve is a fit of the Woodhull equation, I/Io = 1/(1 + [PhTx]/Kde−ZF V/RT). During the fit, the current at a given voltage was normalized to the value at −100 mV. The values of Kd and Z determined from the fits are 21.3 ± 0.4 μM and 2.8 ± 0.1 (mean ± SEM, n = 3), respectively.
Mentions: To test whether the residual current at positive voltages is due to spermine being a permeant blocker, we examined block of the IRK1 channel by PhTx, essentially spermine with a bulky chemical group attached to one end. Fig. 10 A shows the current traces recorded at membrane voltages between −100 and +100 mV in the absence or presence of 10 μM PhTx. The corresponding I-V curves are shown in Fig. 10 B. In Fig. 10 C, we plotted the fractions of unblocked current as a function of membrane voltage. As in the case of TEA, the extent of channel block by PhTx increased monotonically with membrane voltage. Both the hump and the trailing “plateau” phase seen in the spermine-blocking curve are absent. The curve superimposed on the data points in Fig. 10 C is a fit of the Woodhull equation. Thus, PhTx blocks the IRK1 channel very much like a typical (nonpermeant) ionic pore blocker.

Bottom Line: As in other K(+) channels, in the presence of intracellular TEA, the IRK1 channel current decreases with increasing membrane voltage and eventually approaches zero.However, in the presence of intracellular polyamines, the channel current varies with membrane voltage in a complex manner: when membrane voltage is increased, the current decreases in two phases separated by a hump.Furthermore, contrary to the expectation for a nonpermeant ionic pore blocker, a significant residual IRK1 current persists at very positive membrane voltages; the amplitude of the residual current decreases with increasing polyamine concentration.

View Article: PubMed Central - PubMed

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

ABSTRACT
Intracellular polyamines inhibit the strongly rectifying IRK1 potassium channel by a mechanism different from that of a typical ionic pore blocker such as tetraethylammonium. As in other K(+) channels, in the presence of intracellular TEA, the IRK1 channel current decreases with increasing membrane voltage and eventually approaches zero. However, in the presence of intracellular polyamines, the channel current varies with membrane voltage in a complex manner: when membrane voltage is increased, the current decreases in two phases separated by a hump. Furthermore, contrary to the expectation for a nonpermeant ionic pore blocker, a significant residual IRK1 current persists at very positive membrane voltages; the amplitude of the residual current decreases with increasing polyamine concentration. This complex blocking behavior of polyamines can be accounted for by a minimal model whereby intracellular polyamines inhibit the IRK1 channel by inducing two blocked channel states. In each of the blocked states, a polyamine is bound with characteristic affinity and probability of traversing the pore. The proposal that polyamines traverse the pore at finite rates is supported by the observation that philanthotoxin-343 (spermine with a bulky chemical group attached to one end) acts as a nonpermeant ionic blocker in the IRK1 channel.

Show MeSH
Related in: MedlinePlus