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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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Voltage dependence of channel block by a series of intracellular diamines. The fractions of unblocked current in the presence of various diamines (DMC2 through DMC10, labeled C2 through C10) at the indicated concentrations were plotted as a function of membrane voltage. The curves superimposed on the data are fits of . From each fit, the current at a given voltage was normalized to the value at −100 mV.
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Figure 6: Voltage dependence of channel block by a series of intracellular diamines. The fractions of unblocked current in the presence of various diamines (DMC2 through DMC10, labeled C2 through C10) at the indicated concentrations were plotted as a function of membrane voltage. The curves superimposed on the data are fits of . From each fit, the current at a given voltage was normalized to the value at −100 mV.

Mentions: To better understand block of the IRK1 channel by putrescine (a primary diamine) described here, we systematically tested a series of primary diamines of varying methylene chain length. Fig. 4 shows the current traces of the IRK1 channel between −100 and +100 mV, recorded in the absence and presence of nine diamines containing from 2 to 10 methylene groups (DMC2 through DMC10; putrescine is DMC4). Fig. 5 shows I-V curves obtained without and with the nine diamines at various concentrations including those of Fig. 4. Except for DMC2, which blocked the channel slightly more effectively than DMC3, blocking efficacy generally increased with methylene chain length. The effect of each diamine on the I-V curves is qualitatively similar to that of putrescine (DMC4). However, the currents appear to reach a nonzero “plateau” at progressively lower membrane voltages as the methylene chain length increases. In Fig. 6, we plotted the fractions of unblocked current in the presence of the various diamines as a function of membrane voltage. As in the case of putrescine (DMC4), the extent of channel block by each of the diamines tends to a nonzero level at positive membrane voltages, a behavior reminiscent of block of the retinal cGMP-gated (CNG) channel by certain diamines (e.g., DMC8, DMC9, and DMC10), where these diamines act as permeant blockers (Guo and Lu 2000).


Mechanism of IRK1 channel block by intracellular polyamines.

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

Voltage dependence of channel block by a series of intracellular diamines. The fractions of unblocked current in the presence of various diamines (DMC2 through DMC10, labeled C2 through C10) at the indicated concentrations were plotted as a function of membrane voltage. The curves superimposed on the data are fits of . From each fit, the current at a given voltage was normalized to the value at −100 mV.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2232894&req=5

Figure 6: Voltage dependence of channel block by a series of intracellular diamines. The fractions of unblocked current in the presence of various diamines (DMC2 through DMC10, labeled C2 through C10) at the indicated concentrations were plotted as a function of membrane voltage. The curves superimposed on the data are fits of . From each fit, the current at a given voltage was normalized to the value at −100 mV.
Mentions: To better understand block of the IRK1 channel by putrescine (a primary diamine) described here, we systematically tested a series of primary diamines of varying methylene chain length. Fig. 4 shows the current traces of the IRK1 channel between −100 and +100 mV, recorded in the absence and presence of nine diamines containing from 2 to 10 methylene groups (DMC2 through DMC10; putrescine is DMC4). Fig. 5 shows I-V curves obtained without and with the nine diamines at various concentrations including those of Fig. 4. Except for DMC2, which blocked the channel slightly more effectively than DMC3, blocking efficacy generally increased with methylene chain length. The effect of each diamine on the I-V curves is qualitatively similar to that of putrescine (DMC4). However, the currents appear to reach a nonzero “plateau” at progressively lower membrane voltages as the methylene chain length increases. In Fig. 6, we plotted the fractions of unblocked current in the presence of the various diamines as a function of membrane voltage. As in the case of putrescine (DMC4), the extent of channel block by each of the diamines tends to a nonzero level at positive membrane voltages, a behavior reminiscent of block of the retinal cGMP-gated (CNG) channel by certain diamines (e.g., DMC8, DMC9, and DMC10), where these diamines act as permeant blockers (Guo and Lu 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.

Show MeSH
Related in: MedlinePlus