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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 spermidine. (A) Current traces recorded in the absence or presence of various concentrations of spermidine. The voltage protocol was as for Fig. 3. (B) I-V curves in the absence or presence of various concentrations of spermidine. (C) The fractions of current not blocked by spermidine are plotted as a function of membrane voltage. The curves are fits of . For each fit, the current at a given voltage was normalized to the value at −100 mV. The parameters determined from the fits are: Ka1 = 6.7 (±0.6) × 10−6 M, Za1 = 5.0 ± 0.1; ka−2/ka−1 = 3.2 (±0.7) × 10−2, “za−1 + za−2” = 5.1 ± 0.1; Kb1 = 2.9 (±0.3) × 10−5 M, Zb1 = 3.2 ± 0.1; kb−2/kb−1 = 3.0 (±0.6) × 10−3, “zb−1 + zb−2” = 3.4 ± 0.1 (mean ± SEM, n = 9).
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Figure 7: Channel block by intracellular spermidine. (A) Current traces recorded in the absence or presence of various concentrations of spermidine. The voltage protocol was as for Fig. 3. (B) I-V curves in the absence or presence of various concentrations of spermidine. (C) The fractions of current not blocked by spermidine are plotted as a function of membrane voltage. The curves are fits of . For each fit, the current at a given voltage was normalized to the value at −100 mV. The parameters determined from the fits are: Ka1 = 6.7 (±0.6) × 10−6 M, Za1 = 5.0 ± 0.1; ka−2/ka−1 = 3.2 (±0.7) × 10−2, “za−1 + za−2” = 5.1 ± 0.1; Kb1 = 2.9 (±0.3) × 10−5 M, Zb1 = 3.2 ± 0.1; kb−2/kb−1 = 3.0 (±0.6) × 10−3, “zb−1 + zb−2” = 3.4 ± 0.1 (mean ± SEM, n = 9).

Mentions: Fig. 7 A shows the current traces recorded between −100 and +100 mV in the absence or presence of various concentrations of the triamine spermidine. Practically, 30 nM is the lowest concentration of spermidine at which the outward IRK1 current reaches a steady state within 300 ms after a voltage step. In Fig. 7 B, we plotted I-V curves recorded in the absence or presence of spermidine at three concentrations. The I-V curve in the presence of spermidine consists of multiple phases. Fig. 7 C shows the fractions of unblocked current at the three spermidine concentrations as a function of membrane voltage. As with diamines, the extent of channel block by spermidine tends to nonzero levels at positive membrane voltages, but a noticeable hump appears in spermidine-blocking curves that was absent from those of diamines.


Mechanism of IRK1 channel block by intracellular polyamines.

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

Channel block by intracellular spermidine. (A) Current traces recorded in the absence or presence of various concentrations of spermidine. The voltage protocol was as for Fig. 3. (B) I-V curves in the absence or presence of various concentrations of spermidine. (C) The fractions of current not blocked by spermidine are plotted as a function of membrane voltage. The curves are fits of . For each fit, the current at a given voltage was normalized to the value at −100 mV. The parameters determined from the fits are: Ka1 = 6.7 (±0.6) × 10−6 M, Za1 = 5.0 ± 0.1; ka−2/ka−1 = 3.2 (±0.7) × 10−2, “za−1 + za−2” = 5.1 ± 0.1; Kb1 = 2.9 (±0.3) × 10−5 M, Zb1 = 3.2 ± 0.1; kb−2/kb−1 = 3.0 (±0.6) × 10−3, “zb−1 + zb−2” = 3.4 ± 0.1 (mean ± SEM, n = 9).
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Figure 7: Channel block by intracellular spermidine. (A) Current traces recorded in the absence or presence of various concentrations of spermidine. The voltage protocol was as for Fig. 3. (B) I-V curves in the absence or presence of various concentrations of spermidine. (C) The fractions of current not blocked by spermidine are plotted as a function of membrane voltage. The curves are fits of . For each fit, the current at a given voltage was normalized to the value at −100 mV. The parameters determined from the fits are: Ka1 = 6.7 (±0.6) × 10−6 M, Za1 = 5.0 ± 0.1; ka−2/ka−1 = 3.2 (±0.7) × 10−2, “za−1 + za−2” = 5.1 ± 0.1; Kb1 = 2.9 (±0.3) × 10−5 M, Zb1 = 3.2 ± 0.1; kb−2/kb−1 = 3.0 (±0.6) × 10−3, “zb−1 + zb−2” = 3.4 ± 0.1 (mean ± SEM, n = 9).
Mentions: Fig. 7 A shows the current traces recorded between −100 and +100 mV in the absence or presence of various concentrations of the triamine spermidine. Practically, 30 nM is the lowest concentration of spermidine at which the outward IRK1 current reaches a steady state within 300 ms after a voltage step. In Fig. 7 B, we plotted I-V curves recorded in the absence or presence of spermidine at three concentrations. The I-V curve in the presence of spermidine consists of multiple phases. Fig. 7 C shows the fractions of unblocked current at the three spermidine concentrations as a function of membrane voltage. As with diamines, the extent of channel block by spermidine tends to nonzero levels at positive membrane voltages, but a noticeable hump appears in spermidine-blocking curves that was absent from those of diamines.

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