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Intrinsic versus extrinsic voltage sensitivity of blocker interaction with an ion channel pore.

Martínez-François JR, Lu Z - J. Gen. Physiol. (2010)

Bottom Line: To date, no systematic investigation has been performed to distinguish between these voltage-dependent mechanisms of channel block.The most fundamental characteristic of the extrinsic mechanism, i.e., that block can be rendered voltage independent, remains to be established and formally analyzed for the case of organic blockers.Additionally, a blocker generates (at least) two blocked states, which, if related serially, may preclude meaningful application of a commonly used approach for investigating channel gating, namely, inferring the properties of the activation gate from the kinetics of channel block.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physiology, Howard Hughes Medical Institute, University of Pennsylvania, Philadelphia, PA 19104, USA.

ABSTRACT
Many physiological and synthetic agents act by occluding the ion conduction pore of ion channels. A hallmark of charged blockers is that their apparent affinity for the pore usually varies with membrane voltage. Two models have been proposed to explain this voltage sensitivity. One model assumes that the charged blocker itself directly senses the transmembrane electric field, i.e., that blocker binding is intrinsically voltage dependent. In the alternative model, the blocker does not directly interact with the electric field; instead, blocker binding acquires voltage dependence solely through the concurrent movement of permeant ions across the field. This latter model may better explain voltage dependence of channel block by large organic compounds that are too bulky to fit into the narrow (usually ion-selective) part of the pore where the electric field is steep. To date, no systematic investigation has been performed to distinguish between these voltage-dependent mechanisms of channel block. The most fundamental characteristic of the extrinsic mechanism, i.e., that block can be rendered voltage independent, remains to be established and formally analyzed for the case of organic blockers. Here, we observe that the voltage dependence of block of a cyclic nucleotide-gated channel by a series of intracellular quaternary ammonium blockers, which are too bulky to traverse the narrow ion selectivity filter, gradually vanishes with extreme depolarization, a predicted feature of the extrinsic voltage dependence model. In contrast, the voltage dependence of block by an amine blocker, which has a smaller "diameter" and can therefore penetrate into the selectivity filter, follows a Boltzmann function, a predicted feature of the intrinsic voltage dependence model. Additionally, a blocker generates (at least) two blocked states, which, if related serially, may preclude meaningful application of a commonly used approach for investigating channel gating, namely, inferring the properties of the activation gate from the kinetics of channel block.

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Simulated curves of a Boltzmann function (gray curve) and of the three-state ion displacement model (black curve), both for the case of a positively charged intracellular blocker. The Boltzmann curve was generated from Eq. 1 with [B] = 5 mM, appKd = 6.5 mM, and Z = 1. The black curve was generated from Eq. 7 with [B] = 5 mM, [Na+] = 130 mM, KB = 1 mM, KB-Na = 0.05, and Z = 1.
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fig13: Simulated curves of a Boltzmann function (gray curve) and of the three-state ion displacement model (black curve), both for the case of a positively charged intracellular blocker. The Boltzmann curve was generated from Eq. 1 with [B] = 5 mM, appKd = 6.5 mM, and Z = 1. The black curve was generated from Eq. 7 with [B] = 5 mM, [Na+] = 130 mM, KB = 1 mM, KB-Na = 0.05, and Z = 1.

Mentions: Two general model types have been proposed to account for the voltage dependence of ion channel block by charged blockers. One model assumes that the blocker itself traverses (a portion of) the transmembrane electric field to reach its binding site in the pore (Woodhull, 1973), and the voltage dependence is thus a property intrinsic to blocker binding. Characteristic of this intrinsic model is that the fraction of current not blocked (I/Io) by a nonpermeant blocker is described over the entire membrane voltage range by a single Boltzmann function. Thus, for a positively charged intracellular blocker (Fig. 13, gray curve):(1)II0=11+[B]appKd e−ZVFRT,where appKd is the apparent equilibrium dissociation constant for the blocker-binding reaction in the absence of a membrane potential, [B] is the concentration of blocker, Z is the effective valence (sometimes denoted as zδ), V is the membrane voltage, and F, R, and T have their usual meaning. As shown here, the voltage dependence of CNGA1 channel block by extracellular PhTx can be well accounted for by this type of mechanism, as equilibrium channel block varies with voltage from none to complete block according to single Boltzmann functions (Figs. 2 D and 5 B).


Intrinsic versus extrinsic voltage sensitivity of blocker interaction with an ion channel pore.

Martínez-François JR, Lu Z - J. Gen. Physiol. (2010)

Simulated curves of a Boltzmann function (gray curve) and of the three-state ion displacement model (black curve), both for the case of a positively charged intracellular blocker. The Boltzmann curve was generated from Eq. 1 with [B] = 5 mM, appKd = 6.5 mM, and Z = 1. The black curve was generated from Eq. 7 with [B] = 5 mM, [Na+] = 130 mM, KB = 1 mM, KB-Na = 0.05, and Z = 1.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2812505&req=5

fig13: Simulated curves of a Boltzmann function (gray curve) and of the three-state ion displacement model (black curve), both for the case of a positively charged intracellular blocker. The Boltzmann curve was generated from Eq. 1 with [B] = 5 mM, appKd = 6.5 mM, and Z = 1. The black curve was generated from Eq. 7 with [B] = 5 mM, [Na+] = 130 mM, KB = 1 mM, KB-Na = 0.05, and Z = 1.
Mentions: Two general model types have been proposed to account for the voltage dependence of ion channel block by charged blockers. One model assumes that the blocker itself traverses (a portion of) the transmembrane electric field to reach its binding site in the pore (Woodhull, 1973), and the voltage dependence is thus a property intrinsic to blocker binding. Characteristic of this intrinsic model is that the fraction of current not blocked (I/Io) by a nonpermeant blocker is described over the entire membrane voltage range by a single Boltzmann function. Thus, for a positively charged intracellular blocker (Fig. 13, gray curve):(1)II0=11+[B]appKd e−ZVFRT,where appKd is the apparent equilibrium dissociation constant for the blocker-binding reaction in the absence of a membrane potential, [B] is the concentration of blocker, Z is the effective valence (sometimes denoted as zδ), V is the membrane voltage, and F, R, and T have their usual meaning. As shown here, the voltage dependence of CNGA1 channel block by extracellular PhTx can be well accounted for by this type of mechanism, as equilibrium channel block varies with voltage from none to complete block according to single Boltzmann functions (Figs. 2 D and 5 B).

Bottom Line: To date, no systematic investigation has been performed to distinguish between these voltage-dependent mechanisms of channel block.The most fundamental characteristic of the extrinsic mechanism, i.e., that block can be rendered voltage independent, remains to be established and formally analyzed for the case of organic blockers.Additionally, a blocker generates (at least) two blocked states, which, if related serially, may preclude meaningful application of a commonly used approach for investigating channel gating, namely, inferring the properties of the activation gate from the kinetics of channel block.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physiology, Howard Hughes Medical Institute, University of Pennsylvania, Philadelphia, PA 19104, USA.

ABSTRACT
Many physiological and synthetic agents act by occluding the ion conduction pore of ion channels. A hallmark of charged blockers is that their apparent affinity for the pore usually varies with membrane voltage. Two models have been proposed to explain this voltage sensitivity. One model assumes that the charged blocker itself directly senses the transmembrane electric field, i.e., that blocker binding is intrinsically voltage dependent. In the alternative model, the blocker does not directly interact with the electric field; instead, blocker binding acquires voltage dependence solely through the concurrent movement of permeant ions across the field. This latter model may better explain voltage dependence of channel block by large organic compounds that are too bulky to fit into the narrow (usually ion-selective) part of the pore where the electric field is steep. To date, no systematic investigation has been performed to distinguish between these voltage-dependent mechanisms of channel block. The most fundamental characteristic of the extrinsic mechanism, i.e., that block can be rendered voltage independent, remains to be established and formally analyzed for the case of organic blockers. Here, we observe that the voltage dependence of block of a cyclic nucleotide-gated channel by a series of intracellular quaternary ammonium blockers, which are too bulky to traverse the narrow ion selectivity filter, gradually vanishes with extreme depolarization, a predicted feature of the extrinsic voltage dependence model. In contrast, the voltage dependence of block by an amine blocker, which has a smaller "diameter" and can therefore penetrate into the selectivity filter, follows a Boltzmann function, a predicted feature of the intrinsic voltage dependence model. Additionally, a blocker generates (at least) two blocked states, which, if related serially, may preclude meaningful application of a commonly used approach for investigating channel gating, namely, inferring the properties of the activation gate from the kinetics of channel block.

Show MeSH
Related in: MedlinePlus