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Divalent Cation Interactions with Light-Dependent K Channels: Kinetics of Voltage-Dependent Block and Requirement for an Open Pore
The light-dependent K conductance of hyperpolarizing Pecten photoreceptors exhibits a pronounced outward rectification that is eliminated by removal of extracellular divalent cations. The voltage-dependent block by Ca(2+) and Mg(2+) that underlies such nonlinearity was investigated. Both divalents r...
Autores principales: | , |
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Formato: | Texto |
Lenguaje: | English |
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The Rockefeller University Press
1999
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Materias: | |
Acceso en línea: | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2230541/ https://www.ncbi.nlm.nih.gov/pubmed/10532963 |
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author | Nasi, Enrico del Pilar Gomez, Maria |
author_facet | Nasi, Enrico del Pilar Gomez, Maria |
author_sort | Nasi, Enrico |
collection | PubMed |
description | The light-dependent K conductance of hyperpolarizing Pecten photoreceptors exhibits a pronounced outward rectification that is eliminated by removal of extracellular divalent cations. The voltage-dependent block by Ca(2+) and Mg(2+) that underlies such nonlinearity was investigated. Both divalents reduce the photocurrent amplitude, the potency being significantly higher for Ca(2+) than Mg(2+) (K (1/2) ≈ 16 and 61 mM, respectively, at V(m) = −30 mV). Neither cation is measurably permeant. Manipulating the concentration of permeant K ions affects the blockade, suggesting that the mechanism entails occlusion of the permeation pathway. The voltage dependency of Ca(2+) block is consistent with a single binding site located at an electrical distance of δ ≈ 0.6 from the outside. Resolution of light-dependent single-channel currents under physiological conditions indicates that blockade must be slow, which prompted the use of perturbation/relaxation methods to analyze its kinetics. Voltage steps during illumination produce a distinct relaxation in the photocurrent (τ = 5–20 ms) that disappears on removal of Ca(2+) and Mg(2+) and thus reflects enhancement or relief of blockade, depending on the polarity of the stimulus. The equilibration kinetics are significantly faster with Ca(2+) than with Mg(2+), suggesting that the process is dominated by the “on” rate, perhaps because of a step requiring dehydration of the blocking ion to access the binding site. Complementary strategies were adopted to investigate the interaction between blockade and channel gating: the photocurrent decay accelerates with hyperpolarization, but the effect requires extracellular divalents. Moreover, conditioning voltage steps terminated immediately before light stimulation failed to affect the photocurrent. These observations suggest that equilibration of block at different voltages requires an open pore. Inducing channels to close during a conditioning hyperpolarization resulted in a slight delay in the rising phase of a subsequent light response; this effect can be interpreted as closure of the channel with a divalent ion trapped inside. |
format | Text |
id | pubmed-2230541 |
institution | National Center for Biotechnology Information |
language | English |
publishDate | 1999 |
publisher | The Rockefeller University Press |
record_format | MEDLINE/PubMed |
spelling | pubmed-22305412008-04-21 Divalent Cation Interactions with Light-Dependent K Channels: Kinetics of Voltage-Dependent Block and Requirement for an Open Pore Nasi, Enrico del Pilar Gomez, Maria J Gen Physiol Original Article The light-dependent K conductance of hyperpolarizing Pecten photoreceptors exhibits a pronounced outward rectification that is eliminated by removal of extracellular divalent cations. The voltage-dependent block by Ca(2+) and Mg(2+) that underlies such nonlinearity was investigated. Both divalents reduce the photocurrent amplitude, the potency being significantly higher for Ca(2+) than Mg(2+) (K (1/2) ≈ 16 and 61 mM, respectively, at V(m) = −30 mV). Neither cation is measurably permeant. Manipulating the concentration of permeant K ions affects the blockade, suggesting that the mechanism entails occlusion of the permeation pathway. The voltage dependency of Ca(2+) block is consistent with a single binding site located at an electrical distance of δ ≈ 0.6 from the outside. Resolution of light-dependent single-channel currents under physiological conditions indicates that blockade must be slow, which prompted the use of perturbation/relaxation methods to analyze its kinetics. Voltage steps during illumination produce a distinct relaxation in the photocurrent (τ = 5–20 ms) that disappears on removal of Ca(2+) and Mg(2+) and thus reflects enhancement or relief of blockade, depending on the polarity of the stimulus. The equilibration kinetics are significantly faster with Ca(2+) than with Mg(2+), suggesting that the process is dominated by the “on” rate, perhaps because of a step requiring dehydration of the blocking ion to access the binding site. Complementary strategies were adopted to investigate the interaction between blockade and channel gating: the photocurrent decay accelerates with hyperpolarization, but the effect requires extracellular divalents. Moreover, conditioning voltage steps terminated immediately before light stimulation failed to affect the photocurrent. These observations suggest that equilibration of block at different voltages requires an open pore. Inducing channels to close during a conditioning hyperpolarization resulted in a slight delay in the rising phase of a subsequent light response; this effect can be interpreted as closure of the channel with a divalent ion trapped inside. The Rockefeller University Press 1999-11-01 /pmc/articles/PMC2230541/ /pubmed/10532963 Text en © 1999 The Rockefeller University Press This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/4.0/). |
spellingShingle | Original Article Nasi, Enrico del Pilar Gomez, Maria Divalent Cation Interactions with Light-Dependent K Channels: Kinetics of Voltage-Dependent Block and Requirement for an Open Pore |
title | Divalent Cation Interactions with Light-Dependent K Channels: Kinetics of Voltage-Dependent Block and Requirement for an Open Pore |
title_full | Divalent Cation Interactions with Light-Dependent K Channels: Kinetics of Voltage-Dependent Block and Requirement for an Open Pore |
title_fullStr | Divalent Cation Interactions with Light-Dependent K Channels: Kinetics of Voltage-Dependent Block and Requirement for an Open Pore |
title_full_unstemmed | Divalent Cation Interactions with Light-Dependent K Channels: Kinetics of Voltage-Dependent Block and Requirement for an Open Pore |
title_short | Divalent Cation Interactions with Light-Dependent K Channels: Kinetics of Voltage-Dependent Block and Requirement for an Open Pore |
title_sort | divalent cation interactions with light-dependent k channels: kinetics of voltage-dependent block and requirement for an open pore |
topic | Original Article |
url | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2230541/ https://www.ncbi.nlm.nih.gov/pubmed/10532963 |
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