Cell volume changes regulate slick (Slo2.1), but not slack (Slo2.2) K+ channels.

Maria A Tejada,Dan A Klaerke,Sofia Hammami Bomholtz,Kathleen Stople,Asser Nyander Poulsen,Anne-Kristine Meinild,Steven Barnes

doi:10.1371/journal.pone.0110833

Abstract

Slick (Slo2.1) and Slack (Slo2.2) channels belong to the family of high-conductance K+ channels and have been found widely distributed in the CNS. Both channels are activated by Na+ and Cl− and, in addition, Slick channels are regulated by ATP. Therefore, the roles of these channels in regulation of cell excitability as well as ion transport processes, like regulation of cell volume, have been hypothesized. It is the aim of this work to evaluate the sensitivity of Slick and Slack channels to small, fast changes in cell volume and to explore mechanisms, which may explain this type of regulation. For this purpose Slick and Slack channels were co-expressed with aquaporin 1 in Xenopus laevis oocytes and cell volume changes of around 5% were induced by exposure to hypotonic or hypertonic media. Whole-cell currents were measured by two electrode voltage clamp. Our results show that Slick channels are dramatically stimulated (196% of control) by cell swelling and inhibited (57% of control) by a decrease in cell volume. In contrast, Slack channels are totally insensitive to similar cell volume changes. The mechanism underlining the strong volume sensitivity of Slick channels needs to be further explored, however we were able to show that it does not depend on an intact actin cytoskeleton, ATP release or vesicle fusion. In conclusion, Slick channels, in contrast to the similar Slack channels, are the only high-conductance K+ channels strongly sensitive to small changes in cell volume.

Highlights

Na+ activated potassium currents were first described in 1984 by Kameyama et al [1] in mammalian cardiac cells and five years later they were found in brain stem neurons by Dryer et al [2]
To analyse the sensitivity of Slick and Slack channels to cell volume changes, both channels were co-expressed in Xenopus laevis oocytes together with aquaporin 1 (AQP1), to ensure proper water permeability in the otherwise practically impermeable oocyte membrane
The experiments showed a significant expression of Slick channels; under isotonic conditions the current measured at +80 mV was 1.07 mA, whereas the current in non-injected oocytes or oocytes expressing AQP1 alone did not exceed 0.15 mA at +80 mV

Summary

Introduction

Na+ activated potassium currents were first described in 1984 by Kameyama et al [1] in mammalian cardiac cells and five years later they were found in brain stem neurons by Dryer et al [2]. Thereafter two genes, Slick (Sequence Like an Intermediate Conductance K+ channel) and Slack (Sequence Like A Ca2+activated K+ channel), were cloned by Bhattacharjee et al [3] and Joiner et al [4], respectively. These channels have been proposed to have an important role for repolarization of the action potential and for slow afterhyperpolarizations (AHP), which could be of significant importance after episodes of high firing frequency under sustained stimulation [5]. Antiarrhythmic drugs such as clofilium have been found to inhibit Slick and Slack channels, suggesting a potential protective role of these channels in the heart [6].

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Conclusion