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Sodium Permeability of a Cloned Small-Conductance Calcium-Activated Potassium Channel

Department of Life Science, Gwangju Institute of Science and Technology, Gwangju, Korea

Correspondence: Address reprint requests to Chul-Seung Park, PhD, Dept. of Life Science, Gwangju Institute of Science and Technology, 1 Oryong-dong, Buk-gu, Gwangju, 500-712, Korea. Tel.: 82-62-970-2489; Fax: 82-62-970-2484; E-mail: cspark{at}gist.ac.kr.

Small-conductance Ca²⁺-activated potassium channels (SK_Ca channels) are heteromeric complexes of pore-forming main subunits and constitutively bound calmodulin. SK_Ca channels in neuronal cells are activated by intracellular Ca²⁺ that increases during action potentials, and their ionic currents have been considered to underlie neuronal afterhyperpolarization. However, the ion selectivity of neuronal SK_Ca channels has not been rigorously investigated. In this study, we determined the monovalent cation selectivity of a cloned rat SK_Ca channel, rSK2, using heterologous expression and electrophysiological measurements. When extracellular K⁺ was replaced isotonically with Na⁺, ionic currents through rSK2 reversed at significantly more depolarized membrane potentials than the value expected for a Nernstian relationship for K⁺. We then determined the relative permeability of rSK2 for monovalent cations and compared them with those of the intermediate- and large-conductance Ca²⁺-activated K⁺ channels, IK_Ca and BK_Ca channels. The relative permeability of the rSK2 channel was determined as indicating substantial permeability of small ions through the channel. Although a mutation near the selectivity filter mimicking other K⁺-selective channels influenced the size-selectivity for permeant ions, Na⁺ permeability of rSK2 channels was still retained. Since the reversal potential of endogenous SK_Ca current is determined by Na⁺ permeability in a physiological ionic environment, the ion selectivity of native SK_Ca channels should be reinvestigated and their in vivo roles may need to be restated.

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