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Originally published as Biophys J. BioFAST on August 23, 2004.
doi:10.1529/biophysj.103.039487
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Biophysical Journal 87:3122-3136 (2004)
© 2004 The Biophysical Society

Functional Characterization of a Small Conductance GIRK Channel in Rat Atrial Cells

Emil N. Nikolov and Tatyana T. Ivanova-Nikolova

Department of Pharmacology and Toxicology, Brody School of Medicine, East Carolina University, Greenville, North Carolina

Correspondence: Address reprint requests to Tatyana T. Ivanova-Nikolova, Dept. of Pharmacology and Toxicology, Brody School of Medicine, East Carolina University, Greenville, NC 27858. Tel.: 252-744-2757; Fax: 252-744-3203 E-mail: ivanovanikolovat{at}mail.ecu.edu.

Muscarinic K+ (KACh) channels are key determinants of the inhibitory synaptic transmission in the heart. These channels are heterotetramers consisting of two homologous subunits, G-protein-gated inwardly rectifying K+ (GIRK)1 and GIRK4, and have unitary conductance of ~35 pS with symmetrical 150 mM KCl solutions. Activation of atrial KACh channels, however, is often accompanied by the appearance of openings with a lower conductance, suggesting a functional heterogeneity of G-protein-sensitive ion channels in the heart. Here we report the characterization of a small conductance GIRK (scGIRK) channel present in rat atria. This channel is directly activated by Gß{gamma} subunits and has a unitary conductance of 16 pS. The scGIRK and KACh channels display similar affinities for Gß{gamma} binding and are frequently found in the same membrane patches. Furthermore, Gß{gamma}-activated scGIRK channels—like their KACh counterparts—exhibit complex gating behavior, fluctuating among four functional modes conferred by the apparent binding of a different number of Gß{gamma} subunits to the channel. The electrogenic efficacy of the scGIRK channels, however, is negligible compared to that of KACh channels. Thus, Gß{gamma} subunits employ the same signaling strategy to regulate two ion channels that are apparently endowed with very different functions in the atrial membrane.






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Copyright © 2004 by the Biophysical Society.