Abstract
G-protein-gated inwardly rectifying K+ (GIRK) channels control various physiological functions. For example, GIRK1/2 heterotetramers in the brain regulate neuronal excitability; GIRK1/4 heterotetramers in the heart regulate heart rate. GIRK channels are potential therapeutic targets for several diseases, such as atrial fibrillation and addiction. In the present study, we aimed to identify the effect of glycyrrhizic acid (GA), a main ingredient of licorice, and its metabolite glycyrrhetic acid (GRA) on GIRK channel activities. By electrophysiological recordings using Xenopus oocytes expressing different GIRK subunits, we observed that GA inhibits the current of heteromeric GIRK1/2 and GIRK1/4 but slightly activates the current of homomeric GIRK2 and GIRK4. This suggests that the inhibitory effect of GA is GIRK1-dependent. Mutation of a GIRK1-specific amino acid residue in the pore helix, Phe137, to Ser abolishes the inhibition of GIRK current by GA, suggesting that the Phe137 plays important roles in the sensitivity of channels to GA. Unlike GA, GRA activates all GIRK2, GIRK4, GIRK1/2 and GIRK1/4 channels. Taken together, these data indicate that GA and GRA have distinct actions on GIRK currents, and would provide clues to elucidate the diverse mechanisms of GIRK channel regulation by analyzing the difference of these compounds.
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