Effects of ginsenoside on G protein-coupled inwardly rectifying K⁺ channel activity expressed in Xenopus oocytes

Recently, we provided evidence that ginsenoside, the active component of Panax ginseng, uses the pertussis toxin-insensitive Gα_q/11-phospholipase C-β3 signal transduction pathway to increase Ca²⁺-activated Cl^- currents in the Xenopus oocyte. Other investigators have shown that stimulation of receptors linked to the Gα_q-phospholipase C pathway inhibits the activity of G protein-coupled inwardly rectifying K⁺ (GIRK) channels. In the present study, we sought to determine whether ginsenoside influenced the activity of GIRK 1 and GIRK 4 (GIRK 1/4) channels expressed in the Xenopus oocyte, and if so, the underlying signal transduction mechanism. In oocytes injected with GIRK 1/4 channel cRNA, bath-applied ginsenoside inhibited the high K⁺ solution-elicited GIRK current (EC₅₀: 4.9±4.3 μg/ml). Pretreatment of the oocyte with pertussis toxin reduced the high K⁺ solution-elicited GIRK current by 49%, but it did not alter the inhibitory effect of ginsenoside on the GIRK current. Prior intraoocyte injection of cRNA(s) coding Gα_q, Gα₁₁ or Gα_q/Gα₁₁, but not Gα_i2 or Gα_oA, attenuated the inhibitory ginsenoside effect. Injection of cRNAs coding Gβ₁γ₂ also attenuated the ginsenoside effect. Preincubation of GIRK channel-expressing oocytes with phospholipase C inhibitor, {1-[6-((17b-3-Methoxyestra-1,3,5(10)-trien-17-yl) amino)hexyl]-1H-pyrrole-2,5-dione} (U73122), or protein kinase C inhibitor, staurosporine or chelerythrine, blocked the inhibitory ginsenoside effect on the GIRK current. Intraoocyte injection of bis (o-aminophenoxy)ethane-N,N,N′,N′-tetracetic acid (BAPTA), a free Ca²⁺ chelator, had no significant effect on the action of ginsenoside. Taken together, these results suggest that ginsenoside inhibits the activity of the GIRK 1/4 channel expressed in the Xenopus oocyte through a pertussis toxin-insensitive and Gα_q/11-, phospholipase C- and protein kinase C-mediated signal transduction pathway.