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

Abstract

Recently, we provided evidence that ginsenoside, the active component of Panax ginseng, uses the pertussis toxin-insensitive Galpha(q/11) phospholipase C-beta3 signal transduction pathway to increase Ca2+-activated Cl- currents in the Xenopus oocyte. Other investigators have shown that stimulation of receptors linked to the Galpha(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 mug/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 Galpha(q), Galpha(11) or Galphaq/Galpha(11), but not Galpha(i2) or Galpha(oA), attenuated the inhibitory ginsenoside effect. Injection of cRNAs coding Gbeta(1gamma2) also attenuated the ginsenoside effect. Preincubation of GIRK channel-expressing oocytes with phospholipase C inhibitor, {i-[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 Galpha(q/11)-, phospholipase C- and protein kinase C-mediated signal transduction pathway. (C) 2003 Elsevier Science B.V All rights reserved.