Changes in GIRK1/GIRK2 deactivation kinetics and basal activity in the presence and absence of RGS4

The effect of RGS4, a GTPase-activating protein, on the deactivation kineti cs and basal activity of GIRK1/GIRK2 channels activated by the human K-opio id receptor (hKOR) was investigated. Coexpression in Xenopus oocytes of RGS 4 reduces the basal GIRK1/GIRK2 current and strongly increases the percenta ge agonist-evoked K+ conductance. RGS4 reconstitutes the native gating kine tics by accelerating GIRK1/GIRK2 channel deactivation, a phenomenon also se en after activation with other 7 TM receptors (e.g. muscarine type). In the absence of RGS4, the GIRK1/GIRK2 conductance was increased by approx. 50% after hKOR stimulation with the K-selective opioid receptor ligand, U69593; however mon importantly, at the end of the washout period it was dramatica lly reduced to about 60% of the basal conductance as measured before recept or stimulation. Furthermore, we found that repeated receptor stimulation ca uses an increase of the agonist-gated deactivation kinetics, without affect ing the maximal and minimal conductance levels of GIRK1/GIRK2 channels duri ng and after agonist application. Unlike in the absence of RGS4, coexpressi on with RGS4 completely abolished the reduction of basal conductance after agonist washout and the deactivation kinetics remained unaffected upon repe ated agonist application. The results presented here clearly indicate that previous stimulation by agonists activating G protein-coupled receptors may have long-lasting, strong consequences on the following responses. Therefo re, our study provides evidence for a novel modulation of deactivation kine tics of GIRK1/GIRK2 currents in the absence of RGS4. (C) 2000 Elsevier Scie nce Inc. All rights reserved.