ATP-dependent regulation of a G protein-coupled K+ channel (GIRK1/GIRK4) expressed in oocytes

Recent studies suggest that activation of the atrial muscarinic K+ current by acetylcholine (ACh) involves an ATP-dependent process that is then inhibited by a cytosolic protein to result in the rapid desensitization. To obtain further evidence in support of such a dually regulated process, we studied the properties of GIRK1 and GIRK4, which, when coexpressed in oocytes, form a heteromultimer that closely resembles the muscarinic K+ channel. ACh activated an inwardly rectifying K+ current that desensitized slowly. In cell-attached patches with ACh in the pipette, the mean open times (tau zero) of GIRK1/GIRK4 were 1.2 +/- 0.1 (28%) and 6.7 +/- 0.8 ms (72%) and did not change significantly with time. However, in inside-out patches, the tau zero of GIRK1/GIRK4 activated with guanosine 5'-O-(3-thiotriphosphate) was 1.3 +/- 0.1 ms (100%), and the channel activity (NP0) was almost fivefold lower. These changes in channel kinetics did not occur in the presence of sodium orthovanadate (3 mM), an inhibitor of phosphatase. Addition of 1 mM ATP, but not adenylimidodiphosphate, to inside-out patches resulted in increases in NP zero (4.8-fold) and the open-time duration of GIRK1/GIRK4, such that tau zero were 1.2 +/- 0.2 (32%) and 6.2 +/- 0.6 ms (68%). Single channel conductances were unchanged (34 +/- 1 pS). Cytosolic extract from atria, but not oocytes, could reverse these effects of ATP. These results provide further evidence that the antagonistic modulation of G protein-gated K+ channels by ATP and the atrial cytosolic protein produces the early rapid desensitization in atrial cells. In oocytes the ATP-dependent step is dominant and thus provides a major component of the total GIRK1/GIRK4 current activated by ACh.