Slow modal gating of single G protein-activated K⁺ channels expressed in Xenopus oocytes

1. The slow kinetics of G protein-activated K⁺ (GIRK) channels expressed in Xenopus oocytes were studied in single-channel, inside-out membrane patches. Channels formed by GIRK1 plus GIRK4 subunits, which are known to form the cardiac acetylcholine (ACh)-activated GIRK channel (K(ACh)), were activated by a near-saturating dose of G protein βγ subunits (G(βγ); 20 nM). 2. The kinetic parameters of the expressed GIRK1/4 channels were similar to those of cardiac K(ACh). GIRK1/4 channels differed significantly from channels formed by GIRK1 with the endogenous oocyte subunit GIRK5 (GIRK1/5) in some of their kinetic parameters and in a 3-fold lower open probability, P(o). The unexpectedly low P(o) (0.025) of GIRK1/4 was due to the presence of closures of hundreds of milliseconds; the channel spent ~ 90% of the time in the long closed states. 3. GIRK1/4 channels displayed a clear modal behaviour: on a time scale of tens of seconds, the G(βγ)-activated channels cycled between a low-P(o) mode (P(o) of about 0.0034) and a bursting mode characterized by an ~ 30-fold higher P(o) and a different set of kinetic constants (and, therefore, a different set of channel conformations). The available evidence indicates that the slow modal transitions are not driven by binding and unbinding of G(βγ). 4. The GTPγS-activated G(αi1) subunit, previously shown to inhibit GIRK channels, substantially increased the time spent in closed states and apparently shifted the channel to a mode similar, but not identical, to the low-P(o) mode. 5. This is the first demonstration of slow modal transitions in GIRK channels. The detailed description of the slow gating kinetics of GIRK1/4 may help in future analysis of mechanisms of GIRK gating.