Numerous heptahelical receptors use activation of heterotrimeric G proteins to convey a multitude of extracellular signals to appropriate effector molecules in the cell. Both high specificity and correct integration of these signals are required for reliable cell function. Yet the molecular machineries that allow each cell to merge information flowing across different receptors are not well understood. Here we demonstrate that G protein-regulated inwardly rectifying K(+) (GIRK) channels can operate as dynamic integrators of alpha-adrenergic and cholinergic signals in atrial myocytes. Acting at the last step of the cholinergic signaling cascade, these channels are activated by direct interactions with betagamma subunits of the inhibitory G proteins (G betagamma), and efficiently translate M(2) muscarinic acetylcholine receptor (M2R) activation into membrane hyperpolarization. The parallel activation of alpha-adrenergic receptors imposed a distinctive "signature" on the function of M2R-activated GIRK1/4 channels, affecting both the probability of G betagamma binding to the channel and its desensitization. This modulation of channel function was correlated with a parallel depletion of G beta and protein phosphatase 1 from the oligomeric GIRK1 complexes. Such plasticity of the immediate GIRK signaling environment suggests that multireceptor integration involves large protein networks undergoing dynamic changes upon receptor activation.