Synergistic Activation of G Protein–Gated Inwardly Rectifying Potassium Channels by the βγ Subunits of G Proteins and Na⁺ and Mg²⁺ Ions

Native and recombinant G protein–gated inwardly rectifying potassium (GIRK) channels are directly activated by the βγ subunits of GTP-binding (G) proteins. The presence of phosphatidylinositol-bis-phosphate (PIP₂) is required for G protein activation. Formation (via hydrolysis of ATP) of endogenous PIP₂ or application of exogenous PIP₂ increases the mean open time of GIRK channels and sensitizes them to gating by internal Na⁺ ions. In the present study, we show that the activity of ATP- or PIP₂-modified channels could also be stimulated by intracellular Mg²⁺ ions. In addition, Mg²⁺ ions reduced the single-channel conductance of GIRK channels, independently of their gating ability. Both Na⁺ and Mg²⁺ ions exert their gating effects independently of each other or of the activation by the G_βγ subunits. At high levels of PIP₂, synergistic interactions among Na⁺, Mg²⁺, and G_βγ subunits resulted in severalfold stimulated levels of channel activity. Changes in ionic concentrations and/or G protein subunits in the local environment of these K⁺ channels could provide a rapid amplification mechanism for generation of graded activity, thereby adjusting the level of excitability of the cells.