Abstract
Native and recombinant G protein-gated inwardly rectifying potassium (GIRK) channels are directly activated by the betagamma subunits of GTP-binding (G) proteins. The presence of phosphatidylinositol-bis-phosphate (PIP(2)) is required for G protein activation. Formation (via hydrolysis of ATP) of endogenous PIP(2) or application of exogenous PIP(2) 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 PIP(2)-modified channels could also be stimulated by intracellular Mg(2+) ions. In addition, Mg(2+) ions reduced the single-channel conductance of GIRK channels, independently of their gating ability. Both Na(+) and Mg(2+) ions exert their gating effects independently of each other or of the activation by the G(betagamma) subunits. At high levels of PIP(2), synergistic interactions among Na(+), Mg(2+), and G(betagamma) 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.