Regulation of the cardiac delayed rectifier K^+ channel by membrane phosphoinositides

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Regulation of the slowly activating component of delayed rectifier K^+ current (I_<ks>) by the membrane phospholipid phosphatidylinositol 4, 5-bisphosphate (PtdIns(4, 5)P_2) was examined in guinea-pig atrial cells using whole-cell patch-clamp method. Intracellular application of 50 μM wortmannin through a recording pipette evoked progressive increase in I_<ks> over a 10-15 min to 208.5±14.6% (n = 9) of initial magnitude obtained shortly after rupture of the patch membrane, when assessed by measuring the amplitude of tail current elicited upon return to a holding potential of -50 mV following a 2-s depolarization to +30 mV. Intracellular application of anti-PtdIns(4, 5)P_2 monoclonal antibodies also increased the amplitude of I_<ks> tail current to 198.4±19.9% (n = 5). In contrast, intracellular loading of atrial cells with exogenous PtdIns(4, 5)P_2 (100μM) produced a marked decrease in the amplitude of I_<ks> tail current to 44.8±8.2% (n = 5). These results strongly suggest that endogenous membrane PtdIns(4, 5)P_2 produces a potent inhibitory action on I_<ks> channels. Intracellular application of neomycin (50 μM) or aluminum (50 μM) evoked an increase in the amplitude of I_<ks> tail current to 161.0±13.5% (n = 4) and 150.0±8.2% (n = 4), respectively, which suggests that I_<ks> channel is inhibited by membrane PtdIns(4, 5)P_2 through electrostatic interactions with negatively charged head group on PtdIns(4, 5)P_2. Enhancement of I_<ks> by P2Y receptor stimulation with 50 μM ATP was almost totally abolished by exogenously applied PtdIns(4, 5)P_2. This result suggests that depletion of membrane PtdIns(4, 5)P_2 is primarily involved in an enhancement of I_<ks> evoked by stimulation of P2Y receptor in guinea-pig atrial cells.