Regulation of cardiac IKs potassium current by membrane phosphatidylinositol 4,5-bisphosphate

Regulation of the slowly activating component of delayed rectifier K+ current (I-Ks) by membrane phospholipid phosphatidylinositol 4,5-bisphosphate ( PtdIns( 4,5) P-2) was examined in guinea pig atrial myocytes using the whole-cell patch clamp method. I-Ks was elicited by depolarizing voltage steps given from a holding potential of - 50 mV, and the effect of various test reagents on IKs was assessed by measuring the amplitude of tail current elicited upon return to the holding potential following a 2-s depolarization to + 30 mV. Intracellular application of 50 muM wortmannin through a recording pipette evoked a progressive increase in I-Ks over a 10-15- min period to 208.5 +/- 14.6% (n = 9) of initial magnitude obtained shortly after rupture of the patch membrane. Intracellular application of anti-PtdIns(4,5) P-2 monoclonal antibody also increased the amplitude of IKs to 198.4 +/- 19.9% (n = 5). In contrast, intracellular loading with exogenous PtdIns(4,5) P-2 at 10 and 100 muM produced a marked decrease in the amplitude of I-Ks to 54.3 +/- 3.8% ( n = 5) and 44.8 +/- 8.2% ( n = 5), respectively. Intracellular application of neomycin ( 50 muM) or aluminum ( 50 muM) evoked an increase in the amplitude of I-Ks to 161.0 +/- 13.5% ( n = 4) and 150.0 +/- 8.2% ( n = 4), respectively. These results strongly suggest that I-Ks channel is inhibited by endogenous membrane PtdIns( 4,5) P-2 through the electrostatic interaction with the negatively charged head group on PtdIns(4,5) P-2. Potentiation of I-Ks by P2Y receptor stimulation with 50 muM ATP was almost totally abolished when PtdIns( 4,5) P-2 was included in the pipette solution, suggesting that depletion of membrane PtdIns(4,5) P-2 is involved in the potentiation of I-Ks by P2Y receptor stimulation. Thus, membrane PtdIns( 4,5) P-2 may act as an important physiological regulator of I-Ks in guinea pig atrial myocytes.