Effects of PI-103 on I_F in SN cells isolated from rabbit heart. (A) Representative traces of I_F activation. Cells were untreated (control) or incubated for 2 h with 500 nM PI-103. Some cells treated with PI-103 were then infused with 1 µM PI(3,4,5)P₃ in the pipette solution. Inset: The voltage-clamp protocol for I_F activation was to apply 1-s hyperpolarizing voltage steps ranging from −30 mV to −110 mV in −10 mV increments from a holding potential of −30 mV followed by a depolarizing step to +50 mV for 0.5 s to record the tail currents, after which the preparation was stepped back to the holding potential. (B) Boltzmann fit of 1-s IC activation curves for I_F. V_1/2 and K were derived from the curves. Control, V_1/2 = −77.3 mV, K = 11.2 mV; PI-103, V_1/2 = −93.2 mV, K = 8.1 mV; and PI-103 + PI(3,4,5)P₃, V_1/2 = −81.0 mV, K = 11.6 mV. (C) Mean time constants (τ) from a single exponential fit of I_F measured at −110 mV. **, P < 0.01 versus control. (D) Summary data of I_F density measured at −110 mV from a holding potential of −30 mV. Cells were incubated with PI-103 for the times indicated before patching with or without infusion of 1 µM PI(3,4,5)P₃ or the control lipids phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) or phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂). **, P < 0.05 versus control. Numbers above the bars indicate the number of cells examined in C and D. n values for C also apply to B. Summarized data show mean ± SEM.