Fig. 2.

Properties of the IVM potentiation of P2X₄ channel currents. A, Time course of potentiation by IVM. IVM was added at t = 0, just after the first puff of ATP (3 μm). Potentiation by IVM occurs within 15 sec (time of the second ATP pulse) and is maximal by 30 sec. B, The oocyte was exposed once to ATP (3 μm) in the absence of IVM (t = −5 min). Fifteen seconds later, IVM was applied for 5 min. Att = 0, a second ATP application evoked a greater than fourfold larger ATP response. Thus the ATP-evoked current is potentiated in the absence of ATP, showing that IVM action is use-independent. After washout of IVM, the ATP-evoked current returns to baseline levels within 10–20 min. For A andB the period of IVM application is shown by thehorizontal bar, and the bold line is an exponential fit to the data. C, Concentration dependence of IVM action, tested with pulses of 1 μm ATP. IVM causes significant potentiation at concentrations ≥0.1 μm. The superimposed heavy line represents a fitted curve (Hill equation) with an EC₅₀ of 257 nm and a Hill coefficient of 1; note, however, that the IVM potentiation decreases at 10 μm. D. Concentration–effect relationship for ATP in the absence (●) and presence (○) of 3 μm IVM. IVM both increases the maximal current and decreases the EC₅₀ for ATP. E, Leak-subtracted current–voltage relationships for the ATP-evoked current in the absence (●) and presence (○) of 3 μmIVM; note that there is no shift in the reversal potential.F, Data from experiments like that shown inD. The ratio of the IVM potentiated current to that of the ATP-evoked current alone is plotted versus the membrane voltage. IVM can potentiate the ATP-evoked current at all membrane voltages tested; in addition, the potentiation is slightly greater at positive voltages. For this figure the data are plotted as mean ± SEM.