Open channel noise is independent of ATP concentration. (A) Single channel currents recorded at -120 mV from a Xenopus oocyte activated by different concentrations of ATP as indicated in the figure. The data were filtered at 10 kHz and sampled at 40 kHz. The perfusion solution contained no divalent ions. All of the current traces in this figure are from the same patch. This patch contained one channel because there is no overlap of current in all the range of ATP concentrations. (B) The all-points amplitude histograms of the currents from panel A (0.05 pA/bin), with the distributions fit to the sum of two Gaussians. The dot, dash dot, and solid lines are the fits of open peak, closed peak, and the summation of two Gaussians. The fitting parameters are listed in Table 1.