A channel recording showing the effect of 10 μM CrMP on the activity of the 70-pS K⁺ channel in a cell-attached patch. The channel-closed level is indicated by C, and the pipette holding potential was 0 mV. The top trace shows the time course of the effect of CrMP. Three parts of the trace, indicated by numbers 1–3, are extended at fast time resolution. CrMP, chromium mesoporphyrin.