Subtle effects on spermine blocking kinetics after MTSET modification of Kir2.1*-176C channels. Inside-out patches expressing Kir2.1*-176C dimeric channels were pulsed from −80 mV to a range of positive voltages (steps in 20-mV increments are shown for clarity) before (A) and after (B) channel modification with MTSET. Kinetics of block are comparable at positive voltages but slower in MTSET-modified channels at intermediate voltages. (C) Kinetics of spermine block (100 µM) at −10 mV in Kir2.1*-176C channels before and after modification with either MTSEA or MTSET. For comparison, currents have been normalized to the current magnitude at −100 mV in each respective trace. Slow blocking kinetics and more pronounced block are readily apparent in MTSET-modified channels at intermediate voltages. (D) Mean fitted rates of spermine block in Kir2.1*-176C dimeric channels before and after MTSET modification, as determined in 0.01 µM spermine. Straight lines are regression fits or model predictions, as indicated.