Slow spermine unblock after MTSET modification of Kir2.1*-176C dimeric channels. (A) Pulses from +30 to −50 mV (in 100 µM spermine) illustrate the rate of spermine unblock in unmodified Kir2.1*-176C channels and after modification with either MTSEA (blue) or MTSET (red). Currents are normalized for kinetic comparison. Unblock was dramatically slower in MTSET-modified Kir2.1*-176C channels and was characterized in more detail over a wide voltage range (inset). (B) Mean data illustrating the voltage dependence of spermine unbinding from MTSET-modified Kir2.1*-176C channels (n = 6; SEM smaller than symbol size). Straight lines are regression fits and model predictions, as indicated. For comparison, previously determined off-rates from WT Kir2.1 channels are also included, illustrating the relatively shallow voltage dependence of modified channels.