Figure 3.

Measurement of gate closing by a two-pulse procedure and the distinction between gate closing and block. (A) The two-pulse procedure is illustrated at left. K channels were activated by a pulse to 0 mV, and the cell was repolarized to −80 mV for 2–20 ms. (B) The fraction of the channels with gates still open after the interval at −80 mV was determined by measuring the initial current during a second pulse to 0 mV (■). ●, I_K tail amplitude. The tail decays too rapidly to be explained by gate closing as measured by the two-pulse method, and the difference is the result of block by Ca²⁺. Solution: 40 Ca 10 K//150 K.