Fig. 7.
Initial delay in the recovery time course of T-channels over a wide range of membrane potentials. A, The cells was held at −120 mV and pulsed twice to −50 mV (each for 200 msec) every 6 sec, with a gradually lengthened gap between the two pulses at various potentials (the recovery potential,Vr). The average current in the last 2 msec in the first pulse is subtracted from the peak currents in both pulses to make “corrected” peak currents. The fraction of recovered channels is determined by the ratio between the corrected peak current in the second pulse and that in the first pulse. Because the fraction of recovered channels with such short Vr is very small, we try to eliminate the contamination from noises by a smoothing technique. The average value of three consecutive data points is calculated and designated as the final value of the middle point of the three and is plotted against the duration ofVr. It is evident that there is an initial delay in the recovery course with a Vr of −120 mV. The delay becomes shorter with more hyperpolarizedVr and is almost negligible with aVr of −200 mV. Also note that the recovery courses immediately after the delay are approximately linear and are of almost identical slope with Vr of −120, −160, or −200 mV. This is consistent with the findings in Figure 8(see below), which shows that the recovery course after the initial delay can be approximated by monoexponential functions with almost identical time constants at different Vr.B, With the analysis given in A, if one data point is larger than its previous point, the interval between the two points is defined as an “increment.” The initial delay is defined by the first data point that marks the start of four consecutive increments. The initial delays from seven to nine cells are 13.5 ± 2.5, 8.2 ± 1.3, 4.6 ± 0.6, 3.1 ± 0.5, 2.1 ± 0.4, and 1.3 ± 0.3 msec for Vr of −100, −120, −140, −160, −180, and −200 mV, respectively.C, The mean value of the initial delay inB is plotted against Vr in a semilogarithm scale (the longitudinal axis is the natural logarithm of the initial delay in milliseconds). The line is a linear fit of the form: ln(delay) = 4.86 + 0.023V, where V denotesVr in millivolts.