Depolarization time constant

Figure 7. Slow inactivation of Na channels is potentiated by STX. The graph shows the time required for the recovery of Na channels to an activatable state after a long (1 sec, +50 mV) inactivating depolarization. When tested by a brief test pulse, control currents (A) recovered in a fast (r = 233 msec) phase. Addition of STX (q, 2 nM, which approximately halved the currents with no inactivating pulse) approximately doubled the fraction of currents recovering in the slow phase and also increased the time constant of slow recovery. The fast recovery rate was unaffected. (Reproduced with permission from Ref. 47. Copyright 1986 The New York Academy of Sciences).

Berger and Vanderkooi(88) studied the depolarization of tryptophan from tobacco mosaic virus. The major subunit of the coat protein contains three tryptophans. The phosphorescence decay is non-single-exponential. At 22°C the lifetime of the long component decays with a time constant of 22 ms, and at 3°C the lifetime is 61 ms. The anisotropy decay is clearly not singleexponential and was consistent with the known geometry of the virus. 

Where T0 is the initial temperature of the depolarization scan. It is assumed that the relaxation time constant r is related to the barrier height or apparent activation energy Ea in the Arrhenius equation. 

Type II pyrethroids also modify the sodium channel kinetics (20-24) In a squid axon internally perfused with 10 pH deltamethrin a step depolarization from a holding membrane potential of -80 mV to -20 mV produced a peak transient sodium current which was followed by a slow current (Figure 3). With a prolonged, 510 msec depolarization the slow component of sodium current was hardly inactivated. The tail current associated with step repolarization of the membrane decayed very slowly with a dual time constant of 33 msec and 1074 msec. Like the peak... 

Fig. 3. Enhanced rate of veratridine action on the nodal membrane after pretreatment with chloramine-T. Trace 1 shows, after the treatment, the slow and incomplete decay of Na current following the peak (cutoff because of the slow time base) during the first 3 s of a 15-s depolarizing impulse. Arrow, 60 pM veratridine was applied leading to a fast increase in inward current with a time constant of 0.75 s. Trace 2, ca. 20 s later. In either case a large current tail was observed after the pulse. 18.2 C. (With permission from Ulbricht 1990)...

In frog nerve the steady-state current increased with [VT] but not its rate of development (Rando 1989). Likewise, in the more classical approach of suddenly applying alkaloid to the depolarized frog nerve, the time constant (several seconds) of the slowly developing inward current depended only weakly on [VT] (Ulbricht 1972a). These results are presented in more detail below (Sect. 2.4). 

The superfused node of Ranvier is particularly suited to fast perfusate changes. As mentioned above, when veratridine (4.5 or 45 pM at pH 8.1) is suddenly applied to the moderately depolarized nodal membrane, an inward current develops exponentially, with a time constant. Ton of 3.8 and... 

This technique may be further applied to the study of intramolecular motions of large molecules. If there is a change in optical anisotropy (in a laboratory-fixed system) associated with an intramolecular motion (see Pecora, 1968), the relaxation rate of the motion should affect the depolarized spectrum. Schmitz and Schurr (1973) have detected time constants for motions of this type in the depolarized spectra of DNAs in solution. < v... 

The asymmetric current has practically exponential kinetics, the time constant depending on the voltage at the depolarizing stage almost in the same manner as T (p). 

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