Removal of inactivation causes time-invariant sodium current decays.

Abstract

The kinetic properties of the closing of Na channels were studied in frog skeletal muscle to obtain information about the dependence of channel closing on the past history of the channel. Channel closing was studied in normal and modified channels. Chloramine-T was used to modify the channels so that inactivation was virtually removed. A series of depolarizing prepulse potentials was used to activate Na channels, and a -140-mV postpulse was used to monitor the closing of the channels. Unmodified channels decay via a biexponential process with time constants of 72 and 534 #s at 12~ The observed time constants do not depend upon the potential used to activate the channels. The contribution of the slow component to the total decay increases as the activating prepulse is lengthened. After inactivation is removed, the biexponential character of the decay is retained, with no change in the magnitude of the time constants. However, increases in the duration of the activating prepulse over the range where the current is maximal 1-75 ms) produce identical biexponential decays. The presence of biexponential decays suggests that either two subtypes of Na channels are found in muscle, or Na channels can exist in one of two equally conductive states. The time-invariant decays observed suggest that channel closure does not depend upon their past history.

Two open states or two different Na channels in skeletal muscle fibers: Markov models of the decay of Na currents in frog skeletal muscle

“…Depending upon the size of k6 + k7 compared to the eigenvalues of the system, the delay may not be experimentally perceptible. Experimental removal of inactivation using chloramine-T is equivalent to making k5 = k6 = k7 = 0 which will eliminate the third exponential coefficient; thus in this case, the predicted channel closing will remain biexponential after inactivation removal consistent with previously reported observations (Hahin, 1988).…”

“…Recent experiments (Hahin, 1987;Hahin, 1988) have shown that the properties of Na current tails in frog skeletal muscle are consistent with the predictions of Markov models. Using Markov models to obtain predictions of the expected kinetics of the decay as a function of the duration of the pulse used to activate the Na channels, it is possible to evaluate possible alternative models to describe the observed tails of current.…”

“…The time constants reported in that work are 554 • 37 and 77 • 3 ~ for 48 observations and are virtually identical to the time constants reported in this paper (534 • 21 and 72 • 3 its for 29 observations). Hahin (1988) showed that the amplitudes of the fast and slow components of tail currents from chloramine-T treated fibers do not change when the duration of the activating pulse is lengthened. These results suggest that the channel closure process is independent of the inactivation process and pharmacological procedures can be used to dissect out the closure process.…”

“…If the slow tail of currents in muscle arises from thc activation of a subtype of Na channel, its kinetics of inactivation differs from the threshold channels reported in squid axons. Hahin (1988) showed that Na tail currents recorded from muscle fibers exposed to chloramine-T to eliminate Na inactivation exhibit a biexponential decay. The time constants reported in that work are 554 • 37 and 77 • 3 ~ for 48 observations and are virtually identical to the time constants reported in this paper (534 • 21 and 72 • 3 its for 29 observations).…”

“…Hahin (1987;1988) experimentally showed that the elimination of inactivation using chloramine-T treatment does not alter the biexponential character of the decays. Thus simple three-state models can be eliminated from consideration.…”