Interactions of monovalent cations with sodium channels in squid axon. I. Modification of physiological inactivation gating.

Abstract

Inactivation of Na channels has been studied in voltage-clamped, internally perfused squid giant axons during changes in the ionic composition of the intracellular solution. Peak Na currents are reduced when tetramethylammonium ions (TMA+) are substituted for Cs ions internally . The reduction reflects a rapid, voltage-dependent block of a site in the channel by TMA+. The estimated fractional electrical distance for the site is 10% of the channel length from the internal surface. Na tail currents are slowed by TMA' and exhibit kinetics similar to those seen during certain drug treatments . Steady state INa is simultaneously increased by TMA*, resulting in a "cross-over" of current traces with those in Cs' and in greatly diminished inactivation at positive membrane potentials . Despite the effect on steady state inactivation, the time constants for entry into and exit from the inactivated state are not significantly different in TMA' and Cs'. Increasing intracellular Na also reduces steady state inactivation in a dose-dependent manner . Ratios of steady state IN. to peak INa vary from^-0.14 in Cs'-or K''-perfused axons to^-0.4 in TMA+-or Na'-perfused axons. These results are consistent with a scheme in which TMA' or Na' can interact with a binding site near the inner channel surface that may also be a binding or coordinating site for a natural inactivation particle . A simple competition between the ions and an inactivation particle is, however, not sufficient to account for the increase in steady state INa, and changes in the inactivation process itself must accompany the interaction of TMA' and Na' with the channel.

Reconstituted voltage-sensitive sodium channels from eel electroplax: Activation of permeability by quaternary lidocaine, N-bromoacetamide, and N-bromosuccinimide

“…Alternatively, dissociation of the drug from channels in a blocked state (under strongly depolarized membrane voltage) might leave them in an open state that inactivates slowly (cf. Armstrong & Bezanilla, 1977;Oxford & Yeh, 1985;Yeh et al, 1986; see Discussion).…”

“…This was somewhat unexpected because, usually, inactivation mechanisms are thought to reduce sodium channel opening proability to zero during maintained depolarizations (Hodgkin & Huxley, 1952). Nevertheless, under some conditions steady-state inactivation may be incomplete (Shoukimas & French, 1980;Oxford & Yeh, 1985;Patlak & Ortiz, 1985.…”

Structure, function and pharmacology of voltage-gated sodium channels

“…The decay kinetics are faster for the organic cations than for the alkali metal cations. In contrast to the fast inactivation which is rather insensitive to changes of [Na + ] o (Oxford and Yeh 1985;Tang et al 1996), raising extracellular cation concentration inhibits slow inactivation. These findings, together with mutation experiments within the outer channel pore indicate that the conformational change during slow inactivation might depend on the occupancy of the putative "Na + binding site" at the outer mouth of the channel.…”

Internal block of human heart sodium channels by symmetrical tetra-alkylammoniums.

“…Several studies have examined the effects of TAA compounds on Na channels. TMA causes a voltage-dependent block of single Na channels from rat muscle (Horn et al, 1981; ~ = 0.89) and the macroscopic Na currents of squid (Oxford and Yeh, 1985; b = 0.10). In another study of squid Na currents, the apparent ~ decreased from 0.9 to 0.1 with depolarization (Keynes, Meves, and Hof, 1992).…”