The Journal of General Physiology
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Published 22 February 2002. doi:10.1085/jgp.119.3.235
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© Rockefeller University Press, 0022-1295/2002/3/235/ $5.00
Journal of General Physiology, Volume 119, Number 3, March 2002 235-250

Original Article

 Single Ion Occupancy and Steady-state Gating of Na Channels in Squid Giant Axon

Robert F. Rakowski, David C. Gadsby and Paul De Weer

Marine Biological Laboratory, Woods Hole, Massachusetts 02543

Address correspondence to Dr. R.F. Rakowski, Department of Biological Sciences, Ohio University, Irvine Hall, Athens, OH 45701. Fax: (740) 593-0300; E-mail: rakowski{at}ohio.edu

The properties of the small fraction of tetrodotoxin (TTX)-sensitive Na channels that remain open in the steady state were studied in internally dialyzed voltage clamped squid giant axons. The observed Ussing flux ratio exponent (n') of 0.97 ± 0.03 (calculated from simultaneous measurements of TTX-sensitive current and 22Na efflux) and nonindependent behavior of Na current at high internal [Na] are explained by a one-site ("1s") permeation model characterized by a single effective binding site within the channel pore in equilibrium with internal Na ions (apparent equilibrium dissociation constant KNai(0) = 0.61 ± 0.08 M). Steady-state open probability of the TTX-sensitive channels can be modeled by the product pap{infty}, where pa represents voltage-dependent activation described by a Boltzmann distribution with midpoint Va = -7 mV and effective valence za = 3.2 (Vandenberg, C.A., and F. Bezanilla. 1991. Biophys. J. 60:1499–1510) coupled to voltage-independent inactivation by an equilibrium constant (Bezanilla, F., and C.M. Armstrong. 1977. J. Gen. Physiol. 70:549–566) Keq = 770. The factor p{infty} represents voltage-dependent inactivation with empirical midpoint V{infty}= -83 ± 5 mV and effective valence z{infty} = 0.55 ± 0.03. The composite pap{infty}1s model describes the steady-state voltage dependence of the persistent TTX-sensitive current well.

Key Words: inactivation • tetrodotoxin • nonindependence • flux ratio • open probability


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