Biophysical Journal 55: 359-365 (1989)
© 1989 the Biophysical Society

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Influence of negative surface charge on toxin binding to canine heart Na channels in planar bilayers.

Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510.

ABSTRACT

The presence of negative surface charge near the tetrodotoxin/saxitoxin binding site of canine heart Na channels was revealed by analysis of the kinetics of toxin block of single batrachotoxin-activated Na channels in planar bilayers as a function of [NaCl]. The voltage-dependence of toxin binding and the toxin dissociation rate are nearly constant as [NaCl] is varied from 0.05 to 3 M. In contrast, the association rate constant of the toxins is inversely dependent on [NaCl], with the rate for the divalent toxin, saxitoxin2+, affected more steeply than that of the monovalent toxin, tetrodotoxin1+. These results for toxin-insensitive Na channels from canine heart parallel previous findings for toxin-sensitive Na channels from canine brain. The model of Green et al. (Green, W. N., L. B. Weiss, and O. S. Anderson. 1987. J. Gen. Physiol. 89:873-903), which includes Na+ competition and Gouy-Chapman screening of surface charge, provided an excellent fit to the data. The results suggest that the two canine Na channel subtypes have a similar density of negative surface charge (1 e-/400 A2) and a similar dissociation constant for Na+ competition (0.5 M) at the toxin binding site. Thus, negative surface charge is a conserved feature of channel function of these two subtypes. The difference in toxin binding affinities arises from small differences in intrinsic association and dissociation rates.

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