Influence of negative surface charge on toxin binding to canine heart Na channels in planar bilayers.
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.
Ravindran
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A
Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510.
Moczydlowski
E
E
D000818:Animals; D001496:Batrachotoxins / Q000494:pharmacology*; D001921:Brain / Q000502:physiology; D004285:Dogs; D006321:Heart / Q000502:physiology; D007700:Kinetics; D008051:Lipid Bilayers; D008433:Mathematics; D012530:Saxitoxin / Q000494:pharmacology*; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*; D012965:Sodium Chloride / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D001496:Batrachotoxins; D008051:Lipid Bilayers; D015222:Sodium Channels; D012530:Saxitoxin; D013779:Tetrodotoxin; D012965:Sodium Chloride