Lipid surface charge does not influence conductance or calcium block of single sodium channels in planar bilayers.
We have studied the effects of membrane surface charge on Na+ ion permeation and Ca2+ block in single, batrachotoxin-activated Na channels from rat brain, incorporated into planar lipid bilayers. In phospholipid membranes with no net charge (phosphatidylethanolamine, PE), at low divalent cation concentrations (approximately 100 microM Mg2+), the single channel current-voltage relation was linear and the single channel conductance saturated with increasing [Na+] and ionic strength, reaching a maximum (gamma max) of 31.8 pS, with an apparent dissociation constant (K0.5) of 40.5 mM. The data could be approximated by a rectangular hyperbola. In negatively charged bilayers (70% phosphatidylserine, PS; 30% PE) slightly larger conductances were observed at each concentration, but the hyperbolic form of the conductance-concentration relation was retained (gamma max = 32.9 pS and K0.5 = 31.5 mM) without any preferential increase in conductance at lower ionic strengths. Symmetrical application of Ca2+ caused a voltage-dependent block of the single channel current, with the block being greater at negative potentials. For any given voltage and [Na+] this block was identical in neutral and negatively charged membranes. These observations suggest that both the conduction pathway and the site(s) of Ca2+ block of the rat brain Na channel protein are electrostatically isolated from the negatively charged headgroups on the membrane lipids.
Worley
J F
JF
Department of Physiology, University of Maryland School of Medicine, Baltimore 21201.
French
R J
RJ
Pailthorpe
B A
BA
Krueger
B K
BK
D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D001921:Brain / Q000378:metabolism; D002118:Calcium / Q000494:pharmacology; D004553:Electric Conductivity; D004563:Electrochemistry; D066298:In Vitro Techniques; D008051:Lipid Bilayers / Q000378:metabolism*; D008563:Membrane Lipids / Q000378:metabolism; D008954:Models, Biological; D009994:Osmolar Concentration; D051381:Rats; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism*
D008051:Lipid Bilayers; D008563:Membrane Lipids; D015222:Sodium Channels; D002118:Calcium