Ion Channels from Mammalian Brain and Heart, Incorporated into Planar Lipid Bilayers. Regulation by Membrane Potential, Calcium, and Neurotoxins

Sodium and calcium channels from rat brain membranes, have been incorporated into artificial planar phospholipid bilayer membranes and characterized electrophysiologically. Currents through single channels were studied. Sodium channels were activated by batrachotoxin and were blocked by saxitoxin (STX) and tetrodotoxin (TTX). Block by STX and TTX was increased by membrane hyperpolarization. Chemical modification of the STX binding site eliminated block by STX and TTX, reduced single channel sodium currents, and dramatically eliminated block of the channels by calcium ions. Calcium channels were voltage-dependent with depolarizing potentials favoring channel opening. The single calcium channels were selective for divalent cations over monovalent cations and anions. The selectivity sequence for divalent cations, as measured by single channel conductance, was Ba>Ca=Sr>Mn. There was a direct relationship between the transit time for an ion in the channel (the reciprocal of the single channel conductance) and the probability of channel closing (the mean single channel open dwell time) suggesting that only unoccupied channels can close. The data also suggest that there are at lease two (and possibly three) binding sites for calcium ions in the channel pore.