BARIUM MODULATES THE GATING OF BATRACHOTOXIN-TREATED NA-STRENGTH SOLUTIONS( CHANNELS IN HIGH IONIC)

Batrachotoxin-activated rat brain Na+ channels were reconstituted in n eutral planar phospholipid bilayers in high ionic strength solutions ( 3 M NaCl). Under these conditions, diffuse surface charges present on the channel protein are screened. Nevertheless, the addition of extrac ellular and/or intracellular Ba2+ caused the following alterations in the gating of Na+ channels: (a) external (or internal) Ba2+ caused a d epolarizing (or hyperpolarizing) voltage shift in the gating curve (op en probability versus membrane potential curve) of the channels; (b) i n the concentration range of 10-120 mM, extracellular Ba2+ caused a la rger voltage shift in the gating curve of Na+ channels than intracellu lar Ba2+; (c) voltage shifts of the gating curve of Na+ channels as a function of external or internal Ba2+ were fitted with a simple bindin g isotherm with the following parameters: for internal Ba2+, DELTAV0.5 ,max (maximum voltage shift) = -11.5 mV, K(D) = 64.7 mM; for external Ba2+, DELTAV0.5,max = 13.5 mV, K(D) = 25.8 mM; (d) the change in the o pen probability of the channel caused by extracellular or intracellula r Ba2+ is a consequence of alterations in both the opening and closing rate constants. Extracellular and intracellular divalent cations can modify the gating kinetics of Na+ channels by a specific modulatory ef fect that is independent of diffuse surface potentials. External or in ternal divalent cations probably bind to specific charges on the Na+ c hannel glycoprotein that modulate channel gating.