Ion Selectivity and Properties of the Acid Group in Na Channels Modified by Batrachotoxin in Nerve Membrane

Application of 10 5 mol/1 batrachotoxin (BTX) to the voltage-clamped node of Ranvier was accompanied by repetitive pulsing (10 Hz, to +60 mV) until almost all the Na channels become modified. Relative permeabilities of modified channels to Na, K and NH4 ions were calculated according to Goldman-Hodgkin-Katz equation (Hodgkin and Katz 1949) from changes in reversal potential caused by equimolar substitution of external Na + with one of the test cations. Permeability ratios obtained from peak currents measurements are (mean±S. E.):P NHA /PN ! ,=Í 0.47 ±0.02 and P K /P Na = 0.19 ±0.01. The analogous quantities obtained from steady-state current measurements are 0.57 and 0.24 for NH4 and K ions, respectively. These values are considerably lower than those reported earlier (Revenko and Khodorov 1977). According to these new data BTX is less effective in its ability to change Na channels ion selectivity than the alkaloid aconitine. In aconitine-modified Na channels PN^/P^ and PK/PNB ratios are about 1.36 and 0.36 (Mozhayeva et al. 1977). BTX is also less effective in lowering the sensitivity of the channels to hydrogen block. Fig. 1 demonstrates the effects of pH decrease from 7.60 to 5.12 and to 4.40 on sodium currents in BTX-modified channels. There are, at least, two acid groups at the Na channel accessible for protonation from the external solution:one in selectivity filter (Hille 1975) and another just at the external ends of the pore (Mozhayeva et al. 1982 a). But, for the sake of simplicity one can describe channel block in terms of the model assuming only one titrable acid group (Woodhull 1973). In this case calculated pK of this group should be considered as effective one (pKet), which is, generally, not constant when varying pH (Mozhayeva et al. 1982 a). Therefore pK* for different channels should be compared for experiments with close pH values. pKe f of the BTX-modified channels is lower than that of intact Na channels by about 0.40—0.45. BTX does not change markedly the voltage dependence of pK. t .