1. The whole-cell patch-clamp technique was used to determine the actions of n-octyl sulphate (OS-) anions and n-octyl trimethylammonium (OTMA+) cations on sodium current steady-state inactivation and peak amplitude in cells isolated from dorsal root ganglia of neonatal rats and maintained in short-term tissue culture. This paper concentrates on the effects of external addition but the actions of internal OS- and OTMA+ are briefly considered. 2. The main action of external OS- was to cause a hyperpolarizing shift in the voltage dependence of the steady-state inactivation parameter, h infinity. At 1-6 mM OS- caused a shift in the mid-point of the h infinity curve of around - 30 mV. The shape of the h infinity curve was altered in a concentration-dependent manner. Internal OS- had no discernible effect on the shape or position of the h infinity curve. 3. External OS- produced a relatively small (less than 25%) reduction in the maximum current achieved following pre-pulses sufficiently negative to remove resting steady-state inactivation. 4. By contrast, external OTMA+ had little effect on the voltage dependence of h infinity and produced a small, but significant, increase in the maximum sodium current. 2 mM-external OTMA+ moved the mid-point of the h infinity curve (V(h)) 5 mV in the depolarizing direction (relative to the mean of control and reversal curves) and increased the maximum current by 13%. One millimolar internal OTMA+ induced a frequency-dependent current block. 5. Raising the external calcium concentration from 2 to 20 mM (in the presence of 2 mM-magnesium and 5 mM-cobalt) caused an 18 mV depolarizing shift in V(h), consistent with a reduction in the negativity of an external surface charge. The maximum current was reduced by 22%. 6. One millimolar OS- reduced the surface potential of egg phosphatidylcholine (EPC) monolayers (at an air-0.5 m-NaCl interface) by 35 mV but 1 or 2 mM-OTMA+ produced only a 2-3 mV increase. The quantitative agreement between the effects of OS-, on V(h) in the rat and on monolayer surface potential, decreased with increasing concentration. 7. The substantial influence of external OS- on the shape and position of the h infinity curve of rat sensory neurone sodium currents is consistent with a surface charge-mediated increase in the negativity of the external surface potential, involving discrete adsorption of the anion to a site at or near the sodium channel and fluctuations in binding which occur on a time scale longer than the effective time constant of channel inactivation. The small effect of OTMA+ is also in agreement with its action on the surface potential of EPC monolayers. 8. External OS-, at concentrations up to 2 mM, has almost no effect on V(h) in the squid giant axon, while concentrations of 3 mM or greater produce irreversible loss of current. The actions of OS- on the sodium current of rat sensory neurones are thus distinctly different from those previously demonstrated on the squid axon sodium current.
