Sodium-activated potassium current in sensory neurons: a comparison of cell-attached and cell-free single-channel activities

Single-channel currents from Na+-dependent K+ channels (KNa) were recorded from cell-attached and inside-out membrane patches of cultured avian trigeminal ganglion neurons by means of the patchclamp technique. Single-channel properties, such as the high elementary conductance and the occurrence of subconductance levels, were unchanged after the patches had been excised from the cells, indicating that they are not under the control of soluble cytoplasmic factors. In cellattached recordings at the cell resting potential the degree of KNa activity, measured as the probability of the channel being open, Po, was low in most cases (around 0.01) and similar to that observed in the inside-out configuration when the bath solution contained concentrations of Na+ around 30 mM and of K+ close to the physiological intracellular levels. However, in some cell-attached patches Po was high (around 0.2) and comparable to the values measured in cell-free recordings with high Na+ concentrations in the bath (100 mM). The excision of a highactivity patch in the presence of 30 mM Na+ resulted in a fall of Po in about 20 s, which is consistent with the wash-out of a soluble cytoplasmic molecule. After the excision all KNa displayed a similar Na+ sensitivity, irrespective of the degree of activation observed in the cellattached mode. In inside-out patches the Po values observed in the presence of either low or high concentrations of Na+ in bath solutions were not modified by internal Ca2+ (0.8–8.5 μM). The variable degree of KNa activation observed in cell-attached recordings suggests that either internal Na+ concentrations reach very high levels close to the membrane, or soluble factor(s) are involved in the modulation of KNa activity: under such conditions, the Na+-activated K+ current may contribute to the maintenance of the resting membrane potential and to control neuronal membrane excitability.