ANOMALOUS PERMEATION OF NA-GANGLION NEURONS( THROUGH A PUTATIVE K+ CHANNEL IN RAT SUPERIOR CERVICAL)

1. An unanticipated inward tail current was recorded from freshly isol ated adult rat superior cervical ganglion (SCG) neurones using the who le-cell variant of the patch-clamp technique. The tail current was pre sent when Na+ was substituted for tetraethylammonium (TEA) as the prim ary monovalent cation in external solutions designed to isolate Ca2+ c hannel currents (0.5 muM tetrodotoxin present and K+ omitted). 2. The tail current was observed following step potentials positive to -30 mV and reached half-activation near - 9.0 mV. The decay of the tail curr ent was voltage dependent and could be described with two time constan ts. Between potentials of - 120 and - 70 MV, tau(f), the fast componen t, varied from 3 to 8 ms and tau(s), the slow component, changed from 12 to 30 ms, respectively. 3. The tail current was not carried by Ca2, and did not appear to flow through a voltage-gated Ca2+ channel or a Ca2+-dependent channel as it persisted in the absence of external Ca2 + or in the presence of the Ca2+ channel blocker, Cd2+ (0.1 mM). 4. Va rying the external [CI-] did not alter the reversal potential of the t ail current indicating that Cl- was not the charge carrier. 5. The rev ersal potential of the tail current changed in accordance with the Ner nst relationship when [Na+]i/[Na+]o was altered. Our results suggested that this 'unusual or unanticipated current' (I(u)) was carried prima rily by Na+. 6. I(u) was inhibited by the K+ channel-blocking agents q uinidine (0.1 mm), external Ba2+ (5 mm) and internal Cs+ (145 mm). TEA (20 mm either internally or externally) and dendrotoxin (10 muM) were not effective inhibitors of Iu. 7. The decay time constants of the ta il current and parameters of activation and inactivation of Iu were si milar to those of TEA-insensitive delayed rectifier-type K+ channel cu rrents observed in the presence of 145 mm external K+. 8. lu was reduc ed in the presence of either external or internal K+. The interaction of external K+ with Na+ on the Iu tail amplitude was reminiscent of an omalous mole-fraction behaviour. 9. Ion permeability studies revealed that the channel producing Iu had a permeability sequence to monovalen t cations of 3.5:2.5:2:1:0.5 for Rb+, K+, Cs+, Na+ and Li+, respective ly. 10. These data suggest that in the absence of external K+, the ion selectivity of a TEA-insensitive K+ channel in sympathetic neurones i s profoundly diminished. Under these conditions, Na+ traversing a K+ c hannel can generate an unanticipated inward current. Therefore, cautio n should be observed in the interpretation of inward Ca2+ currents whe n recorded in the presence of external Na+.