Distinctly different rates of benzocaine action on sodium channels of Ranvier nodes kept open by chloramine-T and veratridine

Single myelinated nerve fibres of the frog, Rana esculenta, were voltage clamped in a fast-exchange chamber in the presence of 10 mM TEA to block potassium channels. After treatment with 0.6 mM chloramine-T for 1-4 min a sizeable INa component persisted even during a 14-s depolarizing impulse. Changing the perfusate to Ringer solution + 1 mM benzocaine resulted in a fast reduction (half time ca. 0.06 s) of the persistent INa, comparable to the rate of block of peak INa during a series of short impulses before chloramine-T. In the presence of 60 microM veratridine the peak INa was followed by a slow exponential (tau s) reincrease of inward current, Is, that did not appreciably inactivate. Application of 0.25 mM benzocaine during a 14-s depolarizing impulse caused Is to decrease exponentially with a large time constant, tau on of 4.3 s. Recovery on washout proceeded with tau off = 3.4s. Tau on was little dependent on benzocaine concentration and was 4.5 s on the average in 1 mM. Tau on in 25 microM was insignificantly (15%) larger than in 1 mM if tested on the same fibre. After equilibration in 25 microM, 0.25 mM and 1 microM, Is(t = 14s) was reduced to 0.69, 0.30, and 0.10, respectively, of the value without anaesthetic. Cooling by only 4-5 degrees C reduced Is and much increased tau s. tau on (1 mM benzocaine) increased almost in proportion to tau s. Tail currents during a series of pulses (1.1 s every 2.5 s) were reduced by 0.25 mM benzocaine clearly faster (tau on = 1.3 s) than Is during a long pulse of the same amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)