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

Abstract

• 1. 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.

• 2. After treatment with 0.6 mM chloramine-T for 1–4 min a sizeable I_Na 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 I_Na, comparable to the rate of block of peak I_Na during a series of short impulses before chloramine-T.

• 3. In the presence of 60 μM veratridine the peak I_Na was followed by a slow exponential (τ_s) reincrease of inward current, I _s , that did not appreciably inactivate. Application of 0.25 mM benzocaine during a 14-s depolarizing impulse caused I _s to decrease exponentially with a large time constant, τ_on of 4.3 s. Recovery on washout proceeded with τ_off.

• 4. τ_on was little dependent on benzocaine concentration and was 4.5 s on the average in 1 mM. τ_on in 25 μM was insignificantly (15%) larger than in 1 mM if tested on the same fibre. After equilibration in 25 μM, 0.25 mM and 1 μM, I _s (t=14s) was reduced to 0.69, 0.30, and 0.10, respectively, of the value without anaesthetic.

• 5. Cooling by only 4–5°C reduced I _s and much increased τ_s. τ_on (1 mM benzocaine) increased almost in proportion to τ_s.

• 6. Tail currents during a series of pulses (1.1 s every 2.5 s) were reduced by 0.25 mM benzocaine clearly faster (τ′_on = 1.3 s) than I _s during a long pulse of the same amplitude.

• 7. It is concluded that channels kept open by veratridine cannot be blocked directly by benzocaine whose rate of I _s inhibition is mainly determined by the rate with which alkaloid-modified channels close.