Abstract

Mechanosensitive Aβ-fibers (n = 29) and nociceptive Aδ- (n = 6) and C-fibers (n = 10) of the rat sciatic nerve were superfused with lidocaine (LID, 0.1–1.4 mM)in vivo. The [LID] to abolish single electrically stimulated impulses (tonic blockade) in axons was 0.2 to 0.8 mM for Aβ-, 0.1 to 0.6 mM for Aδ- and 0.1 to 1.4 mM for C-fibers. Within each of the fiber groups there was no dependence of blocking [LID] on conduction velocity; slower fibers were no more susceptible than faster ones. Mean blocking concentrations differed between groups, with C-fibers having an IC₅₀ = 0.80 ± 0.32 mM (± S.E.), significantly higher (P < .05, ANOVA) than Aβ-fibers (IC₅₀ = 0.41 ± 0.15 mM) and Aδ-fibers (IC₅₀ = 0.32 ± 0.18 mM). The [LID] causing 50% impulse failure in Aβ-fibers during a 200-Hz, 10-stimulus train (phasic blockade) ranged from 0.2 mM to 0.7 mM; the mean IC₅₀ equaled 0.28 mM (n = 17). Stimulation of nociceptive Aδ-fibers (n = 4) and C-fibers (n = 5) at 5 or 10 Hz for 10 pulses produced no phasic block at [LID]s (0.1–0.5 mM) below those required for tonic blockade. Uptake of ¹⁴C-lidocaine by the nerve, measured in vivo under conditions identical with those for electrophysiology, showed that: a) little drug was in the segments of nerve beyond the superfusion chamber, b) lidocaine was uniformly distributed in the nerve within the chamber, c) the intraneural lidocaine content was identical with that in nerves equilibrated in vitro. The results show a lack of monotonic dependence of sensitivity to local anesthetic on fiber diameter, but do suggest that mean susceptibility to nerve block by lidocaine differs for fibers grouped by, and perhaps according to, function.