Differential contribution of Na +-K + pump and K + conductance to the post-tetanic hyperpolarization of the subtypes of tetrodotoxin-resistant C-fibers in the isolated bullfrog sciatic nerve

Abstract

The post-tetanic hyperpolarizations (PTHPs) of the fast and slow tetrodotoxin-resistant (TTX-R) C-fibers were recorded from isolated bullfrog sciatic nerves using a modified vaseline gap method. Under control solutions, the monophasic fast and slow TTX-R C-fibers compound action potentials (C-CAPs) increased in amplitude up to 133.9±17.9% (mean±SD; n=9) and 167.0±41.5% ( n=7), respectively, shortly after tetanic stimulation. The recovery time constants of the fast and slow ones were 66.5 and 101.2 s, respectively. Significant differences between the rates of amplitude augmentation of the fast and slow TTX-R C-CAPs were found for the most pairs of measured points ( P<0.05 or P<0.01). After application of ouabain solutions, the augmentation of the fast TTX-R C-CAP was inhibited to an insignificant level, while that of the slow TTX-R C-CAP remained significant with the recovery time constant being 90.9 s. After additional application of TEA, the post-tetanic augmentation of the slow TTX-R C-CAP was blocked. These results suggest that Na +-K + pump contributes to the PTHPs of the fast and slow TTX-R C-fibers and that K + conductance contributes only to the PTHP of the slow TTX-R C-fibers, indicating differential modulation of the firing rate of each TTX-R C-fibers group.