CALCIUM POTENTIALS AND TETRODOTOXIN-RESISTANT SODIUM POTENTIALS IN UNMYELINATED-C FIBERS OF BIOPSIED HUMAN SURAL NERVE

Compound action potentials and electrotonic responses to 150 ms curren t pulses were recorded from isolated nerve fascicles of human sural ne rve biopsies. Compound action potentials in normal bathing solution we re characterized by previously described A beta, A delta and C fibre c omponents. In addition, tetrodotoxin-resistant sodium- or calcium-depe ndent potential components were found when a mixture of tetrodotoxin a nd the potassium channel blockers 4-aminopyridine and tetraethylammoni um was added to the bathing solution. In contrast to tetrodotoxin-sens itive action potentials, tetrodotoxin-resistant sodium- or calcium-dep endent potentials could be recorded in the presence of high extracellu lar potassium concentrations (10-20 mM). Calcium action potentials wer e found to be sensitive to specific pharmacological antagonists or ago nists of L-, N- and P type calcium channels. Lidocaine, cadmium, verap amil and capsaicin showed unspecific blocking effects on calcium and t etrodotoxin-resistant potentials. Tetrodotoxin-resistant action potent ials seem to originate from unmyelinated C fibres since a clear correl ation was found between the number of C fibres and the amplitude of te trodotoxin-resistant calcium and sodium spikes in preparations with di fferent axon type composition. The evidence for tetrodotoxin-resistant Na+ and Ca2+ spikes in peripheral human axons offers new possibilitie s for a better understanding and/or treatment of abnormalities in the excitability of damaged or diseased peripheral nerves.