Sodium channels in normal and regenerated feline ventral spinal roots

Regenerated and remyelinated nerve fibers have shorter internodes and thus more nodes than normal mature fibers. This requires either a decrease in the number of sodium channels per node or an increase in the number of channels per fiber or both. The purpose of this investigation was to determine what happens to sodium channel number, as estimated by 3H-saxitonin (STX) binding, in regenerated fibers and to relate this to nodal number. Five adult cats underwent cryoaxotomy of ventral root levels L5, L6, L7, and S1 on the left side. After regeneration for 16-45 weeks, binding parameters were determined. On the right (control) side, binding was consistent with that in unoperated animals (b = 1.3, Bmax = 10.2 +/- 0.4 fmol/mg wet, Kd = 0.6 +/- 0.1 nM). However, the regenerated nerves showed a 3.5-fold increase in maximal binding (b = 1.3, Bmax = 36.1 +/- 0.5, Kd = 0.45 +/- 0.4). Computer-aided histologic analysis of the regenerated roots revealed a decrease in fiber size; a significant decrease in internodal length for fibers in a given size class; and a 1.35-fold increase in total fibers per root. These factors account for a 2.36-fold increase in nodes per milligram (wet). The number of STX binding sites per regenerated node was calculated to be 1.95 X 10(6) (1.31, 3.07, 95% confidence limits), whereas it was 1.26 X 10(6) (0.78, 2.02) for the control roots. The difference was not significant (p greater than 0.05). It is concluded that, in regeneration, the increase in nodal number is accompanied by an increase in sodium channels, so that the number of channels per node is normal or slightly increased. There is a marked increase in channels per fiber and an even greater increase in channels per anterior horn cell. The implications of these data for nodal reorganization in remyelination are discussed.