The disturbance of the fast axonal transport of protein by passive stretching of an axon in Aplysia.

1. Radioactive amino acid, either [3H]leucine or [3H]proline, was injected into neurone R2 in the abdominal ganglion of Aplysia kurodai to investigate the intra-axonal transport of protein in a single axon. 2. Some of the injected amino acid which was not utilized for protein synthesis diffused intra-axonally with a diffusion constant of 6.0 X 10(-6) cm2/s (25 degrees C), which is the value expected from the known diffusion constants of acetylcholine and gamma-aminobutyric acid in the axoplasm (Koike & Nagata, 1979). The true length of the zigzagging axon in the nerve bundle was measured by the diffusion distances. 3. The radioactive proteins newly synthesized in the cell body from either of the injected amino acid were transported axonally in a single axon of R2. 4. Elongation of the axon resulted in a suppression of the fast axonal transport of the proteins and the amount of protein transported. This contrasts with the observation that action potential propagation along the elongated axon never failed. 5. A possible site for the suppression of the axonal transport would be axonal microtubules whose structure is not likely to be resistant to the distortion caused by the elongation.