On the kinetics and maximal capacity of the system for rapid axonal transport in mammalian neurones.

1. Rabbit peroneal nerves were incubated in vitro in two-compartment chambers. Step-gradients of temperature were established so that the proximal part of each nerve was slightly warmer than the distal part. After incubation, the distribution of dopamine-beta-hydroxylase (DBH) activity along the nerves was examined as an indication of the behaviour of rapid transport in adrenergic axons. 2. With temperature gradients of 5 and 8 degrees C, transport velocity in the proximal regions was expected from previous work to be, respectively, 1.5 and 2 times faster than in the distal regions. Exposing nerves to these gradients induced a significant increment in the concentration of DBH activity, beginning at the boundary between regions. This increment was up to 50% of the normal activity and it propagated distally at the velocity expected for transport at the local temperature. 3. A temperature gradient of 13 degrees C was expected to produce a threefold difference in transport velocity between proximal and distal regions. This gradient produced a slightly larger increment of DBH activity propagating distally, again at the expected velocity. However there was also a disproportionate accumulation of enzyme activity at the boundary between regions. Further increases in the temperature gradient did not enhance the size of the propagating increment but only the rate at which enzyme accumulated at the temperature boundary. 4. It was concluded that adrenergic nerves can transport between two and three times as much material per unit time as they normally do. The ability to increase the flux of material appeared to depend on increases in the concentration of material in motion. There was no indication that such increases led to significant changes in the velocity of transport.