Influence of Temperature on the Velocity and on the Isotope Profile of Slowly Transported Labeled Proteins

Slow intra‐axonal flow of [3H]leucine labeled proteins has been studied in the garfish olfactory nerve. Because of the homogeneity of the nerve a very well defined peak of slowly transported radioactivity is observed. The velocity of slow flow increases linearly with temperature. Between 14 and 28°C, the rate of the peak apex increases from 0.26 to 1.57 mm/day and the rate of the leading edge of the wavefront from 0.54 to 2.75 mm/day. Extrapolation of the rate‐temperature function indicates that slow flow should stop at 11°C. However, a velocity of 0.1 mm/day was determined for experiments conducted at 10°C. Between 15 and 25°C a Q10 of 3.7 was determined for the peak apex and of 3.3 for the leading edge of the wavefront. The Q10's are significantly larger than the value of 2.2 found for fast transport (Gross & Beidler, 1975) and support the possibility of at least partial differences between the mechanisms of fast and slow transport. A very small peak was found to migrate in front of the main peak. The positioning of this peak seems to be similar to one found by Lasek & Hoffman (1976) in rat ventral motor neurons. A temperature dependent exponential decrease of the slow moving peak height was measured and it can be estimated that only 1% of the slowly transported radioactivity reaches the synapses. Most of the slow radioactivity appears to remain in the axon behind the peak. The plateau height was also found to decrease exponentially with time. The rate of disappearance greatly affects the profile determined by the slowly transported labeled proteins along the nerve.