Microtubules and the capacity of the system for rapid axonal transport.

Current information favors the view that microtubules are required for rapid axonal transport of proteins and organelles but are normally present in surplus. Different types of axons tolerate losses of between 35 and 65% of their microtubules during exposure to low temperatures or antimitotic drugs before transport is impaired. Greater losses of microtubules are associated with progressive and marked failure of transport. The normal surplus of microtubules may explain why adrenergic axons of rabbit peroneal nerve have spare transport capacity, which enables them to transport between two and three time as much material as they do ordinarily. Spare capacity for transport is diminished or absent when nerves are incubated at temperatures that lead to a partial loss of microtubules. These observations are considered in the light of the hypothesis that the local density of microtubules determines the maximal local concentration of material that can be carried by rapid transport along vertebrate axons.