The viscosity of mammalian nerve axoplasm measured by electron spin resonance.

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Abstract
The microviscosity of the axoplasm of cat sciatic nerve was determined by an in vitro ESR method using the spin label tempone. To identify the spin label signal as one arising only from within the axoplasm, Ni2+ was used as a line broadening agent. In one series of experiments in nerves with sheath intact the Ni2+ was shown to eliminate the tempone signal arising from the surface water, and in another series of experiments with the sheath slit, to eliminate the signal from the extracellular space as well. A microviscosity of less than 5 centipoise (cP), i.e., 5 .times. that of water, was determined for the axoplasm. Changes in the viscosity of the nerve axoplasm as a function of temperature over a range of 38.degree. down to 2.degree. C were seen to follow closely the viscosity change found for a water solution. The microviscosity of nerve axoplasm and its change with temperature were related to axoplasmic transport of material in nerve fibers. The results were used to exclude a large increase in viscosity at low temperatures as the cause for the cold-block of fast axoplasmic transport.