Diffusion of ions and indicator dyes in neural cytoplasm.

The dispersion of dye molecules and small cations injected from a point source in the cytoplasm of molluscan neurons has been measured photometrically and compared with dispersion in aqueous solution. The diffusion of phenol red and arsenazo III was at least a factor of five slower in the cytoplasm than in saline. Movement of both dyes was slowed by about the same factor in a given cell. The dispersion rate of arsenazo III was not significantly affected by preloading the cytoplasm to dye concentrations up to 0.5 mM. Calcium and barium dispersion was measured in neurons and saline droplets preloaded with arsenazo III, while phenol red absorbance changes were used to follow the dispersion of injected protons. Ba2+ and H+ moved very slowly in the cytoplasm compared to aqueous solution. Ca2+ movement in all probability underwent a similar retardation in the neurons but high-affinity buffering of the cytoplasm severely restricted the spread of detectable amounts of this ion away from the injection site.