Certain axonal membranes of crayfish abdominal nerve cord display ultrastructural changes if the axons are fixed, during electrical stimulation, by aldehydes followed by osmium. Such changes are characterized by an increase in electron opacity and thickness of the unit membranes' dense strata in the axon surface, endoplasmic reticulum, and outer mitochondrial membranes. The electron opacity completely disappears if the sections are treated with hydrogen peroxide solutions. This suggests that the changes represent an increase in the membranes' reactivity for osmium. An unmasking of SH groups could explain such increased osmiophilia, since SH groups are very reactive with osmium, while disulfide bonds are considerably less reactive. This hypothesis was tested by treating control, glutaraldehydefixed nerve cords with disulfide reducing agents. In these preparations an increase in electron opacity and thickness was observed to be localized in the same axonal membranes which reacted as a result of electrical stimulation. The phenomenon did not appear if the SH groups were blocked by maleimide or N-ethylmaleimide before treatment with osmium. These findings seem to suggest that certain axonal membranes of crayfish contain proteins rich in sulfur whose SH groups are unmasked as a result of electrical stimulation. In preliminary experiments an increase in osmiophilia localized in the same membranes with the same characteristics and distribution was observed also in axons from nerve cords asphyxiated either in vitro or in the living animal.

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