Effects of the external ions and metabolic poisoning on the constriction of the squid giant axon after axotomy.

After transecting the squid giant axon in the presence of an artificial external medium, which was composed of the ions normally present in squid blood, the cut ends of the axon constrict. This constriction could be completely blocked by cutting the axon in the presence of an artificial internal medium composed of the ions normally present inside the axon. By interchanging the ions in the internal medium with those in the external medium, it was determined that constriction was stimulated by the high concentrations of calcium, chloride, and magnesium ions present in the external medium and inhibited by the high concentrations of potassium ion in the internal medium. Constriction could also be inhibited by 2,4-dinitrophenol and cyanide. Softening of axoplasm and elution of the axoplasmic proteins at the cut end of the axon also occurred in the external medium. This softening and elution may be necessary for constriction since constriction occurred only in those media that also induced axoplasmic softening and elution. Softening and elution are not sufficient for constriction, however, since high potassium, 2,4-dinitrophenol, and cyanide inhibited constriction without inhibiting the softening or elution of axoplasm.