The flow properties of axoplasm in a defined chemical environment: influence of anions and calcium

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Abstract
The flow properties of axoplasm have been studied in a defined chemical environment. Axoplasm extruded from squid giant axons was introduced into porous cellulose acetate tubes of diameter roughly equal to that of the original axon. Passage of axoplasm along the tube rapidly coated the tube walls with a layer of protein. By measuring the rate of flow back and forth along the tube, the rheological properties of the axoplasm plug were investigated at a range of pressures and in a variety of media. Axoplasm behaves as a classical Bingham body the motion of which can be characterized by a yield stress ( θ ) and a plastic viscosity ( η p1 ). In a potassium methanesulphonate medium containing 65 nM free Ca 2+ , θ averaged 109 ± 46 dyn/cm 2 and η p1 146 ± 83 P. † These values were little affected by ATP, colchicine, cytocholasin B or by replacing K by Na but were sensitive to the anion composition of the medium. The effectiveness of different anions at reducing θ and η p1 was in the order SCN > I > Br > Cl > methanesul­phonate. θ and η p1 were also drastically reduced by increasing the ionized Ca. This effect required millimolar amounts of Ca, was unaffected by the presence of ATP and was irreversible. It could be blocked by the protease inhibitor TLCK. E. p. r. measurements showed that within the matrix of the axoplasm gel there is a watery space that is largely unaffected by anions or calcium.