Cytoplasmic gel and water relations of axon

Summary

A previous method of measuring the swelling pressure (ΔΠ _g ) of the cytoplasmic gel of the giant axon ofLoligo vulgaris was refined. The estimates ofΔΠ _g made with the improved method were consistent with those made with the earlier method. In these methods the activity of the solvent in the gel is measured by increasing the activity of the solvent in the internal phase of the gel by application of hydrostatic pressure to the gel directly. Comparable values for the activity of the solvent in the gel were obtained also by an alternate method, in which the deswelling of the gel is measured upon decreasing the activity of the solvent in the external phase by addition of a nonpenetrating high mol wt polymer (i.e., Ficoll).

Additional support was obtained for the earlier suggestion thatΔΠ _g contributes to the swelling and shrinkage pattern of the whole axon. In part, the new evidence involved two consecutivedirect measurements of intraxonal pressure. The first measurement was that of a mixed pressure composed ofΔΠ _g andΔΠ _m (ΔΠ _m being the effective osmotic pressure due to the intra-extraxonal gradient in the activity of mobile solutes). The subsequent measurement was that ofΔΠ _g alone. The latter measurement was made feasible by destroying the axolemma, thereby eliminating the contribution ofΔΠ _m . An estimate ofΔΠ _m was obtained by subtractingΔΠ _g from the total pressure measured initially. TheΔΠ _m determined by the above method was two orders of magnitude smaller than the theoretical osmotic pressure. This is consistent with theΔΠ _m determined previously, where osmotic intra-extraxonal filtration coefficients were compared to the hydrostatic. The mixed pressure experiments lend credence to the idea that the substantial contribution ofΔΠ _g to the water relations of the whole axon is due toΔΠ _g being of the same order of magnitude asΔΠ _m .

The degree of free swelling of axoplasmic gels was studied as a function of pH, salt concentration, and hydration radius of the anion of the salt used. The swelling increased with an increase in the reciprocal of the hydration radius, a decrease in salt concentration, and at pH below or above ∼4.5.

The nature of the constraints to the free swelling of axoplasm in axons immersed in seawater was studied. With the seawater employed, these constraints appeared to be due more to the retractive forces of the sheath than toΔΠ _m .