Cytoplasmic gel and water relations of axon

Abstract

A previous method of measuring the swelling pressure (delta IIg) of the cytoplasmic gel of the giant axon of Loligo vulgaris was refined. The estimates of delta IIg 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 that delta IIg contributes to the swelling and shrinkage pattern of the whole axon. In part, the new evidence involved two consecutive direct measurements of intraxonal pressure. The first measurement was that of a mixed pressure composed of delta IIg and delta IIm (delta IIm being the effective osmotic pressure due to the intra-extraxonal gradient in the activity of mobile solutes). The subsequent measurement was that of delta IIg alone. The latter measurement was made feasible by destroying the axolemma, thereby eliminating the contribution of delta IIm. An estimate of delta IIm was obtained by subtracting delta IIg from the total pressure measured initially. The delta IIm determined by the above method was two orders of magnitude smaller than the theoretical osmotic pressure. This is consistent with the delta IIm 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 of delta IIg to the water relations of the whole axon is due to delta IIg being of the same order of magnitude as delta IIm. 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 similar to 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 to delta IIm.