Components of the plasma membrane of growing axons. III. Saxitoxin binding to sodium channels.

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Abstract
The density of Na channels was measured in growing and mature axons of the olfactory nerve of the bullfrog, using as a probe the drug saxitoxin (STX). The toxin binds to control nerves from adult animals in a saturable manner with a dissociation constant of .apprx. 23 nM at 4.degree. C and a capacity of 72 fmol/mg wet wt, equivalent to .apprx. 5 sites/.mu.m2 of axolemma. In growing nerves, obtained from adult frogs 4-5 wk following removal of the original nerve, the STX-binding capacity per wet wt of tissue is markedly reduced, to .apprx. 25% of control values, and appears to decrease in the proximodistal direction. STX-binding data, expressed as STX/mg wet wt, was converted to STX/.mu.m2 of axolemma using stereologically derived values of membrane area per mg wet wt of nerve. The axolemmal content (area/mg wet wt) of all regions of growing nerve is substantially decreased compared to controls, but increases in the proximodistal direction by 60%. These changes in axolemmal area result in calculated STX receptor densities (per unit axolemmal area) which, in distal regions, are approximately at the level of the mature nerve and, in proximal regions, are actually increased above controls by 50-70%. A correlation between STX receptors and intramembrane particles with diameters of 115.4-14.0 nm was found. The growing axon''s gradient of Na channels and the shift from this gradient to a uniform distribution in the mature axon suggest Na channels are inserted into the perikaryal plasmalemma and diffuse from there into the growing axolemma and the axolemma undergoes functional maturation during growth.