%0 Journal Article %A J. M. RITCHIE %A R. B. ROGART %T Characterization of Exchange-Labeled Saxitoxin and the Origin of Linear Uptake by Excitable Tissue %D 1977 %J Molecular Pharmacology %P 1136-1146 %V 13 %N 6 %X Tritium-labeled saxitoxin, which binds saturably to the sodium channels of excitable tissue, also is taken up linearly. Some of the linear uptake seems to represent intracellular uptake of tritiated water. However, the main component of linear uptake is of true toxin and not of radioactive impurity. It results from an accumulation of the divalent toxin in the immediate vicinity of the axonal (and other cell) membranes because of the fixed negative charges on them. A new and general method for determining the radiochemical purity of the exchange-labeled tritiated saxitoxin is described. The temperature dependence of the kinetics of loss of the label from the methylene hydrogens of carbon 12 of labeled saxitoxin is also reported. %U https://molpharm.aspetjournals.org/content/molpharm/13/6/1136.full.pdf