A Method for Studying Intracellular Movement of Water-Soluble Isotopes Prior to Radioautography

The compartmentalization of cell structure as revealed by electron microscopy has been shown to have a striking parallelism with the segregation of enzyme systems as revealed by differential centrifugation. The biochemical anatomy of the cell is now of great interest, and a powerful technique for its investigation has been provided by the recent application of radioautography at the ultrastructural level (1-3). At present, this technique has been most successful in the localization of proteins (1) and nucleoproteins (4). However, one might also expect smaller compounds and ions to show a pattern of differential localization similar to that of the enzymes mediating the reactions with which they are concerned. Though of great interest, the intracellular localization of diffusible compounds has been a problem of exceptional difficulty, as the careful studies of D. K. Hill (5, 6) have clearly demonstrated. While investigating such a problem, it is essential to have prior knowledge of the movement of the compound, both under the experimental conditions and during the steps of histological preparation. While studying the cellular localization of a small organic compound, serotonin, and a divalent cation, calcium, the authors found that the curves of the washout of radioactivity from the tissues of serotonin-C TM and calcium-45 provided a useful tool for investigating intracellular movement and finding a means of limiting it.