The location of tritiated adenine nucleotide was studied by autoradiography in toad muscle which had been fixed in an acetic acid/ethanol mixture, with lead acetate present to act as the nucleotide precipitant. The muscles were embedded in Araldite. The autoradiographs were examined in both the light microscope and the electron microscope. In confirmation of earlier work, it was found that at least 50 per cent of the adenine nucleotide is concentrated in the I band, near the A-I boundary, and evidence is now available to suggest that this fraction is located in the interfibrillar spaces rather than in the substance of the fibrils. Electron micrographs of unstained sections failed to show any structural features at the site in question, though the triads were made visible by the lead. However, when sections were stained with an alkaline solution containing lead, or with uranyl acetate, "vesicles" were revealed at the appropriate site and it is thought that these may be elements of a transverse network (reticulum) of tubules containing the adenine nucleotide. The location of phosphocreatine could not be investigated because this substance was lost from the muscle during the preparative procedure.
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