Abstract
When sartorius muscle of a frog was fixed with potassium pyroantimonate in osmium tetroxide, localized electron-dense deposits occurred mainly in the lateral cisternae of the triads and in the N-lines of the thin filaments. The nature of these deposits was studied by soaking the muscles, prior to fixation, in solutions of varying ionic compositions. These included fluids devoid of Na+ and others containing calcium-chelating agents. The results obtained suggested that the precipitate in the myofilaments was due to Ca2+ as it was absent after chelation with ethylene glycol bis (β-aminoethyl ether)-N,N'-tetraacetate (EGTA) in an excess of Mg2+, but still occurred after removal of Na+. The deposits in other locations were probably caused by several cations capable of precipitating pyroantimonate. It was also observed that: the fixative caused contraction of the muscles; after extended soaking in NaCl-free media, severe contractions and dispersal of the precipitate occurred; after EGTA pretreatment there was dilation of the lateral cisternae until the formation of microcysts. Thus, conditions before, during and after fixation could affect the presence and localization of the pyroantimonate deposits.