Abstract
Pyroantimonate + osmium fixation was used to study ultrastructurally the frog sartorius in various experimental conditions. At rest there was an accumulation of precipitate in lateral cisterns near the triads, and in I zones. The I zone deposits were linear and parallel to the Z line, the distance between them varying with sarcomere length. In tetanus there was a dispersal of the I zone precipitate along the myofilaments and a diminution of the triadic deposits. No change from resting state localization occurred after stimulation of briefly glycerinated muscles. The following conclusions were drawn: 1. The cisternal aggregation is caused either by fixation procedures or by crowding of cations, probably including sodium and calcium near the triadic junctions. 2. The I band precipitate may be related to the N lines, and it may be an intracellular storage site of rapidly released calcium. 3. Dispersal of the linear pyroantimonate occurs in tetanus plateau. 4. Transmission of depolarization through T tube membranes is necessary for dispersal of the precipitate after nerve stimulation.
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