An electron microscope study of the influence of divalent ions on myosin filament formation in chicken gizzard extracts and homogenates.

Myosin as well as actin filaments could be seen in negatively stained preparations of fresh chicken gizzard homogenized in buffered KCI (I=0.12, pH 6.85), in a 1 : 1 ratio. Myosin filaments were also present when homogenates were diluted (1 : 9) with solutions containing additional Mg(++) and ATP provided naturally occurring traces of Ca(++) had not been chelated with EGTA. There were no myosin filaments when Mg(++), ATP, Ca(++) or Ca(++) +ATP were added respectively to the diluent. It is suggested that in vivo in relaxed muscles myosin is present in dimers, and only aggregates into filaments at the onset of contraction when Ca(++) are released.