The site of action of calcium in producing contraction in striated muscle.

1. Contraction of glycerinated crab myofibrils, suspended in a relaxing medium containing Mg-ATP and EGTA, was induced by iontophoretic application of Ca ions through a micropipette. Contraction was observed by phase-contrast microscopy.2. The large size of the sarcomere (10-15 mu) in the crab Cancer allowed applications of Ca ions at several discrete points along a single sarcomere.3. The threshold iontophoretic current needed to produce just visible contraction was measured for each position of the micropipette along the sarcomere. It was always lowest at the A-I junction.4. When Ca was delivered to the I band, the threshold current increased with the distance of the pipette from the A-I junction. Along the A band, the results depended on the sarcomere length: for short sarcomere lengths, the threshold current remained roughly constant over a large part of the A band; for long sarcomeres, it increased with the distance from the A-I junction in the same manner as in the case of the I band.5. As Ca reached the fibril by diffusion from the micropipette, the influence of the diffusion path on the threshold current was investigated.6. The length of the region of overlap of the thick and thin filaments in the A band was estimated by electron microscopy.7. The results demonstrate that, in order to produce contraction, Ca has to reach the region of overlap of the myofilaments.