Intracellular calcium translocation during the contraction–relaxation cycle in scorpionfish swimbladder muscle

To examine intracellular Ca(2+) translocation during the contraction-relaxation cycle in vertebrate striated muscle, electron probe X-ray microanalysis was performed on the swimbladder muscle (SBM) fibres of a scorpionfish Sebastiscus marmoratus. The SBM fibres were rapidly frozen at rest, during contraction and at various times after the onset of relaxation. Changes in calcium distribution in the components of the sarcoplasmic reticulum (SR) were examined on the SBM fibre cryosections. In resting fibres, the calcium concentration was highest around the boundary between the A and I bands (A-I boundary), where the terminal cisternae (TC) were located. In contracting fibres, the calcium concentration decreased around the A-I boundary, while it increased in all other regions of the sarcomere, indicating Ca(2+) release from the TC into the myoplasm. During relaxation, the calcium concentration first increased around the regions, where the fenestrated collars (FC) and the longitudinal tubules (LT) were located, and then gradually returned to the levels seen in resting fibres. These results support the view that, after the onset of relaxation in the SBM fibres, Ca(2+) in the myoplasm is first taken up by the FC and the LT, and then gradually returns to the TC.