A-band movement and junctional gap dissociation during caffeine-induced contracture of skeletal muscle fibers.

The action of caffeine on the myofibril profile of the sarcomere and the junctional gap of the intracellular membrane was studied ultrastructurally. When a high concentration of caffeine (5 mM) was applied, the movement of the A-band in sarcomeres dissected from the central portion of single muscle fibers was greater than that in sarcomeres dissected from the proximal and distal portions of the fiber. The amount of calcium released from the sarcoplasmic reticulum of the caffeine-treated muscle was greater in the terminal cisternae located on the narrow I band than in the wide I band, as determined by electron-probe analysis of ultrathin cryosections. The junctional gap was clearly expanded after caffeine treatment, resulting in an irreversible change in muscle contractility. These structural alterations may lead to the distinctive development of tension induced by caffeine. The expansion of the gap may also cause excitation-contraction uncoupling.