CONTRACTION-INDUCED MOVEMENTS OF WATER IN SINGLE FIBERS OF FROG SKELETAL-MUSCLE

Although X-ray diffraction measurements imply almost constant filament separation during isometric contraction, such constancy does not hold at the level of the isolated cell; cell cross-section increases subst antially during isometric contraction This expansion could arise from accumulation of water drawn from other fibre regions, or from water dr awn into the cell from outside. To distinguish between these hypothese s, we froze single fibres of frog skeletal muscle that were jacketed b y a thin layer of water. Frozen fibres were freeze-substituted section ed transversely, and examined in the electron microscope. In fibres fr ozen during contraction, we found large amounts of water just beneath the sarcolemma, less in deeper regions, and almost none in the fibre c ore. Such gradients were absent or diminished in fibres frozen in the relaxed state. The water was not confined to the myofibril space alone ; we found large water spaces between myofibrils, particularly near mi tochondria. Accumulation of water between myofibrils and around mitoch ondria implies that the driving force for water movement probably lies outside the filament lattice, and may therefore be osmotic. The fact that the distribution was nonuniform-highest near the sarcolemma and l owest in the core-implies that the water was likely drawn from the thi n jacket surrounding the cell. Thus, the contractile cycle appears to be associated with water entry into and exit from the cell.