Isolation of functional giant smooth muscle cells from an invertebrate: structural features of relaxed and contracted fibers.

The giant smooth muscle fibers of a ctenophore were isolated by enzymatic digestion. These fibers are multinucleated cells, up to 50 micrometers in diameter and 2 cm in length. Their ultrastructure and membrane electrical properties are similar to those of in situ fibers. Relaxed, coiled (partially contracted), and fully shortened states were distinguished in isolated cells and studied by scanning and transmission electron microscopy. Calcium-containing mitochondrial granules were found in the coiled cells but not in either the relaxed or the fully shortened cells. The relaxed cell is characterized in cross section by the density of myosin filaments (457 +/- 15 per micrometer2) and the thin-to-thick filament ratio (5.2 +/- 0.2). In the coiled cell, the muscle lattice does not expand uniformly, as shown by the variability of myosin spacing, and the thin-to-thick filament ratio decreases. Both clockwise and counterclockwise coiling occur along the same fiber. The implications of these findings with respect to the structure of the contractile apparatus are discussed.