Length-tension relation of single smooth muscle cells isolated from the pedal retractor muscle of Mytilus edulis.

Single smooth muscle cells were isolated with an enzymatic dispersion procedure from the pedal retractor muscle (PRM) of Mytilus edulis. The isometric tension of the isolated cell was measured by photoelectrically recording the small, elastic displacement of one of the pair of micropipettes used to hold the ends of the cell by suction. Stimulation with a 0.1 ms rectangular current pulse elicited in the cell a twitch-like contraction which was comparable to that of the whole PRM in both time course an tension per unit cross-sectional area. The length-tension relation for maximal twitches showed that the isolated cell, like the whole PRM, can generate tension over a very wide range of lengths, that is from 0.2 to more that 2.0 Ls (where Ls is the shortest length which the resting cell assumes without any slack). No persistent tension was observed in the cell stretched up to 2.0 Ls, indicating that the passive tension observed in the whole PRM is caused by extracellular material.