Length vs. active force relationship in single isolated smooth muscle cells

The length vs. active force relationship (L-F) may provide information about changes in smooth muscle contractile protein interactions as muscle length changes. To characterize the L-F in single toad stomach smooth muscle cells, cells were attached to a force measurement system, electrically stimulated, and isometric force and elastic modulus (an estimate of the number of attached cross bridges) determined at different cell lengths. Cells generated maximum stress (Pmax = 152.5 mN/mm2) and elastic modulus (Eact = 0.68 x 10(4) mN/mm2) at their rest length (Lcell = 78.0 microns; distance between cell attachments). At shorter lengths, active force and elastic modulus declined proportionally with active force eliminated at 0.4 Lcell. Stretching the relaxed cells up to 1.4 Lcell shifted the subsequent L-F along the length axis by the amount of the stretch but did not change Pmax or the shape of the L-F. In activated cells, force was a function of cell length rather than of shortening history. We interpret these findings as evidence that 1) Lcell is close to the optimum length for force generation, 2) the decline in force at lengths less than Lcell results from a reduced number of attached cross bridges, and 3) stretching relaxed smooth muscle cells may not move the contractile units to new positions on their L-F.