The force generated by a visceral smooth muscle.

1. Strips of taenia coli from the caecum of the guinea-pig were mounted in an organ bath at 37 degrees C; isometric contractions were elicited with 10(5)M carbachol. Each taenia was stretched to the length at which it produced the maximum active tension; it was then fixed and embedded for measurement of the transverse sectional area. 2. The maximal force produced ranged between 96-1 and 138-3 mN. This corresponded to a force of between 251 and 513 mN.mm(2) (mean: 416 +/-28 [n = 10]). Temperature changes in the range 23-38 degrees C had little effect on the maximal force output.3. When allowance is made for the extracellular space (about 32% of the transverse sectional area), for the non-muscular cells present in the taenia (about 5%), and for the non-contractile material present in the muscle cells (about 10%), the maximal force generated was about 734 mN.mm(2) of contractile material (or almost twice as large as in skeletal muscle).4. Electron microscopy revealed terminal apparatuses at the ends of muscle cells, anchoring the cells to the connective tissue, and cell-to-cell junctions (attachment plaques). In addition, many dense patches of dense bands, sites near the cell surface where filaments are seen to end, were scattered along the entire length of the muscle cell and lay close to bundles of collagen fibrils. 5. It is suggested that the production of such a large force by this smooth muscle is partly explained by the lateral attachment of some contractile units to sites along the entire cell length, which in their turn are anchored to the collagen network; the latter may be considered a sort of intramuscular tendon.