3-dimensional configuration of perimysial collagen fibres in rat cardiac muscle at resting and extended sarcomere lengths

1. We have used fluorescence confocal laser scanning microscopy to attain t he three-dimensional (3-D) microstructure of perimysial collagen fibres ove r the range of sarcomere lengths (1.9-2.3 mu m) in which passive force of c ardiac muscle increases steeply. 2. A uniaxial muscle preparation (right ventricular trabecula of rat) was u sed so that the 3-D collagen configuration could be readily related to sarc omere length. Transmission electron microscopy showed that these preparatio ns were structurally homologous to ventricular wall muscle. 3. Trabeculae were mounted on the stage of an inverted microscope and fixed at various sarcomere lengths. After a trabecula was stained with the fluor ophore Sirius Red F3BA and embedded in resin, sequential optical sectioning enabled 3-D reconstructions of its perimysial collagen fibres. The area fr action of these fibres, determined fi om the cross-sections of seven trabec ulae, was 10.5 +/- 3.9% (means +/- S.D.). 4. The reconstructed 3-D images show that perimysial collagen fibres are wa vy (as distinct from coiled) cords which straighten considerably as the sar comere length is increased from 1.85 +/- 0.06 mu m (near-resting length) to 2.3 +/- 0.04 mu m (means +/- S.D., n = 4). These observations are consiste nt with the notion that the straightening of these fibres is responsible fo r limiting extension of the cardiac sacromere to a length of similar to 2.3 mu m.