The mechanically active domain of titin in cardiac muscle.

One of the main contributors to passive tension of the myocardium is titin. However, it is not exactly known what portions of this approximately 1 micron-long molecule are anchored in the sarcomere (hence, are rendered inelastic) and what portions are elastic (hence, are mechanically active in developing passive tension). We assessed the length of the elastic domain of cardiac titin by ultrastructural and mechanical methods. Single cardiac myocytes were stretched by various amounts, and while in the stretched state, they were processed for immunoelectron microscopy. Several monoclonal anti-titin antibodies were used, and the locations of the titin epitopes in the sarcomere were studied as a function of sarcomere length. Only a small fraction (5% to 10%) of the approximately 1000-nm-long molecule behaved elastically under physiological conditions. This mechanically active domain is located close to the A/I junction, and its contour length when unstretched is estimated at approximately 50 to 100 nm. In sarcomeres that are slack (length approximately 1.85 microns), the mechanically active domain is folded on top of itself, and the length of the domain reaches an elastic limit of approximately 550 nm in sarcomeres that are approximately 2.9 microns long.