© 1995 American Heart Association, Inc.
Articles |
The Mechanically Active Domain of Titin in Cardiac Muscle
From the Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology (H.G.), Washington State University, Pullman; the Department of Medical Biochemistry (J.-P.J.), University of Calgary (Canada); and the Central EM Laboratory (K.T.), University Medical School of Pécs (Hungary).
Abstract One of the main contributors to passive tension of
the myocardium is titin. However, it is not exactly known
what portions of this 
1 µm-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 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 50
to 100 nm. In sarcomeres that are slack (length 1.85 µm), the
mechanically active domain is folded on top of itself, and the length
of the domain reaches an elastic limit of 550 nm in sarcomeres that
are 2.9 µm long.
Key Words: cardiac myocytes • immunoelectron microscopy • titin • passive tension • elasticity
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