Title (also known as connectin) is a muscle-specific giant protein found inside the sarcomere, spanning from the Z-line to the M-line. The I-band segment of title is considered to function as a molecular spring that develops tension when sarcomeres are stretched (passive tension). Recent studies on skeletal muscle indicate that it is not the entire I-band segment of titin that behaves as a spring; some sections are inelastic and do not take part in the development of passive tension. To better understand the mechanism of passive tension development in the heart, where passive tension plays an essential role in the pumping function, we investigated titin's elastic segment in cardiac myocytes using structural and mechanical techniques. Single cardiac myocytes were stretched by various amounts and then immunolabeled and processed for electron microscopy in the stretched state. Monoclonal antibodies that recognize different title epitopes were used, and the locations of the title epitopes in the sarcomere were studied as a function of sarcomere length. We found that only a small region of the I-band segment of titin is elastic; its contour length is estimated at ~75 nm, which is only ~40% of the total I-band segment of titin. Passive tension measurements indicated that the fundamental determinant of how much passive tension the heart develops is the strain of titin's elastic segment. Furthermore, we found evidence that in sarcomeres that are slack (length, ~1.85 μm) the elastic titin segment is highly folded on top of itself. Based on the data, we propose a two-stage mechanism of passive tension development in the heart, in which, between sarcomere lengths of ~1.85 μm and ~2.0 μm, titin's elastic segment straightens and, at lengths longer than ~2.0 μm, the molecular domains that make up titin's elastic segment unravel. Sarcomere shortening to lengths below slack (~1.85 μm) also results in straightening of the elastic titin segment, giving rise to a force that opposes shortening and that tends to bring sarcomeres back to their slack length.
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