A SURVEY OF IN-SITU SARCOMERE EXTENSION IN MOUSE SKELETAL-MUSCLE

The giant molecule titin/connectin was demonstrated to connect the end s of thick filaments with the Z-disks and thus to provide an elastic c onnection that seems to be responsible for passive tension in striated muscle. To investigate the physiological limits of I-band titin exten sion in skeletal muscle, we have measured sarcomere lengths of a numbe r of mouse postural and clonal muscles ii? situ under the constraints imposed by the skeletal, ligamentous and tendinous components of the m otile apparatus. These values now give upper limits for the extension of the I-band and therefore for the maximal degree of titin extension under physiological constraints. We find that I-band extension in all muscles investigated does not exceed a factor of approximate to 2.5 in situ, which is well below values obtainable in isolated fibre prepara tions. Approach to the yield-point is therefore prevented by extramusc ular mechanisms. Sarcomere lengths near the tendinous junction and wit hin the muscle are virtually identical in extended muscle, suggesting that a major function of titin in intact muscle is to ensure uniform s arcomere lengths over the entire muscle length and thus to prevent loc alized myofibril overstretch during isometric contraction.