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The location of triads within a mammalian skeletal muscle sarcomere has traditionally been defined as 'at the A-I junction'. We attempted to verify this statement by examining systematically the location of triads within the sarcomere over the physiological range of sarcomere lengths. This study was conducted using intact feline muscle fibres from caudofemoralis and exclusively fast-twitch muscle from the hindlimb. Our results intact fibres indicate that the distance between the Z-band and triad (ZT) is relatively constant over the range of sarcomere lengths (SLs)examined in this study (1.8-3.4 micron). The slope between ZT and SL was measured to be 0.06 +/- 0.01 (r= 0.36, p0.001) while the slope between the M-line to triad distance (MT) and SL was measured to be 0.44 +/- 0.01 (r0.9, p0.001). The mean ZT was 0.52 +/- 0.07 micron, which corresponds to a triad location approximately halfway along the thin filaments. These results do not support the traditional statement regarding triad location. Nor do these results support a similar recent study conducted using chemically skinned muscle fibres from rat extensor digitorum longus (also a homogeneously fast-twitch muscle of the hindlimb), in which a slope of 0.25 was observed between ZT and SL (r0.9, p0.01). These results are, however, in qualitative agreement with results using intact fibres from fast-twitch rat semitendinosus. Based upon known morphology, we suggest that the only structure supporting triad position is the SR itself, and that a non-homogeneous distribution of the SR within the sarcomere might be responsible for maintaining triad location near the mid-region of the thin filaments. We also suggest that there might be optimal design reasons for locating the triads at the mid-region of the thin filaments. |
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