THE EFFECT OF SARCOMERE-LENGTH ON TRIAD LOCATION IN INTACT FELINE CAUDOFEMORALIS MUSCLE-FIBERS

The location of triads within a mammalian skeletal muscle sarcomere ha s traditionally been defined as 'at the A-I junction'. We attempted to verify this statement by examining systematically the location of tri ads within the sarcomere over the physiological range of sarcomere len gths. This study was conducted using intact feline muscle fibres from caudofemoralis - an exclusively fast-twitch muscle from the hindlimb. Our results in 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 mu m) The slope between ZT and SL was measured to be 0.06 +/- 0.01 (r = 0.36,p < 0.001) while the slope between the M-line to triad distance (MT) and SL was measur ed to be 0.44 +/- 0.01 (r > 0.9, p < 0.001). The mean ZT was 0.52 +/- 0.07 mu m, which corresponds to a triad location approximately halfway along the thin filaments. These results do not support the traditiona l statement regarding triad location. Nor do these results support a s imilar recent study conducted using chemically skinned muscle fibres f rom rat extensor digitorum longus (also a homogeneously fast-twitch mu scle of the hindlimb), in which a slope of 0.25 was observed between Z T and SL (r > 0.9,p < 0.01). These results are, however, in qualitativ e agreement with results using intact fibres from fast-twitch rat semi tendinosus. Based upon known morphology, we suggest that the only stru cture supporting triad position is the SR itself, and that a non-homog eneous distribution of the SR within the sarcomere might be responsibl e for maintaining triad location near the mid-region of the thin filam ents. We also suggest that there might be optimal design reasons for l ocating the triads at the mid-region of the thin filaments. (C) Chapma n & Hall Ltd.