Sarcomere length changes during end-held (isometric) contractions in intact mammalian (rat) fast and slow muscle fibres.

The sarcomere length change, within a 2 mm region, during end-held isometric contractions in intact rat fast and slow muscle fibre bundles was investigated at 20 degrees C and an initial sarcomere length of 2.68 microm using He-Ne laser diffraction. In some experiments, the fibre segment displacement was monitored with markers (pieces of human hair) placed at regular intervals on the surface of the muscle fibre bundles. The sarcomere length changes, monitored near the proximal end of the bundle (transducer end), during tetanic contractions were similar to those previously reported in frog muscle fibres. Thus, throughout the tension plateau, sarcomere length remained constant (and shortened) but showed evidence of non-uniform sarcomere behaviour (further shortening) during the rapid tension relaxation phase. Such non-uniform behaviour was not seen during twitch contractions. During a twitch contraction, sarcomeres at the proximal end shortened rapidly at first and continued to shorten--or remained shortened--until the tension had relaxed to between 20-23% of its peak value before lengthening back to the original length. The maximum twitch sarcomere shortening (mean +/- SEM) was 5.9 +/- 0.2% (n = 16) in fast and 5.4 +/- 0.3% (n = 14) in slow fibre bundles at 20 degrees C; sarcomere shortening near body temperature (approximately 35 degrees C) was greater, 8.8 +/- 0.2% (n = 7) in fast and 8.1 +/- 0.2% (n = 5) in slow fibre bundles. Increasing the initial sarcomere length of a preparation decreased the extent of sarcomere shortening and reducing the amount of sarcomere shortening, by sarcomere length clamping, markedly increased the peak twitch tension without significantly altering the twitch time course. When examined at different positions along muscle fibres, a sarcomere shortening was observed along much of the fibre length in most preparations. However, in about a third of the preparations some sarcomere lengthening was recorded in the distal end, but its amplitude was too small to accommodate the fibre shortening elsewhere. Complementary data were obtained using the surface marker technique. The displacement was largest and in opposite--but fibre shortening--direction in the markers placed approximately 0.5-1.0 mm away from the two tendon attachments; the markers placed at or near the centre of the fibre bundle showed the least amount of displacement. The findings suggest that the compliant region, where lengthening occurs, is at fibre ends, i.e. near myotendinous junction.