Stretch activation is known to occur in insect flight muscle fibres (Jewell & Rƒegg, 1966) and to a lesser extent in maximally Ca²⁺-activated mammalian skinned muscle fibres (Rƒegg et al. 1970). Another type of stretch activation is that seen at temperatures above 28 °C in mammalian muscle fibre types under relaxed conditions. This form of stretch activation occurs as a small delayed tension rise ~1-2 % of maximum isometric tension and its rate of development is 3-4 times longer in the slow than in the fast muscle fibres (Mutungi & Ranatunga, 1996). The aim of this study was to investigate the effects of stretch on muscle fibres isolated from neonatal rats.

The experiments were performed at 20 °C on small muscle fibre bundles isolated from 7-day old rats killed with an overdose of sodium pentobarbitone given intra-peritoneally. A preparation was mounted horizontally between two stainless-steel hooks, one attached to a tension transducer and the other to a servo-motor, in a flow through muscle chamber. The average sarcomere length, over a 1 mm region, was monitored using an He/Ne laser and was used to set the initial sarcomere length of the preparation. The preparation was then stretched and released using large triangular length changes (amplitude ~40 % initial muscle length, L₀; rise time 10-100 ms).

As Fig. 1 shows, the length changes are followed by a large tension increase that is maximal at sarcomere lengths of between 2.50 and 2.60 µm. In seven muscle fibres (4 fast and 3 slow), the amplitude of this tension was optimum at sarcomere lengths between 2.4 and 2.6 µm and was ~40-80 % that of the isometric twitch tension. Its rise time was similar to that of the twitch (~200 ms) and in two muscle fibres, exposure to 10 mM BDM led to a ~75 % reduction of tetanic tension and to a 50 % reduction in the stretch-induced tension response. When similar experiments were repeated on adult muscles (toe muscles, n = 3), no evidence of stretch-induced contractile activation was seen. These results suggest that the contractile activation mechanism in neonatal muscles is more sensitive to stretch than that of adult muscles.We thank The Wellcome Trust for financial support.

Figure 1. Tension response to triangular length changes in a slow muscle fibre bundle. The fibre was held at the initial sarcomere lengths shown. Note that the length change is followed by a pronounced increase in tension and that its amplitude is maximal at sarcomere lengths of between 2.50 and 2.60 µm.

Jewell, B.R. & Rƒegg, J.C. (1966). Proc. Roy. Soc. B 164, 428-459.

Mutungi, G. & Ranatunga, K.W. (1996). Biophys. J. 70, 1432-1438.

Rƒegg, J.C., Steiger, G.J. & Schadler, M. (1970). Pflugers Arch. 319, 139-145.