TENSION RELAXATION AFTER STRETCH IN RESTING MAMMALIAN MUSCLE-FIBERS -STRETCH ACTIVATION AT PHYSIOLOGICAL TEMPERATURES

Tension responses to ramp stretches of 1-3% L(0) (fiber length) in amp litude were examined in resting muscle fibers of the rat at temperatur es ranging from 10 degrees C to 36 degrees C. Experiments were done us ing bundles of similar to 10 intact fibers isolated from the extensor digitorum longus (a fast muscle) and the soleus (a slow muscle). At lo w temperatures (below similar to 20 degrees C), the tension response c onsisted of an initial rise to a peak during the ramp followed by a co mplex tension decay to a plateau level; the tension decay occurred at approximately constant sarcomere length. The tension decay after a sta ndard stretch at similar to 3-4 . L(0)/s contained a fast, an intermed iate, and a (small amplitude) slow component, which at 10 degrees C (s arcomere length similar to 2.5 mu m) were similar to 2000 . s(-1), sim ilar to 150 . s(-1), and similar to 25 . s(-1) for fast fibers and sim ilar to 2000 . s(-1), similar to 70 . s(-1), and -8 . s-(1) for slow f ibers, respectively. The fast component may represent the decay of int erfilamentary viscous resistance, and the intermediate component may b e due to viscoelasticity in the gap (titin, connectin) filament. The t wo- to threefold fast-slow muscle difference in the rate of passive te nsion relaxation (in the intermediate and the slow components) compare s with previously reported differences in the speed of their active co ntractions; this suggests that ''passive viscoelasticity'' is appropri ately matched to contraction speed in different muscle fiber types. At similar to 35 degrees C, the fast and intermediate components of tens ion relaxation were followed by a delayed tension rise at similar to 1 0 . s(-1) (fast fibers) and 2.5 . s(-1) (slow fibers); the delayed ten sion rise was accompanied by sarcomere shortening. BDM (5-10 mM) reduc ed the active twitch and tetanic tension responses and the delayed ten sion rise at 35 degrees C; the results indicate stretch sensitive acti vation in mammalian sarcomeres at physiological temperatures.