The visco-elasticity of resting intact mammalian (rat) fast muscle fibres

Tension responses induced by ramp stretches (amplitude of 1-2% fibre length and speeds of 0.01-15 L(0)s(-1)) were examined in resting intact muscle fibre bundles isolated from the extensor digitorum longus (a fast muscle) of the rat; sarcomere length of a 2 mm region was monitored near the tension transducer end by means of a He-Ne laser diffractometer. The experiments were done at 10 degrees C. During a ramp stretch, the tension rose rapidly (P-1) and then slowly (P-2) to reach a peak; after completion of the ramp, the tension decayed in a complex manner to a steady level (P-3) at approximately constant sarcomere length. At stretch velocities higher than similar to 1-2 L(0)s(-1), P-1 tension increased in direct proportion to stretch velocity, indicating that it is due to viscous resistance; in a half sarcomere, the viscous resistance to filament sliding may be about 5 x 10(8) Nsm(-3). The steady tension level after the ramp (P-3 tension) was independent of stretch velocity indicating that it represents an elastic tension. The amplitude of the slow tension rise (P-2 tension corrected for P-3) increased with stretch velocity up to a plateau (as in a visco-elastic component); the calculated relaxation time was 5-13 ms. Amplitudes of all three components were larger at longer sarcomere length (range 2.4-3 mu m) The presence of 5-10 mM BDM which abolished the twitch and markedly depressed the tetanic responses, produced little or no change in the tension components. Our results show that none of the tension components to stretch in relaxed mammalian muscle fibres is due to active, cycling cross-bridges; the possibility that the resting sarcomeric visco-elasticity (net P-2) resides in the connectin (= titin) containing gap filament is discussed.