THE VISCOELASTICITY OF RESTING INTACT MAMMALIAN (RAT) FAST MUSCLE-FIBERS

Tension responses induced by ramp stretches (amplitude of 1-2% fibre l ength 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 f ast muscle) of the rat; sarcomere length of a 2 mm region was monitore d near the tension transducer end by means of a He-Ne laser diffractom eter. 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 manne r 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 tensio n increased in direct proportion to stretch velocity, indicating that it is due to viscous resistance; in a half sarcomere, the viscous resi stance 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 amplitu de 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 de pressed the tetanic responses, produced little or no change in the ten sion components. Our results show that none of the tension components to stretch in relaxed mammalian muscle fibres is due to active, cyclin g cross-bridges; the possibility that the resting sarcomeric visco-ela sticity (net P-2) resides in the connectin (= titin) containing gap fi lament is discussed.