THE EFFECTS OF AGE AND LOW-INTENSITY ENDURANCE EXERCISE ON THE CONTRACTILE PROPERTIES OF SINGLE SKINNED FAST-TWITCH AND SLOW-TWITCH SKELETAL-MUSCLE FIBERS

Aged (25 month old) rats were subjected to a low-intensity exercise pr ogramme, consisting of 10-weeks endurance swimming. At the conclusion of training (at the age of 27 months), fresh single skinned muscle fib res were prepared from the extensor digitorum longus (EDL) and soleus (SOL) muscles of the hindlimb and activated in Ca2+- and Sr+-buffered solutions to measure selected isometric contractile characteristics. T he major fibre population of the SOL from trained aged animals demonst rated a higher threshold [Ca2+] and [Sr2+] for contraction and a reduc ed sensitivity to Sr2+. Few changes in contractile characteristics wer e observed in the EDL muscle fibres from exercised rats, except for fi bre type specific changes in the steepness of the force-pCa and force- pSr relationship. The effect of ageing on the contractile characterist ics of skinned muscle fibres was also investigated by comparing the da ta obtained from aged sedentary rats with that from adult (6-9 month o ld) sedentary rats. Ageing was shown not to have affected the single f ibre contractile properties of the SOL, but did affect one of the two fibre populations of the EDL. This was illustrated by the higher sensi tivity to Ca2+ in the aged muscle fibres with a concomitant decrease i n the cooperative interactions within the thin filament during tension activation.