Effects of age on contractile and enzyme‐histochemical properties of fast‐ and slow‐twitch single motor units in the rat.

Abstract

1. Contractile, enzyme-histochemical and morphometrical properties of muscle fibres were studied in single motor units of tibialis anterior (t.a.) and soleus muscles in young (3-6 months) and old (20-24 months) male albino rats. The technique of measuring glycogen depletion as a marker of previous muscle contraction was used for direct correlation of enzyme-histochemical and contractile parameters within single motor units of the fast- and slow-twitch type. 2. In t.a., the fast-twitch motor units covered 18 .+-. 9 and 22 .+-. 16% (P = not significant, n.s.) of t.a. cross-sections, included 148 .+-. 65 and 162 .+-. 63 muscle fibres per unit (P = n.s.) and had a cross-sectional area of 0.50 .+-. 0.32 and 0.44 .+-. 0.22 mm2 (P = n.s.) in the young and old animals, respectively (means .+-. S.D.). 3. In soleus, the slow-twitch motor units covered 53 .+-. 11 and 71 .+-. 12% (P = n.s.), included 55 .+-. 10 and 83 .+-. 13 muscle fibres per unit (P < 0.01) and had a total cross-sectional area of 0.14 .+-. 0.02 and 0.22 .+-. 0.06 mm2 (P < 0.01) in the young and the old animals, respectively. The calculated number of motor units in soleus accordingly decreased (P < 0.01) from 49 .+-. 10 in the young to 29 .+-. 10 in the old animals resulting in a loss of muscle fibres and an increased innervation ratio in old age (mean .+-. S.D.). 4. Clusters of more than three muscle fibres were rarely seen in any of the glycogen-depleted motor units in either the young or the old animals. However, in the slow-twitch motor units of old animals the muscle fibres were less randomly distributed within the motor unit territory (P < 0.05), indicating a denervation reinnervation process. 5. The contraction and half-relaxation times of the isometric twitch were significantly prolonged in old age. In 274 randomly isolated single motor units of t.a. the contraction time increased from 13 .+-. 1 in young animals to 17 .+-. 3 ms in old ones and the half-relaxation time from 12 .+-. 2 to 16 .+-. 5 ms (P < 0.01 in both cases). In 236 randomly isolated soleus single motor units, the contraction and half-relaxation times increased (P < 0.001) from 24 .+-. 5 to 31 .+-. 7 ms and from 26 .+-. 8 to 35 .+-. 9 ms, respectively (means .+-. S.D.). 6. The results show that the age-related decrease in the number of muscle fibres is due to a loss of whole motor units and that in old age reinnervation of previously denervated muscle fibres is incomplete. 7. It is concluded that the reduced speed of isometric contraction during ageing is primarily related to changes in the contractile properties of both fast- and slow-twitch motor units and that the age-related decrease in the number of type II muscle fibres observed in soleus is of less importance in this respect.