Effects of age on calcium transport activity of sarcoplasmic reticulum in fast‐ and slow‐twitch rat muscle fibres.

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Abstract
1. The calcium transport activity of the sarcoplasmic reticulum (SR) was measured in chemically skinned single fast‐ and slow‐twitch muscle fibres from young (3 months) and old (23‐24 months) rats. Contractile properties, the myosin heavy chain (MHC) composition and enzyme histochemical features were studied in relation to the SR characteristics. 2. In fast‐twitch single motor units, the contraction time of the isometric twitch increased (P less than 0.001) from 13 +/‐ 1 ms in young animals to 18 +/‐ 2 ms in old ones. In the slow‐twitch soleus, the contraction (P less than 0.001) and half‐relaxation (P less than 0.05) times increased from 30 +/‐ 5 and 45 +/‐ 10 ms, respectively, in the young animals to 43 +/‐ 3 and 55 +/‐ 4 ms in the old ones. The proportion of slow‐twitch (type I) fibres increased (P less than 0.05) with age in the soleus from 92 +/‐ 6 to 98 +/‐ 2% and the proportion of fast‐twitch fibres (type IIA) decreased (P less than 0.01) from 6 +/‐ 5 to 0 +/‐ 0%. 3. The Ca2+ accumulation capacity (an index of SR volume), the rate of Ca2+ uptake and the fractional rate of SR filling (an estimate of the specific activity of the Ca2+ pump) were decreased by 18 (P less than 0.05), 32 (P less than 0.01) and 32% (P less than 0.001), respectively, in the old fast‐twitch muscle fibres. In the slow‐twitch muscle fibres, on the other hand, no significant age‐related changes were observed in the Ca2+ transport activity of the SR. Thus, ageing exerts a differential influence on SR volume and function in fast‐ and slow‐twitch fibres. 4. It is concluded that an age‐related impairment of intrinsic SR function and a decrease in SR volume are probable factors underlying the decreased speed of contraction of fast‐twitch muscle fibres in old age. In the slow‐twitch soleus, on the other hand, one or more other mechanisms are responsible for the age‐related decrease in the speed of contraction. The loss of fast‐twitch muscle fibres in old soleus is one mechanism, but not the dominant one.