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

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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.

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