Morphometric Analysis of the Muscle Fibres of the Soleus Muscle of the Ageing Rat: Long-Term Effect of Hypophysectomy and Food Restriction

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Abstract
This communication reports on the effects of ageing, hypophysectomy and food restriction on the fibre sizes of the soleus muscle which is primarily concerned with posture. The results reported here are compared with those previously shown for the gastrocnemius muscle which is involved mainly with locomotion. Muscle fibre size (measured as equivalent circle diameter) in the soleus muscle of the male Wistar rat increased from youth at 130 days(type 1 fibres, 61.2 μm;type 2, 58.3 μm) to middle age at 600 days (type 1 fibres, 70.4 μm; type 2, 68.0 μm) and then underwent atrophy in old age to 1,000 days (type 1 fibres, 54.5 μm; type 2, 38.0 μm). There was no growth or atrophy of these fibres in either hypophysectomized or food-restricted rats. There was an increased range of fibre sizes in control rats between youth and old age being 5-fold for type 1 fibres and 3-fold for type 2 fibres, but there was a loss of type 2 fibres in hypophysectomized rats. This ageing process in the range of fibre size was retarded by hypophysectomy (2-fold increase for type 1 fibres) and to a lesser extent by food restriction (3-fold increase for type 1 fibres). Muscle fibre atrophy and compensatory hypertrophy were not as prominent in the hypophysectomized and food-restricted groups as in the old control groups. In general these effects were similar to those previously shown for the gastrocnemius muscle. However, atrophy of both type 1 and type 2 fibres in old age was more pronounced in the soleus muscle. The retarded ageing of the soleus muscle in hypophysectomized and food-restricted rats was probably due to the absence or reduced circulating levels of hormones known to target muscle cells, such as growth hormone and thyroxine.

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