CHANGES IN FIBER TYPES IN RAT SOLEUS AND PLANTARIS MUSCLES FOLLOWING HYPOPHYSECTOMY AND COMPENSATORY OVERLOAD

We investigated whether hypophysectomy could modify the change in musc le fibre types caused by compensatory overload. Male Wistar strain rat s were assigned to groups of either normal control (NC). hypophysectom ized control (HC). normal compensatory overloaded (NO), or hypophysect omized compensatory overloaded (HO). Compensatory overload was induced by the bilateral removal of the gastrocnemius muscle. Five weeks late r, there were losses in the soleus and plantaris muscle weights as a r esult of hypophysectomy. Compensatory overload increased muscle weight s independently of the hypophysectomy. Growth hormone and 3,5.3'-triio dothyronine levels were significantly decreased following hypophysecto my. In the soleus. hypophysectomy increased the percentage of type I f ibres at the expense of type IIA fibres. Compensatory overload decreas ed type IIA fibres under the hypophysectomized condition. In the plant ris. the percentage of type IIC fibres was increased at the expense of both type IIA and IIB fibres following hypophysectomy. The decrease i n type IIB fibres cause by compensatory overload was induced irrespect ive of hypophysectomy. The changes in muscle fibre types in the HO gro up were equal to the sum of the changes in the HC and NO groups. These results suggest that after a period of 5 weeks hypophysectomy may ind uced decreases in type IIA and IIB fibres in association with the lack of pituitary and thyroid hormones, and that the hypophysectomy could not modify the change in muscle fibre types caused by compensatory ove rload.