Denervation of the gastrocnemius muscle at various stages of development reduces muscle weight and creatine kinase activity. An inverse relationship between the muscle-specific enzyme, creatine kinase, and the lysosomal enzyme, acid phosphatase, was shown. An increased percentage of the BB isoenzyme of creatine kinase is observed after long-term denervation. Apparently the muscle tissue has the ability to regenerate and presumptive myoblasts are formed from satellite cells. When the denervated muscle is treated with dibutyryl-cyclic-GMP a less decreased muscle weight is found. The total creatine kinase activity per muscle has increased which means that after dibutyryl-cyclic-GMP administration new muscle tissue has been formed. Similar effects could not be demonstrated with either cyclic AMP or succinylcholine. The higher percentage of the BB isoenzyme after dibutyryl-cyclic-GMP administration supports the theory that presumptive myoblasts are derived from satellite cells. Succinylcholine also causes an increase of the B-type of creatine kinase. It can be concluded that cyclic GMP, generated via the nerve, has an important role in maintaining muscle weight.

Various approaches have been used in order to determine whether or not a certain hormone is a stimulus for the development of the muscle-specific enzyme, creatine kinase. Both thyroxine and glucocorticoids can be considered as naturally occurring stimuli for the synthesis of creatine kinase. The maximum increase of creatine kinase activity after stimulation by glucocorticoids (about 25%) occurs between 5 and 7 days after birth. A single injection of thyroxine has virtually no effect during this period. However, when a pretreatment with thyroxine is given, cortisone acetate administration increases creatine kinase activity to about 155%. The effect of cortisone acetate is due to de novo synthesis of creatine kinase. The augmentation of the effect of cortisone acetate by thyroxine is dependent on DNA synthesis. Thyroxine administration apparently causes the formation of more competent muscle cells. The effects of both hormones are age-dependent. Thyroxine and cortisone acetate administration to fetuses can prematurely evoke the MM isoenzyme of creatine kinase. Both hormones probably play a role in the activation of the M gene during embryonic development. Sex hormones are able to influence neither creatine kinase activity nor muscle growth. However, castration of male rats immediately after birth causes an impairment of growth at older ages. The androgen production by the testes immediately after birth seems to be of main importance for body growth and development. It can be concluded from these results that creatine kinase in muscle is under multiple hormonal control, just as is observed for a number of enzymes in other tissues.