Influence of compensatory overload on glucocorticoid receptors in rat skeletal muscle

The effect of testosterone on gastrocnemius muscle fibres in growing and adult male and female rats: a histochemical, morphometric and ultrastructural study

In this study, the effect of testosterone on gastrocnemius muscle fibres in growing and adult rats (male and female) was examined using histochemical, morphometric and ultrastructural techniques. After physiological saline (PS), olive oil (OvO) or olive oil + testosterone (OvOT) injections on 72 rats (growing and mature, 36 male and 36 female), the sample tissues of fibre types of the gastrocnemius muscle taken were examined by histochemical [alkaline adenosine triphosphatase (alk-ATPase), acid ATPase (ac-ATPase)], morphometric and ultrastructural techniques. In PS-injected control groups, the gastrocnemius muscle of both sexes contained all the fibre types studied [slow-oxidative muscle fibres (type I), fast-oxidative glycolytic muscle fibres (type IIA) and fast-glycolytic muscle fibres (type IIB)]. The type I fibres had the smallest diameter, type IIA had a medium diameter and type IIB fibres had the largest diameter. In OvO-injected groups, it was observed that the OvO had little effect on the gastrocnemius muscles of either sex, although there was significant enlargement of type IIB fibres. After the injection of OvOT, hypertrophy of muscle fibres was determined by morphometric study. The biggest increase in diameter was on type I fibres. In addition, degenerations on some mitochondria, accumulation of lipid droplets on type I and type II fibres, an increase in glycogen particles, bifurcation of myofibrils, an increase in the number and diameter of units resembling T tubules and an increase in ribosomal content were also observed in the same group by transmission electron microscope. Consequently, it was determined that testosterone can induce protein synthesis in gastrocnemius muscle fibres, and induces changes in shape and size, and also can change the appearance and the number of fibres.

Overload-induced androgen receptor expression in the aged rat hindlimb receiving nandrolone decanoate

This study's purpose was to examine whether functional overload with nandrolone decanoate (ND) administration increased muscle mass and steroid receptor concentration in aged rat soleus (Sol) and plantaris (Plan) muscle. ND (6 mg/kg body wt) was administered once a week for 4 wk, whereas control rats received sesame seed oil injections. Functional overload of the hindlimb Sol and Plan was induced by synergistic gastrocnemius muscle ablation at the beginning of the fourth week. Adult (5 mo of age) and aged rats (25 mo of age) were randomly assigned to four groups: control, overload, control-ND, and overload-ND. Seven days of functional overload increased adult Sol muscle mass 27%, whereas the aged Sol muscle mass did not change. The aged overloaded Sol muscle receiving ND significantly increased muscle weight by 35% and total muscle protein by 24%. Aged Plan muscle did not increase muscle weight with overload or ND treatment. Androgen receptor protein was induced by ND treatment and functional Ov, and combining the two treatments induced Sol androgen receptor protein concentration above either alone. Sol glucocorticoid receptor protein concentration increased in overload groups of both ages. ND administration can increase aged Sol muscle mass and protein content after 7 days of functional overload, and the cooperative induction of androgen receptor may be important for this response.

SRF protein is upregulated during stretch-induced hypertrophy of rooster ALD muscle

Serum response element 1 has previously been reported to be necessary and sufficient for activation of the skeletal alpha-actin promoter during hypertrophy of the anterior latissimus dorsi (ALD) muscle of roosters [J. A. Carson, R. J. Schwartz, and F. W. Booth. Am. J. Physiol. 270 (Cell Physiol. 39): C1624-C1633, 1996]. Serum response factor (SRF) protein is the transcription factor that binds as a homodimer to serum response element 1 and activates the skeletal alpha-actin promoter. An increased expression of exogenous SRF protein in replicating C2C12 myoblasts induced a three- to fourfold activation of the skeletal alpha-actin promoter (L. Wei, W. Zhou, J. D. Croissant, F.-E. Johansen, R. Prywes, A. Balasubramamyan, and R. J. Schwartz. J. Biol. Chem. 273: 30287-30294, 1998). Thus we hypothesized that SRF protein concentration would be increased during hypertrophy of skeletal muscle. In the present study, 10% of the rooster's body weight was attached to the left wing to induce enlargement of the ALD muscle compared with the contralateral muscle. With Western analysis, a significant increase in SRF protein per gram of wet weight of the ALD muscle was noted at 7 and 13 days of hypertrophy. Furthermore, the increase in SRF protein occurred in both crude nuclear protein and cytoplasmic fractions in 7-day stretched ALD muscles. This is the first report showing increased protein concentration for a transcription factor whose regulatory element in the skeletal alpha-actin promoter has previously been shown to be required for the transduction of a hypertrophy signal in overloaded skeletal muscle of an animal.