Sexually dimorphic expression of myosin heavy chains in the adult mouse masseter.

Little is known regarding the role of androgenic hormones in the maintenance of myosin heavy chain (MHC) composition of rodent masticatory muscles. Because the masseter is the principal jaw closer in rodents, we felt it was important to characterize the influence of androgenic hormones on the MHC composition of the masseter. To determine the extent of sexual dimorphism in the phenotype of masseter muscle fibers of adult (10-mo-old) C57 mice, we stained tissue sections with antibodies specific to type IIa and IIb MHC isoforms. Females contain twice as many fibers containing the IIa MHC as males, and males contain twice as many fibers containing the IIb MHC as females. There is a modest amount of regionalization of MHC phenotypes in the mouse masseter. The rostral portions of the masseter are composed mostly of type IIa fibers, whereas the midsuperficial and caudal regions contain mostly type IIb fibers. Using immunoblots, we showed that castration results in an increase in the expression of type IIa MHC fibers in males. Ovariectomy has no effect on the fiber type composition in females. We conclude that testosterone plays a role in the maintenance of MHC expression in the adult male mouse masseter.

Investigation of sexual dimorphism in the rabbit masseter muscle showing different effects of androgen deprivation in adult and young adult animals.

To evaluate the role played by androgens in the development and maintenance of sex differences in the proportion of muscle fibres of different phenotypes, the effects of castration in adult (>6 months old) and in young adult (2-3 months old) male rabbits was examined. Immunohistochemical methods were used to evaluate the proportion of muscle fibres containing different myosin heavy-chain isoforms in 10 different neuromuscular compartments of the masseter. In young adult animals of both sexes, the proportion of fibres of different phenotypes in different compartments was not significantly different from that of normal adult females. In animals castrated as young adults, the development of adult male phenotype proportions was completely blocked in most compartments. In animals castrated as adults, proportions were not significantly different from those of the intact males. For most masseter compartments, androgens produced permanent changes in muscle fibre phenotype during a critical period of postnatal development. However, in the posterior deep compartment, androgen deprivation in young adults had no effect on phenotype proportions, but castration of adults resulted in a striking increase in the proportion of fibres containing the IIa myosin heavy-chain isoform.

Detrimental effects of short-term glucocorticoid use on the rat diaphragm.

BACKGROUND AND PURPOSE: The purpose of this study was to determine the effect of short-term, high doses of glucocorticoids on both body and diaphragm weights as well as contractile characteristics of the rat diaphragm. SUBJECTS: Adult, female Sprague-Dawley rats were divided into 2 groups: a control group (n=16) and a prednisolone group (n=16). METHODS: The prednisolone group received prednisolone at a dosage of 5 mg/kg, and the control group received sham saline injections for 5 days. Animals were weighed prior to and after completion of the drug injection period. At the completion of the drug injection period, the animals were sacrificed, and the diaphragm, soleus, and extensor digitorum longus muscles were removed and weighed. A small strip of the costal diaphragm was connected to a force transducer, and the following contractile characteristics were measured: maximal specific isometric tetanic tension, peak isometric twitch specific tension, one-half relaxation time, and time to peak tension. RESULTS: Both body and diaphragm weights decreased by 15% in the prednisolone group as compared with the control group. Maximal specific isometric tetanic tension was reduced 13% in the prednisolone group as compared with the control group. There was no difference in any twitch contractile characteristics between the 2 groups. CONCLUSION AND DISCUSSION: These data support the hypothesis that glucocorticoid treatment over a 5-day period results in a decrease in specific tension as well as diaphragm and body weight. These results may have implications for the treatment of patients receiving high doses of glucocorticoids for acute medical conditions.

Interaction of glucocorticoids and activity patterns affect muscle function.

The purpose of this study was to determine the effects of glucocorticoids on muscle mass and contractile properties of muscles of similar fiber composition but differing in activity patterns. Rats were divided into two groups and administered prednisolone (5 mg/kg per day) (P; N = 9) or served as controls (C; N = 10) for 10 days. Contractile properties were then determined in the left gastrocnemius-plantaris-soleus (GPS) muscle complex and a strip of costal diaphragm (D). An index of fatigue was also determined in both muscle preparations. Whole-body, GPS, and D weights decreased (P < 0.05) in the P animals (22%, 17%, and 15%, respectively) when compared to C. Specific tension (PO) increased (P < 0.05) in the GPS complex (21%) and decreased in the D (26%). Maximal shortening velocity (Vmax) was not different (P > 0.05) between groups in either the GPS or the D. While the index of fatigue was not different (P > 0.05) between groups in the D, there was a 30% increase (P < 0.05) in the rate of fatigue in the GPS. These data indicate that, although glucocorticoids cause decreased muscle mass in both D and GPS, a change in muscle architecture may prevent a decrease in force-generating ability in some limb muscles. However, glucocorticoids do not increase D fatigability as seen in the GPS.