Meeting Abstract
A single course of prenatal corticosteroids (dexamethasone or betamethasone) has been shown to significantly reduce premature infant mortality, but the use of multi-course prenatal steroids is still under debate. We test the hypothesis that exposure to multi-course betamethasone accelerates the development of fetal ventilatory muscles in a model species, the guinea pig (Cavia porcellus), specifically resulting in less fast-twitch types IIA and IIX muscle fibers and reduced expression of IIA and IIX myosin heavy chains. We also measured fiber size (diameter) of the treated muscles, as other studies have observed atrophy in steroid-exposed muscles. Pregnant guinea pigs were exposed to three courses of betamethasone, which were timed to overlap the last week of myogenesis, and samples of fetal (59-days gestation) ventilatory muscles (diaphragm, rectus thoracis, scalenus, external abdominal obliques, and rectus abdominis) were collected. Standard muscle histochemistry techniques (myosin ATPase) were used to determine the proportions and diameters of each fiber-type. Muscle proteins were separated in SDS-polyacrylamide gels, and scanning densitometry was used to determine the proportions of each type of myosin expressed by the treated and control muscles. The fiber-type profiles (P = 0.069), fiber diameters (fast-twitch: P = 0.175; slow-twitch: P = 0.183), and myosin expression (embryonic: P = 0.068; neonatal / IIA/IIX: P = 0.302; slow: P = 0.475) of the steroid-treated muscles were not significantly different from those of control muscles. These results do not support our hypothesis and suggest that multi-course steroid exposure during the last week of myogenesis has no effect on the development of fetal ventilatory muscles.
