Meeting Abstract

P2.165  Saturday, Jan. 5  Effects of prenatal steroids on the fetal rectus thoracis BROWN, J.M.*; CHUGHTAI, A.; WALKER, R.A.; DEAROLF, J.L.; Hendrix College, Conway, AR brownjm@hendrix.edu

Glucocorticoids accelerate lung development in premature infants and are used in clinical medicine to improve their chances of survival. However, the effects of these steroids on breathing muscle development are undetermined. When glucocorticoids are given to mothers going into premature labor, they cause cell differentiation in tissues to occur sooner than it would during normal development. This switch to differentiation means that cellular proliferation is halted prematurely. We hypothesize that exposure to prenatal steroids will cause IIA fast-twitch fibers in the rectus thoracis, an accessory inspiratory muscle, to be reduced in number. Also, since other studies have shown that steroids lead to muscle fiber atrophy, we hypothesize that exposure to prenatal steroids will result in a decrease in muscle fiber size. To test these hypotheses, the glucocorticoid, betamethasone (0.05 mg/kg), or sterile water, was injected into pregnant guinea pigs twice a week at 65%, 75%, and 85% gestation. Samples of the fetal rectus thoracis muscle were collected 24-hours after the last steroid or water injection. Samples were sectioned and stained with antibodies to differentiate between slow (A4.951) and IIA fast-twitch (2F7) fibers. Digital images of the antibody-stained muscles were taken, and fibers were circled in Scion Image, which measured their mean 2F7 staining densities and diameters. Z-scores were calculated and used to group fast-twitch fibers staining lightly (IIX) or darkly (IIA) for 2F7. The mean proportions and diameters of these fibers were then calculated. If our hypothesis is supported, then infants exposed to glucocorticoids will have smaller and fewer IIA fast-twitch muscle fibers. As a result, the breathing muscles of these infants will be more prone to fatigue from ventilatory activity.