Meeting Abstract

P2.164  Saturday, Jan. 5  The effect of prenatal steroids on the fast-twitch fibers of the fetal guinea pig rectus abdominis O’CONNELL, K.J.*; WALKER, R.A.; DEAROLF, J.L.; Hendrix College, Conway, AR oconnellkj@hendrix.edu

Glucocorticoids are often used to combat premature infant mortality. Although studies have shown an improvement in lung function in premature infants after their mothers were exposed to glucocorticoids during gestation, the effects of these steroids on breathing muscles are not widely known. A study of the rectus thoracis, an accessory inspiratory muscle, demonstrated an increase in the percentage of fast-twitch fibers, as well as their diameters, in response to prenatal steroid exposure. Therefore, we hypothesize that betamethasone injections will result in an increase in the percentage of IIA fast-twitch fibers and larger IIA and IIX fast-twitch fibers in the rectus abdominis (expiratory muscle) of treated fetuses. Betamethasone (0.5 mg/kg) or sterile water injections were given to pregnant guinea pigs twice a week at 65%, 75%, and 85% gestation. Rectus abdominis samples were collected from fetal guinea pigs and prepared for histo- and immunocytochemistry. Sections of the control and steroid-treated muscles were stained for their reaction to antibodies against slow (A4.951) or IIA fast myosin (2F7). Images of the stained sections were taken, and the 2F7 staining densities of the fast-twitch fibers, as well as their diameters, were measured with Scion Image. Staining densities were converted to Z-scores, which were used to group fibers into those darkly (IIA) and lightly staining for 2F7 (IIX). A supported hypothesis would suggest that the rectus abdominis of premature babies exposed to betamethasone would perform more efficiently in ventilation than the muscles of babies not exposed to the steroid. More IIA fast-twitch fibers and larger IIX and IIA fibers would allow the muscles of treated babies to produce greater and more sustained forces when meeting ventilatory demands.