Meeting Abstract

P3.43  Tuesday, Jan. 6  Prenatal steroids: effects of betamethasone on the guinea pig (Cavia porcellus) rectus thoracis muscle BREWINGTON, A.K.**; WEIGAND, K.L.; DEAROLF, J.L.; Hendrix College, Conway, AR brewingtonak@hendrix.edu

Glucocorticoids are commonly injected into mothers expected to give birth prematurely to promote maturation of vital organs. Although these steroids increase the chance of survival, there is little known about their effects on breathing muscles. Based on previous studies of the effects of glucocorticoids on skeletal muscle, we hypothesize two changes in the treated rectus thoracis, an accessory muscle of the ventilatory pump in guinea pigs (Cavia porcellus): less fast-twitch fibers and smaller fibers. Pregnant Hartley guinea pigs (Cavia porcellus) were injected with betamethasone (0.5 mg/kg body weight) or sterile water at 65%, 75%, and 85% gestation. Muscle samples collected from the fetuses were cut and stained using the myosin ATPase assay. Digital images of the stained sections were then captured using an image analysis system. These images were analyzed to obtain fiber-type percentages by number, and up to twenty-five fibers of each type were measured for least diameter and area using Scion Image. There was not a significant difference between the percentage of fast-twitch fibers in the control (92.2%) and treated (92.0%) muscles. However, the fiber diameters of the treated (fast: 12.900.25 m, slow: 14.140.21 m) rectus thoracis were found to be significantly smaller than those in the control muscles (fast: 14.380.25 m, slow: 17.800.24 m). These findings support the hypothesis that steroid treatment results in smaller muscle fibers. Smaller fibers would cause the muscles of treated babies to not be able to produce as much force as muscles in babies not exposed to steroids. Therefore, these results raise concern of possible breathing muscle weakness in babies treated with betamethasone.