 Anabolic Response to Testosterone in Transgenic Mice with Knocked-down Androgen Receptor in Skeletal Muscle Fibers Cindy D. Knaff, Douglas A. Monks, Ph.D., Cynthia L. Jordan, Ph.D. Neuroscience Program and Department of Psychology, Michigan State University INTRODUCTION  Testosterone is an anabolic steroid hormone which increases muscle mass and regulates both size and number of levator ani (LA) muscle fibers  The LA muscle skeletal muscle is sexually dimorphic, very sensitive to the androgen testosterone and provides a useful model for understanding how anabolic steroids affect skeletal muscle  Previous research has shown that testosterone acts on androgen receptors (AR) in skeletal muscle to regulate size and number of muscle fibers  LA muscle fiber, fibroblasts and endothelial cells express AR  This study determines whether the AR in LA muscle fibers themselves regulate LA muscle fiber size and number by studying a line of transgenic mice whose AR expression has been lowered in skeletal muscle fibers. OBJECTIVE METHODS STATISTICAL ANALYSES  Paired t-tests were used to examine mean differences in LA muscle fiber diameter and number of wild type and transgenic mice. RESULTS CONCLUSIONS  No significant differences in mean diameter of muscle fibers and number were observed between animal groups  Although, no significant differences were found, the number of muscle fibers between animal groups borders on significance.  A larger sample size is needed in order to clarify and extend the results from the current study FUTURE DIRECTIONS REFERENCES ACKNOWLEDGEMENTS Figure A. Low magnification view of a cross section of a LA Muscle Figure B. High magnification view of cross sectioned LA Muscle Graph 1. Graph 2. Table 1. Table 2.  To determine whether the AR in LA muscle fibers regulates LA muscle size and number  LA muscle was harvested from three transgenic and three wild type mice  Muscles were sectioned in twelve micron sections with a cryostat, two slides per animal, six LA muscle sections per slide  An H+E stain was performed on all slides  Cross sectional area of LA muscles were measured in micrometers using images captured at 4x and imageJ public domain software  Cross sectional area of individual LA muscle fibers were measured in micrometers using images captured at 40x and the minor axis of a fitted ellipse is used as a measure of muscle fiber diameter  Muscle fiber number was estimated by multiplying cross sectional area by counts of individual muscle fibers sampled from three 40x images  Breedlove, S.M., and A. P. Arnold (1983) Hormonal Control of a developing neuromuscular system. I. Complete demasculinazation of the male rat spinal nucleus of the bulbocavernosus using the anti-androgen flutamide. J. Neurosci. 3:  Breedlove, S.M., and A. P. Arnold (1983) Hormonal Control of a developing neuromuscular system. II. Sensitive periods for the androgen-induced masculinization of the rat spinal nucleus of the bulbocavernosus. J. Neurosci. 3:  Christensen, S. E., S. M. Breedlove, and C. L. Jordan (2000) Sexual Differentiation of a Neuromuscular System. Sexual Differentiation of the Brain:  The results suggest that AR in LA muscle fibers may regulate LA muscle fiber number.  Androgens can increase the number of LA muscle fibers in development and not in adulthood. Thus, the present results suggest that lowering the number of androgen receptors in developing LA muscle fibers will interfere with the effect of androgen on fiber number.  Examine the spinal cords from these same wild type and transgenic mice because the motoneurons that innervate the LA are also sensitive to androgen  Increase our sample size for all measures The Breedlove Jordan Lab - Michigan State University - Neuroscience Program