Specific Aim 1. Evaluate gender-related differences in the phenotype of masticatory muscle fibers in rabbits. This aim addresses the hypothesis that significant gender-based differences in rabbit masticatory muscle fiber phenotype exist. Gender-based differences related to the expression of one MyHC isoform have been demonstrated in muscle fibers of the masseter muscle in the rabbit. However, we hypothesize that other aspects of muscle fiber phenotype are also gender-dependent. We will examine muscle fiber phenotype in male and female rabbits using quantitative histochemical analysis of oxidative and glycolytic enzyme content and immunohistochemical analysis of MyHC isoforms properties of male and female rabbit masticatory muscles.
Specific Aim 2. Determine the androgen sensitivity of male and female rabbit masticatory muscles. Sexual dimorphism in masticatory muscle fiber phenotype could be the result of differences in androgen sensitivity of the muscle fibers, or it could be related only to differences in plasma androgen levels in males and females. The androgen sensitivity of a muscle or fiber is defined by its content of androgen receptors (ARŐs). This aim will address the hypotheses that rabbit masticatory muscles are more androgen sensitive than limb muscles, that the masticatory muscles of male rabbits are more androgen sensitive than those of females and, that fibers expressing some phenotypes (e.g. cardiac alpha MyHC) are more androgen sensitive than others. To test these hypotheses, we will use immunoblotting and immunohistochemical methods to measure the relative androgen receptor content of male and female rabbit masticatory and non-masticatory muscles.
Specific Aim 3. Determine whether rabbit masticatory muscle fiber phenotype is testosterone dependent. If the effect of androgenic hormones on masticatory muscle fibers is to regulate their phenotype, then we hypothesize that administration or removal of a major source of circulating androgens would alter the size and/or expression of contractile and metabolic proteins in those muscle fibers. We will test this hypothesis by evaluating changes in normal muscle fiber phenotype in castrated male rabbits and in normal females treated with dihydrotestosterone (DHT).
Specific Aim 4 Determine the functional significance of androgen-regulated differences in rabbit masticatory muscle fiber phenotype. This aim addresses the hypothesis that gender-based or hormonally-induced differences in masticatory muscle fibers are reflected as differences in the mechanical effects of those muscles on the masticatory apparatus. We will assay the mechanical effects of activating different compartments of the masticatory muscles using a sensitive multi-axis force-moment sensor attached to the mandible. We will then compare the results obtained from male and female rabbits as well as from castrated male and DHT-treated females.