Meeting Abstract
56.3 Thursday, Jan. 6 Testosterone Inhibits Hepatic IGF-I Message in Sceloporus undulatus, a Female-Larger Species of Lizard DUNCAN, C.A.**; JOHN-ALDER, H.B.; Rutgers Univ.; Rutgers Univ. duncan@aesop.rutgers.edu
In many species of animals, adults of one sex are larger than the other. Sexual dimorphism in body size (SSD) can arise when members of one sex grow faster than the other. Thus, as a starting point to understanding how SSD arises within a species, we first need to understand how sexes differ in growth regulation. Sex differences in growth rate are often attributed to the opposite effects of androgenic versus estrogenic hormones on the growth hormone/insulin-like growth factor-I (GH/IGF-I) axis. In this paradigm, testosterone (T) stimulates hepatic IGF-I message, resulting in an increase in plasma IGF-I, while estrogenic hormones have the opposite effect. However, these conclusions are inherently biased because studies focus almost entirely on male-larger species. In lizards, T promotes growth in males of male-larger species and inhibits growth in female-larger species. Thus, the effect of T on IGF-I may not be universal but may depend on a species’ pattern of SSD. In the present study, we investigated the effects of T on the expression and production of IGF-I in Eastern Fence Lizards (Sceloporus undulatus), a species in which females grow faster to become larger than males and T inhibits growth in both sexes. We manipulated circulating T via surgical castration with and without implantation of Silastic tubules loaded with T. Contrary to published studies on male-larger species of mammals and fishes, T decreased hepatic IGF-I message in adult males; although, we found no affect on plasma IGF-I. Further, T decreased hepatic IGF-I message in both male and female juveniles. We attribute our novel findings to our use of a female-larger species for this kind of study. Our results challenge the widespread belief that males grow faster than females because T promotes the production of IGF-I. Supported by SICB, Graduate School-New Brunswick, and School of Environmental and Biological Sciences.
