DURANT, S.E.; HOPKINS, W.A.*; TALENT, L.G.; ROMERO, L.M.; Virginia Polytechnic Institute and State University, Savannah River Ecology Laboratory; Virginia Polytechnic Institute and State University, Savannah River Ecology Laboratory; Oklahoma State University; Tufts University: Effect of exogenous corticosterone on respiration in a reptile

Release of glucocorticoids (GCs) enables organisms to meet energy requirements during stressful situations by regulating intermediary metabolism. In the absence of compensatory mechanisms, increased metabolic activity (e.g., protein catabolism, lypolysis, and gluconeogenesis) should translate to increases in whole animal metabolism, and therefore energy expenditures, by organisms. The few studies that have examined the effect of GCs on whole animal metabolism have focused on birds and fish, and have produced conflicting results. Although some of the discrepancies may be attributable to species-specific responses to GCs, others may be due to the levels of GCs examined (physiological vs. pharmacological doses) or the respiration procedures utilized. Here, we evaluated the effect of exogenous corticosterone (CORT) on metabolism in captive western fence lizards (Sceloporus occidentalis) by frequently measuring changes in respiration over a 48 hour period. Injection of low levels of CORT resulted in circulating plasma CORT levels similar to controls, but 0.40 &microg/g CORT produced a 6-fold increase in plasma CORT compared to baseline levels 3 hrs following injection. Concomitantly, oxygen consumption and carbon dioxide production more than doubled in lizards receiving 0.40 &microg/g CORT, resulting in greater energy expenditures compared to lizards in the other treatment groups. Respiration returned to baseline rates within 6 hrs of CORT administration, which coincided with reductions in plasma CORT levels. Our results suggest that frequent measures of metabolism facilitates detection of subtle and dynamic changes in respiration caused by GCs that may be important for understanding the overall energetic implications of stress.