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, lipolysis, and gluconeogenesis) should translate to increases in whole animal metabolism, and therefore energy expenditures, by organisms. However, to our knowledge, no study has estimated the total energy cost of elevated plasma GCs in any organism. Here we evaluated the effect of exogenous corticosterone (CORT) on metabolism in captive western fence lizards (Sceloporus occidentalis) by conducting two experiments. In experiment I we determined the dynamics of plasma CORT concentrations resulting from CORT injections. In experiment II we frequently measured changes in respiration for 24h before and after CORT injection. Injection of 0.025 microg/g (low CORT) and 0.40 microg/g CORT (high CORT) produced up to 26-fold increases in plasma CORT in lizards 3h following injection compared to baseline levels. Plasma CORT concentrations returned to baseline levels 6h after injection. CORT increased metabolism in both low and high CORT lizards (up to 121% and 245%, respectively) during the first 7.5h after CORT injections compared to pre-treatment trials. Respiration returned to baseline rates 7.5h after CORT administration. A surprising finding was that although high CORT males achieved higher plasma CORT concentrations than high CORT females, the metabolic response of high CORT females was 30% greater than high CORT males. Our results suggest that GC-induced changes in respiration may be important for understanding the overall energetic implications of stress.