Corticosterone and insulin interact to regulate glucose and triglyceride levels during stress in a bird

Captive European starlings (Sturnus vulgaris) were exposed to the stress of handling and restraint while corticosterone, glucose, and triglyceride con centrations were monitored in blood plasma. In saline-injected controls, ba sal samples were taken within 3 min of disturbance with subsequent samples taken at 40, 70, and 150 min. This was repeated at two times during the dai ly cycle (day and night) on two different photoperiods: short and long days . During both photoperiods, corticosterone concentrations approximately tri pled (compared with a sixfold increase in free-living starlings) and trigly ceride concentrations decreased 25-45% in response to stress at both times of the day, whereas an similar to 25% stress-induced hyperglycemia occurred only at night. Exogenous corticosterone (200 mug), 1.0 or 4.0 IU/kg of ins ulin, or a combination of corticosterone with each insulin dose was then se parately administered to alter the above responses. Insulin did not affect corticosterone or triglyceride concentrations but resulted in a dose-depend ent hypoglycemia of 10-40%. Injected corticosterone resulted in supraphysio logical corticosterone concentrations (three- to fivefold higher than norma l), yet it did not affect the already altered plasma glucose or triglycerid e concentrations. This suggests that glucose output and triglyceride decrea ses were already maximal in response to handling and restraint. However, th e low glucose concentrations resulting from exogenous insulin returned to b asal quicker with exogenous corticosterone but only during the day. No resp onse to either hormone showed photoperiodic differences. These data suggest that corticosterone's role in metabolism changes to meet varying energetic demands throughout the day.