Corticosterone and insulin interact to regulate plasma glucose but not lipid concentrations in molting starlings

Captive starlings (Sturnus vulgaris) undergoing a prebasic molt were given exogenous insulin (INS) and corticosterone (CORT), to determine how these counterregulatory hormones would affect glucose and triglyceride concentrations during stress. Experiments were conducted at both morning (11:00) and night (23:00) to monitor daily variation in these responses. Concentrations of CORT, glucose, and triglycerides were measured in blood plasma within 3 min of disturbance (basal) and at 40, 70, and 150 min thereafter (stress-induced) to monitor the effect of injecting saline, INS, CORT, or INS+CORT. Saline injection (which included the stress of handling and restraint) increased CORT concentrations, decreased triglycerides, but had no effect on circulating glucose. Daily variation was not evident in CORT or glucose, but concentrations of triglycerides were higher at night than during the day. INS markedly suppressed glucose concentrations, but had no effect on plasma CORT or triglycerides. Glucose levels did not change in response to stress, but exogenous CORT elicited hyperglycemia during the day. Injected CORT also hastened the recovery of glucose concentrations from INS-induced hypoglycemia at night, and had no effect on circulating triglycerides. Basal concentrations of CORT, glucose, and triglycerides exhibited photoperiodic (mimicking seasonal) changes when combined with data from an earlier study in starlings held on long- and short-day photoperiods. During the prebasic molt, all three measurements were lower compared to other photoperiods. Together, these data suggest that glucose and triglycerides concentrations are regulated differently during molt, but INS and CORT maintain their traditional effects.