Adaptation of corticosterone - but not beta-endorphin - secretion to repeated blood sampling in rats

Effects of shout-term repeated blood sampling on the secretion of corticost erone (CORT) and beta-endorphin (beta-END) were evaluated in male Wistar ra ts. Blood was drawn from the tail vein of conscious rats four times within 2 h both at the peak and trough period of the diurnal corticosterone secret ion cycle. All rats were well accustomed to the procedure. The main finding s were: (1) At both sampling intervals, CORT increased significantly in res ponse to the first sampling and declined to baseline values in successive s amples. (2) beta-END also increased significantly in response to the first sampling but remained elevated in successive samples. (3) Intensities of in itial CORT and beta-END responses correlated positively with each other and with the baseline beta-END values. Feedback inhibition of CORT secretion w ith sustained elevation of beta-END titres suggests a moderate stress inten sity of the repeated blood sampling procedures. In general, due to lack of short-term feedback inhibition, beta-END seems to reflect the effects of re peated administration of moderate intense stressors more closely than CORT.