In Vitro and In Vivo Temporal Aspects of ACTH Secretion: Stimulatory Actions of Corticotropin-Releasing Hormone and Vasopressin in Cattle

The objective of the present study was to evaluate the temporal aspects associated with corticotropin-releasing hormone (CRH) and vasopressin (VP) stimulated bovine adrenocorticotropic hormone (ACTH) secretion in vitro and in vivo. For the in vitro studies, bovine anterior pituitary glands were enzymatically dispersed to establish primary cultures. On day 5 of culture, cells were challenged for 3 h with medium alone (Control) or various combinations and concentrations of bovine CRH (bCRH) and VP. Both CRH and VP each increased (P < 0.05) ACTH secretion. Maximal increases in ACTH secretion occurred in response to 0.1 μ [smallcapital m] CRH (5.5-fold) and 1 μ [smallcapital m] VP (3.7-fold), relative to Control cells. The in vivo portion of the study examined possible temporal differences in the activation of the pituitary-adrenal axis by CRH and VP. Jersey cows were randomly assigned to one of four groups (n = 8 cows/group): (i) Control (saline); (ii) bCRH (0.3 μg/kg BW); (iii) VP (1 μg/kg BW) and (iv) bCRH (0.3 μg/kg BW) + VP (1 μg/kg BW). Jugular blood samples were collected at 15-min intervals for 4 h pre- and for 6 h post-treatment; samples were also taken at 1, 5 and 10 min post-treatment. Plasma concentration of ACTH did not differ among treatment groups for the 4-h pre-treatment period. At 1 min post-treatment, bCRH + VP, VP and bCRH increased ACTH secretion by 22.4-, 9.6- and 2.2-fold, respectively, relative to Control (32.7 ± 7.2 pg/ml). Maximal plasma concentration of ACTH occurred at 5, 10 and 15 min post-treatment for the VP (1017.7 ± 219.9 pg/ml), bCRH + VP (1399.8 ± 260.1 pg/ml) and bCRH (324.8 ± 126.2 pg/ml) treatment groups respectively. Both the in vitro and in vivo data demonstrated that while VP acutely activates the bovine pituitary-adrenal axis, CRH-induced ACTH secretion is slower in onset but of longer duration. The present study also provides insight into the dynamics of ACTH and cortisol (CS) responsiveness to CRH and VP in cattle.