Adrenocorticotropic hormone and cortisol in calves after corticotropin-releasing hormone

The aim for this study was to analyze responsiveness of the hypothalamo-pit uitary-adrenocortical axis to exogenous bovine corticotropin-releasing horm one (bCRH) in calves. Two dose-response studies were carried out, using eit her bCRH alone (dose rates of 0, .01, .03, and .1 mu g bCRH/kg live weight) or in combination with arginine-vasopressin (bCRH:AVP, 0:0, .1:.05, .5:.25 , and 1:.5 mu g kg live weight). The bCRH was administered i.v. to calves ( n = 5 to 7 per dose) housed individually or in groups. Serial blood samples were obtained from before to 300 min after injection and analyzed for plas ma ACTH and cortisol concentrations. The lowest bCRH dose that produced a r esponse in all calves was .1 mu g/kg. In the experiment using bCRH with AVP , increasing the bCRH dose from .1 to 1 mu g/kg resulted in an increase in peak ACTH concentration (321 vs 2,003 pg/mL) but did not significantly affe ct the peak cortisol concentration (37 vs 40 ng/mL). The time to reach the peak cortisol concentration increased with the dose of bCRH with AVP (from 38 to 111 min). The ACTH and cortisol concentrations determined at any time between 20 and 90 min after bCRH injection were correlated to the integrat ed responses calculated as areas under the ACTH and the cortisol curves (r between .61 and .99, P < .05). In comparison with results from studies in h umans, pigs, and sheep, our data showed that the pituitary of calves seems less sensitive to CRH than that of other mammals, despite a greater capacit y to produce ACTH. Moreover, the calfs adrenals seem to have a lower capaci ty to produce cortisol than adrenals of other mammals. As in other species, it seems that AVP enhances the release of ACTH and cortisol. For CRH chall enge to be used in calves, we suggest injecting at least .1 mu g of bCRH/kg live weight either with or without AVP and taking several blood samples be fore injection and between 20 and 90 min after injection.