ADRENOCORTICAL RESPONSES TO AN ADRENOCORTICOTROPIC HORMONE IN BULLS AND STEERS

We hypothesized that the sexual status of male cattle would influence adrenocortical responses to exogenous ACTH. Eight male Holstein-Friesi an cattle, four intact and four castrated at 4 mo of age, were submitt ed to adrenal response tests using .01 mg ACTH(1-24)/100 kg live weigh t at 6, 8, 10, and 15 mo of age. At 11 mo of age, a dose-response stud y using three dose rates of ACTH (.01 to .04 mg/100 kg live weight) wi th an intensive blood sampling regimen was also carried out. In the lo ngitudinal study, the responses to exogenous ACTH were similar in the bulls and steers at 6 mo of age. By 8 mo, the responses of the bulls h ad declined (P < .01), and they remained less than those of the steers thereafter (P < .01). This difference was confirmed in the dose-respo nse study. Plasma cortisol concentrations at 50 min and at peak were 1 .5 times greater in the steers than in the bulls at equivalent dose ra tes of ACTH (P < .001). The results of these studies clearly indicate that adrenocortical function of male dairy cattle, after the age of pu berty, is influenced by the presence of the testicles. The observed ch anges may be a result of direct modulation of steroidogenesis by andro gens, changes in negative feedback to the pituitary, inhibition of the hypothalamo-pituitary adrenal axis from higher centers, or a combinat ion of these factors.