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-Friesian cattle, four intact and four castrated at 4 mo of age, were submitted to adrenal response tests using .01 mg ACTH(1-24)/100 kg live weight at 6, 8, 10, and 15 mo of age. At 11 mo of age, a dose-response study using three dose rates of ACTH (.01 to .04 mg/100 kg live weight) with an intensive blood sampling regimen was also carried out. In the longitudinal 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 had declined (P < .01), and they remained less than those of the steers thereafter (P < .01). This difference was confirmed in the dose-response 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 rates of ACTH (P < .001). The results of these studies clearly indicate that adrenocortical function of male dairy cattle, after the age of puberty, is influenced by the presence of the testicles. The observed changes may be a result of direct modulation of steroidogenesis by androgens, changes in negative feedback to the pituitary, inhibition of the hypothalamo-pituitary adrenal axis from higher centers, or a combination of these factors.