Plasma adrenocorticotropic hormone and cortisol in pigs: effects of time of day on basal and stressor-altered concentrations

An initial study was conducted to establish the presence in plasma of diurnal rhythms of immunoreactive porcine adrenocorticotropic hormone (pACTH) and cortisol in castrated male pigs (barrows). Fourteen barrows with jugular catheters were bled at 6-hr intervals for 24 hr. Significant changes in plasma pACTH were evident with peak levels (61 +/- 6 pg/ml) at 0100-0700 hr and a trough (38 +/- 4 pg/ml) at 1900 hr. Changes (P less than 0.05) in plasma cortisol were also present in barrows with a peak (44 +/- 6 ng/ml) at 0700 hr and a trough (21 +/- 5 ng/ml) at 1900 hr. Plasma norepinephrine and epinephrine were measured at the same time intervals and did not differ among hours. In these unstressed pigs the ratio cortisol/log10pACTH at 0700 hr (25.3 +/- 3.0) was greater than the ratio at 1900 hr (12.9 +/- 2.7). Sequential blood samples were subsequently taken on four of the barrows 12 and 26 days later. Plasma pACTH was variable among pigs and did not differ among hours. Plasma cortisol on both dates was greater (P less than 0.05) in the morning (0100 or 0700 hr) than at 1900 hr. The ratio cortisol/log10pACTH at 0700 hr was repeatedly greater than at 1900 hr. A second study was conducted to determine whether plasma pACTH and cortisol responses to mild (32 degrees C for 2 hr) or strong (20-min restraint) stressors were dependent on the time of day of stressor application (0800 hr, AM; 1600 hr, PM). Response-associated parameters (maximum concentration, maximum incremental concentration, and integrated response) for pACTH and cortisol did not differ between AM and PM. However, a qualitative difference existed between the AM and PM plasma pACTH responses to restraint +32 degrees C wherein the AM response consisted of a single prolonged surge, and the PM response of an initial major peak followed by a second significant minor peak. A suggested explanation is that the initial 20-min restraint stressor potentiated the hypothalamic-hypophyseal response to 32 degrees C. These studies are the first direct measurements which suggest the presence of diurnal changes in plasma ACTH and cortisol in barrows. The studies also indicate for barrows an absence of diurnal changes in plasma epinephrine and norepinephrine. The responsiveness of the pituitary-adrenocortical axis to stressors did not exhibit quantitative diurnal changes at the time periods measured. However, it is hypothesized that the repeatable AM-PM difference in the ratio cortisol/log10ACTH reflects a diurnal change in adrenal responsiveness to ACTH in unstressed pigs.