Hypothalamo-pituitary-adrenocortical axis in stress-susceptible and stress-resistant pigs: endocrine responses to corticotrophin-releasing factor and vasopressin*

Swiss Landrace pigs selected into genetically well-characterized low and high tissue fat lines (f and F respectively) react differently to exogenous and endogenous stressors. Response of the hypothalamo-pituitary-adrenocortical (HPA) axis to i.v. administered ovine corticotrophin-releasing factor (oCRF) and lysine vasopressin (LVP) in young females, intact and pretreated with dexamethasone or metyrapone, leads to the conclusion that different stress susceptibility of the two lines correlates with the sensitivity of pituitary corticotrophs to oCRF stimulation. Total amount of ACTH released after stimulation with submaximal oCRF doses was roughly equal in both lines, and cortisol level is even lower in the f-line, most likely due to the considerably enhanced metabolic clearance rate of cortisol (lower half-life of plasma cortisol compared with F-line). LVP-stimulated ACTH release is comparable with that of oCRF is stronger in the f-line. Combined effect of oCRF and LVP is rather additive than synergistic but the half-life ratio cortisol/ACTH after this stimulation is about four times higher than for stimulation by LVP and oCRF separately. In cases of externally stimulated HPA axis, cortisol plasma concentration tightly cross-correlates with that of ACTH.

Hormonal and stressor-associated changes in porcine adrenocortical cholesterol ester hydrolase activity

Cholesterol ester hydrolase (CEH) activity was characterized in the porcine adrenal gland and experiments conducted to determine the nature of its hormonal regulation. CEH activity was studied in the 14,000 gmax pellet (F4) and in the 192,000 gmax supernatant (F6). Characteristics associated with pH optima, product formation with time, linearity with increasing protein concentration, and equilibration of exogenous cholesterol esters added in acetone with endogenous cholesterol esters were determined. Scatchard analyses of saturation data demonstrated two-site models, which indicated the presence of lower velocity lower Km enzymes (catalytic sites) (L-VKm) and higher velocity higher Km enzymes (catalytic sites) (H-VKm) in both subcellular fractions. Neither ACTH (0.4 micrograms/kg body weight) nor 30-min restraint affected CEH activities at 0.5, 2, and 5 h after injection or initiation of restraint. However, 1 h after a longer restraint period (45 min), F4 H-VKm CEH activity increased concomitantly with decreased F6 L-VKm (P = 0.003). More modest increases in F4 H-VKm (P = 0.03) were still apparent 1 h after the last of nine daily 45-min restraints. Bromocriptine (CB154, a dopamine agonist) administration for 6 days (9.6 mg/daily) reduced plasma prolactin (PRL) by 53% (P < 0.05), but had no effect on CEH activities. ACTH treatment to CB154-induced hypoprolactinemic barrows dramatically reduced F4 (63%) and F6 (49%) L-VKm CEH activity (P = 0.03). These data are the first concerned with regulation of adrenal CEH activity in swine, and are the first to evaluate in vivo treatments on in vitro CEH activity in any species evolutionarily higher than rodents. In vivo regulation of porcine adrenal CEH activity appears complex. Stressor-associated hormonal perturbations apparently must surpass a certain threshold of duration and/or magnitude before they alter CEH activity. Differing Km and Vmax of CEH within and between the two subcellular fractions studied and the differential responses to restraint stressor suggest that as many as four different enzymes with CEH activity are involved. Additionally, the combined effect of ACTH and CB154-induced hypoprolactinemia argues for an interrelated modulatory function of ACTH and PRL (or dopamine) on specific porcine adrenal CEH activities.