Prolonged adrenal stimulation does not affect plasma DHEA in the dairy cow

Introduction: In primates, adrenal DHEA secretion is episodic and displays a diurnal rhythm in parallel to that of cortisol, and it is responsive to ACTH stimulation. In the dairy cow, circulating DHEA originates primarily from the gonads, while the adrenal contribution has been poorly investigated. The aim of this work was to study DHEA release under prolonged adrenal stimulation in the dairy cow. Materials and Methods: Five healthy non-lactating, non-pregnant Friesian cows were used. The experiment included a control phase followed by a treatment phase with a 30-day resting period in between. During each phase, estrus was synchronized with two injections of cloprostenol (PG) administered 11 days apart. The second PG administration was referred as Day 0. From Day –2 to Day 3, animals received i.m. injections of either saline (control phase) or ACTH analogue (Synacthen depot, 0.7 mg/head/administration; treatment phase) every 12 hours. Plasma cortisol and DHEA concentrations were monitored every 8 hours on Days -3, -2, -1, 2, 3, 5 and 9. On Day 4, hormone release and response to naloxone were studied in plasma samples taken every 10 minutes for 8 hours. Naloxone was administered after 4 hours. Results: During the treatment phase, plasma cortisol was significantly higher (P<0.01) from Day –1 to Day 5, while plasma DHEA was not affected by treatment. Frequent sampling performed on Day 4 did not reveal any pattern of episodic DHEA release. Mean DHEA plasma levels were significantly higher during the treatment phase (P<0.01), but no response to naloxone administration was observed. Mean plasma cortisol significantly increased during the treatment phase (P<0.001), and was significantly reduced by naloxone administration (P<0.05). During the treatment phase, cortisol pulse amplitude was significantly greater before naloxone administration (P<0.05). Conclusions: These data suggest that adrenal over-stimulation has a negligible effect on circulating DHEA in the dairy cow.