Comparison of the basal and luteinising hormone-releasing hormone induced luteinising hormone release by perifused hypophyses from turkey hens (Meleagris gallopavo) at different physiological stages.

1. An experiment was performed to investigate the weight of the ovary, the oviduct, the pituitary gland, the plasma concentrations of luteinising hormone (LH), progesterone, oestradiol and the responsiveness of the pituitary gland in vitro to doses of Luteinising Hormone-Releasing Hormone (LH-RH) ranging from 10(-10) to 10(-5) M in laying (L), incubating (I) and out-of-lay (OL) turkey hens. 2. Pituitary weights did not differ between the groups but the weights of the ovary and oviduct and the plasma concentrations of progesterone and oestradiol were lower in I and OL than in L hens. The plasma concentrations of LH were lower in I than in L hens. 3. In vitro, the basal release of LH was similar in L and I hens, but significantly higher in OL hens. A slow and linear increase in basal LH release by the glands from I and OL hens was observed throughout the experiment. 4. No clear dose-response relationship was found in any of the reproductive states with respect to LH release in vitro following LH-RH stimulation, probably as a result of partial cell desensitisation. On the other hand, the amount of LH released over basal level in responses to stimulation with different doses of LH-RH were not significantly different between L and I hens, but they were between 5- to 10-fold higher in OL hens, except at the lowest dose. 5. These findings confirm that there is no correlation between circulating LH in turkey hens and the capacity of the hypophysis to release LH in vitro passively or in response to LH-RH. Therefore, the low circulating concentrations of LH in I and OL turkey hens cannot be accounted for by decreased adenohypophyseal responsiveness to LH-RH. They may indicate a low level of hypothalamic secretion of LH-RH and/or to the existence of an inhibitory mechanism on LH secretion in vivo in both OL and I hens.