Dynamic changes in Plasma Prolactin, Luteinizing Hormone and Growth Hormone in Ovariectomized Ewes

Plasma levels of prolactin (PRL), luteinizing hormone (LH) and growth hormone (GH) were determined in samples obtained at frequent intervals throughout the day from ovariectomized ewes and from ovariectomized ewes which were given a single injection of progesterone (10 mg/kg) or estradiol benzoate (0.5 mg/kg). The steroids (in ethanol) were injected intrajugularly 2 hr. after a control injection of ethanol. Plasma LH levels varied according to regular secretory spikes. Plasma levels of PRL and GH varied with secretory spikes of irregular frequency and magnitude. The injection of progesterone resulted in decreased plasma levels of LH and PRL but appeared to enhance the secretion of GH. Similarly, estradiol benzoate injection caused decreased plasma LH, but enhanced PRL and GH secretory activity. In addition, a nocturnal increase in plasma levels of PRL and GH was observed to occur between 1800 and 2200 hours. The causative factor(s) for the nocturnal PRL and GH secretory activity are unknown but may be related to the change in day-light to darkness and/or perhaps to sleep. These results confirm the inhibitory influence of progesterone and estradiol on plasma LH in the ovariectomized ewe. In addition, the results demonstrate that plasma levels of PRL fluctuate widely in ovariectomized ewes, and that plasma levels of GH occasionally exhibit secretory spikes during the day.