Progesterone Secretion by the Sheep Corpus Luteum After Repeated Infusions of Luteinizing Hormone and Human Chorionic Gonadotrophin

Abstract

SUMMARY The response of the ovine corpus luteum to repeated infusions of luteinizing hormone (LH) or of human chorionic gonadotrophin (HCG) was tested in four ewes with the left ovary autotransplanted to the neck. Constant infusion for 1 h of either LH (100 or 1000 μg/h) or HCG (200 i.u./h) via the ovarian artery stimulated a temporary increase in secretion of progesterone which fell to control levels by 60 min. Ovarian blood flow increased progressively (P < 0·05) throughout the infusion of gonadotrophin in three of the five experiments. A second infusion of either gonadotrophin after a further control hour failed to stimulate progesterone secretion. These results suggest that ovine luteal tissue rapidly becomes refractory to the steroidogenic effect of LH in vivo.

Effect of continuous infusion of gonadotrophin-releasing hormone in ewes at different times of the year

“…Although endoscopy revealed that 10/11 ewes had ovulated during the breeding season as a result of Gn-RH infusion, there was no evidence that the CL were functional (Lamond, Gaddy & Kennedy, 1972). The transient release of progesterone which occurred during each Gn-RH infusion has previously been described by Baird & Collett (1973) during infusion of a large dose of LH or HCG to a ewe with an auto-transplanted ovary containing a CL. According to Bedford, Harrison & Heap (1972), there is an increased metabolic clearance rate for progesterone in the anoestrus of lactation, when compared with the breeding season.…”

Superfusion in Vitro in the Study of Ovarian Steroidogenesis

“…It has been suggested that the ovary becomes refractory to the steroidogenic effect of LH in vivo. Baird & Collett (1973) observed that progesterone secretion from the autotransplanted ovary of the ewe could be stimulated by LH to give a temporary increased secretion, but a second infusion of gonadotrophin gave no response. The rat adrenal, on the other hand, does not become refractory to the action of ACTH under superfusion conditions in vitro (Saffran et al 1971 ;Schulster, 1973) and indeed it is possible to stimulate corticosteroid secretion repeatedly by infusions of ACTH.…”

“…Depletion of cholesterol has been suggested as a possible reason for the refractory behaviour of the ovary in vivo (Armstrong, 1968), and the results of Baird & Collett (1973) indicate that this occurs within 1-2 h. During superfusion, however, the principal response does not occur until almost 2 h after gonadotrophin administra¬ tion, and further stimulation can be achieved after 6 h. These differences between the autotransplanted ovary and the superfused ovary are difficult to explain, but it does seem unlikely that cholesterol depletion will occur within such a short time, and it would also be of interest to know how long the autotransplanted ovary required to recover from its refractory state.…”