| Abstrakt: |
To determine the threshold of prostaglandin F2α(PGF2α)-stimulated oxytocin secretion from the ovine corpus luteum, low levels of PGF2α(5–100 pg/min) were infused into the ovarian arterial blood supply of sheep with ovarian autotransplants. PGF2αwas infused for six sequential 10-min periods at hourly intervals, 6, 12, or 24 days after estrus (n = 3 for each day). Each cycle day was studied during a separate cycle. Oxytocin and progesterone in ovarian venous and carotid arterial plasma was measured by radioimmunoassay, and secretion rates were determined (venous-arterial concentration × plasma flow). In animals treated on Day 6, 5 pg/min PGF2αcaused a significant release of oxytocin (p<0.01), whereas in animals treated on Day 12, this threshold was 40 pg/min (p<0.05). In animals treated on Day 24, the threshold for oxytocin release was >100 pg/min. PGF2αdid not significantly change ovarian blood flow or progesterone secretion rate on any day (p>0.05). To determine residual luteal oxytocin after each threshold experiment, 5 mg PGF2αwas given i.m. to all animals. Significantly more oxytocin was released by Day 6 than by Day 12 and Day 24 corpora lutea, and by Day 12 than by Day 24 corpora lutea (1.2 μg, 0.7 μg, and 0.3 μg, respectively; p<0.01). We conclude that (1) very low arterial levels of PGF2αcompatible with levels secreted by the late lureal phase uterus, can initiate luteal oxytocin release; (2) the corpus luteum has a d4fferential sensitivity to PGF2αfor increased o.rytocin and decreased progesterone secretion; (3) the decline in the releasable pool of oxytocin with advancing age of the corpus luteum is consistent with the proposal that luteal oxytocin is not replenished once discharged; and (4) thus, PGF2αmay be the physiologic stimulus that triggers the release of luteal oxytocin in this species. |
