CONCENTRATIONS OF OXYTOCIN IN THE INTERCAVERNOUS SINUS OF MARES DURING LUTEOLYSIS - TEMPORAL RELATIONSHIP WITH CONCENTRATIONS OF 13,14-DIHYDRO-15-KETO-PROSTAGLANDIN F2-ALPHA

The reproductive tracts of nine thoroughbred mares were examined by ul trasound to determine the day of ovulation (day 0). Mares were fitted with intercavernous sinus cannulae on the day before the start of samp le collection of pituitary venous effluent rich in oxytocin. Intercave rnous sinus blood samples were collected for at least 36 h at 5 min in tervals beginning at noon on day 13 (n = 2), day 15 (n = 5) or day 16 (n = 3) after ovulation. Concentrations of oxytocin and 13,14-dihydro- 15-keto prostaglandin F-2 alpha (PGFM) in plasma were determined by ra dioimmunoassay. Three high-magnitude surges of PGFM (> 1 ng ml(-1)) we re found in these samples. Three high magnitude pulses of oxytocin (> 200 pg ml(-1)) were also observed, one associated with each of the PGF M surges. In each of these cases, the oxytocin pulse appeared to follo w or coincide with the onset of the PGFM surge. Lower magnitude pulses of both hormones were detected throughout the bleeding period in ever y mars. The average interval between these pulses was 122.3 min for ox ytocin and 121.0 min for PGFM. The interval between pulses for individ ual mares varied from 90 to 199 min for oxytocin, and from 87 to 213 m in for PGFM. However, there was no correlation between PGFM and oxytoc in pulse intervals among mares. Within each mare, there was no discern able association between low magnitude pulses of oxytocin and PGFM. Fr om these data, it was concluded that high-magnitude surges of PGF(2 al pha) are associated with similar surges of oxytocin from the posterior pituitary gland, and that PGF(2 alpha) may induce their secretion. Th e posterior pituitary gland also appears to secrete oxytocin in a puls atile manner at a frequency of approximately I pulse every 2 h but the se pulses do not appear to be associated with the low magnitude pulses of PGF(2 alpha) secreted from the uterus.