INTERRELATIONSHIP BETWEEN PLASMA ESTRADIOL CONCENTRATION AND OXYTOCIN-INDUCED PGF(2-ALPHA) RELEASE IN HEIFERS

The objective of this study was to determine if the increase in respon siveness to oxytocin toward the time of luteolysis was correlated with an increase in plasma estradiol in the cow. Six heifers each had a ca nula placed in the jugular vein on Day 14 of the estrous cycle. Then, beginning on Day 15, growth of the largest follicles was determined by ultrasonography, and a blood sample was taken via the canula for the measurement of progesterone and estradiol by radioimmunoassay (RIA). A fter the first blood sample, 3 more samples were taken at 10-min inter vals, 100 IU oxytocin were injected into the vein, and a further 3 blo od samples were taken at 15, 30 and 60 min after injection. The concen tration of 13,14-dihydro-15-keto prostaglandin F-2 alpha (POEM) was me asured in these frequent samplings and was used to determine the abili ty of oxytocin to stimulate PGF(2 alpha) release from the uterus. This procedure was repeated daily for at least 7 d. The results showed tha t the response to oxytocin increased before luteolysis and that there was a significant increase in the response to oxytocin (P<0.05) before any changes in plasma estradiol or progesterone were detected. These data show that an increase in estradiol secretion from the ovulatory f ollicle does not appear to initiate luteolysis.