Effect of a prostaglandin F2α analogue on the cyclic corpus luteum during its refractory period in cows

All cows that responded to PGF given on day 5 ovulated on day 9. In the cows that did not respond on day 5, the luteal cross-sectional area stagnated after treatment, whereas the plasma progesterone concentration continued to increase. In the cows that received PGF on day 3 of the oestrous cycle, the luteal cross-sectional area and the plasma progesterone concentration continued to increase.The results of this study suggest that the transition of the bovine corpus luteum from refractoriness to responsiveness to PGF occurs during day 5 of the oestrous cycle because in 5 of 8 cows given PGF on day 5, early luteal regression was evident during the examination 24 hours after PGF administration.Prostaglandin F2α (PGF2α) and prostaglandin analogues (PGF) are luteolytic in cattle and other domestic species and usually induce oestrus when given during the luteal phase of the oestrous cycle. The interval between administration of the hormone and onset of oestrus ranges from 2 to 6 days [1]. Other than the dose and route of administration [2], follicular dynamics at the time of prostaglandin administration have a significant effect on this interval [1,3].When prostaglandin is given in the first few days after ovulation, it does not induce luteolysis. The reason for this is that the cyclic bovine corpus luteum is refractory to prostaglandin for 5 to 7 days postovulation [4]. A single instillation of PGF2α into the uterine horn ipsilateral to the corpus luteum 1 to 4 days post ovulation did not result in luteolysis, whereas instillation of PGF2α on day 5 induced oestrus within 3 days [5]. When cows with a corpus luteum measuring at least 17 mm in diameter and a blood progesterone concentration of at least 0.8 ng/ml were given PGF2α, there was a continuous and significant decrease in the luteal cross-sectional area on ultrasonograms and the blood progesterone concentration within 24 hours [6]. In another study, the volume of the corpus luteum and the blood progesterone concentra