Role of early luteal phase progesterone in control of the timing of the luteolytic signal in cows.

The effects of the pattern and concentration of early luteal phase progesterone on subsequent prostaglandin F2 alpha release in response to exogenous oxytocin was investigated during simulated luteal phases in ovariectomized cows treated with progesterone and oestradiol in patterns designed to simulate the range of luteal phase concentrations that occur naturally. In the first experiment, three groups of four cows received different concentrations of early luteal phase progesterone to determine the effective concentration in terms of cycle control. The results show that a plasma progesterone concentration early in the luteal phase as low as 0.6 ng ml-1 was sufficient to affect the timing of the subsequent luteolytic signal. In the second experiment, an early (day 1), a normal (day 4) or a late (day 7) postovulatory increase in progesterone was recreated in three groups of four cows. Responsiveness to oxytocin in the early progesterone group developed 3 days earlier than in the normal progesterone group, demonstrating the ability of early progesterone to advance the luteolytic signal. However, in the late progesterone group, there was no delay in the development of responsiveness to oxytocin compared with the normal progesterone group, demonstrating that the luteolytic signal is programmed to occur by a given time, irrespective of the early progesterone pattern. This demonstrates that a factor other than the timing of the early luteal phase progesterone increase ultimately must control the timing of luteolysis in cows.