Delayed effect of low progesterone concentrations on bovine uterine PGF(2 alpha) secretion in the subsequent oestrous cycle

Low progesterone concentrations during the bovine oestrous cycle induce enh anced responsiveness to oxytocin challenge late in the luteal phase of the same cycle. The delayed effect of low progesterone concentrations during on e oestrous cycle on uterine PGF(2 alpha), secretion after oxytocin challeng e on day 15 or 16 of the subsequent cycle was studied by measuring the conc entrations of the major PGF(2 alpha), metabolite (13,14-dihydro-15-keto PGF (2 alpha); PGFM) in plasma. Two experiments were conducted, differing in th e type of progesterone treatment and in the shape of the low progesterone c oncentration curves. In Expt 1, progesterone supplementation with intravagi nal progesterone inserts, with or without an active corpus luteum, was used to obtain high, or low and constant plasma progesterone concentrations, re spectively. In Expt 2, untreated cows, representing high progesterone treat ment, were compared with cows that had low but increasing plasma progestero ne concentrations that were achieved by manipulating endogenous progesteron e secretion of the corpus luteum. Neither experiment revealed any differenc es in plasma progesterone concentrations between the high and low progester one groups in the subsequent oestrous cycle. in both experiments, both grou ps had similar basal concentrations of PGFM on day 15 (Expt 1) or 16 (Expt 2) of the subsequent oestrous cycle, 18 days after progesterone treatments had ended. In both experiments, the increases in PGFM concentrations in the low progesterone groups after an oxytocin challenge were markedly higher t han in the high progesterone groups. These results indicate that low proges terone concentrations during an oestrous cycle have a delayed stimulatory e ffect on uterine responsiveness to oxytocin during the late luteal phase of the subsequent cycle. This resulting increase in PGF(2 alpha) secretion ma y interfere with luteal maintenance during the early stages of pregnancy.