Autonomous progesterone secretion from the bovine corpus luteum in vitro

Abstract

Abstract. While the regulation of progesterone secretion from the corpus luteum by LH has been convincingly demonstrated, the secretory patterns in the absence of any pituitary LH inputs are yet unclear. Consequently, we investigated the progesterone secretion by an in vitro perifusion system to characterize spontaneous progesterone release from the isolated bovine corpus luteum. Slices (120 mg) of midluteal corpora lutea were placed in perifusion chambers and continuously perifused by Medium-199 for 160-320 min. Progesterone was determined by radioimmunoassay in the effluent fractions collected at 2-min intervals. The spontaneous progesterone release from all bovine corpora lutea was pulsatile. Pulses were observed at mean (±sem) intervals of 17.7±1.5 min with amplitudes of 6.7±0.5 ng and release rates of 29.5±2.4 ng · ml−1 · (2 min)−1 (N=5). Addition of 6.7 nmol/l hCG to the perifusion medium appeared to increase the pulse amplitudes and release rates (195±25% over unstimulated conditions), but did not change the pulse frequencies (N=3). Perifusions with calcium-free medium containing 50 μmol/l verapamil and 20 mmol/l EGTA tended to suppress the pulse frequencies and amplitudes of this spontaneous progesterone release, whereas addition of hCG reversed this decrease again (N=3). When prostaglandin activity was inhibited by 100 μmol/l indomethacin added to the perifusion medium, both pulse frequencies and amplitudes of this progesterone release were enhanced (N=4). During perifusions with 100 μmol/l of the anti-progesterone ZK 96.734, both the pulse frequencies and amplitudes increased (N=4). These studies demonstrate an episodic progesterone release from the bovine corpus luteum perifused in vitro. The spontaneous progesterone secretion is independent of, but facilitated by gonadotropin secretion. This pulsatile progesterone secretion from the midluteal corpus luteum and its regulation by intraovarian factors may reflect an autonomous pulse generating mechanism.