Local Release of Progesterone and Oxytocin from Microdialyzed Corpus Luteum in Superovulated Ewes: Characterization during the Non-breeding Season.

Abstract

To characterize the local release of progesterone (P) and oxytocin (OT) within the corpus luteum (CL) formed after superovulation during the non-breeding season in ewes, a microdialysis system (MDS) was implanted in corpora lutea (CcL). Superovulation was induced with a combination of PMSG, FSH and GnRH injections in ewes (n=5) that had been pretreated with P using an intravaginal sponge. On Day 5 after GnRH injection, the microdialysis capillary membrane (cut off Mr=1000 kDa) was surgically implanted into the individual CL of both ovaries, and continuously perfused with a Ringer's solution up to Day 17. Serial fractions were collected every 30 min on Days 6-7, Days 9-10, Days 12-13 and Day 15, to observe in detail the release patterns of P and OT at different ages of the CL. Both P and OT were released into the MDS in a pulsatile manner throughout the experiment. The frequency of P pulses changed by the age of the CL (P<0.05); it decreased temporally on Days 12-13, but gradually increased on Day 15. The percentages of concomitant P release (coincident rate of pulses) decreased on Days 12-13, but extremely increased to 52.3% on Day 15 (P<0.01). On the other hand, the profiles of OT pulses were constant throughout the experiment. The absolute P concentrations gradually decreased as the CL age progressed, while the absolute OT concentrations remained constant. A high concomitant rate of OT pulses with a P pulse within individual lines of the MDS was observed on Days 6-7 when plasma P concentrations were still rapidly increasing, but such a close relationship between OT pulses and a P pulse disappeared thereafter (P<0.05). Furthermore, a 3 h-infusion with LH on Day 10 stimulated P release during the infusion, whereas a PGF2α infusion induced an inhibition of P release after the infusion (P<0.05). These observations characterized the profiles of a local release of P and OT within an individual CL formed after superovulation during the non-breeding season. The characteristics of P release, but not those of OT release, change with the CL age. Collectively, the data appear to be well comparable to those in the cyclic CL during the breeding season. Thus, a local secretory function of the CL formed after superovulation during the non-breeding season is likely to have a close similarity to those of the cyclic CL.