Growth hormone, but not luteinizing hormone, acts with luteal peptides on prostaglandin F2α and progesterone secretion by bovine corpora lutea in vitro

Prostaglandin F2α (PGF2α) is a major physiological luteolysin in the cow. However, injection of PGF2α before day 5 (day 0 = estrus) of the estrous cycle dose not induce luteolysis. On the other hand, the early corpus luteum (CL) actively produces PGF2α. This indicates that luteal PGF2α may play a key role in the refractoriness to PGF2α injected during the early luteal phase when angiogenesis is active in the CL. Thus, this study aimed to investigate the possible interaction between pituitary hormones and local factors (luteal peptides) on secretion of PGF2α and progesterone (P) by the early bovine CL, and to evaluate the effect of growth hormone (GH) as well as its interactions on production of PGF2α in the developing CL. A RT-PCR analysis revealed that mRNA for GH receptor in CL was fully expressed from early in the luteal phase throughout the estrous cycle, while luteinizing hormone (LH) receptor mRNA was expressed less by the early and regressing CL than those at mid or late luteal phases (P < 0.05). For the stimulation test, an in vitro microdialysis system (MDS) was used as a model. Each bovine early CL (days 3-4) was implanted with the MDS, and maintained in an organ culture chamber. The infusion of GH, insulin-like growth factor-1 (IGF-1) and oxytocin (OT) increased (P < 0.05) PGF2α and P release. In contrast, LH had no effect (P > 0.05) on PGF2α secretion and little effect on P release. Unexpectedly, there was no distinct interaction between pituitary hormones and luteal peptides on secretion of PGF2α and P. These results indicate that GH is a more powerful stimulator of PGF2α and P production in the early bovine CL than LH and suggest that GH and luteal peptides, IGF-1 and OT, contribute to maintenance of elevated PGF2α production in the developing bovine CL.