Direct stimulation of bovine ovarian progesterone secretion by low concentrations of alpha-interferon

Demonstration of luteotrophic responses of human recombinant gamma interferon in porcine corpora lutea using an in-vivo microdialysis system

Conceptuses from several mammalian species prior to implantation secrete proteins belonging to the family of interferons. The main species of interferons known to be secreted by the pig blastocyst is interferon gamma (IFNgamma), the precise role of which is unclear. We decided to explore its effects on corpus luteum (CL) function using the novel microdialysis technique in vivo. Six cycling miniature pigs were monitored for estrus by daily plasma progesterone analysis and visual symptoms. On day nine of the cycle (day zero being the day of ovulation) the animals underwent surgery, and microdialysis tubing (vitafiber, Amicon U.S.A, cut off mol. wt. 1 million) were implanted in 17 corpora lutea. The inlets and outlets of all tubings were exteriorized and the entry and exit points of tubings in the CLs sealed with tissue glue. The afferent extension tubings were connected to a fraction collector and the system was continuously flushed with Ringer at a flow rate of 2.4 ml/h. After an initial flushout phase of 8 h, fractions were collected every half hour over 3 days. On days 10, 11 and 12 post estrus 12 CLs were stimulated for 4 h with 10(-7) M, 2 x 10(7) M and 4 x 10(-7) M human recombinant IFNgamma (Pharma Biotechnologie) respectively. Simultaneously, fractions were also collected from the remaining five unstimulated corpora lutea which served as controls. Progesterone concentrations in the dialysates were estimated by a sensitive enzymeimmunoassay (EIA). A significant increase (P < 0.01) in progesterone release was observed in all 3 days following stimulation. The progesterone increase was more marked on the first day of stimulation (1 x 10[-7] M) with the hormone levels rising further even after the end of stimulation. The overall increase in progesterone concentration was 2-fold on day 10 in comparison to 15-30% on subsequent days even though IFN concentrations for stimulation were 2- and 4-fold higher. In the unstimulated CLs, a gradual decline (P < 0.01) in progesterone levels were observed over days. In conclusion, these data provide evidence that the early conceptus signals its presence by way of IFNgamma to maintain the CL in pigs.

Intercellular communication in the bovine corpus luteum

There is a growing body of evidence that intercellular communication is important in the regulation of luteal function. Although the nature of the interactions between small and large luteal cells are not yet clear, it seems likely that they do exist. Many of the substances to which luteal cells respond, such as prostaglandins, growth factors, oxytocin and progesterone, are produced locally. These substances may act as paracrine factors to modulate the response of luteal cells to hormonal signals. Endothelial cells also produce factors that can modify steroidogenesis, and luteal cell-stimulation of endothelial cell proliferation is necessary for the extensive angiogenesis that occurs during luteinization Finally, bidirectional intercellular communication likely occurs between luteal cells and resident immune cells. Immune cells produce cytokines that can modify progesterone and prostaglandin synthesis by luteal cells. Cytokines may also have direct cytotoxic effects on luteal cells, and dead cells are then phagocytized by resident macrophages. Also, factors secreted by luteal cells can serve as chemoattractants for immune cells, and can enhance or suppress immune cell functions. There is little doubt that intercellular communication within the corpus luteum is very complex. One must remember, however, that nearly all evidence collected thus far is based on in vitro studies. Eventually, technology will allow for study of these interactions in vivo, and may lead to new methods for control of luteal function.