The proteolysis of insulin-like growth factor binding proteins in ovine uterine luminal fluid

During days 12–15 after oestrus (day 0), the uterine luminal fluid (ULF) of both pregnant and non-pregnant ewes contains only two prominent insulin-like growth factor binding proteins (IGFBPs) of 16–18 kDa and 22–24 kDa which preferentially bind IGF-2. Immunoblotting with an IGFBP-3 antibody revealed these to be proteolytic fragments of IGFBP-3. In contrast, the ULF from anoestrus and ovariec-tomized ewes contained intact IGFBP-3 (40–44 kDa) and IGFBP-2 (34 kDa). Co-incubation of ULF from an anoestrus ewe with that from a day 12 cycling ewe cleaved the IGFBP-3 present into the two lower molec-ular weight IGFBPs characteristic of ewes in the late luteal phase of the oestrous cycle. The variation in pro-teolytic activity both during the year and during the cycle suggested an influence of progesterone. Supplementation of progesterone to long-term ovariectomized ewes via a CIDR-G™ breeding device for 5, 10 or 15 days induced marked proteolytic activity in all 10-day treated sheep. The ULF from the 15-day treated ewes showed reduced activity and could inhibit the activity present in 10-day ULF, suggesting the induction of an inhibitor after prolonged exposure to progesterone treatment. A possible role of IGFBP-3 proteolysis in the ovine ULF may be to selectively increase the bioavailability of IGF-1 in the uterine microenvironment, which may be crucial for the rapid elongation of trophoblast that begins during days 12–15 after mating.