Uterine protein secretion in postpartum and cyclic mares

Abstract

Proteins secreted by cultured endometria and present in uterine fluids of mares after parturition were compared to uterine secretory products of mares during the estrous cycle. Uterine fluids and endometrial biopsies were obtained from mares at Day 4 (n = 5) and Day 8 (n = 5) after parturition, from cyclic mares at Day 2 of estrus (uterine fluids, n = 4; endometrial biopsies, n = 3) and at Day 13 after ovulation (uterine fluids, n = 6; endometrial biopsies, n = 4). Endometrial explants were cultured and secretion of de novo synthesized macromolecules were estimated. To determine the type of polypeptides secreted, media from representative cultures (n = 11) were analysed by two-dimensional SDS-polyacrylamide gel electrophoresis. The pattern of proteins produced by the endometrium at Day 4 after parturition was similar to the pattern produced by the endometrium during estrus. Total protein secretion in vitro, however, was greater after parturition than during estrus (P < 0.075). The major proteins present at all times were a group (mol wt ∼ 64,000) slightly more acidic than albumin. A basic protein (mol wt ∼ 30,000) was present in the medium of cultures from diestrous mares. This protein was barely detectable in the culture medium from other days. Total protein, IgG, IgA concentration and acid phosphatase were measured in uterine fluids. Except for IgA, concentrations of these were significantly greater after parturition than during the estrous cycle but concentrations decreased significantly between Day 4 and Day 8 after parturition. The high concentration of acid phosphatase activity in postpartum uterine fluids was associated with uteroferrin. Our findings suggest that alteration in uterine environment rather than endometrial protein secretion during the postpartum period is responsible for the lower fertility observed early after foaling.