51 Effects of modulating early luteal phase progestin concentration on endometrial function in early pregnant mares

Progesterone prepares the endometrium for pregnancy. This requires down-regulation of progesterone receptors in the endometrial epithelium as a prerequisite for the expression of pregnancy-associated proteins. We investigated effects of modulated peripheral progestin concentration in early luteal phase mares on endometrial function on Day 14 of pregnancy. Genitally healthy oestrous mares (n=8; age 4 to 14 years) were inseminated until ovulation and treated with either altrenogest (0.044mg kg−1 once daily orally) on Days 5 to 10 after ovulation (ALT), cloprostenol (125mg once daily intramuscularly) on Days 0 to 3 after ovulation (CLO) or left untreated (CON). The ALT and CLO treatments were chosen to increase and decrease total peripheral progestin concentration, respectively. Each treatment was given to every mare in consecutive cycles at random order. On Day 14 after ovulation, endometrial fluid was collected with a cotton roll (Salivette, Sarstedt, Germany) inserted into the uterus and an endometrial biopsy for immunohistological examination was collected. In endometrial fluid, free amino acid concentrations were analysed by ion exchange liquid chromatography with an amino acid analyser (Institut Kuhlmann, Analytik-Zentrum Ludwigshafen, Germany). Cell nuclei staining positive for the progesterone receptor were determined in the luminar and glandular epithelium as well as in the stroma. Statistical analysis was performed by non-parametric Friedman test with subsequent Wilcoxon test. Values are given as mean±standard error of mean. Pregnancy rate was 0.6±0.1 (13 cycles/8 pregnancies), 1.0±0 (8 cycles/8 pregnancies), and 0.7±0.1 (11 cycles/8 pregnancies) in CON, ALT, and CLO cycles, respectively (P=0.062). Conceptus size between Days 10 and 14 did not differ among treatments. The percentage of luminal epithelial cells staining positive for progesterone receptor differed among treatments (CON 72.8±4.1, ALT 70.7±4.7, and CLO 84.1±1.9%; P<0.05) and was higher in CLO than in ALT and CON cycles (P<0.05). Free amino acids glutamic acid and glycine were most abundant in endometrial fluid, but their concentrations did not differ among treatments. The concentrations of the amino acids isoleucine (CON 0.17±0.03, CLO 0.14±0.02, and ALT 0.23±0.04 µmol) and lysine (CON 0.27±0.08, CLO 0.18±0.05, and ALT 0.44±0.13 µmol) were influenced by treatment (P<0.05) and lower in CLO than in ALT and CON cycles. In conclusion, impaired luteal function due to CLO treatment during the early luteal phase of pregnant mares delayed down-regulation of progesterone receptors in the endometrial epithelium on Day 14. This influenced endometrial function as reflected in lower concentrations of the amino acids lysine and isoleucine in endometrial secretions.