Relationship of circulating steroid hormones, luteal luteinizing hormone receptor and progesterone concentration, and embryonic mortality during early embryogenesis in the domestic cat

Serum hormones, corpus luteum (CL) progesterone, and CL LH receptors were characterized and interrelated to ovarian follicle and CL number and preimplantation embryo quality/survival in the cat. Blood samples were collected from queens ovarichysterectomized at 64 (n = 7), 76 (n = 11), 100 (n = 8), 124 (n = 7), 148 (n = 6), or 480 (n = 8) h after first copulation (3-times-daily matings on Days 2 and 3 of estrus). Ovarian CL were enucleated, weighed, and bisected; one hemi-CL was assayed for progesterone and the other for LH receptors. Serum was assessed for estradiol-17 beta and progesterone. Serum estradiol-17 beta concentrations did not return (p > 0.05) to baseline (approximately 20 pg/ml) until 124 h after first copulation, whereas serum progesterone began to increase (> 1 ng/ml) by 76 h, was elevated (p < 0.05) by 124 h, and continued to rise thereafter. Serum progesterone was highly correlated with CL mass and LH receptor and progesterone concentration (range, r = 0.69-0.82; p < 0.01). CL progesterone and LH receptor concentrations were similar (p > 0.05) at 64 and 76 h, and both increased (p < 0.05) at subsequent time intervals and were correlated closely (r = 0.65; p < 0.01). Although ovarian CL were distinct and well organized by 64 h, pronounced elevations in serum progesterone and CL LH receptors and progesterone did not occur until at least 36 h later. The rapid increase in serum progesterone concentrations between 100 and 148 h was related to accelerated LH receptor synthesis and CL progesterone production and not entirely to enhanced CL growth. There were few discernible differences in hormonal and luteal traits between queens with viable embryos or high implantation rates and females with degenerate embryos, unfertilized oocytes, or poor implantation, with one consistent exception. Queens with poor fertility in the 64-, 76-, and 100-h groups had higher (p < 0.05) CL progesterone concentrations than cats with viable embryos, suggesting that altered follicular dynamics (perhaps premature luteinization) adversely impacted oocyte/embryo quality. In summary, there is a direct and significant relationship between circulating progesterone, CL mass, and CL progesterone/LH receptors during preimplantation embryogenesis in the domestic cat. The temporal kinetics of these events are remarkably similar among mated females and, with the possible exception of CL progesterone, appear unrelated to embryonic viability.