Adiponectin in equine reproduction
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Adiponectin, a protein secreted predominately by white adipose tissue, has been a focus of recent research due to its effects on metabolism and reproduction. The following dissertation describes research performed to profile serum adiponectin in horses and to identify and localize adiponectin and its two receptors, AdipoR1 and AdipoR2, within the reproductive tracts of mares and stallions. In the first study, adiponectin, leptin, estradiol, progesterone, and body fat percentage profiles were obtained in broodmares which foaled and became pregnant on the first postpartum estrus. Mares were grouped by lactation status. Adiponectin was not different based on lactation, but was elevated during late gestation and the early postpartum period, which contrasts previous research in other species. Mechanisms behind this increase are unexplored. In the second study, adiponectin and its receptors were localized to the luminal and glandular epithelia of the endometrium of mares 8 and 16 days pregnant and open. Gene expression was increased for adiponectin and both receptors at 16 days of pregnancy compared to 16 days open, suggesting a role for adiponectin in early gestation and embryonic fixation. Serum profiles of mares showed no changes in adiponectin at 16 days open, or 8 and 16 days pregnant. In the third study, adiponectin and its receptors were localized to the Leydig cells and seminiferous tubules of the testis in both prepubertal and adult stallions. Gene expression was increased for both receptors in the testis of adults, whereas adiponectin expression was higher in colts. These findings suggest a role for adiponectin in testis function including steroidogenesis and spermatogenesis. Serum concentrations of adiponectin did not increase in response to challenge with hCG in either age group. Collectively, this research shows that adiponectin and its receptors are present in the male and female reproductive tract and are differentially expressed based on reproductive status. Further studies to determine functionality of this ligand and its receptors in equine reproduction are needed, and may serve to provide further information regarding anovulation and infertility in horses with equine metabolic syndrome, a disease characterized by insulin dysregulation and alterations in adiponectin expression.