Abstract
Female mammalian reproductive functions are closely linked to body condition and metabolic status. Energy homeostasis is regulated by endocrine hormones such as insulin, IGF-I, leptin, and adiponectin via the hypothalamic-pituitary-adrenal axis. These metabolic hormones and their receptors are also expressed in reproductive tissues and the embryo. We investigated the relationship between circulating leptin and the fatty acid (FA) and amino acid (AA) composition of the equine uterine fluid (UF) and peripheral blood plasma (BP) by using a mass spectrometry-based approach. UF and BP were collected from ten broodmares on days 6 and 7 post ovulation, respectively. The mares were retrospectively assigned to two groups according to their BP leptin concentrations (high leptin [> 1.6 ng/mL] versus low leptin [<0.8 ng/mL]). Specific AA and FA compositions for BP and UF were found with different levels of respective metabolite abundances. The main FAs in BP were stearic, palmitic and linoleic acid. In UF, the three most abundant FAs were eicosapentaenoic, arachidonic and stearic acid. The AA profile of BP was dominated by glycine, glutamine, serine and alanine, which were likewise among the highly abundant AAs in UF. In UF, glutamic acid had by far the highest concentration. Therefore, BP leptin concentration within a physiological range does not seem to affect the specific FA nor the AA composition of the UF. The composition of the UF may therefore be mediated by local rather than by peripheral metabolic hormones.
Highlights
Leptin and its receptors are similar in structure to the interleukin 6 family of cytokines [34] and leptin has been shown to act as a pro-inflammatory mediator [35]
And in contrast to other studies [13], in our study, neither Body Condition Score (BCS) nor Fat Thickness (FT) significantly positively correlated with plasma leptin concentrations
This might be attributed to the fact that the BCS is not suitable to differentiate accurately between lean body mass and fat mass [59]
Summary
Leptin and its receptors are similar in structure to the interleukin 6 family of cytokines [34] and leptin has been shown to act as a pro-inflammatory mediator [35]. Leptin increases vascular permeability and stimulates angiogenesis in a paracrine and/or autocrine fashion [45]. Enhanced vascular permeability can potentially increase transudation of plasma components such as FAs into the uterine lumen. Leptin could directly impact on the secretory activity of the endometrium via leptin receptors as the latter is expressed in the endometrial glandular epithelium in humans, swine and dogs [19,21,22]
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