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The Effects of the Nitric Oxide (NO) System and Nutritional Plane on Ovarian Function in Sheep
Numerous factors influence reproduction including hormones, growth and angiogenic factors, nutritional plane, and supplements such as arginine (Arg), a semi-essential amino acid and precursor for proteins, polyamines, and nitric oxide (NO). Nitric oxide is a reactive gas molecule made naturally in the body and is critical for angiogenesis, which is the development of blood vessels, generally capillaries, from pre-existing blood vessels to allow for blood flow to specific tissues. During a female’s reproductive cycle, certain reproductive tissues undergo rapid changes including vascular development and regression, and tissue growth and regression. One of those tissues, the corpus luteum (CL), is critical for pregnancy as the tissue produces progesterone (P4), a pregnancy maintenance hormone. Therefore, the objective of this research was to investigate how manipulation of the NO system via Arg supplementation to nutritionally compromised ewes impacts ovarian function and endocrine activity in non-pregnant sheep.
Feeding non-pregnant sheep to achieve different planes of nutrition (i.e. overfeeding or underfeeding) for a prolonged period of time resulted in different body conditions, ovulation rates in both non-stimulated and FSH-treated ewes, angiogenic factor expression in luteal tissues, serum P4 concentrations, and in vitro luteal cells and oocyte functions when compared to control ewes.
Our current study was performed using non-pregnant animals, and therefore it provides novel information regarding the effects of Arg supplementation on ovarian function in animals fed different nutritional plane. In vivo Arg supplementation to non-pregnant ewes resulted in increased luteal vascularity and selected angiogenic factor expression in luteal tissue, but did not affect ovulation rate, CL weight, and P4 production. Additionally, Arg supplementation was associated with increased vascularization (as noted by increased endothelial cell marker CD31), but did not lead to changes in production of P4.
Supplementation of Arg during in vitro maturation and fertilization resulted in increased lipid droplet accumulation in oocytes and decreased cleavage rates after in vitro fertilization, respectively. Future research explaining how the oocyte, as well as reproductive tissues, utilize(s) Arg may assist in elucidating why Arg supplementation to pregnant mothers is beneficial, while supplementation to non-pregnant females has limited effects.
In summary, these studies contribute to the knowledge of nutritional plane and the NO system and how they influence reproduction in female livestock. The results obtained provide beneficial knowledge for future research regarding the improvement of reproductive efficiency in females, which could additionally assist producers considering therapeutic supplements for reproductively-sound livestock.
Keywords: arginine, nitric oxide, sheep