Effect of the Nutrient Microenvironment of an Ovarian Follicle on Gap Junction Activity in Cumulus-Oocyte Complexes in Post-Partum Cows

For mammalian oocyte maturation it is important for the oocyte to communicate with the surrounding somatic cells in a bidirectional manner. In part, this is achieved through gap junction connections between cumulus cells and the oocyte. These gap junctions permit the transfer of essential metabolites, nucleotides, amino acids and ions from the cumulus cells to the oocyte in order for growth and development to occur. It is hypothesized that one reason for post-partum infertility in dairy cows is poor oocyte quality due to an inappropriate nutrient microenvironment in the developing antral follicle. In turn, this might compromise the nutrient and/or energy delivery by cumulus cells to the oocyte and thus affect oocyte quality. The objectives of this study were to: (1) -develop a reliable in vitro measure of gap junction activity for bovine cumulus–oocyte-complexes (COC) using a fluorescent (calcein) dye technique, and (2)- assess the effects of different amino acid compositions, cholesterol and fatty acid concentrations on gap junction activity in COC. The development and validation of the bovine COC gap junction assay was undertaken using a standard commercial medium (M199). Thereafter, media were prepared representing the amino acid concentrations in follicular fluid of both non-lactating and lactating dairy cows as measured in a NZ pastoral situation. These media formed the basis for subsequent measures of COC gap junction activity with differing concentrations of cholesterol and the essential fatty acids- oleic acid, stearic acid and palmitic acid. The major findings from this study were that the different amino acid concentrations typical of that in the follicular fluid of the non-lactating and lactating dairy cow and M199 do not significantly affect the gap junction activity in COC. Given that the amino acid compositions in lactating and non-lactating cows were approximately 50% of those in M199; this indicates that the amino acid concentrations within the ovarian follicle are not a critical factor affecting oocyte quality. However, high concentrations of cholesterol or of the essential fatty acids significantly compromised gap junction activity in bovine COC. High concentrations of the fatty acids were identified in a number of lactating dairy cows during the post-partum interval. In times of negative energy balance, which is common in pasture fed NZ dairy cows during the early post-partum interval and lactation, a significant mobilisation of fatty acids occurs. In turn, this is known to result in an increase in lipid concentrations within the ovarian follicle. The results from this study support the hypothesis that the nutrient environment of the ovarian follicle during early lactation is detrimental to the quality and developmental competence of the oocyte and thus may account for post-partum infertility in some animals.